1. Circuit Switching Circuit switching networks,
Circuit switches-space division switches,
Time division switches,
Time-space-time switches,
Routing in circuit switching networks,
Control signaling, SS7

2. Switching Networks
Long distance transmission is typically done by a network of switched nodes
Nodes not concerned with content of data
End devices are stations
Computer, terminal, phone, etc.
Data routed by switches from node to node

3. Nodes
Nodes may connect to other nodes only, or to stations and other nodes
Node to node links usually multiplexed
Two different switching technologies
Circuit switching
Packet switching

4. Simple Switched Network

5. Types of switched networks

6. CIRCUIT-SWITCHED NETWORKS
A circuit-switched network consists of a set of switches connected by physical links. A connection between two stations is a dedicated path made of one or more links. However, each connection uses only one dedicated channel on each link. Each link is normally divided into n channels by using FDM or TDM.
CONNECTION ORIENTED SERVICE

7. Note
A circuit-switched network is made of a set of switches connected by physical links, in which each link is divided into n channels.

8. Circuit Switching Dedicated communication path between two stations
Three phases
Establish
Transfer
Disconnect
Must have switching capacity and channel capacity to establish connection
Must have intelligence to work out routing

9. Note
In circuit switching, the resources need to be reserved during the setup phase; the resources remain dedicated for the entire duration of data transfer until the teardown phase.

10. Circuit-switched network used in Example
Telephone 1 is connected to telephone 7; 2 to 5; 3 to 8; and 4 to 6. Of course
the situation may change when new connections are made.
The switch controls the connections.

11. Switching in the traditional telephone network uses
Note
Switching in the traditional telephone network uses
the circuit-switching approach.

12. Circuit Switching Concepts
Digital Switch
Provide transparent signal
path between devices
Network Interface
Control Unit
Establish connections
Generally on demand
Handle and acknowledge
requests
Determine if destination
is free
construct path
Maintain connection
Disconnect

13. Delay in a circuit-switched network

14. Circuit Switch Types Space-Division switches Time-Division switches
Crossbar switches
Multistage switches
Time-Division switches
Time switches
Time-space-time switches :Hybrids Time & Space switching

15. Space division switches
Developed for analog environment
Separate physical paths
Connection oriented service
Suitable to voice signals

16. Crossbar Space Switch N x N array of cross points
Connect an input to an output by closing a cross point
Non blocking: Any input can connect to idle output
Complexity: N2 cross points N 1 2
N –1 …

17. Crossbar switch with three inputs and four outputs

18. Why crossbar switch can’t be used practically?
So many numbers of cross points are impractical. Such a switch is also inefficient because statistics show that, in practice fewer than 25 percent of the cross points are in use at any given time.

19. Multistage Switch Reduced number of cross points
More than one path through network
Increased reliability
More complex control
May be blocking

20. Three Stage Switch

21. Multistage switch
Large switch built from multiple stages of small switches
The n inputs share k paths through intermediate switches
Larger k (more intermediate switches) means more paths to output
nk
N/n  N/n
kn 1 2
N/n N
inputs 3
outputs k
2(N/n)nk + k (N/n)2 crosspoints …

22. Note
In a three-stage switch, the total number of cross points is 2kN + k(N/n)2
which is much smaller than the number of cross points in a single-stage switch (N2).

23. 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 cross points is 2kN + k(N/n)2, or 2000 cross points. This is 5 percent of the number of cross points in a single-stage switch (200 × 200 = 40,000).

24. In 1950s, Clos asked, “How many intermediate switches required to make switch nonblocking?” &
Introduced Non blocking criteria for Multistage space switch.

