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H,-+6
w|re|ess Access 8ystem
0ecember 2000
Copyr|g|l (C) 2000 V|das Corrur|cal|or Tec|ro|og|es Pr|vale L|r|led
No. 15, First Avenue, Shastri Nagar, Adyar, Chennai, India 600020
http://www.midascomm.com
No material from this publication may be used in full or in part without the prior written permission of
the copyright holder.
TalIe of Conlenls
6hapter 1 |ntroduct|on
6hapter 2 w|re|ess Access Network: 8ome Key |ssues
2.1 lrlroducl|or.......................................................................................................................................................... 2 - 1
2.2 Access NelWor| ................................................................................................................................................... 2 - 1
2.3 T|e lrlerrel Targ|e ............................................................................................................................................. 2 - 2
2.1 w|re|ess Loca| Loop vs. Voo||e w|re|ess Access 3ysler ...................................................................................... 2 - 1
2.5 Capac|ly ard 3peclra| Ell|c|ercy ........................................................................................................................... 2 - 1
2. 3urrary................................................................................................................... .......................................... 2 -
6hapter 3 cor0E6T w|re|ess Access 8ystem
3.1 lrlroducl|or.......................................................................................................................................................... 3 - 1
3.2 Corceplua| Access 3ysler .................................................................................................................................. 3 - 1
3.3 cor0ECT w|re|ess Access 3ysler ....................................................................................................................... 3 - 2
3.1 3uo-syslers ol cor0ECT w|re|ess Access 3ysler ............................................................................................... 3 - 3
3.5 cor0ECT Access Cerlre Furcl|ora||ly ard lrlerlaces ........................................................................ ................... 3 - 8
6hapter 4 cor0E6T 0ep|oyment Examp|es
1.1 lrlroducl|or.......................................................................................................................................................... 1 - 1
1.2 cor0ECT 0ep|oyrerl W|l| 0lu |r Exc|arge Prer|ses ........................................................................................ 1 - 1
1.3 0lu lrlegraled W|l| Access Cerlre ....................................................................................................................... 1 - 1
1.1 Rura| 0ep|oyrerl ............................................................................................................................................... 1 - 5
1.5 Frarc||se Access Prov|der .................................................................................................................................. 1 - Z
6hapter 5 cor0E6T Features at a C|ance
5.1 lrlroducl|or.......................................................................................................................................................... 5 - 1
5.2 vo|ce 0ua||ly ........................................................................................................................................................ 5 - 1
5.3 0ala 3erv|ces ...................................................................................................................................................... 5 - 1
5.1 lrlerrel Access 3peed ......................................................................................................................................... 5 - 1
5.5 Payp|ore/PC0................................................................................................................................................... 5 - 1
5. 3ysler Capac|ly ................................................................................................................................................. 5 - 1
5.Z A|r lrlerlace Trarsr|l PoWer ................................................................................................................................ 5 - 1
5.8 Typ|ca| C83 Coverage ...................................................................................................... .................................. 5 - 1
5.9 Typ|ca| R83 Coverage ...................................................................................................... .................................. 5 - 2
5.10 Aul|erl|cal|or ard 3uoscr|pl|or .......................................................................................................................... 5 - 2
5.11 Vajor 3uoscr|oer 3erv|ces ................................................................................................................................. 5 - 2
5.12 Vajor 3W|lc| Fealures ....................................................................................................................................... 5 - 3
5.13 0VC Fealures .................................................................................................................................................. 5 - 3
5.11 Vax|rur C83-0lu Copper 0|slarce ................................................................................................................ 5 - 1
5.15 Vax|rur C83-830 Copper 0|slarce ............................................................................................................... 5 - 1
5.1 0lu PoWer 3upp|y ............................................................................................................................................ 5 - 1
5.1Z wa||sel ard Vu|l|Wa||sel PoWer 3upp|y ............................................................................................................... 5 - 1
5.18 wa||sel ard Vu|l|Wa||sel Ta|| T|re/3lardoy T|re ................................................................................................ 5 - 1
5.19 R83 PoWer 3upp|y ........................................................................................................................................... 5 - 1
5.20 830 PoWer 3upp|y ........................................................................................................................................... 5 - 1
5.21 0l|er Fealures .................................................................................................................................................. 5 - 5
6hapter 6 8ystem 0|mens|on|ng |n cor0E6T
.1 lrlroducl|or.......................................................................................................................................................... - 1
.2 3ysler Capac|ly ........................................................................................................... ....................................... - 1
.3 Trall|c Capac|ly ol C83 ................................................................................................... ..................................... - 1
.1 Trall|c Capac|ly ol l|e 0lu ............................................................................................... .................................... - 2
.5 Trall|c 0|rers|or|rg lror RA3 lo lrlerrel ............................................................................................................. - 3
. Re-use Ell|c|ercy |r cor0ECT ............................................................................................................................. - 3
.Z Capac|ly ol Vu|l|-Ce||u|ar ToWer-Vourled C83 C|usler ........................................................................................ - 3
.8 loW l|g| Capac|ly |s Ac||eved ........................................................................................................................... - 1
.9 Capac|ly |r l|g|-R|se Velropo||lar 0ep|oyrerl ............................................................................. ...................... - 5
.10 Capac|ly ol R83 ................................................................................................................................................ -
.11 3urrary ........................................................................................................................................................... -
6hapter 7 A|r |nterface L|nk udgets and 6e|| P|ann|ng
Z.1 lrlroducl|or.......................................................................................................................................................... Z - 1
Z.2 cor0ECT Er|arcererls ...................................................................................................... ............................... Z - 1
Z.3 L|r| 8udgels W|l| cor0ECT ................................................................................................................................. Z - 3
Z.1 Pal| Loss Vode|s ................................................................................................................................................ Z - 3
Z.5 Fade Varg|rs |r cor0ECT .................................................................................................................................. Z - 3
Z. Cao|e Losses ...................................................................................................................................................... Z - 1
Z.Z L|r| Ara|ys|s lor cor0ECT ................................................................................................................................... Z - 1
Z.8 lrsla||al|or ard 3urvey Too|s ............................................................................................................................... Z - Z
Z.9 3urrary................................................................................................................... .......................................... Z - Z
6hapter 8 0perat|on and Ha|ntenance
8.1 lrlroducl|or.......................................................................................................................................................... 8 - 1
8.2 cor0ECT 0VC |r 3W|lc| Vode ........................................................................................................................... 8 - 1
8.3 cor0ECT 0VC |r RLu Vode .............................................................................................................................. 8 - 3
8.1 RA3 Varagererl ............................................................................................................................................... 8 - 1
8.5 corv|eW user lrlerlace ........................................................................................................................................ 8 - 1
8. NV3 lor Vu|l|p|e 0lu's ......................................................................................................................................... 8 - 5
8.Z 0ala Varagererl ........................................................................................................... .................................... 8 - Z
6hapter 9 Future Roadmap
9.1 lrlroducl|or.......................................................................................................................................................... 9 - 1
9.2 ToWards A|Ways-or lrlerrel Access .................................................................................................................... 9 - 1
9.3 Pac|el-3W|lc|ed l|g| 3peed lrlerrel 0oWr|oad|rg ........................................................................... .................. 9 - 1
9.1 Vore lrlegral|or lor Cosl-Ellecl|veress................................................................................................................. 9 - 2
9.5 NeW Vu|l|Wa||sel 0eve|oprerls ........................................................................................................................... 9 - 2
9. lrcreased 3ca|ao|||ly ............................................................................................................................................ 9 - 2
9.Z volP |r cor0ECT ................................................................................................................................................. 9 - 2
9.8 NeW A|r lrlerlace ................................................................................................................................................. 9 - 2
9.9 lrsla||al|or P|arr|rg ............................................................................................................................................. 9 - 2
9.10 3urrary.................................................................................................................. ......................................... 9 - 3
Append|x 0|g|ta| Enhanced 6ord|ess Te|ecommun|cat|ons
A.1 0ECT: 3ore 3a||erl Fealures ............................................................................................... ............................. A - 1
A.2 0yrar|c C|arre| 3e|ecl|or ................................................................................................................................ A - 3
A.3 cor0ECT P|ys|ca| Layer 3pec|l|cal|ors ..................................................................................... ......................... A - 5
Abbrev|at|ons
corDECT
corDECT Wireless Access System
5
corDECT is an advanced, field proven, Wireless
Access System devel oped by Mi das
Communication Technologies and the Indian
Institute of Technology, Madras, in association
with Analog Devices Inc., USA.
corDECT provides a complete wireless access
sol uti on for new and expandi ng tel e-
communication networks with seamless
integration of both voice and Internet services. It
is the only cost-effective Wireless Local Loop
(WLL) system in the world today that provides
simultaneous toll-quality voice and 35 or 70 kbps
Internet access to wireless subscribers.
corDECT is based on the DECT standard
speci fi cati on from the European Tel e-
communication Standards Institute (ETSI). In
addition, it incorporates new concepts and
innovative designs brought about by the
collaboration of a leading R & D company, a
renowned university, and a global semiconductor
manufacturer. This alliance has resulted in many
breakthrough concepts including that of an
Access Network that segregates voice and
Internet traffic and delivers each, in the most
efficient manner, to the telephone network and
the Internet respectively, without the one choking
the other. Chapter 2 discusses this.
Chapter 3 contains a brief description of the
various corDECT sub-systems that make it
scalable and modular. Next, Chapter 4 describes
the several ways in which corDECT can be
deployed to cater to a wide variety of subscriber
densities and teletraffic levels, to suit both
incumbent and greenfield operators.
Chapter 5 presents at a glance the key features
and services provided by the corDECT system.
The topic of Chapter 6 is the dimensioning of the
corDECT system to cater to the required voice
and Internet traffic levels. Chapter 7 highlights
the coverage achieved by different configurations.
A system with active elements at each subscriber
location, apart from several Base Station sites,
requires a sophisticated and user-friendly Network
Management System (NMS) for monitoring and
maintenance. Chapter 8 discusses the NMS
available for corDECT. Chapter 9 gives a glimpse
of the future, as corDECT evolves to a full-fledged
3G+ system with advanced features such as fast
download from the Internet.
Finally, there is an Appendix that gives a brief
overview of the DECT standard. The main aspects
of DECT are dealt with here, in particular MC-
TDMA medium-access and Dynamic Channel
Selection. A short list of key DECT physical
parameters is also included.
Chapler 1 Inlroduclion
1 - 1
corDECT
corDECT Wireless Access System
6
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corDECT
corDECT Wireless Access System
7
2.1 |ntroduct|on
Till around the mid-eighties, a local loop or an
Access Network (AN) used to consist of a pair
of copper wires connecting the subscriber at
home or office to the nearest exchange. The local
loop length in urban areas would be typically as
long as 6 to 8 km and the diameter of the copper
used was 0.5 mm to 0.6 mm. The loop was
designed to carry 0 - 4 kHz voice and was difficult
to maintain, with almost 85% of all faults found
in the local loop. Above all it was expensive, as
well as difficult and time-consuming, to deploy.
With copper and digging costs increasing every
year, if one were to continue with such an
approach, the per-line local loop cost itself would
today amount to over 80% of the total cost of
putting up a telecom network.
2.2 Access Network
Fortunatel y, an uncel ebrated but maj or
technological innovation changed the Access
Network from the mid-eighties onwards. As
Chapler 2 WireIess Access NelvorI: Sone Key Issues
shown in Figure 2.1, the AN now consists of an
optical fibre from an exchange to a Remote Line
Unit (RLU) and typically a 3 - 4 km copper loop
from the RLU to the subscriber premises. The
signals carried on the fibre are time-multiplexed
digital voice and signaling information. A RLU
typically serves 1000 to 4000 subscribers and
the links from RLU to exchange consist of
4 - 16 E1s. Since the loop length is reduced to
3 - 4 km, a wire gauge of 0.4 mm is sufficient and
this brings down the cost considerably. The rising
cost of copper, however, continues to push up
the cost of even this solution every year. Today,
the per-line copper cost (3 - 4 km long, including
laying charges) and the shared cost of fibre and
RLU, again amounts to almost two-thirds of the
total per-line cost.
The signaling protocol on the AN (in the signaling
slots on the E1 links between the RLUs and
exchange) initially remained proprietary. However,
access signaling was standardized internationally
in the early nineties, in the form of the V5.1 and
V5.2 protocols. The V5.2 interface makes the
Figure 2.1 Access Network in the 90s
2 - 1
corDECT
corDECT Wireless Access System
8
AN appear as an RLU to any exchange,
overcoming the earlier restriction of having a
proprietary RLU for each exchange.
Telephone traffic is concentrated at the AN-
exchange interface and one channel is not
requi red for every subscri ber l i ne. The
dimensioning of the links from the AN to the
exchange is determined by an estimate of the
traffic. For example, 1000 subscribers with an
estimated traffic of 0.1 E per subscriber require
no more than four E1 links (120 channels) to the
exchange at 0.5% GOS (Grade of Service). This
not only implies savings in bandwidth, but also
implies that only four E1 interfaces are required
at the exchange (as compared to the 1000 two-
wire interfaces that would have been required if
all 1000 lines were brought to the exchange), thus
reducing the cost.
The open V5.2 interface permits the AN to be
procured and deployed independently of the
exchange vendor. The AN can use innovative
technologies on media like fibre, wireless, Digital
Subscriber Line (DSL) on copper, hybrid fibre-
coaxial cable, or even power-lines. As the AN
dominates the cost, is the most fault-prone, and
is the most time consuming to deploy, availability
of new access solutions is becoming the key to
cost-effective expansion of the telecom network.
Wireless ANs, just like any other access network
of today, must connect to an exchange using
the V5.2 access protocol.
Even as the issue involving interface to the
telecom network was resolved, another important
issue cropped up about three years ago. Today
a telecom network can no longer just focus on
providing telephone service at homes and offices,
but must provide Internet service too. We now
examine some of the factors involved in providing
Internet connections using the existing telephone
network.
2.3 The |nternet Tang|e
The Internet has emerged as second only to the
telephone in connecting people and may
tomorrow subsume the telephone network. But
today, Internet access at homes and offices
largely rides on the telephone network. Internet
access appears to be simple: just get a telephone
line, connect a modem and computer and dial
an Internet Service Provider (ISP). The ISP has a
bunch of telephone lines and an equal number of
modems connecting the users to a router as
shown in Figure 2.2. This router is connected to
other routers on the Internet. A dial-up connection
to an ISPs router gives a user access to everyone
and everything on the Internet.
This straightforward-looking approach to
accessing the Internet, is however, beset with
problems. The telephone network is typically
designed to handle 0.1 E traffic per subscriber
on the average. This is generally adequate for
voice telephony. However, Internet sessions are
Figure 2.2 Internet access using the telephone network
2 - 2
corDECT
corDECT Wireless Access System
9
usually of longer duration, often even exceeding
an hour. As a significant percentage of telephone
users start accessing the Internet, the load on
the telecom network will far exceed 0.1 E per
subscriber, resulting in severe congestion and
eventual network collapse. If this has not
happened yet, it is because only a small
percentage of telephone users are also accessing
the Internet.
The second problem is associated with the local
call charges for accessing the Internet in this
manner. In many countries the telephone call
made for accessing the Internet is usually
charged based on the call duration. In addition, a
subscriber may also have to pay the ISP for
connection to the Internet.
Thirdly, the analog modem-to-modem link
between the subscriber and the ISP is often
unreliable. One does not always get connected
at 33.6 kbps and the speed can go down to
9.6 kbps and even 4.8 kbps at times, especially
in rural areas. Further, the connection often drops.
Finally, an ISP with N telephone lines, N modems
and a N-port router could serve at most N
subscribers at a time. If the connection drops,
one may not get an immediate reconnection
during busy hours.
This Internet tangle requires a new approach in
order to support future growth. Though an Internet
connection is kept on for long hours, a peculiarity
of computer-to-computer communications is that
the connection is not in continuous use, but in
bursts. Packets are transmitted to and from the
Internet in bursts, with the link remaining
practically idle most of the time. A circuit-switched
connection on a telephone network, however, is
unable to take advantage of this and dedicates
network resources throughout the duration of the
connection thereby congesting the network.
An ideal solution to this problem is to have packet-
switched access. However, the local loop is often
a separate physical line to each subscriber and
packet access on this dedicated line gives little
advantage, as no one else can use this resource
anyway. In such a situation, it is advisable to
separate the Internet data at the network node
nearest to the subscriber, where data from
multiple subscribers can be multiplexed. This is
shown in Figure 2.3, where separation of Internet
data and voice traffic takes place at the Access
Centre (AC), located typically at a street-corner.
As shown in the figure, both wired and wireless
i nterfaces to the AC are possi bl e. DSL
technology and narrowband ISDN equipment can
provide reliable, high-speed, simultaneous, voice
and Internet access on a single copper pair. If
wired access is used, there is strictly no
restriction on the bitrate between the subscriber
and the AC, as long as the physical medium can
support it.
Figure 2.3 Access Centre (AC) separates voice and Internet data; SU1 and SU2 provide
simultaneous voice and Internet services to subscribers using wired and wireless connectivity
respectively
2 - 3
corDECT
corDECT Wireless Access System
10
However, wireless access makes use of an
important shared resource, namely, the frequency
spectrum. It is this resource which limits the
capacity of a wireless system. Thus, access
strategies which assign a channel to a subscriber
only when he/she wishes to transmit a packet
would significantly enhance capacity for Internet
access. Wireless access networks which can
share the frequency spectrum and utilize it
efficiently during packet bursts are obviously very
attractive candidates for rapid expansion of
Internet access in the future.
