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CSE-342
B.Eng. (Electrical Engineering)
TelecommunicationNetwork
Lecture 5
Telecommunication Switching
Muniba Q. Zaman
EVOLUTION
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Switching
If we could have point to point lines between all senders and all receivers, then we would not need switching
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Switching
Imagine point to point lines between all pairs of people:-
# of people # of lines2 13 3
10 45 100 4950 1000 0.5 million
In general, for n people ?Advantage ?Disadvantage ?
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Switching
A switch provides temporary path between end users in a communication network
Traffic engineering is science of designing switching network so that circuits are optimally used & are highly available, most important consideration is cost
Shehryar
MoezUmar
Arsalan
Rohan
Umair
Usama
5CSE-342
Introduction Definition
Switching the process of directing a signal from its source to its destination the establishing, on demand, of an individual connection from a desired inlet to a
desired outlet within a set of inlets and outlets for as long as is required for the transfer of information
Directing Data From Source to Destination involves: involves: Dedicated lineDedicated line – a telephone line leased by a firm and used exclusively by that firm SwitchSwitch – a special purpose computer that directs incoming messages along a path Private branch exchange (PBX)Private branch exchange (PBX) – a special-purpose computer that distributes calls
within a customer’s site Switch Switch a special purpose computer that directs incoming messages along a path
Switching example – PSTN, PDN, PSPDN
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Exchange Structure
Elements of an exchange include following :-1. Concentrator
Concentrates calls from terminations to links2. Expander
Expands terminating calls to terminations & trunks3. Distributor or group switch
For interconnecting switches, # of links decided on statistical basis Types of calls could be :-
Own exchange calls Outgoing junction calls Incoming junction calls Transit calls
Signaling System To pass call connection / disconnection information between termination &
exchange and between exchanges Channel associated or common channel signaling
Control System To interpret signaling information and take appropriate measures
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Switching Types
Point-to-Point Network Switching Circuit Switching, Message Switching, Packet Switching, Cell Switching Connection-Oriented versus Connectionless Virtual Circuit versus Datagram Networks Internal/External Abstractions
Point-to-Point Switching Circuit Switching Store-and -Forward Networks
Message Switching Packet Switching
connection-oriented vs connectionless virtual circuit vs datagram
Cell Switching
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Switch System Classification
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Message Switching
Message is first stored in buffer and then sent forward as and when resources become available or sufficient messages have been collected
Also called store and forward It is example of queuing system Example is telegraph message Advantage
Better utilization of bandwidth
Disadvantage Delay
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Circuit Switching
A path is established between caller and destination for that call
If all the ccts are busy then call is lost therefore it is an example of lost call system
Advantage No delay , real time communication
Disadvantage Low bandwidth efficiency, ccts used for fraction of time
speech contains pauses and silent interval
Example is PSTN
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Circuit Switching
Circuit Switching (Telephone network) One physical link is established for each connection
Seeking out and establishing a physical copper path end-to-end [historic definition] .
Circuit switching implies the need to first set up a dedicated, end-to-end path for the connection before the information transfer takes place.
Once the connection is made the only delay is propagation time. Advantage
No delay , real time communication Disadvantage
Low bandwidth efficiency, ccts used
for fraction of time speech contains
pauses and silent interval
Packet Switching
Message is broken down into packets and then each packet is sent separately (possibly by different routes)
If outgoing routes are busy then they are kept in queue therefore packet switch is an example of queuing system
Switches should be intelligent to choose best routing path
Example is internet Advantage
Better bandwidth efficiency Disadvantage
Packet switching introduces latency 13CSE-342
Packet Switching
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Comparison
Characteristics Circuit Packet
Origin Voice tel Data NW
Connection less or oriented
Connection- Oriented
Both
Delay Low High
NW intelligence
centralized De-centralized
BW efficiency Low High
Information Loss
Low High15CSE-342
Circuit Switching vs. Packet Switching
CIRCUIT SWITCHINGCIRCUIT SWITCHING: The method used in telephone networks Sets up a temporary circuit between the source and the destination Resources are reserved for the duration of the session (call)
PACKET SWITCHINGPACKET SWITCHING: Appropriate when data are transmitted infrequently from a large
number of nodes Used on the Internet
The message is divided into packets containing control information No circuit is established
Provides better sharing of resources Multiple users share the same resources
A store-and-forward network where the block of transfer is a complete packet. A packet is a variable length block of data with a tight upper bound.