25. Clos Non-Blocking Condition: k=2n-1
Request connection from last input to input switch j to last output in output switch m
Worst Case: All other inputs have seized top n-1 middle switches AND all other outputs have seized next n-1 middle switches
If k=2n-1, there is another path left to connect desired input to desired output
nxk
kxn
N/n x N/n 1 1 1 … …
n-1
busy
N/n x N/n
Desired
input
nxk
kxn
Desired
output
n-1 j m
n-1
busy
N/n x N/n …
n+1 …
N/n x N/n
2n-2
nxk
kxn
N/n
N/n x N/n
Free path
Free path
N/n
2n-1

26. Minimum Complexity Clos Switch
C (n) = number of cross points in Clos switch
= 2Nk + k( )2 = 2N(2n – 1)+(2n – 1)( )2
Differentiate with respect to n:
0 = = 4N – ≈ 4N – ==> n ≈ √
The minimized number of cross points is then:
C* = (2N )(2( )1/2 – 1) = 4N [(2N)1/2 – 1]
This is lower than N2 for large N
N n
N n
dC dn
2N2 n2
2N2 n3
2N2 n2
N2 N/2
N 2

27. Example: Clos Switch Design
Circa 2002, Mind speed offered a Crossbar chip with the following specs:
144 inputs x 144 outputs
Clos Nonblocking Design for 1152x1152 switch
N=1152, n=8, k=16
N/n=144
8x16 switches in first stage
x144 in centre stage
x8 in third stage
8x16
144144
144x144
16x8 1 2
144
1152 inputs 3
N/n
1152 outputs 16 …

28. According to the Clos criterion: n = (N/2)1/2 k > 2n – 1
Note
According to the Clos criterion:
n = (N/2)1/2
k > 2n – 1
Cross points ≥ 4N [(2N)1/2 – 1]

29. Example
Redesign the previous three-stage, 200 × 200 switch, using the Clos criteria with a minimum number of cross points.
Solution
We let n = (200/2)1/2, or n = 10. We calculate k = 2n − 1 = 19. In the first stage, we have 200/10, or 20, crossbars, each with 10 × 19 cross points. In the second stage, we have 19 crossbars, each with 10 × 10 cross points. In the third stage, we have 20 crossbars each with 19 × 10 cross points. The total number of cross points is 20(10 × 19) + 19(10 × 10) + 20(19 ×10) = 9500.

30. Time Division Switching
Partition low speed bit stream into pieces that share higher speed stream
e.g. TDM bus switching
based on synchronous time division multiplexing
Each station connects through controlled gates to high speed bus
Time slot allows small amount of data onto bus
Another line’s gate is enabled for output at the same time

31. TIME DIVISION SWITCH Time-slot interchange

32. Time-Space-Time Hybrid Switch
Use TSI in first & third stage; Use crossbar in middle
Replace n input x k output space switch by TSI switch that takes n-slot input frame and switches it to k-slot output frame
nxk
kxn
N/n x N/n 1 1 1
nxk N
inputs 1 2
   n
Time-slot interchange
Input TDM frame with n slots
Output TDM frame with k slots
n …
k … 2
nxk 3 …
nxk
N/n

33. Flow of time slots between switches
First slot
First slot
n  k
N/n  N/n
k  n 1 1 1
k  n
n  k 2 2
N/n  N/n 2 … … …
k  n
n  k
N/n
N/n
N/n  N/n
kth slot k
kth slot
Only one space switch active in each time slot

34. Time space time Switch using Time multiplexed space switch
nxk
N/n x N/n
Time-shared
space switch
kxn 1 2
N/n N
inputs 3
outputs
TDM
n slots
k slots
TSI stage
Space stage …
Very compact design: fewer lines because of TDM & less space because of time-shared crossbar

35. Time-space-time switch by using time multiplexed space switch

36. Example: T-S-T Switch Design
For N = 960
Single stage space switch ~ 1 million cross points
T-S-T
Let n = N/n = 8 TSIs
k = 2n – 1 = for non-blocking
Pick k = time slots
Need 8x8 time-multiplexed space switch

37. Circuit Switching - Applications
Inefficient
Channel capacity dedicated for duration of connection
If no data, capacity wasted
Set up (connection) takes time
Once connected, transfer is transparent
Developed for voice traffic (phone)