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 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

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.

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

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.

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

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 Time-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

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 …

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 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 …

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).

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).

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

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

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

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 144144 144x144 16x8 1 2 144 1152 inputs 3 N/n 1152 outputs 16 …

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]

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.

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

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 … 2 1 k … 2 1 2 nxk 3 … nxk N/n

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

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

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)