2.4 w|re|ess Loca| Loop Vs.
Hob||e w|re|ess Access 8ystem
Today, there is little doubt that wireless access
systems should provide digital and not analog
access. Wireless connectivity to subscribers
today is provided by mobile communication
systems as well as wireless local loop systems.
These two appear to be similar and are often
confused with each other. However, the
requirements for the two systems are significantly
different.
2.4.1 Hob||e Te|ephone 8ystem
Mobile telephone systems, often called cellular
systems, are meant to provide telephony for
people on the move. The handset is primarily
meant to keep the subscriber connected while
he/she is away from the home or office. The key
here is total coverage of the city/state/country.
The mobile telephone must be reachable wherever
the subscriber is in the car, on the street, or in
a shopping mall. Other requirements are
secondary. Modest voice quality is acceptable
as the user may often be speaking from a location
with high ambient noise, such as a street or a
car. Data communication is not very important.
Fax communication is highly unlikely to be used.
Internet access will be provided on the next-
generation systems. The important applications
will normally be email and web browsing with
small displays. The data rate needed is much
less than for accessing the Internet from a desktop
PC. Finally, the traffic per subscriber will not be
high: typically, it will be only 0.02 E traffic per
subscriber. Additional air-time charges for such
value-added services are generally acceptable to
users.
2.4.2 w|re|ess Loca| Loop 8ystem
A Wireless Local Loop (WLL) system, on the
other hand, is meant to serve subscribers at
homes or offices. The telephone provided must
be at least as good as a wired phone. Voice
quality must be good a subscriber carrying on
a long conversation must feel comfortable. One
must be able to use speakerphones, cordless
phones and parallel phones. The telephone must
support fax and modem communications and
should be usable as a Public Call Office (PCO).
The ability to support at least medium-speed
(about 64 kbps) Internet access is a must.
Further, the traffi c supported shoul d be
reasonably high at least 0.1 E per subscriber.
Besides, the ability to support a large number of
subscribers in an urban area (high teledensity)
with a limited frequency spectrum is required.
Finally, for the system to be viable, the cost of
providing this wireless access should be less than
that of a wired telephone connection. Air-time
charges are totally unacceptable.
Therefore, even though mobile communication
systems and WLL systems appear to be similar
and are someti mes even referred to
interchangeably, the requirements in the two
applications are quite distinct.
2.5 6apac|ty and 8pectra|
Eff|c|ency
Having looked at the PSTN-AN interface and the
requirements that a WLL system has to fulfill, let
us now take up the most important issue that
governs the choice of a WLL technology. One
2 - 4
corDECT
corDECT Wireless Access System
11
has to recognize that the frequency spectrum
available will always be limited. Since a telephone
or an Internet connecti on i s not used
continuously, channels must obviously be
assigned to a subscriber on demand. But this is
not enough. The key focus has to be efficient
use and spatial re-use of the spectrum.
What governs the re-use of spectrum? The use
and re-use of spectrum is governed by multiple
factors including:
i. channel pay load (bitrate)
ii. signaling overhead
iii. modulation efficiency
iv. cell radius (range)
v. multiple access method
vi. interference reduction techniques
vii. spatial diversity and space-time processing
|. 6hanne| Pay Load
It is obvious that a higher payload will require
more frequency resources. Therefore, for voice
communication on wireless systems, it may be
desirable to have efficient voice compression and
lower bitrate voice codecs. The resulting reduction
in quality, however small, is quite acceptable for
mobile communications. But for telephones at
homes and offi ces, tol l -qual i ty voi ce
communications at 32 kbps/64 kbps (ADPCM/
PCM) is desirable. Besides, PCM and ADPCM
are transparent to other communication services
like fax. For Internet access, high bitrate
communication is obviously desirable. As the
frequency resource used per channel is directly
proportional to the payload, high bitrate Internet
access implies use of more frequency resources.
||. 8|gna||ng 0verhead
As signaling is the key to the setting up,
monitoring, and tearing down of a call, it needs
to be carried out on air between the subscriber
equipment and the Base Station. Signaling is an
overhead that takes away part of the frequency
resources but plays an important role in improving
the overall efficiency of spectrum usage.
|||. Hodu|at|on Eff|c|ency
The modulation technique employed has a direct
bearing on the efficient use of spectrum. Highly
spectrum-efficient techniques have been
developed over the years. For example, 16-QAM
is more spectrally efficient than 8-QAM, which
in itself is more efficient than QPSK and MSK
modulation techniques. But more efficient
techniques are usually expensive to implement
and may sometimes require larger power
margins. For a WLL system, cost is an important
consideration and the power margin available is
usually not large. QPSK, MSK or even BFSK
techniques are often used, even though their
spectral efficiency is modest.
|v. 6e|| Rad|us
Cell radius is perhaps the most important factor
governing spectrum utilization in a wireless
system. Let there be N independent channels
available for use in a cell of radius r. It is the re-
use efficiency which determines the re-use of
channels in neighboring cells. Leaving this issue
aside for the moment, let us concentrate on the
N channels available within a cell. Let us also
assume that the traffic per subscriber is e
Erlangs. The number of subscribers that can be
served in the cell works out to N/e and the
Subscriber Density (SD) that can be served in
this cell is approximately,*
Thus, subscriber density is inversely proportional
to the square of the cell radius. The implication
of this can be seen by an example. If N = 200
* For a large number of servers N, from the Erlang-B formula,
at 1% blocking probability, the offered load can be nearly
N Erlangs.
2 - 5
corDECT
corDECT Wireless Access System
12
and e = 0.1 E, the capacity (subscriber density)
varies with cell radius as follows:
Therefore, cell radius plays the dominant role in
determining the subscriber density given a certain
amount of frequency spectrum. In other words, a
smaller cell radius (microcell) is the key to
efficient use of spectrum and one may have to
have cells as small as 500 m in radius, if a high
subscriber density is desired.
v. Hu|t|p|e Access Techn|que
A key parameter determining the efficient re-use
of spectrum is the multiple-access technique
used. The access technique defines how
frequency spectrum is divided into channels and
affects re-use. FDMA, TDMA, CDMA, and MC-
TDMA are different multi-access techniques
which affect the re-use factor (extent to which
the spectrum can be re-used in every cell). Re-
use further depends on the number of sectors
used in a cell and also on whether Fixed Channel
Allocation (FCA), or Dynamic Channel Selection
(DCS) is used. For CDMA and MC-TDMA, the
re-use factor varies from 0.25 to 0.5 per sector.
v|. |nterference Reduct|on Techn|ques
The re-use distance is primarily determined by
the minimum Signal to Interference Ratio (SIR)
requirement. The target SIR is based on the
minimum sensitivity required at the receiver input
in order to obtain a particular Bit Error Rate (BER).
The required BER is typically 10
-3
for voice
appl i cati ons (and 10
-6
or l ower for data
applications, obtained by using error control
coding and/or ARQ). Depending on the choice of
multiple access technique, the modulation
scheme, and the particular application (mobile
or fixed wireless), the target SIR will differ.
Interference reduction techniques are widely used
in wireless systems to increase re-use efficiency
while retaining the target SIR requirement.
2.6 8ummary
A Wireless Access System today must provide
simultaneous voice and medium-rate (at least)
Internet connectivity at homes and offices. To
serve dense urban areas, the system should
support a microcellular architecture, whereas for
rural areas, larger range is desirable. At the
Access Unit, the voice and Internet traffic from
subscribers should be separated and delivered
to the tel ephone and Internet networks
respectively.
r (km) SD (per sq. km)
25 1
10 6
3 70
1 640
0.5 2550
2 - 6
corDECT
corDECT Wireless Access System
13
3.1 |ntroduct|on
The corDECT Wireless Access System (WAS)
is designed to provide simultaneous circuit-
switched voice and medium-rate Internet
connectivity at homes and offices. The Access
System model, which the corDECT emulates, is
shown in Figure 3.1.
3.2 6onceptua| Access 8ystem
In this conceptual model, there is a Subscriber
Unit (SU) located at the subscriber premises.
The SU has a standard two-wire interface to
connect a telephone, fax machine, PCO (Public
Call Office), speakerphone, cordless phone, or
modem. It also provides direct (without a modem)
Internet connectivity to a standard PC, using
either a serial port (RS-232 or USB) or Ethernet.
The Access System allows simultaneous
telephone and Internet connectivity. The SUs are
connected to an Access Centre (AC) using any
convenient technology like wireless, plain old
copper, DSL on copper, coaxial cable, optical
fibre, or even power lines.
The AC must be scalable, serving as few as 200
subscribers and as many as 2000 subscribers.
In urban areas, the AC could be located at a
street corner, serving a radius of 700 m to 1 km.
This small radius in urban areas is important for
wireless access, in order to enable efficient re-
use of spectrum. When cable is used, the small
radius ensures low cost and higher bitrate
connectivity. However in rural areas, the distance
between the AC and the SU could easily be
10 km and even go up to 25 km in certain
situations.
The AC is thus a shared system catering to
multiple subscribers. The voice and Internet traffic
to and from subscribers can be concentrated here
and then carried on any appropriate backhaul
transport network to the telephone and Internet
networks respectively.
At the AC, the telephone and Internet traffic is
separated. The telephone traffic is carried to the
telephone network on E1 links using access
protocols such as V5.2. The Internet traffic from
multiple subscribers is statistically multiplexed,
taking advantage of the bursty nature of Internet
traffic, and carried to the Internet network. As
use of Voice-over-IP (VoIP) grows, voice traffic
from subscriber units could also be sent to the
Internet, gradually making connectivity to the
telephone network redundant. However, for
connecting to the legacy telephone network, the
Chapler 3 corDLCT WireIess Access Syslen
Figure 3.1 Conceptual Access System providing simultaneous voice and Internet connectivity.
SU: Subscriber Unit ; AC: Access Centre
3 - 1
corDECT
corDECT Wireless Access System
14
voice port of the AC may be required for some
time to come. An AC could also incorporate
switching and maintenance functions when
required. Further, it is possible to co-locate
Internet servers with the AC.
3.3 cor0E6T w|re|ess Access
8ystem
Following the conceptual model, the corDECT
Wireless Access System uses a similar
architecture to provide telephone and Internet
service to a subscriber, as shown in Figure 3.2.
The subscriber premises equipment, Wallset IP
(WS-IP) or Wallset (WS), has a wireless
connection through a Compact Base Station
(CBS) to an Access Switch, called a DECT
Interface Unit (DIU). The air interface is compliant
to the DECT standard specified by ETSI. The
DIU switches the voice traffic to the telephone
network using the V5.2 protocol to connect to
an exchange. The DIU also switches the Internet
calls to a built-in Remote Access Switch (RAS)
which then routes the traffic to the Internet
network. The RAS has an Ethernet interface,
which is connected to the Internet using any
suitable routing device.
The CBS is normally connected to the DIU using
three twisted-pair wires, which carry signals as
well as power from the DIU to the CBS.
Alternatively, it can be connected to the DIU
through a Base Station Distributor (BSD). The
BSD is a remote unit connected to the DIU using
a standard E1 interface (on radio, fibre, or copper)
as shown in Figure 3.3. A BSD can support up
to four CBSs.
For long-range communication, a WS-IP or WS
can also be connected to the CBS using a two-
hop DECT wireless link, one between WS-IP or
WS and a Relay Base Station (RBS) and another
between the RBS and CBS, as shown in Figure
3.4. The wireless range supported between a WS-
IP or WS and the CBS or RBS is 10 km in Line-
Figure 3.2 corDECT Wireless Local Loop
Figure 3.3 CBS remoted to DIU through BSD
3 - 2
corDECT
corDECT Wireless Access System
15
of-Sight (LOS) conditions. The range supported
between a CBS and RBS is 25 km in LOS
conditions.
A typical system consists of one DIU with one
or two RAS units, up to 20 CBSs, and up to a
1000 WS-IPs or WSs. The BSD and RBS units
are used as required by the deployment scenario.
3.4 8ub-systems of cor0E6T
w|re|ess Access 8ystem
Before we get into more details at the system
level, we take a brief look at each of the sub-
systems.
3.4.1 wa||set |P and wa||set
As shown in Figure 3.5, the Wallset with Internet
Port (WS-IP) provides voice connectivity to the
subscriber using a RJ-11 interface, enabling one
to connect a standard DTMF or decadic
telephone, G3 fax machine, PCO (battery reversal
and 12/16 kHz metering are standard features),
speakerphone, cordless phone, or modem. In
addition, the WS-IP has a RS-232 port to directly
connect a PC (obviating the need for a telephone
modem). The PC establishes a dial-up PPP
(Point-to-Point Protocol) Internet connection
using a standard dial-up utility. Internet access
is supported at 35 or 70 kbps. In fact, the WS-IP
can support simultaneous voice and 35 kbps
Internet connections.
Besides these two user interfaces, the WS-IP
has an antenna port where either a whip antenna,
or an externally mounted antenna (through
cable), can be connected. The power to the WS-
IP is provided by a 12 V adaptor connected to
Figure 3.4 WS-IP connected to DIU using a two-hop radio link through a Relay Base Station
Figure 3.5 WS-IP (Wallset with Internet Port)
3 - 3
corDECT
corDECT Wireless Access System
16
the AC mains and optionally by a solar panel
which can be connected in parallel. The WS-IP
has a built-in battery and battery charger. The
built-in battery provides 16 hours stand-by time
and more than 3 hours talk time for voice calls.
A Wallset (WS) is a similar terminal without the
Internet port.
3.4.2 Hu|t|wa||set
The Multiwallset (MWS), shown in Figure 3.6,
provides simultaneous voice service to four
subscribers. It has all the features of the WS,
but at a significantly lower per-line cost.
The Multiwallset has a DECT Transceiver Module
(DTM), which is an outdoor unit with a built-in
antenna with 7.5 dB gain. It is connected to an
indoor Subscriber Interface Module (SIM) unit,
which has four RJ-11 ports for telephones. Each
port supports all the terminals a WS supports
(see section 3.4.1).
The connection between the DTM and the SIM
uses a single twisted-pair wire, obviating the need
for RF cable and connectors. The MWS has a
built-in battery for backup and is powered through
the AC mains.
3.4.3 Hu|t|wa||set |P
The Multiwallset with Internet Port (MWS-IP) is
a MWS with four telephones and an additional
Ethernet interface to provide dial-up Internet
connectivity. Multiple PCs can be connected to
the Ethernet port and provide a shared 35/70 kbps
Internet connection. The PPP-over-Ethernet
protocol is used to set up individual connections.
It is to be noted that at any time, either four
simultaneous telephone calls with no Internet
connection, or three telephone calls and a 35
kbps shared Internet connection, or two
telephone calls and a shared 70 kbps Internet
connection, can be made. Depending on usage,
this may introduce some blocking for voice calls.
3.4.4 6ompact ase 8tat|on
The Compact Base Station (CBS), shown in
Figure 3.7, provides the radio interface between
the DIU and the corDECT subscriber terminal. It
supports up to 12 simultaneous voice calls. It is
a small, unobtrusive, weatherproof unit that is
remotely powered from the DIU or a BSD.
The CBS has two antennas for diversity. A
directional antenna with significant gain can be
used when coverage is required to be confined
to certain directions. For example, if the coverage
area is divided into sectors, each sector can be
covered by a different Base Station with
directional antennas. For 360
0
coverage using a
single CBS, omni-directional antennas are used.
More than one CBS can be deployed to serve a
single sector or a cell.
The maximum LOS range between a subscriber
unit and a CBS is 10 km. An isolated CBS
Figure 3.6 Multiwallset
3 - 4
corDECT
corDECT Wireless Access System
17
supports approximately 5.8 E of traffic with a
Grade of Service (GOS) of 1%, typically serving
30 - 70 subscribers. Multiple CBS's serving the
same sector or cell increase the traffic in Erlangs
handled by each CBS (see Chapter 6).
The CBS is connected to a DIU or a Base Station
Distributor (BSD) with three twisted-pair copper
wires, each of which carry voice/data traffic,
signaling and power. The maximum loop length,
with a 0.4 mm diameter wire, can be 4 km
between the DIU and the CBS and 1 km between
the BSD and the CBS.
3.4.5 0E6T |nterface Un|t
The DECT Interface Unit (DIU) shown in Figure
3.8, implements the functions of a switch (or a
Remote Line Unit), Base Station Controller, and
the Operations and Maintenance Console (OMC).
System reliability is guaranteed by a redundant,
hot stand-by architecture. The OMC allows
exhaustive real-time monitoring and management
of the entire corDECT system. A fully-configured
DIU with an in-built Remote Access Switch (RAS)
only occupies a single 28U, 19" cabinet and
consumes less than 600 W.
Up to 20 CBS's can be supported by a DIU,
directly or through the BSD. The DIU provides up
to eight E1 links to the telephone network and/or
RAS. The signaling protocol used is either V5.2,
which parents the DIU (as an RLU) to an
exchange, or R2-MF, in which case the DIU acts
as a 1000-line exchange. There is a third option,
Figure 3.7 Compact Base Station
Figure 3.8 DECT Interface Unit (DIU) with in-built RAS
3 - 5
corDECT
corDECT Wireless Access System
18
wherein the corDECT system, using additional
equipment, appears to an exchange simply as a
number of twisted-pair lines (see section 3.5.1.2.).
