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
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
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
Simple Switched Network
Types of switched networks
CIRCUIT-SWITCHED NETWORKSCIRCUIT-SWITCHED NETWORKS
A circuit-switched network consists of a set of switches A circuit-switched network consists of a set of switches connected by physical links. A connection between two connected by physical links. A connection between two stations is a dedicated path made of one or more links. stations is a dedicated path made of one or more links. However, each connection uses only one dedicated However, each connection uses only one dedicated channel on each link. Each link is normally divided channel on each link. Each link is normally divided into n channels by using FDM or TDM.into n channels by using FDM or TDM.
CONNECTION ORIENTED SERVICECONNECTION ORIENTED SERVICE
A circuit-switched network is made of a set of switches connected by physical
links, in which each link is divided into n channels.
Note
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
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.
Note
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.
Switching in the traditional telephone network uses
the circuit-switching approach.
Note
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
Delay in a circuit-switched network
Circuit Switch Types
Space-Division switches Crossbar switches Multistage switches
Time-Division switches Time switches Time-space-time switches :Hybrids Time & Space
switching
Space division switches
Developed for analog environment Separate physical paths Connection oriented service Suitable to voice signals
N
1 2
1
N
2
N –1
…
…
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
Crossbar switch with three inputs and four outputs
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.
Multistage Switch
Reduced number of cross points More than one path through network
Increased reliability More complex control May be blocking
Three Stage Switch
Multistage switch
Large switch built from multiple stages of small switchesThe n inputs share k paths through intermediate switches
Larger k (more intermediate switches) means more paths to output
nk
nk
nk
nk
N/n N/n
N/n N/n
N/n N/n
kn1
2
N/n
Ninputs
1
2
3 3
N/n
Noutputs
1
2
k
2(N/n)nk + k (N/n)2 crosspoints
kn
kn
kn
… … …
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).
Note
Design a three-stage, 200 × 200 switch (N = 200) with k = 4 and n = 20.
SolutionIn 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).
Example
In 1950s, Clos asked, “How many intermediate switches required to
make switch nonblocking?” & Introduced Non blocking criteria
for Multistage space switch.
nxk
nxk
nxk
N/n x N/n
N/n x N/n
N/n x N/n
kxn1
N/n
Desiredinput
1
j m
N/n
Desiredoutput
1
2n-1
kxn
kxn
n-1
N/n x N/nn+1
N/n x N/n2n-2
Free path Free path
n-1busy
n-1busy
…… …
…
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
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
Minimum Complexity Clos Switch
N2
N/2
2N2
n2
2N2
n32N2
n2
N
2
C n
N
nN
n
2
N
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 16 144x144 in centre stage 144 16x8 in third stage
8x16
8x16
8x16
8x16
144144
144x144
144x144
16x81
2
14411
52 in
puts
1
2
3 3
N/n
1152 outputs
1
2
16
16x8
16x8
16x8
… … …
According to the Clos criterion: n = (N/2)1/2
k > 2n – 1 Cross points ≥ 4N [(2N)1/2 – 1]
Note
Redesign the previous three-stage, 200 × 200 switch, using the Clos criteria with a minimum number of cross points.
SolutionWe 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.
Example
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
TIME DIVISION SWITCH Time-slot interchange
nxk
nxk
nxk
nxk
N/n x N/n kxn1
2
N/n
Ninputs
1
3
1
…
1
2
n
Time-slot interchange
Input TDM frame with n slots
Output TDM frame with k slots
n … 2 1 k … 2 1
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
n k N/n N/n
N/n N/n
N/n N/n
k n1 1
2
N/n
1
2
k
k n
k n
n k2
n kN/n
First slot
kth slot
First slot
kth slot
… … …
Flow of time slots between switches
Only one space switch active in each time slot
nxk
nxk
nxk
nxk
N/n x N/nTime-sharedspace switch
kxn1
2
N/n
Ninputs
1
2
3 3
N/n
Noutputs
TDMn slots
n slots
n slots
n slots
kxn
kxn
kxn
TDMk slots
TDMk slots
TSI stage TSI stageSpace stage
… …
Time space time Switch using Time multiplexed space switch
Very compact design: fewer lines because of TDM & less space because of time-shared crossbar
Time-space-time switch by using time multiplexed space switch
Example: T-S-T Switch Design
For N = 960 Single stage space switch ~ 1 million cross points T-S-T
Let n = 120 N/n = 8 TSIs k = 2n – 1 = 239 for non-blocking Pick k = 240 time slots Need 8x8 time-multiplexed space switch
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)