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Circuit Switching vs Packet Switching
(a) Circuit switching. (b) Packet switching.
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CSE-342 18 Circuit switching Message switching Packet switching
Event Timing
Event Timing
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DCC 6th Ed., W. Stallings, Figure 10.3 Circuit switching Virtual circuit packet switching Datagram Packet switching
Connection-Oriented
A setup stage is used to determine the end-to-end path before a connection is established.
Data flow streams are identified by some type of connection indicator (e.g. OSI, X.25, SNA).
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Connectionless Oriented
No set up is needed Each packet contains information which allows the packet to be
individually routed hop-by-hop through the network.
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Datagram vs Virtual Circuit
Datagram Each datagram packet may be individually routed.
Virtual Circuit Virtual circuit set up is required. All packets in a virtual circuit follow the same path.
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External Virtual Circuit/Datagram
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Internal Virtual Circuit
And Datagram
Operation
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Connection Strategies
Circuit switching. A permanent physical link is established for the duration of the communication (i.e., telephone system).
Message switching. A temporary link is established for the duration of one message transfer (i.e., post-office mailing system).
Packet switching. Messages of variable length are divided into fixed-length packets which are sent to the destination. Each packet may take a different path through the network. The packets must be reassembled into messages as they arrive.
Circuit switching requires setup time, but incurs less overhead for shipping each message, and may waste network bandwidth. Message and packet switching require less setup time, but incur more overhead per message.
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Circuit switched and packet-switched networks
CSE-342
B.Eng. (Electrical Engineering)
TelecommunicationNetwork
Switching Technology
Categorizing switching
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SPC: Stored program control
Switch System Classification
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Manual Switching 1876 A. G. Bell telephone patent 1878 The first exchange constructed
in La Porte, the US connect any two of the 21
subscribers using manual switching A ringing signal sounded at the
operator's switchboard when any of the subscribers turned the crank of his telephone
Upon answering the signal, the operator was asked to connect the call
End-to-end physical path or connection
Sequencing preserved: ABCDE in implies ABCDE out
Nothing is lost or inserted
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switchboard
Plug-cord
Manual Switching
A simple manual switchboard has subscribers’ terminations connected to jacks (sockets)
The “links” consist of a pair of plugs on flexible cords (wires) which can be used to interconnect any pair of jacks
Signaling consists of calling indicators (e.g. relays and lamps) and clearing indicators
Call routing instructions are passed verbally
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Manual Switching
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Manual Switching
Manual exchange demonstrate following features: Central-battery operation Loop/disconnect signaling The multiple Busy testing Concentration Metering and ticketing Classes of service Common control Scanning Stored-program control Common-channel signaling
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Manual Switching
Advantages Human control Services such as advice of duration and charge, transfer of calls
when absent, wake up calls etc., which are so complex to provide automatically, pose no problem on manual exchanges
Disadvantage The weakness of the manual exchange, which has resulted in its
almost complete disappearance, was essentially its slowness Long distance calling involved chain of operators
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Functions of a Switching System
1. Attending
2. Information receiving (address signal)
3. Information processing
4. Busy testing
5. Interconnection
6. Alerting
7. Supervision
8. Information sending
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Switch System Classification
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Strowger
In 1891, first automatic telephony system was introduced by Strowger
Strowger’s business was being taken away by his competitor’s wife who was working as telephone operator
Stepping relay was used to allow a subscriber dialing a telephone to select one of ten lines
