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:

Types of Circuit switches  1) space division switch  2)Time division switch 1) space division switch :  paths r separated from one another spatially.  Used both in analog and digital network  Sub Types : crossbar, multistage

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 outputs

Which combines crossbar switch in several stages. Adv : reduced no. of cross points Dis adv: causes blocking Figure Multistage switch

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/O

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). Example

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

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 interchange

Combination of time and space division switch: space switch: Dis adv: no of crosspoints required Adv: no delays Time switch: Dis adv: delays Adv: needs no crosspoints. So Combination of time and space division switch combines advantages of both  Eg TST switch shown on next slide

Figure Time-space-time switch

Packet switch: Used in packet switched networks

Figure Packet switch components It has four parts 1) Input port 2) Output port 3)Routing processor and 4) switch fabric

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 port

Reverse order of function of i/p ports Figure Output port

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 switch

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 ?

Figure Examples of routing in a banyan switch

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.