TELECOM SWITCHING SYSTEMS

[16TE54]

UNIT 1
Evolution of Switching systems

Neethu S

Assistant Professor

Dept. of TCE.
 Introduction
 Message Switching
 Circuit Switching
 Register-Translator Senders
 Distribution frames
 Cross bar systems
 Need of trunking
 Electronic Switching
 Reed-electronic systems
 Digital systems.
 Introduction
 Transmission of telegraphic signals over wires was the first
technological development in the field of modern
telecommunications
 Telegraphy was introduced in 1837 by Wheatstone and Morse
in Great Britain and in 1845 in France.
 In March 1876, Alexander Graham Bell invented the telephone
and the first telephone exchange at New Haven and he
demonstrated a point to point telephone connection.
 In such a network, a calling subscriber chooses the appropriate
link to establish connection with the called subscriber.
 In order to draw the attention of the called subscriber before
information can begin, some form of signaling is required with
each link.
 If the called subscriber is engaged, a suitable indication should
be given to the calling subscriber by means of signaling.
 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
 In order to connect the first entity to all other
entities, we require (n-1) links. With this 2nd
entity is already connected to the 1st, we now
need (n-2) links to connect the 2nd entity to the
others. For the 3rd entity we need (n-3) links and
so on...
 Therefore total number of links
L = (n-1) + (n-2) + (n-3) + ….. + 1 + 0
= n(n-1)/2
 Networks with point to point links among all the
entities are known as Fully connected networks
 With introduction of switching systems, the subscribers are not
connected directly to one another; instead they are connected to
the switching system as shown in the figure.

 Signaling is now required to draw the attention of the switching

system to establish or release a connection.
 It should also enable the switching system to detect whether
called subscriber is busy and if so indicate the same to the calling
subscriber.
 The functions performed by a switching system in establishing
and releasing connections are known as control functions.
 Automatic switching systems can be classified as electro
mechanical and electronic as
 Telecommunication network may be considered to be the
totality of transmission links and the nodes which are of
the following types:
  Customer nodes
 Switching nodes
 Transmission nodes
 Service nodes

 A telecommunication network may therefore be

considered as a system consisting of the following three
interacting sub-systems:
 Transmission systems
 Switching systems
 Signaling systems
Message switching

 Telegraph – torn tape system - Morse code

 Dynamic link establishment
 Priority based done
 Traffic congestion reduced
 Store and forward facility
 Large buffer space to store the message.
 Not applicable for real time
 Modified form is packet switching
 Delay/Queuing system
 Message switching

Message switching is still used for telegraph traffic & modified

form of it, known as packet switching, is used in data
communication. Dividing long message into smaller unit, known
as pocket. Packet switch sends each of these as a separate
Fig: evolution of msg switching a) manual transfer of hardcopy b)
manual transfer of paper tape c) manual transfer of paper tape
with automatic route selection d).automatic msg switching
system. T- Teleprinter R/P- reperferator A/T- automatic transfer S-
Circuit Switching

 Telephone :to maintain the connection during the

call to connect the circuit between two callers.
 If line –busy ,no store and forward facility, call lost
 Lost call system
• Circuit switching (Example ) - dedicated communication
path(circuit) between an O-D pair - data are transmitted
along the path with pre-negotiated rate - path (i.e., the
link capacity/bandwidth) is occupied for the entire
lifetime of communication - Three phases of the CS:
1) circuit/connection establishment (call setup)
2) data transfer
3) circuit disconnect (release the granted capacity) - only
propagation delay while transmission Ex. Telephone
network: dial .Æ talk Æ hang up
Manual exchange
Features

 Cord type of switch board and automatic systems

 Central battery operation
 Loop/disconnect signalling
 Multiple – check the status of the customer
 Busy test-tip ,ring and sleeve.
 T, R and sleeve-busy condition –switch in automatic
 Metering and ticketing
 Class of service –ordinary telephones & pay phones (coin)
 Stored program control in modern
 Features demonstrated by Manual
System
 Central-battery operation
 Loop/ disconnect signalling
 The multiple
 Busy testing
 Concentration
 Metering and ticketing
 Class of service
 Common control
 Scanning
 Stored-program control
 Common-channel signalling
 Functions of switching
system
Manual exchange –
 Attending
 Information receiving
 Information processing
 Busy testing
 Interconnection
 Alerting
 Supervision
 Information sending
Strowger system

