Chapter # 3Switching

Switching

• If we could have point to point lines between all senders and all receivers, then we would not need switching

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 ?

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

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Exchange Structure• Elements of an exchange include following :-• Concentrator

– Concentrates calls from terminations to links• Expander

– Expands terminating calls to terminations & trunks• 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

Switching System Classification

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

Circuit Switching

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

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

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 High

Manual Switching• In 1878, the first manual exchange was

constructed in La Porte, in the United States• The first telephone exchange was operated by

hand• It served 21 subscribers and could connect any

two of them together• 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

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

Manual Switching

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

Strowger• In 1892, first automatic system was introduced

by Strowger• Strowger business was being taken away by his

competitor wife who was working as telephone operator

• Steping 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

Uni-Selector

• In theory, the system could be expanded by placing ten more relays in series with each of the ten contacts from the first relay

• When a pause between dialing pulses is sensed the system would transfer the next set of pulses to the second relay

Strowger (two motion Selector)

• Two motion selectors typically have 10 rows with 10 contacts each

• Two motion selector can move vertically and horizontally

• A two-motion selector 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

Two motion Selector

Two motion Selector

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

Strowger• Disadvantages

– Maintenance

– Flexibility

– Power

– Size

– Noise

Crossbar • A crossbar switch connects multiple inputs to

multiple outputs in a matrix manner • If the switch has N inputs and M 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 N inlets can be connected to any of the M 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

• Inlet 1 to outlet  2  by  closing  contact ?

• Inlet 4 to outlet  3  by  closing  contact  ?

Crossbar (relay)

Crossbar

Source:M. P. Clark, Networks and Telecommunications Design and Operation – 2nd Edition, John Wiley & Sons Ltd, pp. 96, 1997.

Crossbar switch

Crossbar Switch OrganizationDial tone marker & register

Connection establishment marker & register

Trunk Link FrameLine Link Frame

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 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

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

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

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.

4 x 4

4 x 4

4 x 4

4 x 4

4 x 4

4 x 4

4 x 4

4 x 4

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

Multistage Switching

Multistage Switching

• 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]

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)

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]

Stored Program control

• Crossbar switches were slow in processing calls• Relays were replaced by vacuum tubes,

transistors, CRT etc to improve speed • 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

Stored Program control

• SPC allowed several features to be extended to users like

• 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

Central Control• In Central control central processor controls every

thing• 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

Reed Relay

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 received to data to processor for

action• Processor establishes connection through reed

relay switching network using marker • More then one processors are employed in actual

exchanges

Reed Relay with Central Control

Registers

Processors Scanner

Markers

LC Reed Relay

SW NW

Junctions

Reed Relay with Central ControlTXE-4(Telephone Exchange

Electronics)

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