Switching

Fourth Meeting

Switching Modes

Circuit Switching Continuous link Exclusive

Packet Switching No continuous link Data is divided into

packets Not exclusive

Circuit Switching` Packet Switching`

Incurred Delay

Circuit-switched fixed delay of transferring

data

Packet-switched Queuing delays Variable transmission

delay because of the capacity of the link. Hi Mom

Multiplexing It improve the efficient use of the transmission capacity of physical

media Two types of multiplexing

Deterministic Statistical

Deterministic Multiplexing transmission medium is divided between a fixed number of

communication channels

Statistical Multiplexing The multiplexer and demultiplexer do not perform any switching The multiplexer stores the incoming packets in a buffer. Why?

Data rates of the input channel and the multiplexed channel are different; and Packets may arrive simultaneously on several input channels.

In the multiplexer, the packets are stored with a header with input channel number.

In the demultiplexer, the packets are retrieved in reverse order The packet header identify the output channel. Buffers are required for each output channel

The demultiplexer, over a short period of time, receive packets at a faster data rate than it can transmit.

Switching Structures

Cross-points It connect input lines to output

lines with a dedicated cross-point

It is non-blocking Switching Arrays

single-stage switch connects one input line to an output line using a single cross-point

Three stage-switching Blocking occurs N inputs = m groups * n inputs N outputs = m groups * n outputs

The first stage m array switches. Each array switches

n input lines k output lines

The second stage k array switches Each array switch

m input lines m output lines

The third stage m array switches. Each array switches

k input lines n output lines

Switching Structures:3-Stage-Switching

How Many Cross Points The total number of cross-points =

number of cross-points in the first stage plus

number in the second stage plus number in the third.

First stage Total = n × k × m Since N = n × m, then Total = N × k

Second stage Total = k × m × m Since m = N/n, Then Total = k × (N/n)2.

Third stage (same as first stage) Total = N × k

All three stages .

22

2)()()(

n

NkNkkN

n

NkkNTotal

Three Stage Switching Example

6 input lines that is blocking. four first-stage arrays

m = 4 Each array hasfour input

lines n = 4

Five second-stage arrays k = 5

16 × 16 three-stage configuration has 240)165(160)

4

165()5162(

2

Digital Space Switching

Two types Digital space switching

and Digital time switching.

Digital space switching The first automatic

telephone exchanges Paths were set up using

electromagnetic devices

Space Switch Carries A Time Division Multiplexed (TDM)

Space Switch Multiplexer

Space Switch Multiplexer

Example

727170

626160

525150Input 0

Multiplex X

Multiplex Y

Input 1

Input 2

Selects0 1

0 0

Control

0

Frame

Selects 0 = 0

Selects 1 = 0

Time Slot 0 Output

0

Example

727170

626160

5150Input 0

Multiplex X

Multiplex Y

Input 1

Input 2

Selects0 1

0 1

ControlFrame

1Time Slot 1 Output

0

Example

727170

6160

5150Input 0

Multiplex X

Multiplex Y

Input 1

Input 2

Selects0 1

1 1

ControlFrame

3Time Slot 2 Output

0

Example

727170

6160

5150Input 0

Multiplex X

Multiplex Y

Input 1

Input 2

Selects0 1

1 0

ControlFrame

2Time Slot 3 Output

0

Time Switching

Time Switching Implementation

Use two memory devices Speech store

All incoming data octets are stored in their sequence of arrival, Octet 0 → location 0 Octet 1 → location 1 : Cyclic counter

Connection store, Contains the destination

outgoing lines Has a cyclic counter

Example

0

0

3

01234 0

1234

Counter 1

Counter 20

Input Time Slot

Output Time Slot

0 3

1 2

2 1

3 4

0

Time

Example

01

1

32

01234 0

1234

Counter 1

Counter 21

Input Time Slot

Output Time Slot

0 3

1 2

2 1

3 4

0 1

Time

Example

012

2

321

01234 0

1234

Counter 1

Counter 22

Input Time Slot

Output Time Slot

0 3

1 2

2 1

3 4

0 1 2

Time

Example

0

23

3

3214

01234 0

1234

Counter 1

Counter 23

Input Time Slot

Output Time Slot

0 3

1 2

2 1

3 4

4 0

0 1 2 3

Time

Example

234

4

32140

01234 0

1234

Counter 1

Counter 24

Input Time Slot

Output Time Slot

0 3

1 2

2 1

3 4

4 0

0 1 2 3 4

Time

Space Time Space Switching

727170

6160

5150Input 0

Multiplex X

Multiplex Y

Input 1

Input 2

Selects0 1

1 0

ControlFrame

2Time Slot 3 Output

0

727170

6160

5150Input 0

Multiplex X

Multiplex Y

Input 1

Input 2

Selects0 1

1 0

ControlFrame

2Time Slot 3 Output

0

727170

6160

5150Input 0

Multiplex X

Multiplex Y

Input 1

Input 2

Selects0 1

1 0

ControlFrame

2Time Slot 3 Output

0

k = 1

n = 3

234

4

32140

01234

01234 0

1234

01234

Counter 1

Counter 24

72 71 70

61 60

51 50Input 0

Multiplex X

Multiplex Y

Input 1

Input 2

Selects 01

1 0

ControlFrame

2 Time Slot 3Output

0

Packet Switching In the past, the delay

associated with packet switching meant that it was not considered suitable for real-time applications such as voice communication.

Packet-switching exchanges (PSEs) have used the concept of stored program control (SPC) since the first generation of exchanges, in the late 1960s

2nd and 3rd Generation PSE

Cell Switching

Cell switching ≈ packet switching breaks a data stream into packets

Cell switching ≠ packet switching Cells have a fixed size

Cell switching ≈ circuit-switching connection-oriented service virtual circuit created.

Cell switching ≠ circuit-switching circuit is virtual No reserving of network resources

Virtual Path Switch 1

Virtual Path Switch 2

Picture source http://www.sce.carleton.ca/netmanage/dcommTutorials/scan06/scan0603.gif