DIGITAL SWITCHING SYSTEM
Module 1
Development of telecommunications
DEEPASHREEA P
EC DEPT
Communications
Communications
The message (data and information) is
communicated via the signal
The tra s issio
ediu carries the sig al
Transmission
medium
Sender
Signal
Receiver
Co
u icatio s Discussio
The transmission of data from one computer to
another, or from one device to another. A
communications device, therefore, is any machine
that assists data transmission.
For example, modems, cables, and ports are all
communications devices.
Communications software refers to programs that
make it possible to transmit data.
Telecommunications
Telecommunications
The electronic transmission of signals for communications,
including such means as:
Telephone
Radio
Television
Telecommunication medium
Anything that carries an electronic signal and interfaces between a
sending device and a receiving device
Communications and Telecommunications
In human speech, the sender transmits a signal
through the transmission medium of the air
In telecommunications, the sender transmits a
signal through the transmission medium of a cable
Schematic
Types of communication transmission
mode:
Simplex : one way communication ex: Radio
Half Duplex : Two way communication shared by
single channel ex: walkie Talkie
Full Duplex : Two way communication simultaneously
ex: Telephone
Term in switching
Traffic:In telecommunication system, traffic is defined as the
occupancy of the servering the network.
There are two types of traffic viz. voice traffic and data traffic.
For voice traffic, the calling rate is defined as the number of
calls per traffic path during the busy hour.
In a day, the 60 minutes interval in which the traffic is highest
is called busy hour (BH).
Development of telecommunication
Telecommunication:
Transporting information by converting it to
electrical signals & transmitting these over a
distance
Electrical communication began with telegraphy
invented by WHEANSTONE & MORSE
It consisted of point to point
Telephony: ALEXANDER GRAHM BELL
2 way communication
9
In telephone conversation,
Initiates call calling subscriber
Call destinated called subscriber
If there are n entities & n(n-1)/2 point to point
links
Network with point to point links among all
entities known as fully connected network
10
Bandwidth requirement of various
application
11
A network using point to point connection is shown.
In the figure, there are 5 entities and 10
point to point links. In general case with
n entities,
there are n(n-1)/2 links.
e.g., n=5 then 5(5-1)/2 = 10 links
Total number of links L = n(n-1)/2
12
If 50 entities then 1225 links which is not
practical due to
Large demand for wires
Expensive
Switching system came into existence
13
Figure 1.4 Categories of topology
1.14
Figure 1.5 A fully connected mesh topology (five devices)
1.15
Figure 1.6 A star topology
In telephony two way communication is required
A line from each user station to a central switching centre
which connects the lines together as required
1.16
Figure 1.7 A bus topology connecting three stations
This cannot be used for telephony
Only one communication can be possible at a time
1.17
Figure 1.8 A ring topology connecting six stations
Bus & ring are used for data communication
1.18
Tree topology
Links are formed in a
tree structure.
