1B. Communication System - Review - Circuit Switching

COMMUNICATION SYSTEM REVIEW

CIRCUIT SWITCHING

The N2 Problem    

For N users to be fully connected directly Requires N(N – 1)/2 connections Requires too much space for cables Inefficient & costly since connections not always on 1

N = 1000 N(N – 1)/2 = 499500 N

2

4

3

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Circuit Switching Patchcord panel switch invented in 1877 Operators connect users on demand

 



Establish circuit to allow electrical current to flow from inlet to outlet

Only N connections required to central office

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1

N

N–1 3

2

Manual Switching

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Switch Network 

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

A switched network consists of a series of interlinked nodes, called switches. Switches are hardware and/or software devices capable of creating temporary connections between two or more devices linked to the switch but not to each other. Methods of switching 

Circuit switching, packet switching, and message switching 5

Circuit Switch 



Circuit switching creates a direct physical connection between two devices such as phones or computers. We can use switches to reduce the number and length of links.

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A Circuit Switch 

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A circuit switch is a device with n inputs and m outputs that creates a temporary connection between an input link and an output link. The number of inputs does not have to match the number of outputs.

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A Folded Switch 



An n-by-n folded switch can connect n lines in full-duplex mode. For example, it can connect n telephones in such a way that each phone can be connected to every other phone. Circuit switching uses space-division switch [paths in the circuit are separated from each other spatially] or timedivision switch.

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Circuit Switch Types 

Space-Division switches 

 

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Time-Division switches  

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Provide separate physical connection between inputs and outputs Crossbar switches Multistage switches Time-slot interchange technique Time-space-time switches

Hybrids combine Time & Space switching

Space Division Switch   



Paths in the circuit are separated from each other spatially. Crossbar Switch Crossbar switch connects n inputs to m outputs in a grid, using electronic micro-switches (transistors) at each crosspoint. Limitation is the number of cross-points required

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Multistage Switch 

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

Multistage switch combines crossbar switches in several stages. Design of a multistage switch depends on the number of stages and the number of switches required (or desired) in each stage. Normally, the middle stages have fewer switches than do the first 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 for the same output. The output port is blocked.

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Multistage Space Switch Large switch built from multiple stages of small switches The n inputs to a first-stage switch share k paths through intermediate crossbar switches Larger k (more intermediate switches) means more paths to output In 1950s, Clos asked, “How many intermediate switches required to make switch nonblocking?”

   

2(N/n)nk + k (N/n)2 crosspoints 1

1

3

N/n  N/n 2

…

2

nk

1

2

kn 3

N outputs

kn

nk N/n

kn kn

nk

…

N inputs

N/n  N/n

…

nk

N/n  N/n

N/n

k

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



Request connection from last input to input switch j to last output in output switch m Worst Case: All other inputs have seized top n-1 middle switches AND all other outputs have seized next n-1 middle switches If k=2n-1, there is another path left to connect desired input to desired output

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

j

1

1

n-1 busy

…

… nxk

…

Desired input

kxn

N/n x N/n

1

N/n x N/n n-1 N/n x N/n n+1

kxn m

n-1 busy

# internal links = 2x # external links

N/n x N/n 2n-2 nxk N/n

x N/n Free path N/n2n-1

Desired output

…

nxk

Free path

kxn N/n

Minimum Complexity Clos Switch C(n) = number of crosspoints in Clos switch = 2Nk + k( N )2 = 2N(2n – 1)+(2n – 1)( N )2 n n Differentiate with respect to n: 2 2 2 0 = dC = 4N – 2N + 2N ≈ 4N – 2N ==> n ≈ √ N 3 2 2 2 dn n n n

The minimized number of crosspoints is then: N2 N C* = (2N + N/2 )(2( 2 )1/2 – 1) ≈ 4N √ 2N = 4 √ 2N1.5

This is lower than N2 for large N

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Example: Clos Switch Design

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1

16x8 1

16x8

144x144

2

8x16 3

2

16x8

…

8x16 2

144144

… 8x16 144

3

…

Clos Nonblocking Design for 1152x1152 switch  N=1152, n=8, k=16  N/n=144 8x16 switches in first stage  16 144x144 in centre stage  144 16x8 in third stage  Aggregate Throughput: 3.6 Tbps!

8x16 1

1152 outputs

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Circa 2002, Mindspeed offered a Crossbar chip with the following specs:  144 inputs x 144 outputs, 3.125 Gbps/line  Aggregate Crossbar chip throughput: 450 Gbps 1152 inputs

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16x8

144x144

N/n

16

Note: the 144x144 crossbar can be partitioned into multiple smaller switches

Time Division Switch 

Time-division switching uses time-division multiplexing to achieve switching. Two methods used are: 

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

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Time-Slot Interchange (TSI) 

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TSI consists of random access memory (RAM) with several memory locations. The size of each location is the same as the 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 the order received. Slots are then sent out in an order based on the decisions of a control unit.

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Time-Slot Interchange (TSI) Switching   

Write bytes from arriving TDM stream into memory Read bytes in permuted order into outgoing TDM stream Max # slots = 125 msec / (2 x memory cycle time) 1

a

2

b

3 c … 23

Incoming

TDM stream

b 2

a 1



d 24

Write 22 slots in order of 23 arrival

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Read slots according to connection permutation

c d

b 24

a … 23

d 2

c 1

Outgoing TDM stream

Time-slot interchange

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TDM Bus   

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Input and output lines are connected to a high-speed bus through input and output 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 of gates allows a burst of data to be transferred from one specific input line to one 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: 

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Disadvantage: 

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Instantaneous. Number of cross points required.

TDM 

Advantage: 

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No cross points.

Disadvantage: 

Processing delay. 22

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TST Switch  

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

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Time-Space-Time Hybrid Switch 

Use TSI in first & third stage; Use crossbar in middle Replace n input x k output space switch by TSI switch that takes n-slot input frame and switches it to k-slot output frame nxk 1

kxn

N/n x N/n

1

1

nxk N inputs

2

nxk 3

nxk

Input TDM frame with n slots

1 2

n … 2

1



…



Output TDM frame with k slots k … 2

1

n

N/n

Time-slot interchange

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Flow of time slots between switches First slot

First slot

nk

N/n  N/n

1

1

kn 1

kn

nk 2

2

N/n  N/n

…

…

…

2

kn

nk N/n

kth slot 

N/n

N/n  N/n k

kth slot

Only one space switch active in each time slot

Time-Share the Crossbar Switch Space stage

TSI stage TDM n slots

nxk

n slots

nxk

N inputs

n slots

1

2

nxk

 

kxn 1

kxn N/n x N/n Time-shared space switch

2

kxn

N outputs

3

…

n slots

TDM k slots

TDM k slots

…

3

TSI stage

nxk

kxn

N/n

N/n

Interconnection pattern of space switch is reconfigured every time slot Very compact design: fewer lines because of TDM & less space because of time-shared crossbar

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Example: A→3, B→4, C→1, D→3 (a) A B

C

C D

D

A

3-stage

Space Switch B

(b) B2 A2 B1 A1

2x3

B1 A1

C1 A1

3x2

A1 C1

1

1

Equivalent

TST Switch D2 C2 D1 C1

2x3

D1 C1

D1 B1

2

3x2

B1 D1

2

Example: T-S-T Switch Design For N = 960  Single stage space switch ~ 1 million crosspoints  T-S-T    

Let n = 120 N/n = 8 TSIs k = 2n – 1 = 239 for non-blocking Pick k = 240 time slots Need 8x8 time-multiplexed space switch

For N = 96,000  T-S-T   

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

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