 Automatic exchange to improve speed and carry more

subscribers
 Several electromechanical exchanges invented
 1880 to 1890
 Strowger step  step system was most popular
 The 1st electromechanical was developed by Almon
B Strowger , an undertaker in Kansas city ,USA
Basic Elements of Strowger Switching System:

 Two types of basic elements which perform

most of the functions of the strowger
switching system
 (a) Uniselectors and
 (b) Two motion selectors
Uniselector
Two motion selector
If calling party dial 5831
Strowger step by step
system
Strowger system
Disadvantages
 4 digit numbering scheme
 Junction calls- separate digits for call and routing between
exchanges
 No alternate routing possible, since the call process is step by
step
 Strowger system can handle only slow speed dialing, suitable
for pulse dialing not for DTMF[Dual Tone – Multi Frequency]
 Lifetime of moving parts is finite and limited
 Lots of mechanical components and requires non-stop
maintenance
 Congestion could arise when the Switching system is heavily
loaded.
 The capacity of switching system reduces drastically by 20%
to 40% due to special service numbers such as Fire service,
Police, Ambulance etc.
 1xx- to provide access to various services
 ‘0’ as trunk prefix,1,7,9,and 0 of 1 st group selectors cannot
be used for obtaining access to customers of the exchange
 Different dial numbers for same destination
 Register-Translator Sender
 Route for a call should not depend on the digits dialled to
establish a connection. Solution is Register-translator
sender, simply termed as
 Receives the number dialled by customer and stores it,
which are later analysed to determine routing.
 If necessary part of the number is translated into a different
number which is sent out to establish the connection.
 Register is used only for a short period at the beginning of
the call. (fraction of a minute)
 Register is connected to a trunk by an auxiliary switch
when it receives a calling signal, performs its functions
and is then released for use on other calls.
 Post-dialling delays are inherent when registers
are employed.
 Translation function consists essentially of looking up
a table of data.
 The dialled digits stored in the register are used to
access an address in a store and the number read out
from that address is Translation i.e., the digits to be
used for establishing the required connection through
the network.
 Since the charge for a call depends on the destination,
the translation may also contain the charging rate for
the call.
 Time required to obtain a translation is much less than
that taken to receive and send digits
Distribution frames
 Growth of traffic may require additional switches in the
exchange and more junctions to other exchanges.
 Great flexibility is therefore required in the trunking of an
exchange.
 This is obtained by inserting distribution frames into the
permanent exchange cabling.
 Frames contain an array of terminal blocks and the terminals
are linked in a less permanent fashion by wires called jumpers.
 The main distribution frame (MDF) is the place where the
cables of the customers’ distribution network terminate.
 The arrangement of terminals on the line side of the MDF
corresponds to the street cabling and so reflects the geography
of the area.
 In MDF the exchange terminals are arranged in directory
number order. To guard the exchange apparatus against any
high voltage surges on the external lines, protectors and fuses
are mounted on the MDF.
 The MDF also provides a convenient point of access for testing
lines and private circuits and through junctions are strapped
together at the MDF.
 The intermediate distribution frame (IDF) is used to distribute
incoming traffic evenly over the group of first selectors.
 On the multiple side of the IDF, lines are arranged in the
directory number order.
 On the local side, the order can be arbitrary to obtain the
desired result.
 The terminals on the local side of the IDF are corresponds to
equipment number (EN) of the lines and customers