Multiple DIUs are managed through a centralized
Network Management System (NMS).
3.4.6 |K0N Remote Access 8w|tch
The iKON Remote Access Switch (RAS), shown
in Figure 3.9, is a 19" 1U unit normally integrated
within the DIU cabinet. It terminates the PPP
connections from Internet subscribers using
corDECT WS-IP or MWS-IP. It is connected to
the DIU using up to two E1 ports and does IP-
based routing for up to 60 simultaneous corDECT
Internet calls. The RAS has a 10BaseT Ethernet
port to connect to the Internet. It supports
RADIUS for accounting and authentication, PAP
for user authentication and is managed using
SNMP.
3.4.7 ase 8tat|on 0|str|butor
The Base Station Distributor (BSD) is a compact,
remotely located, locally powered, rack-
mountable unit that supports up to four CBSs
(with power feed). The E1 interface between a
DIU and the BSD can be on copper, fibre, or radio
and link distance depends only on the link design.
The BSD is designed to extend corDECT
coverage to pockets of subscribers located far
away from the DIU.
3.4.8 Re|ay ase 8tat|on
A Relay Base Station (RBS), as shown in Figure
3.11, extends the range of the corDECT system
by relaying DECT packets between the CBS and
subscriber units. The RBS can handle 11 calls
simultaneously.
The RBS consists of two units. The RBS Air Unit
is typically mounted on a tower/mast and houses
the baseband and the RF sub-system. The RBS
Ground Unit supplies power and provides
maintenance support to the Air Unit and is
mounted at the bottom of the tower.
The RBS uses three antennas. One antenna
(usually a directional antenna with high gain),
referred to as the RBSWS antenna, points
towards the CBS with which the RBS is
communicating. The other two antennas, called
Figure 3.9 iKON RAS
Figure 3.10 Base Station Distributor
3 - 6
corDECT
corDECT Wireless Access System
19
RBSBS antennas are used for communication
with the subscriber units (two antennas are used
for diversity). These antennas are similar to those
used by the CBS.
The maximum LOS range between a CBS and a
RBS is 25 km, while the maximum LOS range
between the RBS and corDECT subscribers is
10 km.
3.4.9 Network Hanagement
corDECT provides comprehensive operation and
maintenance through the corView OMC console.
Its repertoire includes hardware and software
configuration, subscriber administration,
accounting, fault notification, and traffic
management. Figure 3.12 depicts the corView
GUI for configuring the DIU. Commands range
from a birds-eye view of the operational status
of a network of corDECT systems to probing the
internals of an individual Wallset.
This easy-to-use, menu-driven console can be
run either locally or remotely. When used
remotely, a single corView workstation serves as
an NMS for a number of corDECT systems.
corView can also be used with the CygNet NMS
to provide integrated management of a network
of corDECT and other systems.
corView supports the SNMP protocol and can
be connected to the corDECT system by any IP
network. In the future, corView will also support
Figure 3.11 Relay Base Station
Figure 3.12 corView 200 GUI for configuring the DIU
3 - 7
corDECT
corDECT Wireless Access System
20
TMN/Q3. When used as a switch, detailed billing
records are maintained and can be exported to
the billing centre via several media.
3.5 cor0E6T Access 6entre
Funct|ona||ty and |nterfaces
The corDECT Access Centre, consisting of a DIU
and iKON RAS, is designed to provide interfaces
to the telephone network and to the Internet.
3.5.1 The Te|ephone 6onnect|on
The telephone connection provided to a corDECT
subscriber is a circuit-switched one. The DIU
switches the connection to the telephone
network. The interface to the telephone network
is provided in three different ways:
i. RLU mode, with V5.2 protocol on E1
interfaces to a parent exchange,
ii. Transparent mode, with two-wire interface to
a parent exchange and
iii. Switch mode, with R2-MF protocol on E1
interfaces to the telephone network.
3.5.1.1 RLU Hode
The DIU has up to six E1s that can be connected
to a parent exchange using V5.2 signaling. The
DIU in this case works as a 1000-line Remote
Line Unit of the parent exchange, as shown in
Figure 3.13. Even calls between two corDECT
subscribers belonging to the same DIU are
switched by the parent exchange.
The numbering and all subscriber facilities are
provided by the exchange and billing too is carried
out at the exchange. The DIU does some limited
subscriber administration, such as authenticating
a subscriber (as per the DECT standard). The
DIU console, however, provides management
functions for managing corDECT DIU, CBS, RBS,
BSD, WS, WS-IP, MWS and MWS-IP, and also
carries out wireless traffic monitoring. The
management functions can also be carried out
centrally for multiple DIUs, as discussed in
Chapter 8. The 1000-line DIU in this mode
consists of three 6U 19" racks in one cabinet,
leaving additional space for up to three RAS units.
3.5.1.2 Transparent Hode
In this mode, the DIU is parented to an exchange
using two-wire interfaces. Each subscriber line
is mapped to an unique two-wire port on the
exchange. Hook status and digits dialed at WS/
WS-IP/ MWS are mapped by the DIU to reflect
at the corresponding exchange port. All services
of the exchange are available to the subscriber.
Billing is carried out at the exchange. However,
as in the RLU mode (section 3.5.1.1), the DIU
carries out subscriber authentication and system
management functions.
To provide two-wire interfaces at the DIU, a
Concentrating Subscriber Multiplexer (CSMUX)
is used. Each CSMUX, housed in one 6U 19"
rack, can provide up to 240 two-wire ports
(grouped as 2 x 120 two-wire ports). The CSMUX
is connected to the DIU typically using two E1
Figure 3.13 DIU parented to an exchange in RLU mode
3 - 8
corDECT
corDECT Wireless Access System
21
ports, providing 4:1 concentration. Thus, using
eight E1s and four CSMUX units and a DIU
integrated in two cabinets, one can serve up to
960 subscribers in transparent mode, as shown
in Figure 3.14. A concentration of 4:1 is normally
acceptable since wireless channels are anyway
being shared. Sharing an E1 port among 120
subscribers, one can serve nearly 0.2 Erlang
traffic per subscriber at 1% GOS. However, it is
possible to avoid concentration at CSMUX and
connect eight E1s to a single CSMUX rack. In
this case, one DIU will be limited to serve a
maximum of 240 subscribers.
The transparent mode is the quickest way to
interconnect corDECT to an existing telephone
network. However, it is not a preferred mode for
operation. In order to serve 960 subscribers, 960
two-wire ports are required on the exchange side
connected to four CSMUX units. In contrast, only
four to six E1 ports are required at the exchange
in the RLU mode and the CSMUX is avoided.
Thus, in the RLU mode, the size of the exchange
as well as the DIU is much smaller and the power
required is also less when compared to the
transparent mode.
A more serious problem in the transparent mode
comes from a signaling anomaly that can emerge
in some specific situations. For example, when
an incoming call comes to the exchange for a
subscriber, the exchange signals ring-back to the
calling subscriber if it finds from its database that
the called subscriber is free. The exchange
simultaneously feeds ring to the corresponding
two-wire port. This is detected by the CSMUX in
the DIU and the DIU then attempts to page the
correspondi ng WS/WS-IP and ri ng the
subscriber. However as wireless channels are
shared, it is possible that sometimes the DIU
finds no free channel and fails to feed ring to the
subscriber. The anomaly develops when the
called port gets ring-back tone, but the called
party does not get a ring. Such a situation can
sometimes become problematic. The transparent
mode is therefore not the most desirable mode
Figure 3.14 DIU parented to exchange in transparent mode
Figure 3.15 DIU as an independent medium-sized exchange
3 - 9
corDECT
corDECT Wireless Access System
22
of operation. Nevertheless, it is the quickest way
to integrate a wireless system to the existing
telephone network anywhere in the world.
3.5.1.3 8w|tch Hode
The DIU is designed to be a 1000-line, full-fledged,
medium-sized exchange for corDECT wireless
subscribers. It interfaces to the telephone network
on up to six E1 lines using R2-MF protocol as
shown in Figure 3.15. All the exchange functions,
including subscriber administration, billing, and
management, are carried out at the DIU itself. To
serve 1000 subscribers in this configuration, a
DIU uses three 6U 19" racks. The advantage of
this mode is that the cost of an exchange is
totally saved.
The DIU can also serve as a Direct In-Dialing
(DID) PBX.
3.5.2 |nternet 6onnect|on
A corDECT subscriber connects to the WS-IP
using a PPP dial-up connection on the RS-232
port. The port is programmed at 38.4 kbps rate for
a 35 kbps Internet connection and at 115.2 kbps
rate for a 70 kbps Internet connection. The PC
connected to the RS-232 port on WS-IP dials a
pre-designated number using a standard dial-up
routine. The DIU sets up a circuit-switched
connection between the WS-IP and the iKON
RAS connected to the DIU on an E1 port.
The Internet connection employs the wireless link
between the WS-IP and the CBS and the wired
3 - 10
links between the CBS and the DIU and between
the DIU and the RAS. Since the BER on the
wireless link could occasionally be high, the PPP
packet is fragmented and transmitted with an
error detection code on the link from the WS-IP
to the DIU. ARQ is performed on this link to obtain
error-free fragment transmission. The PPP
packets are re-assembled from these fragments
before transmitting it to the PC (on the WS-IP
side) and to the RAS (on the DIU side).
The connection between the WS-IP and the DIU
is at 32 kbps or 64 kbps (using one or two DECT
slots on air). The start/stop bits received at the
RS-232 port are stripped before transmission on
air. This enables 35 kbps Internet throughput
between the user PC and the RAS on the
32 kbps connection in an error-free situation.
Similarly, 70 kbps Internet throughput is possible
between the user PC and the RAS on the
64 kbps connection. Bit errors on the link will
temporarily bring down the throughput.
Each RAS has two E1 ports for connecting to
the DIU and thus can support Internet connection
for up to 60 subscribers at a time. The PPP
connections are terminated at the RAS and IP
packets are routed to the Ethernet port of the
RAS for onward transmission to the Internet. The
Ethernet ports from multiple RASs would
normally be connected to an Ethernet switch.
The Ethernet switch in turn would be connected
to an Internet router, completing the connection
to the Internet.
corDECT
corDECT Wireless Access System
23
4.1 |ntroduct|on
We saw in Chapter 3 that the corDECT DIU can
be deployed as an access system, parented to
an exchange using either the V5.2 access
protocol, or transparently using two-wire
interfaces. Alternatively, the corDECT DIU itself
can act as a Local Exchange, or even as a direct-
in-dialing PBX. This chapter presents a few
deployment scenarios for the corDECT Wireless
Access System.
4.2 cor0E6T 0ep|oyment w|th
0|U |n Exchange Prem|ses
In one of the most widely deployed scenarios,
the corDECT DIU is placed in the local exchange
premises, parented to an exchange in a
transparent manner or using the V5.2 protocol,
or as an independent Local Exchange. This
scenario will be widely used by an incumbent
operator with existing infrastructure. The
exchange building (usually one of the taller
buildings in the area) would have a tower to deploy
Compact Base Stations as shown in Figure 4.1.
The tower could be a short 15 m rooftop mast,
but in some cases, could be a self-supporting
25 - 35 m tower on the ground. Multiple CBSs
could be mounted on this tower using omni-
directional antennas, but more often, using
directional antennas providing sectorized
coverage. A commonly-used sectorization plan
provides six-sector coverage as shown in Figure
4.2(a) and Figure 4.2(b). Figure 4.2(c) shows a
close up of a CBS and directional antennas.
One or more CBSs are mounted with antennas
having a typical gain of 12 dB to provide coverage
in a 60
0
sector. However as discussed in Chapter
6, one or two CBSs with omni directional
antennas could be additionally mounted on the
same tower, enabling these CBSs to handle
overflow traffic from all sectors. All these CBSs
are connected to the co-located DIU using
twisted-pair cables.
These CBSs provide connectivity to subscribers
as far as 10 km away in Line-of-Sight (LOS)
conditions. However depending on the built-up
environment and in order to re-use the spectrum
Figure 4.1 DIU in exchange premises with co-located CBS
Chapler 4 corDLCT DepIoynenl LxanpIes
4 - 1
corDECT
corDECT Wireless Access System
24
more often, coverage should normally be limited
to 1 to 1.5 km radius in urban environments (see
Chapter 7). Wallsets would typically require an
external (rooftop or window-mounted) antenna,
but in some cases, within a 400 m radius, an
internally-mounted antenna could also be used.
4.2.1 Remote Locat|on of 68
At times, it may be desirable to cover a distant
locality using the same DIU. It is possible to
connect a CBS remotely from the DIU using three
pairs of twisted-pair wires, which carry the voice,
signaling, as well as power, to the CBS. The CBS
could be as far as 4 km away, when 0.4 mm
diameter copper wire is used. If the buried cable
plant in an area is serviceable, it is easy to take
three/six/nine pairs of these wires and mount one/
two/three CBSs remotely, a few kilometers from
the DIU, as shown in Figure 4.3.
The CBSs could then be mounted on a tall
building using a 3 - 6 m pole on the roof and
provide coverage to 30 - 150 subscribers in the
neighborhood of this remote location. It is
important, however, that the buried cable plant
be in reasonable shape and not fail during rain, if
this option is to be used.
A more appropriate way of connecting a multi-
CBS cluster remotely is to use the Base Station
Distributor (BSD). A BSD is connected to the
DIU by a standard E1 link, using an optical fibre,
point-to-point microwave radio, or even copper
(for example, using HDSL). The BSD with a small
48 V power supply unit could then be placed in a
remote building (say, under a staircase landing)
where an optical fibre connection or a cable link
with HDSL, is available. Up to four CBSs can
now be connected to the BSD and mounted on a
pole or small tower on the building as shown in
Figure 4.4. These CBSs could provide coverage
to almost 200 subscribers in the vicinity.
Alternatively, the tower could also support the
antenna for a digital microwave point-to-point E1
Figure 4.2 Six sector coverage by CBS
Figure 4.3 Remote CBS connected using copper twisted-pair wire
(a)
(b)
(c)
4 - 2
corDECT
corDECT Wireless Access System
25
link from the exchange and the BSD could be
connected to it. Again, up to four CBSs could
be mounted on this tower and provide service in
its neighborhood.
It is to be noted that remoting of Base Stations
enables better frequency re-use. The CBSs
mounted at the exchange tower and the CBSs
mounted remotely can often use the same DECT
channels simultaneously.
4.2.2 |nternet 6onnect|on
An iKON RAS, integrated with the DIU,
terminates the PPP connections for all Internet
subscribers at WS-IP (see section 3.5.2, Chapter
3). The IP packets are then routed to the Internet
by the RAS. The RAS could be connected to the
Internet in two different ways. The RAS could be
connected to a Local Area Network (LAN), or to
a switched LAN, on its 10BaseT Ethernet
Interface. A small Internet router (for example,
an Intel 9300 or a CISCO 2610) could be
connected to the LAN as shown in Figure 4.5.
The Internet router is connected to the Internet
using any convenient leased connection. The
router could also carry Internet traffic from other
access systems.
Alternatively, the traffic between the Internet and
RAS could be carried on n x 64 kbps switched
(or leased) circuits. This option can be used only
if the DIU is connected to the telephone network
on E1 lines (using V5.2, or as an independent
LE). The circuits are established between the
DIU and a remote router using the telephone
network. The RAS traffic (IP packets) could then
be routed on such a connection through the DIU,
as shown in Figure 4.6. Since the RAS is
connected to the DIU on E1 lines, a few 64 kbps
slots could be used for this. The maximum
number of subscriber connections that a RAS
(with two E1s) could then support would be less
than 60.
Figure 4.4 Remote CBS deployment using BSD
Figure 4.5 Internet connection using a local router at the exchange
4 - 3
corDECT
corDECT Wireless Access System
26
In certain situations, it is possible to locate the
RAS remotely, using E1 links to the DIU. This is
useful if an operator wishes to install all Internet-
related equipment at one place and optical fibre
is available between different exchanges and the
ISP location. While the DIUs could be located
at different exchanges, all the RASs connected
to various DIUs could be at one place along with
the routers, servers, and other equipment used
by the Internet Service Provider.
The advantage accruing from the RAS statistically
multiplexing bursty traffic from different
subscribers is not availed here. This may not
pose a constraint as fibre typically provides
sufficient bandwidth between exchanges at
marginal cost. Figure 4.7 shows this scenario.
4.3 0|U |ntegrated w|th Access
6entre
In an alternative deployment scenario, an Access
Centre (AC) is deployed to provide the last-mile
connectivity to the subscriber. The AC is deployed
away from the exchange and near the
subscribers.
The DIU along with the RAS acts as an AC,
providing wireless telephone and Internet services
to the subscribers. It could also be integrated
with other similar access equipment using DSL
on copper, cable modem, or even plain old analog
telephony on copper to provide service to
subscribers in the vicinity. In a typical deployment,
the DIU and RAS would be placed at a street
Figure 4.6 n x 64 kbps Internet connection between RAS and remote router
Figure 4.7 Co-location of RASs
4 - 4
corDECT
corDECT Wireless Access System
27
corner to serve urban subscribers in a 1 to 2 km
radius, or placed in the centre of a small town to
serve subscribers in a 10 km radius.
