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Structure of a switch We use switches in circuit-switched and packetswitched networks. In this section, we discuss the structures of the switches used in each type of network. Topics discussed in this section: Structure of Circuit switches Structure of Packet switches http://www.youtube.com/watch?v=hjVNKeVdKcs&list=PL374944B232C0B48E 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 Figure Crossbar switch with three inputs and four outputs 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 Multistage switch Which combines crossbar switch in several stages . Adv : reduced no. of cross points Dis adv: causes blocking Note 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 (N2). K is no of cross pts,n is individual input /output, N is Combined I/O Example 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). TDM switches Use TDM inside a switch. One of the most popular technology is Time-slot interchange Figure 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. 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 Figure Input port 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 Output port Reverse order of function of i/p ports 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.