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)