The voice and Internet traffic are separated at
the DIU and the voice traffic is carried on E1 lines
to an exchange using the V5.2 access protocol
(the DIU acting as a RLU). The Internet traffic is
statistically multiplexed at RAS and carried on
E1 lines to the Internet network. Both these
connections are provided using a backhaul
network built using optical fibre or point-to-point
microwave links, as shown in Figures 4.8(a) and
4.8(b) respectively.
It is possible for the Access Centre to extend its
reach by remoting some Base Stations using
either twisted-pair wires or using the BSD, just
as described in section 4.2.1. This approach,
while increasing the subscriber reach of the AC,
also enables better re-use of frequency spectrum
by creating more CBS sites.
4.4 Rura| 0ep|oyment
Providing telecom and Internet service to
subscribers in rural areas is a major application
of the corDECT Wireless Access System. It can
cost-effectively provide this service to areas where
subscriber density is as low as 0.2 subscribers
per sq. km. For a subscriber density lower than
this, corDECT may not be the most cost-effective
system.
Line-of-Sight (LOS) between a subscriber antenna
and Base Station/Relay Base Station is
necessary for the corDECT system to provide
service to subscribers in sparse (low subscriber
density) areas. It is therefore necessary to choose
sites for CBS and RBS towers carefully, so that
subscribers in a 10 km radius can be provided
service. Similarly, antennas have to be mounted
at subscriber premises using poles, so that LOS
to CBS/RBS is available. The availability of light
and compact antennas for the Wallset makes
this task somewhat easier.
Further, subscribers in rural areas may not have
reliable power and solar panels may have to be
used. A compact solar panel can be connected
to the WS or WS-IP to power the unit and charge
the built-in battery, with solar power taking over
when the mains is off/low.
Figure 4.8(a) Fibre backhaul carrying voice and Internet traffic
4 - 5
Figure 4.8(b) Microwave digital radio backhaul carrying voice and Internet traffic
corDECT
corDECT Wireless Access System
28
A DIU along with a RAS could be located either
in a rural exchange building or a RLU building,
adjacent to a tower (typically 15 m to 35 m high).
CBSs mounted on the tower can directly serve
rural subscribers in a 10 km radius (or 300 sq. km
area), as shown in Figure 4.9. This deployment
scenario is adequate for a subscriber density
higher than 1 subscriber per sq. km.
To serve a pocket of subscribers in a remote area,
a BSD could be used. The BSD could then
connect to up to four CBSs on a remote tower
and serve subscribers in a 10 km radius around
it, as shown in Figure 4.10. The BSD requires
power back-up at the remote location. This
deployment could be cost-effective for a
subscriber density as low as 0.2 subscribers per
sq. km, provided a digital microwave or fibre link
to the BSD is available.
If such E1 links are not available, a cost-effective
rural deployment would use Relay Base
Stations. The RBS could be mounted on a tower
up to 25 km away from the CBS tower, providing
a LOS link between the RBS and the CBS. To
overcome the problem of larger propagation delay
from the RBS to the CBS, the RBS transmission
is appropriately advanced as discussed in section
7.2.2 of Chapter 7.
Each RBS serves subscribers in a 10 km radius,
as shown in Figure 4.11. The RBS has 11
channels and can be used to establish 11
simultaneous calls. The two-hop radio link
provides the same voice and Internet services to
the subscribers as a single-hop link. To the
subscriber, the connection through the RBS is
transparent. The RBS does require a power
supply with appropriate back-up, which is provided
Figure 4.9 Deployment for a subscriber density greater than 1 subscriber per sq. km
4 - 6
Figure 4.10 Rural deployment using BSD
corDECT
corDECT Wireless Access System
29
by a mains supply or a solar panel. The RBS
can effectively cater to a subscriber density as
low as 0.2 subscribers per sq. km.
Use of the RBS therefore enables a corDECT
system to provide service in a 25 km radius. With
the DIU (along with the RAS) deployed at the
centre of a circle, the CBS's would be typically
deployed in six sectors. While subscribers in a
10 km radius would be served directly by these
CBSs, an RBS tower deployed in each of the
surrounding cells, as shown in Figure 4.12, would
enable 25 km coverage. One or more RBSs
could be deployed in each cell, depending on
the number of subscribers that need to be served
in the cell.
Thus, we see that by properly engineering the
deployment, it is possible to cost-effectively
provide telephone as well as Internet service to
rural subscribers in an area with a very low
subscriber density.
4.5 Franch|se Access Prov|der
As the Access Network is the most difficult part
of the telecom network to deploy, and the most
expensive and difficult part to maintain, it may
make sense for an operator to use Franchise
Access Providers (FAPs) to install and maintain
the last-mile access network. A FAP would
provide service in a locality and would connect
to the operators backbone network.
The corDECT system could provide an ideal
solution for such FAPs. The DIU acts as an in-
dialing PBX, with billing and subscriber
management available at the DIU itself. The DIU
would be given a level in the numbering plan for
switching incoming calls to it. The connection to
Figure 4.11 RBS serving remote subscribers in a 10 km radius
4 - 7
Figure 4.12 Sectorized RBS deployment
corDECT
corDECT Wireless Access System
30
the Local Exchange (of the FAP) would be an E1
trunk with R2-MF signaling for incoming calls.
All the incoming calls meant for the DIU would
be switched by the LE on this trunk interface.
The DIU would then complete the switching to
the subscriber. For outgoing calls, either the trunk
lines with R2-MF signaling, or subscriber lines
(using CSMUX), could be used.
In all other ways, this deployment scenario
appears similar to that of an Access Centre. The
CBSs would typically be co-located with the DIU;
yet some CBSs could be remotely mounted
using either twisted-pair wires or a BSD. The
Internet traffic is separated at the DIU and is sent
4 - 8
to the RAS. The statistically-multiplexed IP traffic
at the RAS is then output to an Internet router
through the Ethernet interface at the RAS and
one of several possible ways of establishing a
leased connection from the Ethernet port to the
Internet router could be used.
A FAP could also connect Internet servers at the
Ethernet interface (co-located with the RAS and
DIU) and provide services such as mail server,
web-server, etc. It is also possible to co-locate a
RADIUS server, used for Internet billing and
accounting, at this place. An integrated billing
software for voice calls and Internet service is
available (see Chapter 8).
corDECT
corDECT Wireless Access System
31
5.1 |ntroduct|on
The corDECT WLL system provides features and
services comparable to the best wireline
systems. In the Switch (Local Exchange) Mode,
it boasts of all the features of a large digital
exchange. The Wallset IP provides simultaneous
voice and Internet access (like an ISDN line) as
a basic feature that all subscribers can have.
Base Stations can be deployed in a multitude of
ways, some suited to an incumbent operator,
some to a greenfield operator, and others that
enable coverage of sparsely populated rural
areas. The system also has sophisticated
Operations and Maintenance support and a
Network Management System for managing a
corDECT network. The next few sections
describe some key features of the corDECT
system.
5.2 Vo|ce 0ua||ty
corDECT delivers the same toll-quality speech
performance as a good copper-based local loop.
Toll-quality voice is ensured by using 32 kbps
ADPCM for voice digitization as per the ITU-T
G.726 standard. ADPCM al so ensures
transparency to DTMF signals for Interactive
Voice Response Systems.
5.3 0ata 8erv|ces
The employment of 32 kbps ADPCM permits all
voice-band data services available from a
conventional wired connection. It is also possible
to occupy a double time slot on air to transmit at
64 kbps with error correction. This can be used
for data connectivity at speeds similar to the best
wireline speed. The speed of a modem/G3 fax
supported using 32 kbps ADPCM is 9600 bps,
but with a double slot connection V.34 and V.90
modems can operate at full speed.
5.4 |nternet Access 8peed
Internet Access is possible simultaneously with
a voice call using the Wallset IP. There are two
access rates: 35 kbps and 70 kbps, using one
and two time slots respectively.
5.5 Payphone|P60
The system supports payphone with battery
reversal as well as 12 kHz/16 kHz metering
pulses. The pulses are provided by the Wallset
for an external charge meter. The system also
supports a CCB payphone (battery reversal only).
5.6 8ystem 6apac|ty
Each corDECT system supports up to 1000
subscribers. Its Base Stations can evacuate more
than 150 E of traffic and funnel it to the telephone
network and Internet using up to eight E1 links.
5.7 A|r |nterface Transm|t Power
The power transmitted by a Wallset or Base
Station nominally is 250 mW during the burst, or
about 10 mW on the average. This ties in with
the need for small cells to enhance frequency
re-use and also conserves battery power.
5.8 Typ|ca| 68 6overage
The coverage achieved by corDECT is 10 km in
Line-of-Sight (LOS) conditions, made possible
by enhanced receiver sensitivity, a patented
timing adjustment feature and compact high gain
antennas. The non-LOS (NLOS) coverage varies
Chapler 5 corDLCT Iealures al a CIance
5 - 1
corDECT
corDECT Wireless Access System
32
from 400 m to 1 km depending on the way the
CBSs are installed. Chapter 7 provides further
details.
5.9 Typ|ca| R8 6overage
The Relay Base Station (RBS) can be at a
maximum distance of 25 km from the CBS and
it can serve subscribers in a 10 km radius around
it. The RBS is primarily meant to be used in rural
or sparsely populated areas. It also finds
occasional use in urban areas for covering regions
in shadow.
5.10 Authent|cat|on and
8ubscr|pt|on
Authentication is the process by which a
corDECT subscriber terminal is positively verified
as belonging to a legitimate subscriber of a
particular DIU. It is invoked during call setup for
every call. It can also be invoked during other
circumstances like termination of access of a
Wallset by the DIU. Authentication involves an
Authentication Key which is never transmitted
on air. The keys are maintained securely in the
system and are inaccessible to anyone.
Subscription is the process by which a subscriber
is added/deleted from the system and the
features the subscriber desires to have are
enabled. It is also the process by which the
system formally transfers the identity, such as
subscriber number, to the Wallset. The DECT
standard specifies the usage of On-Air Access
Rights procedures for the Wallset to obtain
access rights to the system.
The Wallset can use this to:
(i) gain access to the system and make calls
and
(ii) recognize the system in order to receive
calls.
The DIU can use this to:
(i) validate service requests from WS,
(ii) limit access to classes of service, and
(iii) recognize calls for valid Wallsets in order to
route calls to them.
5.11 Hajor 8ubscr|ber 8erv|ces
The corDECT system when operating in Switch
Mode provides all the services of a large modern
exchange. All the features and services specified
by major telecom administrations (like the Indian
Department of Telecommunications) in their Large
Exchange Specifications are supported. Some
of the important services are:
Standing Alarm Call Service
Occasional Alarm Call Service
Call Completion Supplementary Services
Absent subscriber
Do not disturb subscriber
Call waiting
Dual telephone number
Call Offering Supplementary Services
Call diversion on no reply
Call diversion on busy
Call diversion unconditional
Call Restriction Supplementary Services
Outgoing only lines
Incoming only lines
Outgoing call restriction service
Charging and Charge Debiting Supplementary
Services
Subscriber call charge meter
Subscriber bulk meter
Non metered lines
Automatic transferred charge
call (collect call)
5 - 2
corDECT
corDECT Wireless Access System
33
Three-Party Conference Calling
Billing for conference call
Rapid Call-Setup Supplementary Services
Abbreviated dialing
Fixed destination call on time-out
Non-Supplementary Services
Payphone service
Malicious call identification
Ring-back facility
Interception of calls
Priority lines
CLI and CLI restriction
5.12 Hajor 8w|tch Features
The corDECT system when operating as a Local
Exchange, provides the operator extensive
numbering, routing, traffic monitoring, and testing
facilities. The major features are:
Exchange Code Numbering Plan
Digit Analysis Access Check
Digit Analysis Routing
Digit Analysis Charging
Operator Trunk Offer
Temporary Out-of-Service Subscriber
Hunting for a Group of Subscribers
Subscriber Line Supervision
Speech monitoring by intelligence
agency
PSTN line supervision
Total exchange meter and junction
metering
Measuring subscriber
supplementary service utilization
Measuring BHCA (regular
measurement)
Measuring Erlang for a period
(occasional measurement)
Measuring call attempts (regular
and occasional)
Logs for congestion
Periodic testing of subscribers
Periodic testing of junctions
Facility for multiple printers
Facility to execute commands from
calendar
Copy switching in hot standby mode
5.13 0H6 Features
The corDECT system s Operati on and
Maintenance Console supports the following:
System Administration Features
Subscriber administration
E1 line administration
Traffic measurements
Billing database
PSTN ports and CBS administration
System Maintenance Features
Health monitoring of all DIU cards and
sub-systems
Facility to test E1 interface
Monitoring of CBS/BSD interface
CBS software upgradation
Alarm conditions
Log files
Silent polling of Wallsets from the DIU
Traffic Analysis
Exchange traffic
CBS traffic
Subscriber traffic
Total number of call attempts
5 - 3
corDECT
corDECT Wireless Access System
34
Total number of successful calls
Call failures
Holding time of calls
Traffic on CPU of OMC
Traffic on printer
5.14 Hax|mum 68-0|U 6opper
0|stance
Two versions of the CBS are available: one
supporting a maximum loop resistance of 540
(3 km copper) and the other a maximum loop
resistance of 820 (4 km copper). In both cases,
a mix of 0.4 mm and 0.5 mm diameter copper
wire can be used.
5.15 Hax|mum 68-80 6opper
0|stance
The BSD supports a maximum loop resistance
of 200 , with a mix of 0.4 mm and 0.5 mm
diameter copper wire.
5.16 0|U Power 8upp|y
The DIU works off a -48 V DC exchange power
supply. The current requirements are very
modest. A fully loaded DIU typically requires, at
most, 14 A and significantly less if the CBSs
are at short distances from the DIU. If the CSMUX
is employed in the Transparent Mode of operation,
an additional 3 A is needed for every 240 lines.
5.17 wa||set and Hu|t|wa||set
Power 8upp|y
Wallset IP: The Wallset (or WS-IP) is powered
from the mains through an external 12 V adapter
drawing a maximum of 500 mA. The backup
battery is a 6 V/1.3 Ah sealed lead-acid
rechargeable type.
Multiwallset: The Multiwallset is powered from
the mains (85 V - 265 V AC, 45 - 65 Hz) and has
a 12 V/7.2 Ah sealed lead-acid rechargeable
battery for back-up. The Multiwallset draws a
maximum of 50 VA from the mains.
5.18 wa||set and Hu|t|wa||set Ta|k
T|me|8tandby T|me
The Wallset IP has a talk time of 3.5 hrs and a
standby time of 16 hrs.
The Multiwallset has a talk time of 4 hrs/line and
a standby time of 16 hrs.
5.19 R8 Power 8upp|y
The RBS is a stand-alone unit. The required
supply is drawn from any one of three sources:
i. 95 to 265 V AC mains
ii. 40 W solar panel (of approximate size 88 x
44 cm)
iii. 12 V/40 Ah rechargeable maintenance-free
lead-acid battery
This design ensures 36 hrs operation on any one
of the three power sources and the battery can
be charged by any of the other sources.
Alternatively, if a -48 V DC battery-backed supply
is available, it can be used to power the RBS.
5.20 80 Power 8upp|y
The BSD is powered by -48 V DC and requires a
maximum current of 1.3 A.
5 - 4
corDECT
corDECT Wireless Access System
35
5.21 0ther Features
5.21.1 Phys|ca| 0|mens|ons
5.21.2 we|ghts
5.21.3 Env|ronmenta| 6ond|t|ons
Al l the sub-systems meet the Indi an Department of
Telecommunications environmental specification QM333. They are
also compliant to the relevant ETSI/IEC/CISPR EMI/EMC
specifications.
DIU 145 cm (H) x 55 cm (W) x 33 cm (D)
CBS 24 cm (H) x 16.5 cm (W) x 9.5cm (D)
Wallset 20 cm (H) x 20 cm (W) x 4 cm (D)
BSD 8 cm (H) x 45 cm (W) x 18.5 cm (D)
Multiwallset DTM 10.5 cm (H) x 10.5 cm (W) x 7.5 cm (D)
SIM 27cm (H) x 20 cm (W) x 12 cm (D)
RBS Air Unit 23 cm (H) x 30 cm (W) x 8.5 cm (D)
Ground Unit 23 cm (H) x 30 cm (W) x 8.5 cm (D)
DIU 90 kg approx.
CBS 1 kg approx. without fixtures
Wallset 1.3 kg
BSD 2 kg
Multiwallset DTM 0.4 kg
SIM 3.2 kg
RBS Air Unit 1.5 kg
Ground Unit 1.5 kg (without battery)
5 - 5
corDECT
corDECT Wireless Access System
36
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corDECT
corDECT Wireless Access System
37
6.1 |ntroduct|on
The corDECT systems modular design, along
with its add-on sub-systems like the Base Station
Distributer (BSD) and Relay Base Station (RBS),
permits the operator to dimension the system to
cater to a wide range of subscriber densities,
teletraffic levels, and deployment scenarios. In
this chapter, we discuss the traffic-carrying
capacity of the system for voice and Internet
services. The discussions below consider the
case of 35 kbps Internet access.
6.2 8ystem 6apac|ty
Each DIU can support up to 1000 subscribers,
irrespective of whether they are connected using
Wal l set, Wal l set IP, or Mul ti wal l set. Al l
subscribers can have both voice and Internet
access, thus providing the equivalent of 2000
lines in a conventional wireline system.