Cheaper than partially
connected;
19
Variation of n/w cost with no. Of exchanges
Average le gth of custo er s li e,& thus total li e cost, decreases
with no. Of exchanges
But cost of providing the exchange increases
20
Conversation between customer with two
exchanges is done using junction circuits & they form
junction network
Junction network has mesh configuration
Cost of junction circuit is high & it is uneconomical to
connect all exchanges directly
So Connection between the customer local
exchanges via central switching centre called tandem
exchange
Junction network has star configuration
21
Indirect routing via tandem exchange is cheaper when
traffic is small & distance is short
Multi exchange area usually has direct junctions
between some exchanges, but traffic between other is
routed via tandem exchange
Network of area is a mixture of star network, joining all
local exchanges to tandem exchanges, & mesh network
connecting some of local exchanges together
22
Fig multi exchange area
23
Different areas of country are interconnected by long
distance circuit which form trunk network or toll
network
In national network these may not be fully
interconnected one or more high level of switching
centre are introduced, this produce concatenation of
star network
Direct routes are provided where traffic is high
24
National tele communication network
25
There is a direct route between two exchanges at the
same level, there is also a possible alternative route
between them via exchange
If direct circuit is not available, it is possible to divert
traffic to the indirect route
In older system such changes can be made by
manual ,but in modern switching system provide
automatic alternative routing(AAR)
26
If an originating exchange is unable to find free
circuit on direct route to a destination exchange
Then it automatically routes the call through the
higher level exchange
This happens not only when there are no direct
circuit available due to breakdown but also when
they are all busy
27
A national PSTN network
1. Local network ,which connect customer stations to
their local exchanges
2. Junction networks, which interconnects a group of
local exchanges serving an area & tandem & trunk
exchange
3. Trunk network or toll network, which provides long
distance circuits between local areas through the
country
28
Totality of 2& 3 is sometimes called the core
network
Inner core trunk network
Outer core junction network
Above this hierarchy there is the international
network which provide circuits linking the national
networks of different countries
The national network is connected to international
network by one by one international gateway
exchange
29
Network services
The customer of PTO require many different
services
PSTN
Telex
Private network for voice & data
Cellular & radio network
PDN
Special service network, introduced to meet
special demands of customer
30
Relationship of service & bearer network
31
These services may use separate switching centre &
the private circuits use transmission links semi
permanently connected together at network nodes
without switching
Different services use a common transmission bearer
network consisting of junction & trunk circuit
Customers are connected to their local exchange via
the local acess network
32
Service provider over telecommunication network
divided into
Teleservices: provision of service depends on particular
apparatus(telephone or teleprinter
Bearer services: which presents the customer with
transmission capacity that can be used for any desired
function(eg. Private circuit)
33
Terminology
34
Terminology
A switching centre is also called exchange in U.K, but
CO in North America.
An exchange that switches long distance traffic is
called trunk exchange in U.K , but a toll office in
North America
In U.K the customer access network of a local
exchange is usually called local network
In some other countries local area network is used to
describe the computer network of multi exchange
area
35
Regulation
It deals with how the connection & services provided
to the customer through an exchange
In some countries telecommunication operating
companies are privately owned
In USA customer can obtain local service from the
regional operating company
Traffics are regulated by federal communication
commission for long distance traffic
36
standards
Standardisation which has made an effective international
network possible is carried out through ITU
ITU classified into 2 main bodies
ITU sector(ITU-T): it includes the study of technical questions,
operating methods& traffics for telephony ,telegraphy ,data
communication
ITU radio communication sector (ITU-R): it studies all technical &
operating questions relating to radio communication, including
point to point communication, mobile services and broadcasting
ISO produces standards in information technology
37
INTRODUCTION TO TRANSMISSION IN
TELECOMMUNICATION SYSTEM
Transmission systems provide circuits between the nodes
of telecommunication network
If a circuit uses separate transmission path for each
direction these are called channels
In general complete channel passes through sending
equipment at a terminal station ,a transmission link ,
which may contain repeaters & receiving station
Signals classified into
Analog
Digital
38
For digital signal no. Signal elements transmitted per
seconds is called bauds
Multiplexing :At transmitting end signals of different
channel are combined to form a composite signal of
wider bandwidth
At receiving terminal signals are separated &
retransmitted over the channels de-multiplexing (or
demuxing).
39
Multiplexing
ANALOG & DIGITAL transmission
To transmit analog signal
without distortion the
channel must be linear
Analog signals contain a
range of frequencies called
bandwidth to convey
information
Analog signals can be
transmitted over digital
channel by A to D
conversion
Is relative to niose &
interference
Does not be linear
The information carried is
no. Of signal elements
transmitted/s is called
signalling rate
Digital signals can be
transmitted over analog
circuit
Has immunity to noise &
interference
41
POWER LEVEL
It is convenient to use Logarithmic unit of power
decibel(db)
If output power P2 is greater than input power P1 then
gain in decibels is given by
G = 10 log10 (P2 / P1 )
If P2 < P1 then the loss is
L = 10 log10 (P1 / P2 )
Gain & loss in terms of voltage & current
G= 20 log10 (V2 / V1 ) = 20 log10 (I2 / I1 )
42
RELATIVE & ABSOLUTE POWER
Decibel is a unit of relative power level
To measure absolute power in decibels it is necessary
to specify a reference level usually 1mW
Eg: to express 1W in dbm we have
10log10 (1W/1mW) =+30dbm
: to express 1W in dbm we have
10log10 (1W/1mW) = -30dbm
43
Power level in analog transmission
44
FOUR WIRE CIRCUITS
term four wire implies that there are two wires
carrying the signals in one direction (go) and two
wires carrying them in opposite direction (return).
the amplifiers (repeaters) are necessary to
compensate the attenuation
As the amplifiers are unidirectional, for two-way
communication, four-wire transmission is necessary.
