1C- Communication Networks - Circuit Switching - Copy

8/14/2019 1C- Communication Networks - Circuit Switching - Copy

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COMMUNICATION SYSTEMREVIEW

CIRCUIT SWITCHING

The N 2 ProblemFor N users to be fully connected directly Requires N (N 1)/2 connectionsRequires too much space for cablesInefficient & costly since connections not always on

N = 1000N (N 1)/2 = 499500

1

2

34

N


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Circuit SwitchingPatchcord panel switch invented in 1877Operators connect users on demand

Establish circuit to allow electrical current to flowfrom inlet to outlet

Only N connections required to central office1

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N 1

N

Manual Switching


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Switch Network A switched network consists of a series of inter-linked nodes, called switches.Switches are hardware and/or software devicescapable of creating temporary connectionsbetween two or more devices linked to theswitch but not to each other.

Methods of switchingCircuit switching, packet switching, and messageswitching

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Circuit Switch

Circuit switching creates a direct physicalconnection between two devices such asphones or computers.

We can use switches to reduce the number andlength of links.

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A Circuit Switch A circuit switch is a device with n inputs and moutputs that creates a temporary connectionbetween an input link and an output link.The number of inputs does not have to matchthe number of outputs.

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A Folded Switch

An n-by-n folded switch can connect n lines in full-duplexmode. For example, it can connect n telephones in sucha way that each phone can be connected to every otherphone.Circuit switching uses space-division switch [paths in thecircuit are separated from each other spatially] or time-division switch.

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Circuit Switch Types

Space-Division switchesProvide separate physical connection betweeninputs and outputsCrossbar switchesMultistage switches

Time-Division switchesTime-slot interchange techniqueTime-space-time switches

Hybrids combine Time & Space switching

Space Division Switch

Paths in the circuit are separated from each other spatially.Crossbar SwitchCrossbar switch connects n inputs to m outputs in a grid,using electronic micro-switches (transistors) at each cross-

point.Limitation is the number of cross-points required

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Multistage SwitchMultistage switch combines crossbar switches in severalstages.Design of a multistage switch depends on the number ofstages and the number of switches required (or desired) ineach stage.Normally, the middle stages have fewer switches than do thefirst and last stages.

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Multiple Switching Paths

Multiple paths are available in multistage switches.Blocking refers to times when two inputs are looking forthe same output. The output port is blocked.

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n k

n k

n k

n k

N/n N/n

N/n N/n

N/n N/n

k n1

2

N/n

N inputs

1

2

3 3

N/n

N outputs

1

2

k

2(N/n)nk + k (N/n)2 crosspoints

k n

k n

k n

Multistage Space Switch

Large switch built from multiple stages of small switchesThe n inputs to a first-stage switch share k paths through intermediatecrossbar switchesLarger k (more intermediate switches) means more paths to outputIn 1950s, Clos asked, How many intermediate switches required tomake switch nonblocking?

Clos Non-Blocking Condition:k=2n-1

Request connection from last input to input switch j to last output in output switch mWorst Case: All other inputs have seized top n-1

middle switches AND all other outputs haveseized next n-1 middle switchesIf k=2n-1 , there is another path left to connectdesired input to desired output


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nxk

nxk

nxk

N/nx N/n

N/nx N/n

N/nx N/n

k xn1

N/n

Desiredinput

1

j m

N/n

Desiredoutput

1

2n -1

k xn

k xn

n-1

N/nx N/nn+1

N/nx N/n2n-2

Free path Free path

n-1busy

n-1busy

Clos Non-Blocking Condition:

k=2n-1

# internal links =2x # external links

C(n) = number of crosspoints in Clos switch

= 2Nk + k ( )2 = 2 N (2n 1)+(2 n 1)( ) 2

Differentiate with respect to n:

0 = = 4 N + 4 N ==> n

The minimized number of crosspoints is then:

C* =(2N + )(2( ) 1/2 1) 4 N 2N = 4 2N 1.5

This is lower than N 2 for large N

Minimum Complexity Clos Switch

N 2N /2

2N 2n2

2N 2n3

2N 2n2

N 2

d Cd n

N n

N n

N 2


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Example: Clos Switch Design

Circa 2002, Mindspeed offered a Crossbarchip with the following specs:

144 inputs x 144 outputs, 3.125Gbps/line

Aggregate Crossbar chip throughput:450 Gbps

Clos Nonblocking Design for 1152x1152switch

N=1152, n=8, k=16

N/n=144 8x16 switches in first stage16 144x144 in centre stage144 16x8 in third stage

Aggregate Throughput: 3.6 Tbps!