The maximum number of Compact Base Stations
supported by a DIU is 20. A BSD provides remote
support to four CBSs, in lieu of four connected
directly to the DIU. Each DIU currently supports
up to eight E1 links, which can be used to carry
voice traffic to PSTN and Internet traffic to the
Chapler 6 Syslen Dinensioning in corDLCT
6 - 1
RASs. There is no limit to the number of RBSs
that can be deployed; however, a RBS cannot
be daisy-chained to another RBS.
6.3 Traff|c 6apac|ty of 68
A CBS can support 12 simultaneous voice/
Internet calls. At 1% Grade of Service (GOS), 12
channels can support 5.8 E traffic. If all 20 CBSs
in a system are deployed in distinct coverage
areas, a total of 116 E traffic can be supported.
These distinct coverage areas can be circular cells,
or angular sectors, as illustrated in Figure 6.1.
The former uses omnidirectional antennas, while
the latter requires directional antennas.
When two CBSs are deployed covering the same
area (cell or sector), a subscriber can access
one of 24 channels, since a call can be placed
through any CBS. With 24 channels, 15.3 E traffic
can be supported at 1% GOS. Thus there is a
significant gain in capacity (7.6 E per CBS now)
when two CBSs share a common coverage area.
If the number of CBSs servicing a cell or sector
is increased to three, 36 channels carry 25.5 E
at 1% GOS. This is equivalent to about 8.4 E per
CBS, amounting to an even more efficient use of
the 12 channels available per CBS.
Figure 6.1 Circular and sectoral deployment
corDECT
corDECT Wireless Access System
38
Thus, when a cell is divided into six sectors and
each sector is serviced by three CBSs each, a
total of 153 E can be evacuated from just one
site, assuming that there are sufficient DECT
channels available to do so. We shall see in a
later section that this is indeed possible.
If a CBS with omnidirectional antennas is
deployed in addition to those serving the six
sectors, as depicted in Figure 6.2, the capacity
goes up significantly. This is because the omni
CBS handles the overflow traffic from all the
sectors. Only if a call cannot be serviced either
by the CBS's in a sector or by the omni CBS, is
it blocked. Since the omni antenna will have lower
gain than the sectoral antenna, a call will be
usually serviced by the sectoral CBSs. With
three CBSs per sector in six sectors and two
omni CBSs (20 CBSs in all), the total traffic
that can be evacuated is approximately 176 E,
amounting to 9 E per CBS.
6.4 Traff|c 6apac|ty of the 0|U
The traffic evacuated by the CBS is delivered to
the DIU, which is a non-blocking switch. That is,
all 240 channels from 20 CBSs (12 x 20) and all
240 channels from eight E1 links (30 x 8), can
be switched without blocking. Hence, the DIU
can be ignored while determining traffic capacity.
The DIU supports up to eight E1 ports towards
the RAS and PSTN. The maximum number of
calls that can be set up through 20 CBSs is
240, which is the same number of circuits
supported by eight E1 ports. The E1 ports can
be apportioned between voice and data as
desired, depending on an estimate of the relative
proportion of voice and Internet calls. For example,
of the 150 E that can be evacuated by 18 CBSs
(in a six-sector cluster), let the voice and Internet
traffic be 75 E each. If we generously assign four
E1 ports for voice traffic and four ports to the
RASs, each set of four E1

ports support 75 E
traffic with negligible blocking.
Hence, the overall GOS, taking into account
blocking on air as well as blocking at the E1
ports, is close to 1%. However, only three E1
ports each need be assigned to voice and Internet
traffic, as the traffic supported by three E1s is
72 E with 0.5% GOS. The CBSs support a total
of 144 E at 0.5% GOS, giving an overall GOS
of 1%.
If the voice traffic component is 100 E instead
(say, 0.1 E per subscriber), we need to assign
four E1 ports for voice. These can carry 100 E
traffic with 0.5% GOS. The Internet calls are
assigned two E1 ports, which support 45 E traffic
with 0.5% GOS. The CBSs can evacuate a total
of 144 E with 0.5% GOS, resulting once again in
an overall GOS of 1%.
Thus, the E1 ports can be assigned to voice and
Internet traffic in the proportion needed, depending
Figure 6.2 Three CBSs per sector and two omni CBSs
6 - 2
corDECT
corDECT Wireless Access System
39
on the requirements. The upper limit to the traffic
carried in the corDECT system comes from the
maximum limit of 20 CBSs, which provide 240
active voice/Internet channels at any given time.
6.5 Traff|c 0|mens|on|ng from
RA8 to |nternet
The RAS statistically multiplexes the bursty IP
packet traffic from several subscribers. The bitrate
of the link from the LAN port of the RASs to the
Internet can be significantly less than the total
bitrate of the E1 ports connected to the RASs.
The concept of activity factor plays a role here.
The activity factor is the fraction of the peak
bitrate a subscriber uses on the average. For
example, a user may have only 3 kbps average
throughput on a 35 kbps link due to the bursty
nature of traffic, representing an activity factor of
around 10%. Since each RAS has two E1 ports
(60 channels) towards the DIU, with 3 kbps
throughput per channel, the total throughput of
the RAS is only 180 kbps.
During a burst of packets, however, the bitrate
can reach the peak value. Of the 60 calls
simultaneously supported by a RAS, a certain
small fraction may pump data at the peak rate at
the same time. For the subscribers to get quick
response, the link between the RAS and the
Internet router needs not only to have a capacity
equal to the total throughput (activity factor x 30
x number of E1 ports carrying Internet calls), but
also close to the total simultaneous peak traffic.
For example, if 10 out of 60 calls are expected
to simultaneously have peak traffic of 35 kbps,
the peak rate would be 350 kbps and the link
from the RAS to the Internet needs to have a
bitrate of 350 kbps.
6.6 Re-Use Eff|c|ency |n cor0E6T
The corDECT system employs the DECT
standard, one of whose stellar features is
Dynamic Channel Selection (DCS). A brief
introduction to the DECT standard is given in the
Appendix. The main aspects of DCS relevant to
our discussion at this point are (i) no frequency
planning is required, (ii) at any point in space
and time the Wallset (WS) chooses the strongest
CBS to lock to and the quietest channel to set
up a call (the WS selects the channel for both
outgoing and incoming calls), (iii) with 10
frequencies, 120 channels (in a 17.28 MHz band)
are available for each WS to choose from, and
(iv) a WS hands over the call seamlessly to
another channel even while a call is going on, if a
significantly better channel is found. The WS
continuously scans all channels and updates its
table of channel quality.
The DECT standard has excellent specifications
for receiver performance in the presence of co-
channel and adjacent channel interference. The
corDECT system has significantly better
performance than specified by DECT in many of
these categories. The net result is that a
subscriber can establish a call even with a Carrier-
to-Interference (C/I) ratio as low as 10 dB.
In corDECT WLL deployment, extensive use is
made of directional antennas. These give several
advantages apart from high gain. Being
directional, the transmitted power is primarily
focussed in one direction and the antenna has a
high front-to-back ratio. These contribute towards
reducing interference.
All these factors contribute to making corDECT
a very high capacity system with excellent
frequency re-use.
6.7 6apac|ty of Hu|t|-6e||u|ar
Tower-Hounted 68 6|uster
We have seen that three CBSs serving one 60
0
sector support 25.5 E traffic at 1% GOS, provided
DECT channels are available to set up the calls.
The question, therefore, is: what traffic level can
6 - 3
corDECT
corDECT Wireless Access System
40
actually be supported per cell in a multi-cellular
deployment, when each cell has six sectors with
multiple CBS's in each sector?
This question has been investigated extensively,
taking all types of interference (co-channel,
adjacent channel and intermodulation) and non-
idealities (e.g., non-ideal antenna radiation
pattern) into account.
Figure 6.3 depicts the sectorized multi-cellular
deployment. It also indicates, for one sector, the
few regions in which the interference is significant.
The use of directional antennas at CBS and WS
restricts the interference-generating regions to a
minimum. It must also be remembered that all
the CBSs transmit together in one 5 ms period,
as do the Wallsets in the next 5 ms period. Thus,
when a Wallset is receiving from a CBS,
interference is present only from other CBS's in
the vicinity. Likewise, interference to a CBS can
come only from WS transmissions.
Simulations indicate that about 200 E can be
evacuated per cell (i.e., 33 E per sector) at 1%
GOS in a six-sector multi-cellular deployment.
This will, of course, require more than three CBSs
per sector. Thus, the traffic of 25 E that three
CBSs can handle per sector is easily supported
on air. Even with a couple of additional omni
CBS's, DECT channels can be found for
evacuating the maximum capacity of 176 E per
cell.
The total capacity of a cell for a different number
of CBSs is shown in Table 6.1.
We see that a configuration can be found for any
traffic level from 6 E to 180 E. It is possible to
deploy additional CBSs as the subscriber base
and consequently the traffic, grows. The omni
CBS, apart from increasing capacity, also
provides redundancy in case of CBS failure. A
DIU located in one cell which is only partially
loaded with CBSs can, for example, support
eight CBSs (six-sector deployment with one CBS
per sector and two omni CBSs) and evacuate
65 E traffic from an adjacent cell. Two BSDs
can be used to link the eight CBSs at the
adjacent site to the DIU. Thus the corDECT
system is highly modular, enabling great flexibility
in deployment.
6.8 how h|gh 6apac|ty |s
Ach|eved
Simple calculations help establish how the
capacity of 153 E per site (in a six-sector, three
CBS/sector configuration) is possible. To service
25 E per sector, we need 25 channels on the
average in each sector. It is highly improbable
that the number of channels used will be more
than this average number simultaneously in all
sectors. That is, the total number of channels
needed for six sectors will be close to 150 most
of the time and not 216 (18 CBS x 12 channels
per CBS), which is theoretically possible. With
a front-to-back ratio of more than 20 dB for the
CBS antennas, the same channel can be re-used
6 - 4
Table 6.1 Capacity of six-sector cell
for different CBS configurations
CBS
configuration
Capacity at 1%
GOS
1 omni 5.8
2 omni 15.3
3 omni 25.5
1 x 6 sectors 34.8
1 x 6 sectors + 1 omni 50.0
1 x 6 sectors + 2 omni 65.0
2 x 6 sectors 91.8
2 x 6 sectors + 1 omni 102.0
2 x 6 sectors + 2 omni 131.0
3 x 6 sectors 153.0
3 x 6 sectors + 1 omni 158.0
3 x 6 sectors + 2 omni 176.0
corDECT
corDECT Wireless Access System
41
in sectors 180
0
apart. With this re-use within a
cell, a cell needs only 75 channels to handle the
total of 150 E traffic. Adjacent cells can also
employ the same channels, DCS ensuring that
a channel is re-used in a region where interference
is low (such regions are shown unshaded in
Figure 6.3). In any case, 45 more channels are
available to choose from in case a channel
cannot be re-used in adjacent cells at certain
times.
An alternative way of understanding how such a
high capacity is obtained, is to compute an
equivalent re-use factor. In a system that requires
frequency planning, the re-use factor is the
number of sets into which the total number of
channels are grouped for allocation to different
sectors, so that the interference is within
permissible limits. The smaller the factor, the
more efficient the re-use, the larger the number
of channels available per sector. In TDMA
systems like GSM, a re-use factor of four is now
common. In a six-sector corDECT cell supporting
176 E traffic (see last entry in Table 6.1), we need
30 channels per sector. This implies an equivalent
re-use factor of four, given that we have 120
channels in all. Since, the equivalent re-use factor
with DCS is typically much higher than in a
frequency-planned system, a factor of four is not
surprising at all.
6.9 6apac|ty |n h|gh-R|se
Hetropo||tan 0ep|oyment
When the buildings in an area are very high (eight
storeys or more), the modular CBS along with
the BSD can be exploited to support very high
subscriber density.
The CBS can be deployed on the corners of the
roof of a high-rise building, with directional
antennas. The CBS in each corner illuminates
one or more faces of several buildings in the
vicinity. Each building will have at least several
10s of potential subscribers. One or more CBSs
can serve the corDECT subscribers in these
illuminated buildings. Since sufficient signal
strength will be available inside the building on
all floors on the illuminated sides, WS deployment
is easy. The number of CBSs required per corner
will depend on the traffic.
DECT channel availability is hardly an issue in
this type of deployment. The channels can be
re-used on every street, since the large buildings
themselves block interference from neighboring
streets. DCS will ensure that the channels are
optimally chosen for each call.
This type of deployment will have CBS sites every
400 m or so on a street and typically on alternate
streets. The CBS can be connected to the DIU
Figure 6.3 Multi-cellular deployment showing interfering regions
6 - 5
corDECT
corDECT Wireless Access System
42
using three pairs of copper (per CBS), if copper
is available. Alternatively, if fibre connectivity is
available, the BSD can be used to connect the
CBS to the DIU by using an E1 link provided by
the optical fibre system. The BSD can also be
deployed with HDSL modems, which support an
E1 link on a single copper pair.
6.10 6apac|ty of R8
A RBS can support up to 11 simultaneous calls.
At 1% GOS, this gives a capacity of 5.1 E. Thus,
for example, one RBS can easily support nearly
75 rural subscribers at 0.07 E per subscriber.
The CBS dimensioning is done by adding the
traffic handled by the RBS in a sector, to the
traffic emanating from WSs directly served by
the CBS. The GOS for subscribers served by an
RBS is determined by adding the GOS of the
CBS's to the GOS of the RBS. For example, if
the RBS as well as the CBS in a sector provide
1% GOS, the overall GOS for an RBS subscriber
is 2%. This can, however, be made close to 1%
by increasing the number of CBSs (typically one
more CBS will do) so that the blocking probability
of the CBS becomes negligible.
If the area served by an RBS site happens to
have high subscriber density, or heavy traffic,
multiple RBSs can be deployed on the same
tower. Two RBSs, with 22 channels, gives a traffic
capacity of 13.7 E at 1% GOS. Once again, the
flexibility of corDECT comes to the fore in enabling
an operator to meet any situation in the field.
A typical rural deployment using RBS is shown
in Figure 4.12 of Chapter 4. If we assume that
each of the cells surrounding the central cell has
about 75 subscribers generating a total of 5.1 E
traffic per cell, one RBS will be sufficient in each
cell. The central cell may have about 200
subscribers (since it also possibly contains the
main town of the area) generating, say, 20 E
traffic among them. The total traffic is 50.6 E (6 x
5.1 E + 20 E), indicating that one CBS per sector
along with one omni CBS (having a capacity of
50 E, as per Table 6.1) will suffice. High traffic
levels in any of the surrounding cells can be
handled by adding a second RBS in that cell, as
well as an extra CBS in the corresponding sector
of the central cell serving that RBS.
Finally, an RBS site may occasionally be
deployed such that its coverage area overlaps
partially with that of a CBS, in order to serve a
shadow area created by a topographical feature.
This is illustrated in Figure 6.4.
6.11 8ummary
We have seen that, with just 120 channels in a
17.28 MHz band, corDECT can support both a
large coverage area (25 km radius) and traffic
density as high as 2000 E/sq. km (20 E in a
200 m x 50 m high-rise area). The corDECT
system can be easily dimensioned and deployed
in a modular and cost-effective manner.
6 - 6
Figure 6.4 RBS providing coverage to shadow area
corDECT
corDECT Wireless Access System
43
7.1 |ntroduct|on
The DECT standard, on which corDECT is based,
originally focussed on high subscriber density,
high traffic capacity, low cost, microcellular
deployment, in applications such as wireless
local loop, wireless PBX, pedestrian PCS, and
home cordless systems. The standard supports
toll-quality voice service and high bitrate data
services, with a number of features that are
available only in 2.5G and 3G mobile cellular
standards. All these capabilities make DECT
immensely attractive for the WLL application,
provided one finds ways to deploy a DECT- based
WLL system with a wide range of cell sizes. The
desired cell radius could be anywhere from
0.5 to 3 km in urban areas and up to 20 to
25 km in rural areas.
7.2 cor0E6T Enhancements
7.2.1 |mproved 8ens|t|v|ty
The DECT standard requires the sensitivity
(defined as received power level at which the Bit
Error Rate is 10
-3
) to be only -83 dBm for the
Wallset and -86 dBm for the Base Station. In
corDECT, the sensitivity achieved is typically
-91 dBm. While the sensitivity indicates the
minimum received signal level that is acceptable,
one would want to have a Bit Error Rate (BER)
lower than 10
-5
most of the time. The DECT
standard requires that at -73 dBm, the BER
should be better than 10
-5
. In corDECT, this has
been improved to -86 dBm. Thus, the link quality
in corDECT will be very robust as long as the
received level is more than -86 dBm. An
occasional reduction in received power due to
fading is acceptable, as long as the level rarely
drops below -91 dBm.
7.2.2 T|m|ng Adjustment
As in any TDMA standard, a guard time is
introduced in DECT between time slots. This
allows for the transmission in one slot from a
distant WS (which travels longer) to arrive a little
late, without interfering with the signal in the next
slot which may arrive at the slot edge. This is
illustrated in Figure 7.1.
The guard time provided in the DECT standard
allows for the WS to be only a maximum of 5 km
from the CBS. This was considered adequate for
the intended applications and for the sensitivity
specified.