When the subscriber dials the telephone a series of electrical pulses are generated on the line (at a maximum rate of ten per second)
Each pulse causes the uni-selector (which starts at the 'home' position) to be advanced by one step 37CSE-342
Strowger (Two motion Selector)
Two motion selectors typically have 10 rows with 10 contacts each
It can move vertically and horizontally It can therefore accept two dialed digits from
a subscriber and route the call to any of 100 numbers
The first digit moves the selector vertically up to the corresponding level
The second digit moves the wipers around the contacts of that level
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Strowger’s Step By Step Switch
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Rotating shaft
Slidingarm
10 X 10 Array of contacts
Two motion Selector
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Two motion Selector
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Strowger
In an 100 line exchange each user has to be provided with his own 100 outlet 2 motion selector
This can be avoided by using stage of uni selectors at the beginning
If we want to increase the size of exchange then we can use more banks of 2 motion selectors
Director is used to translate the digits dialed by user into actual routing digits for calls outside exchange
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Strowger Switch
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Strowger switch
Uniselector & Group selector
Uniselector one which has a single rotary switch with a bank of contacts Depending upon the number of switching contacts, uniselectors
are identified as 10 outlet or 24 outlet uniselectors Several uniselectors can be graded together so that multiple
incoming circuits can be connected to multiple outgoing circuits cheaper act as a concentrator to enable a large number of lines to share
a smaller number of two-motion selectors. Operation: self-drive mode of operation performed by a drive
mechanism of a rotary switch. This mechanism contains an armature, electromagnet, Pawl, and Ratchet wheel. The wiper is attached to the ratchet wheel. When the line relay detects a calling signal, the magnet is energized and operates the ratchet wheel, pawl and its associated wiper. When the electromagnet is de-energised the armature is released and returns to its rest position.
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Uni-Selector
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Strowger Switch
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Uniselector
0
9
8
7
6
5
4
3
2
1
0
9
8
7
6
5
4
3
2
1
0
9
8
7
6
5
4
3
2
1
0
9
8
7
6
5
4
3
2
1
1st 2nd 3rd Final
A
B
C
Group Selector
Step by Step Switching
In general, the Strowger switching system consists of: subscriber’s line circuit, line finder & alloter circuit Group selector and final selector
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Subscriber line circuit (SLC) Every subscriber is connected to his local exchange by one
pair of wires This single pair carries the voice in both directions and the
ring current to ring the bell when a call is received At the exchange, every subscriber line terminates into its
own SLC This consists of a pair of relays dedicated to that subscriber Remaining switching circuits are shared by all the
subscribers. When the subscriber lifts his handset, current starts to flow
on the line. This is detected by the SLC.
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Line Finder & Alloter Mechanism to find a free selector and to connect the calling
subscriber to that free selector
Group selector used in the exchange that has more than one rank of two-
motion selector. The selector each step vertically in response to a dialed digit. However, during the inter-digit pause they operate in a self-drive mode like a uniselector. Thus, the wiper hunt for a free outlet on the selected level, which leads to a selector in the next switching stage. The selector in the last stage respond to two dialed digits and is called final selectors.
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Routing of a Local call in Strowger System
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Why step by step? Disadvantages:
regular maintenance by the skilled technicians are necessary It is not feasible to select an alternate route for interoffice calls, if all the
trunks are busy limited to dial pulses If calling rate is high, heavy operation is performed by the system and
the life time of the system is less Congestion could arise when the switching system is heavily loaded capacity of switching system reduces because of fire service, police
ambulance, fault reports, directory enquiry, operator assistance etc can accept only 7 to 9 pulses in 1 second hence not suitable for fast
dialing Large in size Noise
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Circuit . . .