Note: the 144x144 crossbar can bepartitioned into multiple smaller switches

8x16

8x16

8x16

8x16

144 144

144x144

144x144

16x8 1

2

144

1 1 5 2 i n p u

t s

1

2

3 3

N/n

1 1

5 2 o u t p u t s

1

2

16

16x8

16x8

16x8

Time Division Switch

Time-division switching uses time-division multiplexing toachieve switching. Two methods used are:

Time-slot interchange (TSI) changes the order of the slots based on thedesired connection.

TDM bus


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Time-Slot Interchange (TSI)TSI consists of random access memory (RAM) with severalmemory locations. The size of each location is the same asthe size of a single time slot.The number of locations is the same as the number of inputs.The RAM fills up with incoming data from time slots in theorder received. Slots are then sent out in an order based onthe decisions of a control unit.

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1

2

3

22

23

24

Writeslots inorder ofarrival

Read slotsaccording toconnectionpermutation

24 23 12

Time-slot interchange

24 23 12abcd b a d c

a

b

c

d

Time-Slot Interchange (TSI)Switching

Write bytes from arriving TDM stream into memoryRead bytes in permuted order into outgoing TDM streamMax # slots = 125 msec / (2 x memory cycle time)

IncomingTDMstream

OutgoingTDMstream


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TDM BusInput and output lines are connected to a high-speed bus through input andoutput gates (microswitches)Each input gate is closed during one of the four slots.During the same time slot, only one output gate is also closed. This pair ofgates allows a burst of data to be transferred from one specific input line toone specific output line using the bus.The control unit opens and closes the gates according to switching need.

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Comparison of SDM and TDM

SDM Advantage:

Instantaneous.

Disadvantage:Number of cross points required.

TDM Advantage:

No cross points.

Disadvantage:Processing delay.

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TST SwitchCombine Space division and time division switching.This results in switches that are optimized both physically (thenumber of crosspoints) and temporally (the amount of delay).Various types are: time-space-time (TST), time-space-space-time (TSST), space-time-time-space (STTS), etc.

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nxk

nxk

nxk

nxk

N/nx N/n k xn1

2

N/n

N inputs

1

3

1

12

n

Time-slot interchange

Input TDMframe withn slots

Output TDMframe with k slots

n 2 1 k 2 1

Time-Space-Time Hybrid Switch

Use TSI in first & third stage; Use crossbar in middleReplace n input x k output space switch by TSI switch that takes n-slotinput frame and switches it to k-slot output frame


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n k N/n N/n

N/n N/n

N/n N/n

k n1 1

2

N/n

1

2

k

k n

k n

n k 2

n k N/n

First slot

k th slot

First slot

k th slot

Flow of time slots betweenswitches

Only one space switch active in each time slot

nxk

nxk

nxk

nxk

N/nx N/nTime-sharedspace switch

k xn1

2

N/n

N inputs

1

2

3 3

N/n

N outputs

TDMn slots

n slots

n slots

n slots

k xn

k xn

k xn

TDMk slots

TDMk slots

TSI stage TSI stageSpace stage

Time-Share the Crossbar Switch

Interconnection pattern of space switch is reconfiguredevery time slotVery compact design: fewer lines because of TDM &less space because of time-shared crossbar


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2x3

2x3

3x21

2

1

23x2D1

B1 A1B 2 A2

C 1D2 C2

B 1 A1

C1D1

A1

B 1

C 1

D1

A1 C 1

B1 D1

(b)

AB

CD

(a)C

A

D

B

Example: A3, B4, C1, D3

3-stageSpaceSwitch

EquivalentTST Switch

Example: T-S-T Switch DesignFor N = 960

Single stage space switch ~ 1 million crosspointsT-S-T

Let n = 120 N/n = 8 TSIsk = 2 n 1 = 239 for non-blockingPick k = 240 time slotsNeed 8x8 time-multiplexed space switch

For N = 96,000T-S-T

Let n = 120 k = 239N / n = 800Need 800x800 space switch

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1C- Communication Networks - Circuit Switching - Copy