Chapler 7 Air Inlerface LinI Budgels and CeII IIanning
7 - 1
Figure 7.1 Time-of-arrival of signals from two WSs at differing distance from CBS
corDECT
corDECT Wireless Access System
44
With the improved sensitivity in corDECT and the
availability of low-cost, high-gain antennas (see
section 7.2.3), it is easy to get the required signal
level at a distance much greater than 5 km, under
Line-of-Sight (LOS) conditions. In order to take
advantage of this, a patented timing adjustment
scheme is incorporated in corDECT. In this
scheme, the subscriber terminal (WS/WS-IP/
MWS) and RBS adjust their transmission based
on the distance from the CBS, thus ensuring that
the signals are received by the CBS within their
respective time slots. Using this technique, the
WS can be as far as 10 km and the RBS as far
as 25 km, from the CBS. The difference in
maximum range in these two cases comes from
the respective link budget constraints, as we will
see in the following sections.
7.2.3 Low-6ost, 6ompact, h|gh-Ca|n
Antennas
The frequency of operation in DECT is around
1.9 GHz, at which the wavelength is only 15 cm
(approximately). Compact, but high-gain, patch
antennas that are also inexpensive, can be
designed for such high frequencies. Several
antennas have been specifically developed for the
corDECT system. A few are shown in Figure 7.2.
A 7.5 dB patch antenna (Figure 7.2 (a)) is most
commonly used for the WS/WS-IP. This antenna
weighs a mere 150 gm, has a horizontal beam
width (HBW) of approximately 80
0
and a vertical
beam width (VBW) of approximately 50
0
. This
means that the antenna can be installed without
any special aids, simply by visual alignment of
the antenna towards the CBS. Its minimum Front-
to-Back Ratio (FBR) is 15 dB, ensuring good
rejection of signals from other CBSs in the
vicinity. The MWS has a similar antenna built
into its outdoor unit (called DTM).
A 12 dB antenna, with a HBW of 60
0
and VBW
of 25
0
, is shown in Figure 7.2(b). This antenna is
primarily used for sectorized deployment of the
CBS. Like the 7.5 dB WS antenna, it is light (~300
gm) and small. Its minimum FBR is 20 dB,
ensuring good frequency re-use when deployed
with CBSs in sectors 180
0
apart.
If a CBS is to be deployed with 360
0
coverage,
the omnidirectional antenna shown in Figure
7.2(c) can be used. It has 6 dB gain and 10
0
VBW.
Similar antennas with 9 dB and 11 dB gain are
available. Their VBW is smaller, necessitating
accurate verticality during mounting.
A small and light grid antenna with a gain of
20 dB is available for use with the RBS, for the
RBS-CBS link. This high-gain antenna ensures
a sufficient signal level even at a distance
of 25 km.
These low-cost, specialized antennas and
associated low-cost cables, are key components
(a) (b)
(c)
7 - 2
Figure 7.2 Three different antennas
corDECT
corDECT Wireless Access System
45
of the corDECT system that make it a versatile
WLL system. The antennas, cables, and
accessories are robust and weatherproof,
designed to function for several years without
deterioration.
7.3 L|nk udgets w|th cor0E6T
The transmit power and sensitivity for all corDECT
sub-systems are nearly the same and as
mentioned above, significantly better than the
minimum specified in the DECT standard. For
the purpose of link analysis, we take the transmit
power as 23 dBm and the sensitivity as 90 dBm.
Thus, the link budget is 113+G
B
+G
w
, where G
B
and G
w
are the antenna gains at CBS and WS,
respectively.
In the case of the RBS, G
B
and G
w
represent the
RBS omnidirectional and WS antenna gains,
respectively for the RBS-WS link, while they are
the CBS and RBS grid-antenna gains respectively
for the CBS-RBS link (see Figure 3.4 and Figure
3.11, Chapter 3).
7.4 Path Loss Hode|s
The loss 1 m from an antenna at 1.9 GHz is
38 dB (=20log
10
4). To this, we add a loss of
20log
10
d (where d is in metres), when the link is
Line-of-Sight (LOS). When the link is non-LOS
(NLOS), the path l oss model i s more
complicated. For the case when the CBS is
mounted on the rooftop (say, on a 3 m pole on
the roof) a loss model recommended by ETSI
[ETR 139, Nov. 1994] is 38+20+35log
10
(d-10) for
d>10 m. Here, it is assumed that the loss is as
in free space (proportional to d
2
) up to 10 m and
proportional to d
3.5
thereafter. The model is
presented in an equivalent simplified form in
Table 7.1.
When the CBS is mounted on a mast/tower, rising
high above the buildings, the loss model is even
more complex. We denote by the term skyline
the typical height of the buildings in the coverage
area. For example, in an area where many
buildings are three-storeyed (G+2), the skyline
would be 10 m. When the tower is significantly
higher than the skyline (e.g. if the tower height is
double the skyline), the free-space component
is significantly larger than in the NLOS model for
rooftop CBS deployment. The loss is a function
of the proportion of the path that is free-space
which, in turn, is a function of (i) tower height, (ii)
skyline and (iii) distance between CBS and WS.
A computer model, based on ray tracing, is used
for this case. Table 7.1 summarizes the models
discussed above.
In addition to the above loss models, it is useful
to note that the loss at 1.9 GHz through a
9" brick wall or 5" RCC floor is about 7 dB. Thus,
the signal strength outside a window is often
7 dB higher than that inside. Further, the loss
through foliage at 1.9 GHz can be anywhere from
6 - 16 dB depending on the foliage density.
Signals passing through the foliage of a tall but
relatively thin tree (like a coconut or eucalyptus
tree) undergo only about 6 - 7 dB loss, while a
big, but shorter tree of, say, 10 m height gives a
loss of 10 - 15 dB. However, it will generally be
possible to get LOS by rising above the treeline
in the latter case.
Table 7.1 Path loss models
7.5 Fade Harg|ns |n cor0E6T
The corDECT system employs antenna diversity
at the Base Station (at both the CBS and RBS),
incorporating an important option provided by the
DECT standard. Since DECT is a Time Division
Duplex (TDD) standard and the same frequency
Type of link Loss Model
LOS 38+20log
10
d, d>1 m
NLOS, roof-top 23+35log
10
d, d>10 m
NLOS, tower-
mounted
Computer-based ray
tracing
7 - 3
corDECT
corDECT Wireless Access System
46
is employed in both directions of a link, it is
sufficient to employ diversity at the Base Station
alone. Antenna diversity reduces the fade margin
needed by half. According to ETSI [ETR 139],
the fade margin needed with diversity for the
NLOS link is between 8 - 10 dB and that for the
LOS link is between 4 - 5 dB. The lower limits
are sufficient when the antennas employed have
high gain. In corDECT deployment, we provide a
fade margin of 10 dB for non-LOS links and 5 dB
for LOS links. This fade margin ensures that the
BER is less than 10
-6
most of the time, going up
to 10
-3
only occasionally during fades.
7.6 6ab|e Losses
The corDECT CBS is designed for outdoor
(rooftop/tower) mounting, very close to the
antenna. The RF cables connecting the CBS to
its antennas (two, for diversity) are nominally
0.5 m long. The loss is therefore less than 1 dB.
The same is true for the RBS antennas.
In the case of the WS the cable length will vary,
depending on the subscriber location. The WS,
in general, will be inside the subscriber premises,
close to a window. In case the subscriber is able
to get NLOS coverage, the antenna will be
mounted outside the window and the RF cable
will be 1 - 3 m long. If the link is LOS, the WS
antenna will be on the rooftop and the RF cable
will be 3 - 10 m long.
Two types of RF cable are recommended for WS
installation: a flexible (0.25 inches dia.) low-cost
(LC) cable with a loss of around 0.5 dB/m and a
low-loss (LL) cable with a loss of around
0.2 dB/m. The LC cable is sufficient for most
WS installations and also for the CBS and RBS.
7.7 L|nk Ana|ys|s for cor0E6T
Let us now analyze corDECT links of different
types and for different scenarios. In our
computations below, we denote the total cable
losses as LC (dB). Further, we refer to the excess
signal strength available over the sensitivity limit
of 90 dBm as the link margin. The cable losses
L
C
also have to come from this link margin.
7.7.1 L08 L|nk
The link budget in this case (see section 7.3) is
113+G
B
+G
W
, the path loss (see Table 7.1) is
38+20log
10
d and the fade margin is 5 dB. Thus,
the maximum distance d is obtained from the
constraint: 70+G
B
+G
W
-(L
C
+20log
10
d) > 0.
Let us consider the case of a 10 km LOS link
between the CBS and WS. For a 12 dB sectoral
antenna at CBS and a 7.5 dB patch antenna at
WS, (L
C
+20log
10
d) can be at most 89.5. At
10 km distance, this implies that the link margin
is 9.5 dB, which is ample for even the LC cable.
In case the lower-gain omni antenna (6 dB gain)
is used for the CBS, the link margin is reduced
to 3.5 dB at 10 km. We may now need to use
either the LL cable or 12 dB antenna for a WS
which is installed 7 - 10 km from the CBS.
Figure 7.3 shows the link margin available on a
LOS link as a function of d, for G
W
= 7.5 dB and
for various values of G
B
. The curves tell us how
much margin we have (including cable loss) at
various distances for various CBS antenna gains.
The RBS-WS link is similar to the one discussed
above. We typically employ omni antennas with
6 or 11 dB gain for the RBS. The above analysis
holds and Figure 7.3 can be used to determine
the link margin at various distances from the RBS.
In the case of the MWS, L
C
= 1 dB at the most
(cable loss only on the CBS side). Therefore, a
MWS can be deployed up to 10 km away
irrespective of the CBS antenna type.
7.7.2 NL08 L|nk from Rooftop 68
In the case of a NLOS link from a rooftop CBS,
mounted just 3 - 6 m above the roof of a much
taller building (12 - 20 m), the NLOS loss model
23+35log
10
d applies for d > 10 m (see Table 7.1).
7 - 4
corDECT
corDECT Wireless Access System
47
Let us consider a 6 dB omni antenna at CBS
and 7.5 dB patch antenna at WS. The link budget
from section 7.3 is therefore 126.5 dB. The fade
margin needed for a NLOS link is 10 dB and the
total cable loss for an indoor NLOS installation
is less than 2 dB, giving a total loss of 12 dB.
We are now ready to determine the maximum
NLOS link distance for this case. It is given by
the constraint 23+35log
10
d > 126.5-12, or
35log
10
d > 91.5. This gives d
max
as 400 m. At
distances less than about 250 m, even a whip
antenna directly screwed on to the WS/WS-IP
(thus avoiding an RF cable) will be sufficient.
In the case of a rooftop CBS deployment, the
coverage area increases significantly if the
building, on whose roof the CBS is installed, itself
does not create a shadow for the other buildings
in the coverage area. If the roof area is large, this
can be ensured by mounting multiple CBS,
preferably with sectoral antennas, on the corners
of the roof.
7.7.3 NL08 L|nk from Tower-
Hounted 68
Let us now consider the case of the CBS on a
tower. Typically, the skyline for this type of
deployment will be 7 - 14 m, (two to four storeyed
buildings) and the tower will be 10 - 15 m above
the skyline. This is illustrated in Figure 7.4.
The path taken by the RF signal between the
CBS and WS has a significant free-space
component in this case. The path loss is
obtained using a computer model developed by
the TeNeT Group, IIT Madras. Figure 7.5 shows
the contour for which the mean signal strength
received outside the window is -77 dBm, when
Figure 7.3 Link margin for LOS link vs. distance
7 - 5
Figure 7.4 CBSs mounted on a tower above the skyline
corDECT
corDECT Wireless Access System
48
the skyline is 10 m and the tower height above
the skyline is 15 m. The CBS antenna gain is
taken as 12 dB and the WS antenna gain as 7.5
dB. The contour shown allows for a fade margin
of 10 dB and 3 dB for cable loss L
C
.
The region above the contour is the NLOS
coverage region. The distance from the CBS is
indicated on the bottom x-axis and the height of
the WS antenna from the ground is indicated on
the y-axis. We can see that sufficient signal
strength will be available on the ground up to a
distance of 750 m and at the first floor (5 m) up
to about 900 m. The distances will increase
somewhat if a significant number of buildings
are lower than the skyline.
Once we reach the skyline, the signal strength
is more than sufficient even at a distance of
10 km, since we have a LOS link. The LOS signal
levels are indicated as a function of distance along
the x-axi s on the top of Fi gure 7.5, for
convenience.
When the skyline is less than 12 - 14 m (four-
storeyed buildings, or less), this means that at
most an RF cable of 10 - 12 m will be needed to
reach the skyline. The low-cost (LC) cable of this
length will entail a loss of 5 - 6 dB, for which
there is more than ample margin, given the strong
signal level as one approaches the skyline.
Thus, a WS antenna can be installed outside a
window at various heights all the way up to the
skyline, provided one is able to rise above the
contour. If a building happens to be shorter than
the skyline and it does not get NLOS coverage
on its roof, one will need a pole (typically of length
2 - 6 m) on the roof to reach the skyline.
7.7.3.1 Tower he|ght and NL08
6overage
The contour moves to the right as the ratio of
tower height to skyline increases and to the left
as the ratio decreases. As an approximate rule-
of-thumb, a small change in the ratio of total
tower height (from the ground) to the skyline,
produces a proportionate shift in the contour. That
is, a change of -10% (tower height of 12.5 m in
Figure 7.5) in the ratio produces a leftward shift
of around 10% (about 75 m). For larger changes,
the shift is not proportional, and the contour has
to be re-computed using the computer model.
7.7.3.2 8hadow|ng
In case there are a few tall trees higher than the
skyline, they will typically cause a shadow loss
of about 7 dB. In most cases one will continue to
get sufficient signal above the skyline. However,
if there is a large obstruction taller than the skyline
(say, a big monument), there may be a region
behind the obstruction that is without coverage.
In most situations like this, coverage will be
available from another CBS site a few kilometers
away on the far side of the obstruction.
7 - 6
Figure 7.5 NLOS coverage as a function of distance for tower-mounted CBS
corDECT
corDECT Wireless Access System
49
In summary, one can conclude that for tower-
mounted CBSs in areas with a skyline less than
four storeys high, NLOS coverage will be available
typically within a radius of 1 km, provided the
tower height above the skyline is at least as much
as the skyline. The area beyond this can be easily
covered by LOS links to the roof of the subscriber
premises. Typically, one would expect to have a
DIU site every 2 - 3 km in order to support sufficient
subscriber density. The coverage provided by
corDECT with a tower-mounted CBS is well
suited for this type of deployment.
7.7.4 68-R8 L|nk
The CBS-RBS link is LOS. The total cable loss
in this link is not more than 2 dB as the antennas
in both cases are mounted close to the
equipment. The antenna gain G
w
for the RBS grid
antenna is 20 dB. For the case of 12 dB sectoral
antennas at the CBS, the link budget is
145 dB (113+12+20 dB, from section 7.3). The
LOS path loss for a 25 km link is obtained from
the LOS model of Table 7.1 as 38 + 20log
10
25000,
which is equal to 126 dB. A cable loss of 2 dB
leaves a link margin of 17 dB.
The CBS-RBS link thus has a very large margin
and the overall BER for a WS to CBS link is thus
unaffected when the call goes through a RBS. It
should be noted that the RBS sets up calls to
the CBS on independent channels, each
possibly on a different frequency. If any channel
deteriorates, the call is handed over seamlessly
to a better channel by the Dynamic Channel
Selection (DCS) algorithm specified in DECT.
There are 10 frequencies to choose from. The
frequency spacing varies between 0.1% (for the
adjacent channel) and 1% (10 channels away)
of the carrier frequency, providing good frequency
diversity to the DCS algorithm. In addition, there
is antenna diversity at the CBS. Thus, the CBS-
RBS link with the generous link margin is very
robust to fading.
7.8 |nsta||at|on and 8urvey Too|s
The corDECT Wallset provides a simple built-in
Received Signal Strength Indicator (RSSI). The
signal strength is quantized with 6 dB steps and
converted to a digit between one and ten,
corresponding to a received power level between
-93 dBm and -33 dBm. The digit is converted to
an equivalent number of audio beeps, which can
be heard using a DTMF telephone, by dialing a
specified code. No other special instrument or
meter is needed.
Thus, two beeps which indicates a nominal level
of -85 dBm, is the minimum one needs for an
LOS link (sensitivity of -90 dBm and fade margin
of 5 dB). For an NLOS link, we would require at
least three beeps (fade margin of 10 dB), though
there may be one beep less at times due to
fading. The number of beeps may occasionally
drop even by two.
A Field Survey Tool (FST) is available, which can
measure RSSI with a resolution of less than
1 dB. In addition, it can indicate a number of
other interesting parameters such as Bit Error
Rate, identity of the CBS it is locked to,
frequency, and time slot it is using, etc. It has a
visual display and a serial port for downloading
measured data to a PC for off-line processing.
The FST operates off a built-in rechargeable
battery and is portable.
A portable CBS Emulator is also available. This
transmits a DECT signal on a fixed frequency
and time slot and is useful for surveying the
coverage area of a potential CBS site using the
Survey Tool. The CBS Emulator is self-contained
and portable, obviating the need for a DIU and
associated paraphernalia when conducting a
survey.
7.9 8ummary
In conclusion, we see that using a combination
of improved receiver sensitivity, a novel timing
7 - 7
corDECT
corDECT Wireless Access System
50 7 - 8
adjustment technique, low-cost antennas, and
innovative deployment strategies, corDECT
provides good coverage for small, high-density
areas as well as large, sparsely populated ones.
