Download Figure Time-space-time switch Packet switch

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
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Types of Circuit switches
1) space division switch
2)Time division switch
1) space division switch :
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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
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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
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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).
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TDM switches
Use TDM inside a switch.
One of the most popular technology is Time-slot interchange
Figure Time-slot interchange
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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.
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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
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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
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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
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Reverse order of function of i/p ports
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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.