Structure of a switch - PowerPoint Presentation

Slide1

Structure of a switch

We use switches in circuit-switched and packet-switched networks. In this section, we discuss the structures of the switches used in each type of network.

Structure of Circuit switches

Structure of Packet switches

Topics discussed in this section:

http://www.youtube.com/watch?v=hjVNKeVdKcs&list=PL374944B232C0B48ESlide2

Types of Circuit switches1) space division switch

2)Time division switch1) space division switch : paths r separated from one another spatially.

Used both in analog and digital networkSub Types : crossbar , multistageSlide3

Connects ‘n’ i/p to ‘m’ o/ps using electronic switches( transistors) at each crosspoint.It requires total n*m crosspoints. For 1000 i/p and 1000 o/p it requires 10,00,000 cross points, and worst , 75% r idle statistically.

So multistage is used.

Figure

Crossbar switch with three inputs and four outputsSlide4

Which combines crossbar switch in several stages . Adv : reduced no. of cross points

Dis adv: causes blocking

Figure

Multistage switchSlide5

In a three-stage switch, the total

number of

crosspoints

is

2kN + k(N/n)

2

which is much smaller than the number of

crosspoints

in a single-stage switch (N

2

).

Note

K is no of cross

pts,n

is individual input /output, N is Combined I/OSlide6

Design a three-stage, 200 × 200 switch (N = 200) with

k = 4 and n = 20.

Solution

In the first stage we have N/n or 10 crossbars, each of size 20 × 4. In the second stage, we have 4 crossbars, each of size 10 × 10. In the third stage, we have 10 crossbars, each of size 4 × 20. The total number of crosspoints is 2kN + k(N/n)

2

, or

2000

crosspoints. This is 5 percent of the number of crosspoints in a single-stage switch (200 × 200 = 40,000).

ExampleSlide7

TDM switches

Use TDM inside a switch. One of the most popular technology is Time-slot interchange Slide8

Suppose four i/p lines r to be connected any one of four o/p lines.In following pattern : 1 to 3 , 2 to 4 ,3 to 1 and 4 to 2

TSI has TDM mux, TDM demux and TSI having control unit and RAM , incoming packet is put in Ram and passed to appropriate o/p line using table in control unit.

Figure

Time-slot interchangeSlide9

Combination of time and space division switch:space switch:

Dis adv: no of crosspoints required Adv: no delaysTime switch: Dis adv: delays

Adv: needs no crosspoints.So Combination of time and space division switch combines advantages of bothEg TST switch shown on next slideSlide10

Figure

Time-space-time switchSlide11

Packet switch: Used in packet switched networks

Slide12

Figure

Packet switch components

It has four parts

1) Input port 2) Output port 3)Routing processor and 4) switch fabricSlide13

Converts EM signal to digital data . (Phy layer)

Detects error and corrects( DL layer) The packet is then buffered in Q for processing by next stage i.e. routing processor and switch fabric.

Figure

Input portSlide14
Slide15

Reverse order of function of i/p ports

Figure

Output portSlide16

Routing processor:

Performs function of network layer, ( finds addr of nest hop and corresponding o/p port)

Types of switches used: Cross bar, banyan switch, Batcher-banyan switchSlide17

Figure

A banyan switch

Q. If control bits are 101 , the input from 2 will be sent to which port?

Q. If input at port 1 is to be sent to port 6 , what should be the control bits ?Slide18

Figure

Examples of routing in a banyan switchSlide19

Figure

Batcher-banyan switch

The problem with the banyan switch is

the possibility

of internal collision even when two packets are not heading for same o/p port. So Batcher banyan switch sorts incoming Packets according to their final destination. Trap module prevents packets with same destination to pass to banyan tree and it allows only one packet at a time for such Destinations.