It is perhaps the only WLL system providing toll-
quality voice and simultaneous 35/70 kbps
Internet access, while at the same time
supporting a wide range of subscriber densities
and coverage areas.
corDECT
corDECT Wireless Access System
51
8.1 |ntroduct|on
corDECT provides comprehensive operation and
maintenance through the corView OMC console.
Its repertoire includes hardware and software
configuration, subscriber administration,
accounting, and traffic management.
This easy-to-use, menu-driven console can be
run either locally or remotely. When used
remotely, a single corView workstation can be
used as a NMS for a number of corDECT
systems. corView can also be integrated with
the CygNet NMS to provi de i ntegrated
management of a network of corDECT and other
systems.
corView 100 uses the proprietary CDMP protocol
to communicate with the DIU. corView 200 and
the CygNet NMS uses SNMP V1 or V2. In future,
the TMN/Q3 protocols will also be supported.
8.2 cor0E6T 0H6 |n 8w|tch Hode
8.2.1 6onf|gurat|on
The corDECT OMC provides for configuration of
hardware, software and traffic routing in the
corDECT system.
8.2.1.1 8ub-system 6onf|gurat|on
E1 Line Administration: thi s i ncl udes
configuration of signaling, taking a line into/out
of service, designation as incoming/outgoing/
both.
Software Upgrade: software in the various cards
in the DIU and in the CBS can be downloaded
from the OMC.
8.2.1.2 8ystem hea|th
The OMC continually monitors the health of the
sub-systems and cards in the corDECT system.
Any failure is indicated on the corView console
and may also result in an audio alarm. The
following are monitored:
i. health of all DIU cards
ii. health of E1 interfaces, tested through
answering circuits
iii. health of CBS lines and CBS's
iv. health of BSDs
v. silent polling of Wallsets, without disturbing
the subscriber, to determine Wallset health,
including battery voltage
8.2.1.3 Number|ng P|an and 6a|| Rout|ng
Each DIU can be configured with one or two
exchange prefixes. Each subscriber is assigned
a number within one of these prefixes. corView
200 GUI for configuring the DIU is shown in Figure
3.12 of Chapter 3.
The DIU performs digit analysis on incoming calls
to determine in which of four directions the call
should go: STD/ISD (long-distance), special
service, local, or outgoing. Up to eight digits are
examined.
Next, the route number is determined. Each route
number represents a logical group of up to 48
physical ports for the purpose of physical routing
and charging. Up to 1024 route numbers can be
defined.
The digit analysis also determines the access
level of the call. Up to 32 access levels can be
Chapler 8 peralion and Mainlenance
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corDECT
corDECT Wireless Access System
52
defined and for each subscriber, certain access
levels can be restricted.
8.2.2 8ubscr|ber Adm|n|strat|on
Service class, malicious call tracing, addition to
hunt group, and other subscriber features can
be administered from the OMC.
8.2.2.1 Reg|strat|on and Authent|cat|on
The Wallset (WS) must be registered with the
DIU before it can make/receive calls through that
DIU.
8.2.3 8upp|ementary 8erv|ces
The corDECT system supports a number of
supplementary services. The operator can
program the system to measure the usage of
each service by each subscriber.
Automatic Alarm Call: rings the subscribers
telephone at a registered time, daily or
occasional.
Call Completion Supplementary Services:
Absent Subscriber, Do Not Disturb, Call
Queueing, and Dual Telephone Number services
are present as part of this service. These services
complete the call even in case calls could not be
terminated at the called subscriber's end.
Call Offering Supplementary Services: Call
Diversion on No Reply, Busy, and Unconditional
are available as part of this service.
Call Restriction Supplementary Services:
these services offer restrictions on calls made
and received by the subscriber for outgoing calls,
incoming calls, and dynamic barring of calls. This
could be subscriber controlled or administrator
controlled.
Charging: printed Call Detail Record (CDR) of
any call(s) on demand. Provides battery reversal
and 12/16 kHz metering pulses by the Wallset
for CCB pay-phones.
Multi-party: three-party conference calling.
Rapid Call-setup: abbreviated dialing; fixed
destination on time-out after going off-hook (hot-
line number).
8.2.4 6harg|ng and ||||ng
The corDECT system when used in the Switch
Mode provides complete and flexible support for
charging and billing of individual calls.
Charge Calendar: this specifies the charge rate
(in units of tens of milliseconds) based on time
of day, day in the year, and mode of the call
(whether local or long-distance). A number of
different charge calendars can be defined and
associated with each outgoing route.
Call Duration Metering: if the DIU is connected
to an exchange and the exchange provides
metering pulses, these are used for call duration
metering. Otherwise, the DIU has its own timer
for each call. The time is stored in the call duration
records with a resolution of one second.
Billing Support: corDECT can maintain up to
32 meters for each subscriber. The first meter
contains the total pulse count. The remaining
meters can be programmed to count pulses for
calls based on route code. For example, one
meter could be dedicated to national long-
distance (STD) calls, while another could be used
for international long-distance (ISD) calls.
When used as part of a larger network, the billing
files can be exported to the central billing centre.
This can be done in any of the following ways:
i. tape or floppy disk
ii. FTP over TCP/IP to the Billing Centre.
Any IP network can be used, including:
i. an X.25 connection
ii. a dial-up connection with PPP
iii. Ethernet
8 - 2
corDECT
corDECT Wireless Access System
53
When the corDECT system is used as a stand-
alone switch, for example by a Group PBX
operator, the BlueBill software developed by Nilgiri
Networks provides all necessary billing services
for voice and Internet calls (Figure 8.1). BlueBill
and corDECT can be used to implement a deposit
scheme in which the subscriber can make calls
until his/her deposit is used up.
8.2.5 Traff|c
The corDECT system maintains a number of traffic
statistics. These include Erlangs, call attempts,
call success, call failure with a breakup of various
causes of failures, and CPU utilization.
These statistics can be maintained for each
subscriber, for each CBS, for each call type
(incoming, outgoing, special service), and for each
PSTN port. The DIU can be programmed to
collect the statistics periodically or on demand.
8.2.6 H|sce||aneous
The following services are also available:
Malicious Call Identification: record the calling
number/junctor and not allow calling party
disconnect.
PSTN/Subscriber Line Supervision: any PSTN
port or subscriber line can be put under
supervision for outgoing or incoming calls for
analysis of problems. Details of all calls are
logged in a text file.
Ring-back: subscriber-initiated, to check
telephone.
Operator Trunk-offer: operator can cut into a
call in progress.
Temporary Out-of-service: announcement for
incoming calls, outgoing calls barred.
Hunt Group: for incoming calls.
Monitoring by Intelligence Agency: any
Wallset can be configured as a Monitored
Number. All activity on the Monitored Number is
relayed to the Monitoring Station. Up to 10 such
numbers can be monitored simultaneously.
8.3 cor0E6T 0H6 |n RLU Hode
The corDECT OMC in RLU (V5.2) mode is similar
to that in the Switch Mode, except that certain
functions present in the switching mode are not
present in the RLU mode. The OMC console is a
look-alike of the easy-to-use, menu-driven
console of the corDECT switching system based
on corView.
8.3.1 6onf|gurat|on
The corDECT OMC in the RLU mode provides
hardware and software configuration. The
Figure 8.1 BlueBill GUI showing detailed monthly bill
8 - 3
corDECT
corDECT Wireless Access System
54
Numbering Plan and Call Routing are to be
configured at the Local Exchange to which the
corDECT system is interfaced.
8.3.2 8ubscr|ber Adm|n|strat|on
Every subscriber in the RLU must have an unique
subscriber number and this need not match the
telephone number (Layer 3 Address) of the
subscriber. It is the Local Exchange that will
provide the Layer 3 address for a RLU subscriber.
There are no configurable subscriber features at
the RLU, as these are provided by the Local
Exchange. Similarly, supplementary services and
charging and billing are not the responsibility of
the corDECT system in the RLU mode.
8.3.2.1 Reg|strat|on and Authent|cat|on
The WS is registered and authenticated as in
the Switching Mode.
8.3.2.2 Traff|c
The corDECT system in the RLU mode maintains
traffic statistics as in the Switching Mode.
8.4 RA8 Hanagement
The integrated iKon RAS does the switching of
IP traffic. The RAS has its own SNMP agent with
a separate IP address and the RASview GUI for
management.
8.4.1 |P 6onf|gurat|on
The iKon RAS has its own IP address. In addition,
it assigns an IP address to each subscriber upon
establishment of the PPP connection from the
subscriber PC. These addresses can be
assigned in several ways:
i. one address statically assigned to each RAS
port. This address can be global, or a
local one masqueraded by a NAT server
ii. one IP address statically assigned to each
subscriber, stored in the RADIUS server
The IP addresses of the RADIUS and NAT servers
can be changed dynamically.
8.4.2 |nternet 8ubscr|ber
Adm|n|strat|on
Subscriber information is stored in a RADIUS
server. The RAS acts as a client to obtain
authentication and other information from this
server.
8.4.3 |nternet ||||ng
At the start and end of each session, records
are written to the RADIUS server.
At the start of each session, the RAS gets the
subscribers credit (session time) from the
RADIUS server. It ensures that the session does
not exceed the credit.
The BlueBill billing software (see section 8.2.4)
also computes charges and generates bills for
IP usage, using the RADIUS records. Charges
can be based on session time and/or IP traffic,
with the provision to define a number of tariff plans
and discount schedules. BlueBill updates the
credit while computing charges soon after
completion of each session.
8.4.4 Traff|c
The RAS maintains packet and byte counts for
each subscriber. These can be used to compute
traffic rates. From the RADIUS records, the
utilization of RAS ports can be computed.
8.5 corV|ew User |nterface
corView 100 runs on the console of a Linux PC
in character mode. It has a command-line
interface. While this is intended for experienced
operators, the command-completion facility and
the graphical display make it usable even for
novice operators (see Figure 8.2).
corView 200 is implemented in Java and runs on
X-Windows. It has a sophisticated, flexible
8 - 4
corDECT
corDECT Wireless Access System
55
graphical user-interface (GUI). This can be
customized to the needs of different operators
and includes support for local languages.
For management of the iKon RAS and IP-related
configuration, RASview is provided. RASview is
also implemented in Java and has a look-and-
feel similar to that of corView 200.
8.6 NH8 for Hu|t|p|e 0|U's
An operator with multiple corDECT systems is
likely to want a Network Management System
(NMS). This can be provided in several ways.
8.6.1 corV|ew as NH8
A single workstation running either corView 100
or corView 200 provides remote consoles for
multiple DIU's. At any time, the operator is
interacting with only one of the DIU's. However
at all times, the operator can see the fault status
of all the DIU's. All the functions that are available
at the local console are also available at the
remote console.
In order to support many operators
simultaneously managing a network of DIU's,
corView is partitioned into two sub-systems. The
Figure 8.2 corView 100 console screen
8 - 5
Figure 8.3 corView Manager
corDECT
corDECT Wireless Access System
56
corView GUI runs on each operator workstation.
All traffic between the GUIs and the agents on
the DIU's is routed via a single corView manager
(Figure 8.3) that normally resides on one of the
NMS workstations.
This serves two purposes. First, it improves
security and efficiency as each agent need only
accept requests from one manager. Second, it
simplifies the implementation of the GUI as the
manager provides a high-level interface. Each GUI
request to the manager may be translated into a
series of SNMP requests from the manager to
the agent.
This is useful when the network consists only of
DIU's, or the operator has an independent
management system for other network elements.
8.6.2 6ygNet as NH8
The CygNet NMS product developed by the
TeNeT Group can be used for multiple corDECT
systems. corView and RASview run within
CygNet to provide complete remote OMC
functions. The operator has two options:
i. a restricted version of CygNet is available
which permits management only of corDECT
systems. Besides the management of
individual corDECT systems using corView,
the operator can also monitor aggregate
traffic of two or more corDECT systems and
can use the CygNet database facilities for
storage of traffic and other statistics (Figure
8.4). These are useful for network analysis
and planning of future growth.
ii. the full version of CygNet provides the above
management of corDECT systems and also
integrated management of a wide variety of
other telecom and Internet devices. CygNet
provides single-point integrated management
of the entire voice+Internet network.
8.6.2.1 The 8NHP Agent
The corDECT SNMP Agent supports MIB-II and
the proprietary corDECT MIB. This latter provides
support for the full range of OMC commands.
The corDECT MIB requires password-based user
authenti cati on. For protecti on agai nst
eavesdropping, passwords are encrypted before
being transmitted on the network.
For further security, the IP addresses of legitimate
managers must be configured in the agent. This
can only be done through local commands.
Figure 8.4 Monitoring a network of corDECT systems using CygNet
8 - 6
corDECT
corDECT Wireless Access System
57
8.6.3 |nterconnect|ons
There are several options for interconnection of
one or more corDECT DIU's to a single NMS
station. In all cases, the IP protocol is used over
the physical links.
Single DIU
i. Connect to a co-located NMS over Ethernet.
ii. Connect to a remote NMS with dial-up
modem, or through a 64 kbps leased line
through an E1 to the PSTN
Multiple Co-located DIU's
i. If the NMS is in the same exchange building
as the DIU's, they can be connected on a
single Ethernet LAN.
ii. If the NMS is remotely located the DIU's are
interconnected via an Ethernet and this
Ethernet is connected to the NMS through
a router, via dial-up modem, or through a
leased 64 kbps line.
Remotely located DIU's
In this case, each DIU has a leased 64 kbps line
(one slot on an E1 link) to the NMS location. At
the NMS, all the incoming leased lines are
combined into an E1. This is routed to the NMS
via Ethernet by an iKon RAS (Figure 8.5).
Figure 8.5 DIU's connected on independent 64 kbps leased lines to an NMS
8 - 7
8.7 0ata Hanagement
The OMC maintains four files related to billing.
These are:
bulk.dbs: the bulk billing information for every
subscriber.
normal.dbs: the Call Detail Records (CDR) of
all calls.
com.dbs: the commercial information such as
addition/deletion of subscribers.
tim.dbs: the record of all changes made to the
system time by the operator.
The CygNet NMS can be configured to periodically
collect various statistics related to the operation
of the systems and the network.
These are stored in a R-DBMS for future use.
The default R-DBMS is MySQL but this could be
replaced by any SQL-92 compliant R-DBMS.
The CygNet NMS also maintains logs of all events
such as failure of a system and voice or IP traffic
crossing a threshold.
8.7.1 Redundancy
A hard disk may fail at any time. Hence, all
important data, especially commercial data and
historical statistics, should be stored on at least
corDECT
corDECT Wireless Access System
58
two separate hard disks. In corDECT, this
redundancy can be achieved in several ways:
i. one hard disk on each of two local OMC PCs
ii. one hard disk on a local OMC PC and one
on the NMS PC
iii. one hard disk on the NMS PC and one on a
back-up NMS PC
8.7.2 ackup
The billing and commercial databases (*.dbs) can
be backed up selectively onto tape or floppy or
to a local directory in the machine. Backup to a
tape or a local directory can be done manually
or automatically at pre-programmed times (such
as specific days of the week, specific dates of
the month, or every n
th
day).
8 - 8
corDECT
corDECT Wireless Access System
59
9.1 |ntroduct|on
The corDECT system today provides a rich suite
of services and features. These include
simultaneous voice and Internet access at 35/
70 kbps, a variety of interfaces to the PSTN
including V5.2, segregation of Internet traffic
bypassing the PSTN, several deployment
confi gurati ons that cater to a range of
teledensities from dense urban to sparse rural,
modularity and scalability that make it cost-
effecti ve, and a sophi sti cated Network
Management System. The corDECT system,
however, continues to grow in capabilities. On
the anvil are new products that will keep corDECT
ahead of other WLL systems, as the 3G WLL
system of choice for operators worldwide.
ETSI has standardized the DECT Packet Radio
Service (DPRS) to enable DECT to meet 3G
requirements for fixed and portable applications.
DPRS leverages the high bitrate of DECT
(1.152 Mbps) and i ts ri ch control -pl ane
functionality to provide 3G services. The DECT
physical layer has been upgraded to include the
higher bitrates of 2.304 and 3.456 Mbps. The
modulation has also been upgraded in a
backward-compatible fashion so as to allow
improved link budgets. DECT, with its established
base and new upgrades, is thus a front-runner
for cost-effective 3G fixed (i.e., WLL) applications.
The next few sections describe briefly the
advanced features corDECT will provide in the
near future.
9.2 Towards A|ways-on |nternet
Access
Internet access is characterized by bursts of
packets with long periods of inactivity. If the
wireless connection is suspended during inactive
periods and resumed quickly when there is a
burst of traffic, the available wireless channels
can be used by a much larger number of
subscribers. DECT provides for such suspension
and quick resumption of connections, using its
powerful control-plane signaling protocols.
Development is in progress to build this new
capability into the corDECT system. When it is
available, a very large fraction of the 1000
subscribers in each system can be logged onto
the Internet simultaneously and remain logged
on for as long as desired.
9.3 Packet-8w|tched h|gh 8peed
|nternet 0own|oad|ng
It is highly desirable for a user to have the ability
to download from the Internet at a high peak
bitrate, even if the download-channel is shared
by many users, each accessing it when needed.
The bursty nature of Internet access ensures that
a user can get a significant fraction of the peak
bitrate whenever he needs it.
The high bitrate of the DECT air interface is
eminently suited for providing this type of service.