uniselectors are connected to this side.
 IDF provides equipment number (EN) to directory number (DN)
translation.
 Incoming calls for a customer terminate at final selectors on an
outlet corresponding to the directory number.
 The final selectors multiples are therefore connected to the
multiple side of the IDF.
 A modern system provides DN to EN translation, in order to
enable customers’ incoming traffic to be redistributed in
addition to their outgoing traffic.
 Between the ranks of selectors there are trunk distribution
frames (TDF). These are used in the telecommunication system
to cater growth in traffic.
Cross bar switch
CROSS BAR SWITCH
 Strowger switches require regular maintenance. The banks need
cleaning, mechanisms needs lubrication and adjustment and
wipers and cords wear out.
 This disadvantages lead to the development of other forms of
switch, namely matrix of telephone relays as shown in figure2.3
with their contacts multiplied together horizontally and vertically.
 Since a switch with N inlets and N outlets requires N² relays for
its crosspoint, this was uneconomic for larger exchanges. A more
economic solution was provided by the invention of the crossbar
switch by G.A Betulander in 1917
 The crossbar switch retains a set of contacts at each crosspoint,
but these are operated through horizontal and vertical bars by
magnets at the sides of the switch. Thus, a switch with N inlets
and N outlets only needs 2N operating magnets and armatures,
instead of N².
 The magnets which operate the horizontal bars are called select
magnets and those operating the vertical bars are called hold
magnets or bridge magnets
 Operation of a select magnet tilts one of the horizontal bars up
or down. This causes flexible fingers to engage with the contact
assemblies of one row of crosspoints and provides the link which
was missing from their operating mechanisms.
 One of the bridge magnets is then operated and this closes the
contacts of the crosspoint at the coordinates corresponding to
the horizontal and vertical magnets.
 The select magnets are then released, but the finger remains
trapped and the cross point contacts remain closed for as long as
the bridge magnet is energized.
 Current flows in this magnet for as long as the P wire is at earth
potential. This persists until a ‘clear’ signal causes the earth to
be removed at the end of a call.
 Strowger selectors perform counting and searching. However,
the crossbar switch has no intelligence. Something external to
the switch must decide which magnets to operate. This is called
a marker. Since it takes less than a second to operate the switch,
a marker can control many switches and serve many registers
 Example link 23 connects outlet 3
of the primary switch 2 to inlet 2 of
the secondary switch 3.
 In the above figure there is only
one link from a primary switch to a
secondary switch.
 When a connection is required, this
path may be busy because it is
already in use for a connection
from another incoming trunk on
that primary switch to another
outgoing trunk on that secondary
switch.
 The call attempt fails, although the
outgoing trunk is free. This
situation is known as blocking.
Marker:
 Marker is used to setup a connection from a given incoming
trunk to a given outgoing trunk, this also defines the link to
be used and select the bridge magnets to be operated to
make the connection.
 Marker first test the condition of the outgoing trunk for
busy/free condition of the relevant link. Only when two
sides are free it operates the switches. This is called as
conditional selection.
 A marker has access to both ends of a connection that it
sets up through a network, it can test the connection for
continuity before it releases. It reports the faults in the path
and attempt to set up the connection over a different path
through the network.
 A concentrator can be constructed by multiplying
together the horizontals of a number of primary switches
 This shows a network with 500 incoming trunks and 100 outgoing trunks
using switches of size 10 X 10.