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FOUR WIRE CIRCUITS
46
ECHOES
47
Echo suppresser
Echo canceller
48
Stability
BRL is sufficiently small & gain of its amplifier are suffiently
high,
Net gain around the loop may exceed zero & singing will occur
Net loss Ls = 2[ B+6-G4 ]db =2(B+L2)
Necessary condition for stability is Ls >0 this requires that
L2+B> 0
G2< B where G2= -L2
Gain can be obtained over 4 wire circuit is thus limited by BRL
BRL is zero when either Z or N is zero or infinite
Then the loss between the return & go channel is only 6db
49
Singing point
The singing point of a circuit is defined as the maximum gain
S that can be obtained without producing singing
In practise the attenuation of singing path is deliberately
made greater than zero
This provides a safety margin & avoids the attenuation
distortion caused by echoes when circuit is operating close to
signing point, thus from equation
Ls = 2(B+L2)db, (*)
S=B
Singing point given by BRL
50
STABILITY MARGIN
it is defined as the max amount of additional gain M,
that can be introduced in each direction of
transmission without causing singing
Ls-2m=0 from *
M = B+L2db
Thus it is sum of 2 wire to 2 wire loss & BRL
In practise stability margin of 3db is found to
adequate Ls=6db
51
Digital transmission
Bandwidth & equalisation
The minimum bandwidth needed to transmit a digital
signal at B bauds was shown by nyquist to be Wmin = (1/2)B
52
If the signal sent through ideal LP n/w with this cutoff
frequency it is possible to detect every pulse without error,ie
no ISI
In practise it is not possible
Nyquist also showed that zero ISI can be obtained if the
channel charges from unity to zero over a band of frequencies
with a gain of frequency response symmetrical about f=1/2 B
digital transmission system can use gain & phase equalisation
to obtain negligible ISI
An adaptive equaliser automatically adjust itself to
characteristic of transmission path
53
NOISE & JITTER
Here v is the detection level The receiver
compares the signal voltage Vs with threshold
voltage of v
If a noise voltage Vn is added, an error occur if Vn >
V
If bipolar signal is used, there 0v is decision level
An error occurs if Vn > v
The error rate obtained with a 3db lower SNR
54
For telephone transmission an error rate of 1
in 10^3 is intolerable but an error rate of 1 in
10^5 is acceptable
error rate of the link is inadequate it is
necessary to use an error detecting or error
correcting codes
55
Regenerative repeaters
In digital transmission it is possible to use
regenerative repeaters instead of analog
amplifiers
regenerative repeaters samples the received
waveform at intervals corresponding to digital
rate
If the received voltage at sampling instant exeeds
a threshold voltage this triggers a pulse
generator which sends a pulse to next section
56
Jitter
Variations in the extracted frequency can
cause a periodic variations of the times of the
regenerated pulses known as jitter
The tolerance to jitter of any subsequent
equipment in a link must therefore exceed the
amount of jitter produced by proceeding
equipment
57
The variation of error probability with SNR for a unipolar
binary signal distributed by Gaussian noise shown below
58
Frequency Division Multiplexing
59
11.4 Frequency Division Multiplexing
60
FDM
61
FDM
FDM is an analog multiplexing technique that combines
analog signals.