A major new development of the corDECT system
underway is a packet-switched shared downlink
Internet channel at 384 kbps. It will be possible
for each sector in a cell to have one such shared
download channel. A subscriber terminal
accessing this channel picks off the data meant
for itself. With this service, a subscriber will be
able to download web pages and files at the peak
bitrate of 384 kbps. Further, he will be sharing
this fast channel only with the subscribers in the
sector he belongs to.
A new subscriber terminal with a high-speed
Chapler 9 Iulure Roadnap
9 - 1
corDECT
corDECT Wireless Access System
60
(10BaseT/USB) data interface port is also under
development to support this service.
9.4 Hore |ntegrat|on for 6ost-
Effect|veness
A next-generation subscriber terminal is under
development which is more integrated and
compact. It will provide several options: one voice
line, two voice lines, or one voice line + one
Internet port. A variant of this new product that
has some archi tectural si mi l ari ty to the
Multiwallset (MWS) is also on the anvil. In this
product, there is an outdoor unit similar to the
DTM of MWS. A small indoor unit connected to
it using one copper pair provides the same three
options listed above, while obviating the need for
RF cabling.
9.5 New Hu|t|wa||set
0eve|opments
Under development is a MWS that will permit
one to serve 8/12/16 subscribers, with blocking
whenever four simultaneous calls are in progress.
This will reduce the per-line cost dramatically and
enable an operator to serve the hitherto unviable
low-usage subscribers.
9.6 |ncreased 8ca|ab|||ty
The corDECT system is unique today in the
respect that the cost of the DIU, representing
the up-front investment, is a small fraction of the
total cost. This ensures that the per-line cost is
modest even for a 250-line corDECT system. A
new cost-effective, highly integrated mini-DIU will
be available soon for a 50-line system and also
for a 150-line system. These versions will also
reduce significantly the physical infrastructure
requirements for housing the DIU.
9.7 Vo|P |n cor0E6T
The corDECT system employ DSPs extensively.
As there is a powerful DSP in every Wallset, the
voice signals can be converted to/from packets
at the Wallset themselves, transmitted on air in
packetized form and thence to the Internet
through a gateway at the DIU. Thus, the corDECT
system can be made VoIP-compatible in a very
efficient and cost-effective manner.
9.8 New A|r |nterface
The new DECT air interface supports a maximum
bitrate of 3.456 Mbps with fall-back options of
2.304 Mbps and 1.152 Mbps. The link budget is
also better due to improved sensitivity. The new
air interface enables the use of sophisticated
techniques like sequence estimation and turbo-
coding to achieve superior link performance.
This new air interface will enable corDECT to
increase traffic capacity and Internet access
speed, without increasing bandwidth and with the
same types of deployment. It will also give better
coverage due to the improved link budget.
This development effort is also underway. When
it is available, corDECT will surpass the
performance of all other 3G systems, which will
typically support only 384 kbps Internet access
for fixed applications and at most 2 Mbps when
one is sufficiently close to the Base Stations.
9.9 |nsta||at|on P|ann|ng
For planning of an access network based on
corDECT and other products, the TeNeT Group
will soon release CygPlan which will be available
from Midas Communication Technologies Ltd.
This GIS-based tool runs on MS-Windows and
the plans are stored in an MS-Access database.
Given the expected subscriber base, CygPlan
computes the number of CBSs, DIUs and other
components required, the backhaul bandwidth
for voice and IP, the bill of quantities, and costs.
CygPlan ensures that various hardware
constraints are not violated. If the operator enters
the building heights and other topological details
of the area, CygPlan will predict the coverage
9 - 2
corDECT
corDECT Wireless Access System
61
area of each CBS. The operator can then re-
position CBSs to ensure 100% coverage of the
service area. The propagation model can also
use measured signal levels from a survey, where
available.
9.10 8ummary
The corDECT development team will continue to
make available new products to take corDECT
to ever higher levels of performance. At the same
time, existing versions will be maintained and
upgraded. This will ensure that corDECT remains
the most versatile and cost-effective WLL system
with the best suite of features and services.
9 - 3
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62
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corDECT
corDECT Wireless Access System
63
The DECT standard proposed by the European
Telecommunication Standards Institute (ETSI) is
meant for providing wireless access to networks
of various types, from the PSTN to LANs. It deals
only with the task of defining the air interface
between subscriber terminal and Base Station.
The mode of connecting the DECT-based
Wireless Local Loop system to the PSTN and
Internet is left to the service provider.
DECT has been specified to make possible low-
cost subscriber terminals, high subscriber
density with heavy call-traffic levels, wireline-
quality voice, modem/fax capability, 32/64 kbps
and higher-rate data services, all with a modest
spectral allocation of 20 MHz. The key technical
advances incorporated in DECT when compared
to prior standards that make all this possible are:
(i) dynamic channel selection, (ii) microcellular
architecture, (iii) channels with multiple data rates
and (iv) cost-effective modulation/demodulation
techniques. The next two sections focus on some
of the key features of the DECT standard.
A.1 0E6T: 8ome 8a||ent Features
i. Frequency Band: The RF band originally
allotted to DECT is 1880 1900 MHz, though
the entire 20 MHz need not be employed by
each system. All DECT-based systems
including private and public systems operate
on the common band with no requirement
for regulation. An extended DECT band that
includes the band 1900 1935 MHz is also
defined.
ii. Mode of Access: The DECT standard
employs a version of Time Division Multiple
Access (TDMA). There are 10 frequencies
of operation in a 20 MHz band, with a
spacing of 1.728 MHz. The burst-rate is
1.152 Mbps, accommodating 24 slots. The
communication is Time Division Duplex
(TDD). This not only ensures that propagation
conditions are identical at any time in both
directions of transmission, but also simplifies
transceiver design. The 24 slots in a TDMA
frame are divided into two groups of 12 slots
each, one group for each direction of
transmission. The frame structure is shown
in Figure A.1. The frame duration is
10 ms and a TDD slot-pair is separated
by 5 ms.
iii. Multi-Carrier TDMA: A very important
difference that sets DECT apart from
conventional TDMA systems is that all the
sl ots i n a TDMA frame need not be
transmitted on the same frequency. Each of
the 12 slots could be on a different frequency,
though the pair of slots used for each TDD
link must be on the same frequency. This
variation of TDMA is called Multi-Carrier
TDMA (MC-TDMA) and is the key to the high
Appendix DigilaI Lnhanced CordIess TeIeconnunicalions
Figure A.1 DECT frame structure
A - 1
corDECT
corDECT Wireless Access System
64
capacity achieved by DECT. The 12 slot-pairs
and 10 frequencies give rise to 120 channels,
as if they were independent of one another.
A Wallset can operate on one or more of
these 120 channels, while a Base Station
receives and transmits on a maximum of 12
of them at a given time. The concept of MC-
TDMA is illustrated in Figure A.2 for a
hypothetical frame of three slots, with each
slot employing a different frequency.
iv. Transmit Power: The power transmitted by
Wallset or Base Station is 250 mW during
the burst, or about 10 mW average power.
This ties in with the need for small cells to
increase frequency re-use and conserves
battery power.
v. Voice Digitization: DECT employs 32 kbps
ADPCM. This ensures toll quality and
permits all the data (fax/modem) services
avai l abl e from a conventi onal wi red
connection. It is also possible to occupy a
double-slot to transmit at 64 kbps with error
connection. This can be used for PCM or for
data connectivity.
vi. Modulation: DECT employs Gaussian
Frequency Shift Keying (GFSK) with a
Gaussian Filter (BT=0.5). Only 75% of the
burst rate of 1.152 Mbps is used for voice.
DECT employs ADPCM for its high voice
quality and GFSK because transceiver cost
is reduced. By throwing in generous signaling
capacity, DECT is able to employ a very
sophisticated channel selection procedure.
This is the most important aspect of DECT
which sets it apart from existing cellular
systems and is discussed below.
vii. Channel Allocation: Mobile Cellular
Systems hitherto employ the so-called Fixed
Channel Allocation (FCA) approach. Here,
the available channels are distributed among
neighboring cells in a planned fashion,
depending on traffic needs. Channels are re-
used at appropriate distances based on the
terrain, transmit-power, antenna height, etc.
Channels are allocated from the allotted set
to users on demand by the Base Stations
and hand-off is controlled by the network of
Base Stations as the mobile user crosses
over into neighboring cells.
Systems like GSM employ Mobile-Assisted
Hand-Off (MAHO) but the hand-off is still
centrally controlled. When deciding the re-
use distance in an FCA-based system, one
needs to make allowance for shadowing (due
to obstructions). Re-use is decided based
on worst-case scenarios, assuming the best
propagation path for the interference and
worst-case shadowing of the desired signal.
The DECT standard employs a completely
decentralized channel allocation procedure
called Dynamic Channel Selection (DCS) or
Adaptive Channel Allocation (ACA). In this
approach, the available set of channels is
not distributed a priori among the cells. Any
Wallset can set up a call on any of the
channels, deciding on the one it will use at a
given time by measuring the signal strength
in that channel at its geographical location.
The so-called received signal strength
indication (RSSI) is used for this purpose.
Based on a table of RSSI measurements for
all channels, which is continuously updated,
Figure A.2 MC-TDMA
A - 2
corDECT
corDECT Wireless Access System
65
the Wallset selects the strongest Base
Station signal received at the given location
at that time to lock onto, and the quietest
channel to communicate with the Base
Station. This scheme requires that Base
Stations transmit some signal even if no calls
are in progress, i.e., a beacon, or dummy
bearer in DECT parlance, is a must when
the Base Station is idle.
In section A.2, we take a closer look at DCS.
viii. Encryption and Authentication: DECT
provides encryption of the voice signal or
data, to prevent eavesdropping. Authenti-
cation allows one to curb unauthorized use
of the Wallset.
A.2 0ynam|c 6hanne| 8e|ect|on
In a MC-TDMA system, a channel is specified
by a time-slot/frequency combination. Thus, each
Wallset must make RSSI measurements on
each of the 10 frequencies in each time slot.
There are thus 120 channels in DECT (for a 20
MHz band) to choose from. Each channel is
specified by a frequency and pair of time-slots
(for TDD communications). Figure A.3 depicts
the available choice as a matrix. The shaded
boxes indicate channels that may be in use at a
given time and place.
The time slots are synchronized to the frame of
the Base Station the Wallset is currently locked
to, or to a local frame clock if the Wallset is not
locked to any Base Station yet. In a TDMA
system, the transceiver is idle when not receiving
or transmitting a burst. So, RSSI measurements
can be performed in all other slots on all
frequencies. With DCS, hand-over may become
necessary even if the Wallset under consideration
does not move, because of the autonomous
decisions taken by other Wallsets. In DECT, the
switch-over to another channel is made as soon
as a better channel i s found by RSSI
measurements, without waiting for the current
channel to deteri orate. The cal l i s then
simultaneously transmitted on both channels
(which is easily accomplished in a TDMA system
by transmitting in two slots) and a seamless
switch-over is accomplished. In order to facilitate
this type of self-organizing, Wallset-arbitrated
hand-over, a fair amount of control information
has to be transmitted between Wallsets and Base
Stations. This is one reason why DECT has
generous signaling capacity.
Figure A.3 120 DECT channels
A - 3
corDECT
corDECT Wireless Access System
66
The capacity gain from the use of DCS, made
possible by the generous flow of control
information, is enormous. Firstly, by not splitting
the available set of radio channels and making
the entire set available to every user, a high
trunking efficiency is obtained. This refers to the
ability of the system as a whole to handle
statistical variations in call traffic, while still
maintaining the blocking probability at the desired
level. It is well known that the Erlang capacity
goes up when the available radio channels are
pooled. Thus, a Base Station can handle a
maximum of 12 simultaneous calls without any
limitations imposed by prior frequency allocation.
While it would have been better to have even more
slots/frame from the point of view of trunking
efficiency, this also implies a higher burst rate. It
is however, possible to achieve higher trunking
efficiency, where needed, by co-locating multiple
Base Stations with overlapping coverage areas.
A second gain from the use of DCS is that
channels are re-used based on the instant
situation and re-use distance can sometimes be
very small. Consider the example shown in
Figure A.4. The Wallsets and Base Stations are
so located that either Wallset, when operating
alone, could communicate with either Base
Station. However, even in the situation when both
are simultaneously active, it is possible for each
Wallset to communicate to the Base Station
nearer to it on the same channel. This is because
it is the carrier signal-to-interference (C/I) ratio
that determines whether the channel is good
enough. Even though the signal from the farther
Wallset is good enough for communication in the
absence of any other transmission on the
channel, the interference that this causes to the
signal from the nearer Wallset is too small to
matter. Thus, a channel can be re-used even at
short distances depending on the interference
profile as seen by each Wallset.
Finally, Base Stations may be added to the
system as and when needed to cater to increased
traffic and no co-ordination or planning is needed.
Indeed, multiple Base Stations can even be co-
located. More than one Base Station can be
reached from any location and the trunking
efficiency goes up. Incidentally, DECT systems
belonging to different operators, public or private,
can co-exist and operate over a common
frequency resource without co-ordination.
While DCS is the key to high capacity with small
cells, the use of DECT in large cells with low
subscriber density is not precluded. The improved
sensitivity, compact antennas, and timing
adjustment scheme (see Chapter 7) implemented
in corDECT permit coverage up to 10 km under
line-of-sight conditions. Also, the range can be
extended to as much as 25 km in the case of
the RBS employing high gain antennas that
increase the link budget.
Figure A.4 Frequency re-use at short distance
A - 4
corDECT
corDECT Wireless Access System
67
In summary, DCS
is the key to the high capacity of systems
like DECT
more than makes up for the inefficient
bandwidth utilization due to other constraints
A - 5
effects channel allocation based on the
actual traffic interference situations
gives significant capacity gain when
compared to other channel allocation
schemes.
1. RF Channel Centre Frequencies : 1897.344 - (n - m) x 1.728 MHz,
channel number n = 0, 1, 9
channel offset m = 0 21
e.g.,
m = 0 for 1880 1900 MHz
m = 12 for 1900 1920 MHz
m = 17 for 1910 1930 MHz
2. TDMA Frame Duration : 10 ms
3. Transmission Bitrate : 1.152 Mbps
4. TDMA Slot Length : 480 bits, with 32 bits for synchronization,
64 bits for signaling and 324 bits for voice and CRC.
A double slot of 960 bits is also defined.
5. Modulation : Gaussian Frequency Shift Keying
6. Frequency Deviation : +288 kHz (nominal) for all-ONE bit pattern -288 kHz
(nominal) for all-ZERO bit pattern
7. Transmit Power : +24 dBm nominal
8. Spurious Emission : < -8 dBm in adjacent channels
< -30 dBm, 2 channels away on either side
< -44 dBm, 3 channels away on either side
< -47 dBm in all other channels except for one
instance of -33 dBm
9. Sensitivity : at -85 dBm (typical), BER better than 10
-5
at -90 dBm (typical), BER better than 10
-3
10. Interference Performance : At -73 dBm, with a co-channel interferer at
-83 dBm, or with an adjacent channel interferer
at -60 dBm, or an interferer 2 channels away on
either side at -39 dBm, or an interferer on any other
channel at -33 dBm, BER better than 10
-3
A.3 cor0E6T Phys|ca| Layer 8pec|f|cat|ons
corDECT
corDECT Wireless Access System
68
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AC Access Centre
ADPCM Adaptive Differential Pulse Code Modulation
ARQ Automatic Repeat Request
BSD Base Station Distributor
CBS Compact Base Station
CCB Coin Collection Box
CDMA Code Division Multiple Access
CSMUX Concentrating Subscriber Mux
DCS Dynamic Channel Selection
DECT Digital Enhanced Cordless Telecommunications
DID Direct In-Dialing
DIU DECT Interface Unit
DPRS DECT Packet Radio Service
DSL Digital Subscriber Line
DSP Digital Signal Processor
DTMF Dual Tone Multi-Frequency
ETSI European Telecommunication Standards Institute
FCA Fixed Channel Allocation
FDMA Frequency Division Multiple Access
FTP File Transfer Protocol
GFSK Gaussian Frequency Shift Keying
HDSL High-speed Digital Subscriber Line
IS-95 CDMA Cellular Standard
ITU-T International Telecommunication Union - Telecommunication
Standardization Sector
LOS Line-of-Sight
Allrevialions
corDECT Wireless Access System
corDECT
corDECT Wireless Access System
70
MC-TDMA Multi-Carrier TDMA
MWS Multiwallset
MWS-IP Multiwallset with Internet Port
NAT Network Address Translation
N-LOS Non Line-of-Sight
NMS Network Management System
OMC Operation and Maintenance Console
PAP Password Authentication Protocol
PBX Private Branch Exchange
PPP Point to Point Protocol
PSTN Public Switched Telephone Network
RADIUS Remote Access Dial-in User Service
RAS Remote Access Switch
RBS Relay Base Station
RLU Remote Line Unit
RSSI Received Signal Strength Indicator
RSU Remote Switching Unit
SNMP Simple Network Management Protocol
STD/ISD Subscriber Trunk Dialing/International Subscriber Dialing
TCP/IP Transmission Control Protocol/Internet Protocol
TDD Time Division Duplex
TDM Time Division Multiplexing
TDMA Time Division Multiple Access
V5.2 Interface protocol for connecting an access network to a PSTN
exchange
VoIP Voice-over-IP
WAS Wireless Access System
WS Wallset
WS-IP Wallset with Internet Port