 To obtain larger network, four stages can be used. Figure 2.8 shows a Four
stage network, constructed from 400 switches of size 10 X 10 to serve 1000
incoming and outgoing trunks.
 A large crossbar exchange needs several markers in order to handle its
traffic and this introduces a complication. Here it is essential to prevent two
or more markers from attempting to set up connection in the same link
frame at the same time. To overcome this marker in a crossbar system is
using the quite complex lock out circuits.
Multistage switch
A general trunking diagram
Electronic Switching

 Electronic techniques- more reliable

 Electromechanical –relays and switches –wear out –
frequently used
 Electronics-not dependent on frequency of operation
 Central processor to common control –SPC
 Users data –altered –class of service
 Electronic techniques prove economic for common controls.
Disadvantages in electromechanical exchanges:
i) They mainly use switches and relays which are originally
designed for use in switching networks.
ii) They are operated much more frequently and wear cut
earlier.
 This gave an incentive for developing electronic common
control and resulted in electronic replacements for registers,
markers etc. which have greater reliability than
electromechanical devices.
 Advances in computer technology led to the development of
Stored Program Control (SPC). This enables a digital
computer to be used as central control and perform different
functions with the same hardware by executing different
programs.
 Class of service data stored in a central electronic memory,
since the processors stored data can be altered
electronically, some of these facilities can be controlled by
customers
Electronic Switching
 Electronic ???
 Diode cross point –multipled elements –one bit memory
for duration of a connection-
 Multiplexing done –FDM-TDM
 SDS-TDS
 Too Expensive- high voltage to line
 In order to develop a complete electronic exchange, we
replace electromechanical switches in the speech path
by electronic common control.
 If A is positive the diode is reverse biased and the
crosspoint is open.
 When A is negative the diode conducts and the
crosspoint is closed. These electronic devices are used in
the speech path.

 In order to implement a crosspoint, a one bit memory

along with switching element is required. Cold - Cathode
gas tubes and PNPN semiconductor devices are used to
implement crosspoint because two functions are
provided in these devices.
 But cost and implementation was high and complicated which
lead to the development of FDM and TDM system as a switch.
➢ In FDM system can be used as switch by bringing the two ends of
its transmission path together. Here made the modem to one end of
the path to operate at fixed frequencies, but those at another end to
operate for any frequencies. Now any trunk at one side of the switch
can be connected to any trunk at the other side. This method is too
expensive.
➢ A TDM system can also be used as a switch. If any of the N
receiving gates is operated by a train of pulses coincident with
those applied to one of the N sending gates, then a transmission
path is provided from the incoming trunk to outgoing trunk via a
common highway.

 For a transmission system, fixed pulse timings are used. By

altering the pulse timings any incoming trunk is connected to any
outgoing trunk i.e., N X N switch is obtained. It is cheaper
method.
 Consequently switching systems may be
classified as:
1. Space Division (SD) systems: Each connection is
made over a different path in space which exists for
the duration of the connection.
2. Time Division (TD) system: Each connection is
made over the same path in space, but in different
instants in time.
Time Division Switch
 Reed electronic systems
 Reed relay
 +,-,P wire, memory
 Matrix of reed relays == cross
bar switch –single coil.
 Magnetic material used – no P
wire and memory-released by
opposite current – no P wire
 Free path- found by store-one
marker-lead to SPC-processor
control all the switches
 TXE4 reed electronic exchange
 Semi electronic exchanges
 DIGITAL SWITCHING SYSTEMS:

 TDM transmission was being introduced for trunk and junction

circuits in the form of pulse-code modulation (PCM). If time
division transmission is used with space division tandem
switching as shown in fig (a) it is necessary to provide
demultiplexing equipment to demodulate every channel to
audio before switching and multiplexing equipment to
retransmit it after switching.
 If time division switching is used as shown in figure 2.10 b no
multiplexing and de-multiplexing equipment is needed. A
considerable economy is thereby obtained.
SUBSCRIBER LINE-INTERFACE CIRCUIT
(SLIC):

 As a result it became possible to implement all the

necessary functions economically on a Subscriber line-
interface circuit (SLIC), as shown in figure 2.12.
 The function can be summarized by the acronym
BORSCHT as follows
 ➢ Battery feed
 ➢ Over-voltage protection
 ➢ Ringing
 ➢ Supervisory signaling
 ➢ Coding
 ➢ Hybrid
 ➢ Testing
 Consider for data transmission, if there is an analog
customer’s line, a modem must be added and data can
only be transmitted at relatively slow speeds. If line is
digital, digital data can be transmitted by removing the
codec at higher speed. Say 64 Kbit/s instead of 2.4Kbit/s.
 this can include high-speed fax and slow-scan TV, in
addition to speech and data.
 In ISDN there are two methods of access are present.
➢ Basic rate access - 64 Kbits/sec Speech channel, 10
Kbits/sec common channel signalling
➢ Primate rate access - 1.5 Mbits/sec,1.2Mbits/sec
 The difference between these two is the speed at which
the digital data is being transformed.