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SxSlinklink
SxSlink
Could be 10 or more links
Example
Construct a switch with 50 outlets of 3 wires each from a 25-way uniselector with 6 wipers
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Linked Numbering Scheme (LNS)
For junction calls, dialed digits are used to route a call through each exchange
This requires different dialing codes to reach the same destination from different originating exchanges
Example: call from exchange A to A, B to A, C to A, D to A via C
This is Disadvantageous Hence use Linked Numbering scheme – numbering
scheme of an area and the trunking of its exchange are arranged so that a customer can be reached by dialing the same digits from any exchange in the area
Example: 10,000-line main exchange and several 1000-line satellite exchange can use 5 digit linked numbering scheme. This require an extra rank of selectors
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Register-translator-senders
Although LNS used widely but larger cities require a more general solution
Ideally, route for call should be completely separate from the digits dialled to establish the connection
Solution: Register-translator-sender equipment Number dialled by the customer is stored (registered) in
the register Stored digits are then analyzed to determine the routing If necessary, part of the number is translated into a
different number which is sent out to establish the connection
By using different translations all customer can dial same digits to reach a terminating exchange
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Register used only at the beginning of call Time taken to complete the whole process is only fraction of a
minute Therefore, registers are provided as common controls,
connected to a trunk by an auxiliary switch In electromechanical systems, register function is performed by
relays or switches Modern systems use more rapid signaling methods in order to
reduce the post-dialing delay Translator function consists of looking up a table of data
Dialed digits stored in the register are used to access an address in a store and the number read out from that address is the translation
Since charge for a call depends on its destination, translation may also contain the charging rate for the call
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Register-translator-senders
Registers were added to step-by-step exchange for use in large cities
Uniform 7-digit numbering scheme is used throughout the area
The first three digits (ABC) are an exchange code (which is translated) and the last four digits are the customer’s number on that exchange (which is not translated)
These registers are called Directors
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Switch System Classification
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Crossbar Strowger switches require regular maintenance; banks
need cleaning, mechanisms need lubrication and adjustment, and wipers & cords wear out
Replace manually operated switch by a matrix of telephone relays, with their contacts multiplied together horizontally and vertically.
This means a switch with N inlets and N outlets will require N2 relays for its crosspoint Uneconomic solution
More economic solution was provided by G. A. Betulander in 1917 by the invention of crossbar switch
Crossbar switch retains a set of contacts at each point but these are operated through horizontal (select magnets) and vertical (hold or bridge) magnets at the sides of the switch
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Crossbar
A crossbar switch connects multiple inputs to multiple outputs in a matrix manner
If the switch has M inputs and N outputs, then a crossbar has a matrix with M x N cross-points
At each cross-point some form of switch contact is needed to complete the connection between horizontal and vertical lines
Any of the M inlets can be connected to any of the N outlets by closing the appropriate switch contacts
Maximum number of simultaneous connections that can be carried by a matrix switch is given by which ever of the number of inlets or outlets is smaller
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CSE-342 63
Inlet 1 to outlet 2 by closing contact ? Inlet 4 to outlet 3 by closing contact ?
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Crossbar
Source:M. P. Clark, Networks and Telecommunications Design and Operation – 2nd Edition, John Wiley & Sons Ltd, pp. 96, 1997.
Crossbar switch
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Crossbar Switches
Crossbar exchanges are example of common/register controlled systems i.e. control circuit is shared
Basic building blocks are :- Link frame (consist of number of crossbar) Marker (Controls connection between inlets and outlets) Register (decodes dialed number)
Crossbar has no ‘intelligence’ A Marker must decide which magnets to operate a single marker can control many switches and serve
many registers Thus even a large exchange needs few markers This is called centralized control
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Crossbar Switches
Customer line circuit detects calling condition Signals relevant marker to establish
connection through concentration stage to free transmission bridge
Transmission bridge uses another marker to select free register
Register will receive and analyze number and then seize Marker to set appropriate switches in group selector or concentration stage
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Crossbar Switches
Once connection has been established then marker and register are released
Less numbers of markers and registers are needed as they are being used during setting up of connection and dialing only
Supervision of call is then provided by transmission bridge
Dialed number and number allocated to particular line need not to have any relation due to use of register for translation
Number of switching stages need not to be dependent on exchange numbering scheme
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Crossbar
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horizontals
Crosspointdetail
verticals
Crossbar
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Multistage switching
Three possible situation for crossbar could be :- m=n (non blocking), m>n (concentrating), m<n (expanding)
As N increases the number of required cross points can be excessively high but fewer than 25% of the cross points are used at a given time.
In order to save cross points, crossbar can be arranged in stages
It can suffer internal blocking if sufficient number of stages are not provided
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Multistage switching
How does it work?