It uses the concept of modulation
At TXR each incoming baseband signal (0< fm< Fm)from
an AF circuit to a balanced modulator supplied with
appropriate carrier frequency
o/p of this is called DSBSC
Signal sent to BPF it suppresses fc+fm & transmits only
lower side band
62
At receiving end the incoming signal is applied to
a BPF each of which selects frequency band
containing the signal of one channel
Then signal fed to modulator supplied with
appropriate carrier & output of this modulator
consists of baseband signal & unwanted high
frequency components
LPF used to suppress unwanted components
Baseband signal transmitted to AF circuit
63
64
TDM
Figure 6.12 Time Division Multiplexing (TDM)
6.66
TDM
67
TDM
TDM is a digital multiplexing technique for
combining
several
low-rate
digital
channels into high-rate one.
Each baseband channel is connected to
transmission path by a sampling gate which is
opened for short interval by means of train of
pulses
Pulses with same repetition frequency but
staggered time shown in fig
68
Common transmission path receives interleaved pulses
modulated by signals of different channels
At receiving end gates are opened by pulses coincident with
those received for transmission path
So that demodulator of each channel is connected to the
transmission path for its allotted intervals
Pulse generator at receiving terminal must be synchronized
with that at sending terminal
69
Complete waveform transmitted during each
repetition period thus contains time slot for each
channel & one for sync signal & is called a frame
PLM & PPM not used for line transmission because
of delay distortion & attenuation causes dispersion
of transmitted pulses
They spread in time & interfere with pulses of
adjacent channel causing channel crosstalk
70
PCM
Group of bits representing one sample is called a word or a
byte
8 Bit byte is called an octet
For telephony sampling is carried out at 8khz & 8 bit
coding is used.
Signalling rate is 8*8=64k bauds
Min b.w required is half the pulse rate 32khz is needed
71
Quantising in PCM
PCM produces quantizing distortion which is not present in
analog transmission
This arise because the system can only transmit finite no. Of
samples
If the input signal is large compared with quantizing step ,the
error in successive samples are nearly random
The spectrum of the distortion products approximated to
Gaussian noise & called quantising noise
72
If the coder uses quantising steps of uniform size, then large
amplitude is introduced by large step size & is reproduced with
little distortion
73
A small amplitude signal will range over few steps & a
percentage distortion will present in output signal
Quantising SNR varies with the level of input signal
The effect of quantisation noise can be reduced by smaller
quantising steps for small input voltages & larger step for
large input voltages
This process including non uniform decoding process called
instantaneous companding
74
PCM primary multiplex group
PCM system first developed for telephone
transmission over cables originally designed for
audio frequency
Telephone channel are combined by TDM to form an
assembly of 24 or 30 channels this is called primary
multiplex group
This can be used as a building block for
assembling larger no. Of channels in high
order multiplex systems
75
30 channel PCM SYSTEM
76
Length of frame is 125micro sec
It contain one speech sample from each channel, together
with additional digitals used for synchronising & signalling
Frame 30 channel system divided into 32 time slots, each of 8
digit
Total bit rate is 8khz*8*32=2.048Mb/s
Time slot 1 to 15 & 17 to 31 are each allotted to a speech
channel
Time slot 0 is used for frame alignment & time slot 6 is used
for signalling
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24- channel frame format
78
Digit rate is 193*8kb/s= 1.544Mb/s
First bit is used for framing is called F bit
The other form 24, 8 bit time slot for speech signal
on 0dd numbered frames F bit takes alternating
pattern 1,0,1,0,....
Even frames carry the pattern 0,0,1,1,1....
On 6 & 12 of multiframe bit D8 of each channel time
slot is used for signalling for that channel
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PDH(Plesiochronous digital hierarch)
Primary multiplex group of 24 or 30 channel is use as
a building block for larger no. of channels in higher
order multiplex system
At each level in the hierarchy several bit stream
known as tributaries are combined by a multiplexer
In transmission network, which has not designed for
synchronous operation, the i/p to digital multiplexer
will not be exactly synchronous
80
Although they have same bit rate they
originate from different crystal oscillator & can
vary within clock tolerance & called
plesiochronous
Plesiochronous digital hierarchy is the first
generation of higher order digital multiplex
system
81
BIT & WORD INTERLEVING
82
BIT & WORD INTERLEVING
If i/p to mux are synchronous ie, have same bit
rate & are in phase they can be interleaved by
taking a bit or group of bits from each in turn
The interleaving can be done by switch that
samples each input under the control of
multiplex group
2 methods of interleaving
Bit interleaving
Word interleaving
83
Bit interleaving
One bit is taken from each tributary in turn
If there are n i/p signal ,each with rate of Ft
bits/s, then combined rate will be NFt
each element will have a duration equal to
1/N of an i/p digit.