Divide the 16 inlets into groups of 4. 1st outlet of each Stage 1 block is connected to an inlet of
the 1st Stage 2 block. 2nd outlet of each Stage 1 block is connected to an inlet of
the 2nd Stage 2 block. 3rd outlet of each Stage 1 block is connected to an inlet of
the 3rd Stage 2 block… ith outlet of each Stage 1 block is connected to an inlet of
the ith Stage 2 block.
1
4 x 4
2
4 x 4
3
4 x 4
4
4 x 4
1
4 x 4
2
4 x 4
3
4 x 4
4
4 x 4
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Multistage switching
100 x 100 using 10 by 10 matrices
10 x 10
1
10
1
10
10 x 10
1
10
1
10
(10)
10 x 10
1
10
1
10
10 x 10
1
10
1
10
(10)100 Inlets 100 Outlets
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It may not always be possible to make a connection even when the required outgoing trunk is free???
Blocking – there are free outgoing trunks and free links available but they cannot be used together
Traffic capacity is therefore affected by internal blocking as well as congestion of the external trunks
When marker is instructed to set up a connection, this also defines the link to be used and the select and bridge magnets to be operated
The connection is not made until the marker has checked the busy/free condition of the outgoing trunk and the relevant link
The connection is made only if both are found to be free This is called conditional selection
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Multistage Switching
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Multistage Switching
Let N = number of inlets and outlets Let n = size of each inlet-outlet group Let k = number of center-stage groups There are N/n groups in the first and last stages There are (N/n) (nk) cross points in the first and
last stages There are k middle groups with (N/n)2 cross
points in each middle array There are Nx = 2Nk+k(N/n)2 cross points total
which is much smaller than the number of cross points in a single-stage switch (N2)
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Multistage Switching
Example: For N = 100, The square matrix will require 100 x 100 = 10,000 cross points Assuming n = K = 10; # of cross points are: = 10(100/10)2 + 2 x 100 x 10 = 2200 Saving = 78%. The saving will increase as N
increases Advantage: The number of cross points Disadvantage: blocking Clos criterion: condition of non blocking
n = (N/2)1/2
k > 2n – 1 Cross points ≥ 4N [(2N)1/2 – 1]
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Switch System Classification
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Stored Program control
In electromechanical exchanges, common control mainly use switches and relays
Since they are operated much more frequently therefore wear out earlier
Also, Crossbar switches were slow in processing calls Therefore, relays were replaced by vacuum tubes,
transistors, CRT etc to improve speed, whose life is almost independent of its frequency of operation
After invention of modern digital computers they decided to use them for controlling switches also
Computer use the store program concept i.e. instructions are stored in memory and are executed one by one
SPC also carries out exchange control function through program stored in memory
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Stored Program control
SPC allowed several features to be extended to users like: Call barring Repeat last call Reminder calls e.g. wake-up call Call diversion Three way calls Charge advice
Computer has to be highly tolerant to faults In 1965, first SPC exchange was installed in USA by
AT&T at New Jersey There are two approaches of implementing SPC
Centralized Distributed
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Central Control
In Central control, central processor controls every thing and performs different functions with the same hardware by executing different programs
Central control uses reed relay for cross points Reed relay consist of sealed contacts inside an
operating coil and glass They are faster and more reliable as there are
sealed and there are no external moving parts Each cross point has its own reed relay This system is similar to manual exchange i.e.
single processor is controlling every thing
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Reed Relay
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Reed Relay with Central Control
A scanner is continuously monitoring all the connections
This record is accessible to processor Processor uses marker to establish path
between calling line and register Register sends data to processor for action Processor establishes connection through reed
relay switching network using marker More then one processors are employed in
actual exchanges
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Reed Relay with Central Control
Registers
Processors Scanner
Markers
LC Reed Relay
SW NW
Junctions
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Reed Relay with Central ControlTXE-4 (Telephone Exchange Electronics)
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Distributed Control
Control functions are shared by many processors in an exchange
Offers better availability and reliability and was possible because of low cost of processors
Control functions can be divided horizontally or vertically
Exchange control functions are divided among number of processors and each processor is responsible to do all the tasks
In horizontal decomposition, each processor perform only one or some function
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