84
WORD INTERLEAVING
Group of bits are taken from each tributary in turn &
this involves the use of storage at each i/p to hold
the bits waiting to be sampled
Bit interleaving is simpler & chosen for PDH
Word interleaving was chosen for SDH
There are 3 incompatible set of standards for
Plesiochronous digital multiplexing centre on
EUROPE,N. AMERICA ,JAPAN
85
EUROPEAN PDH
It is based on 30 channel primary multiplex
Uses bit interleaving
Frame length is 125micros
When N tributeries are combined ,the no. digits contained in
higher order frame > n times no. Digits in tributary frame, this is
because it is necessary to add extra overhead digits for 2 reasons
86
1)frame alignment
Higher order demux recognise the start of each
frame in order to route subsequent received digits to
correct outgoing tributaries
A unique code is sent as a frame alignment word,
which is recognised by demux
And used to maintain its operation in synchronism
with incoming signal
87
2)Adding extra digits to the frame to
perform justification
Justification process enables MUX& DEMUX to maintain
correct operation ,although the signal of the tributary
entering the MUX may drift relative to each other
If an input is slow a dummy digit is added to maintain correct
o/p digit rate
if the i/p tributary signal of the tributary speeds up no
justification digit is added
These justification digit must be removed by DEMUX in order
to send correct sequence of signals to the o/p tributary
88
AMERICAN PDH
It is based on 30 channel primary multiplex
89
PDH multiplex mountain
To separate one channel from aggregate bit stream a total
demuxing process is required called multiplexing mountain
90
SDH
The introduction of integrated digital network has
resulted in transmission system being fully synchronised
& this has led to the emergee of a new synchronous
digital hierarchy
It employs byte interleaving, this allows add/drop
muldexes to insert or remove lower order assemblies ,
This new multiplex hierarchy is also called as
synchronous optical n/w[SONET]
SDH uses digit rate 155.52Mb/s & multiples by factor 4n
91
SDH
92
93
Basic SDH signal called synchronous transport
module at level 1 (STM-1)
IT has 9 segments with overhead byte at the start of
each
Remaining byte consists of mixture of traffic &
overhead depending on traffic is carried
Total length is 2430bytes with each overhead using 9
bytes
Overall bit rate is 155.52Mb/s
94
Frame is represented as 9 rows & 270 coloumns of 8
byte
First 9 coloumns are section overhead[SOH] such as
frame alignment error monitoring & data
Remaining 261 coloumn comprise payload into
whicha variety of signal can be mapped
Each tributary to the multiplex has its own payload
area known as tributary unit or virtual tributary
95
In multiplexing process the byte from a tributary are
assembled into a container & a path overhead is
added to form a virtual container[VG]
VG travels through the network as a complete
package untill it is demultiplexed
Start point of VG indicated by a pointer
The VG together with its pointer constitute the TU
96
REMOTE SWITCHING UNITS
Because of SDH provides interface for network
management messages in a standard format, it can
lead to a managed transmission bearer network
The n/w can be reconfigured under s/w control from
remote terminals
The ability of SDH to provide add & drop mux can
lead to n/w structure
97
Transmission performance
98
telephony
In telephone connection the complete path includes the air
path from talkers mouth to a telephone TXR &
from telephone receiver to listeners each in addition to
telephones & switched connection between them
The overall attenuation of such path expressed in overall
loudness rating(OLR) in db
OLR is measured by comparing the perceived loudness of the
received sound with that from a standard speech path called
intermediate reference system defined by CCITT
99
It is possible to plot curves of % of connections
considered unsatisfactory against overall loudness
rating of the connection
In analog n/w 4 wire connection has nominal loss
When the component losses are greater than the
nominal value overall loss of connection is increased
100
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