Circuit Switching

Circuit switching creates a direct physical connection between two devices such as phones or computers. For example, in Figure 9.1-1, instead of point-to-point connections between the three phones on the left (A, B, and C) to the four phones on the right (D, E, F, and G), requiring 12 links, we can use four switches to reduce the number and the total length of the links. In Figure 9.1-1, phone A is connected through switches I, II, and III to phone D. By moving the levers of the switches, any phone on the left can be connected to any phone on the right.

A circuit switch is a device with n inputs and m outputs that creates a temporary connection between an input link and an output link (see Figure 9.1-2). The number of inputs does not have to match the number of outputs.

An n-by-n folded switch can connect n lines in full-duplex mode. For example, it can connect n telephones in such a way that each phone can be connected to every other phone (see Figure 9.1-3).

Circuit switching today can use either of two technologies: space-division switches or time-division switches (see Figure 9.1-4).

Circuit Switching

Fig. 9.1-1 Circuit-switched network

Fig. 9.1-1 Circuit-switched network

Fig. 9.1-2 A circuit switch Switch

Fig. 9.1-3 A folded switch

Space Division Switch

Fig. 9.1-3 A folded switch

Circuit Switching

Space-division Switching Time-division Switching

Fig. 9.1-4 Switching

9.1.1 Space-division Switches

In a space-division switch, the paths in the circuit are separated from each other spatially. This technology was originally designed for use in analog networks but is used currently in both analog and digital networks. It has evolved through a long history of many designs. Today, however, the only type used is the crossbar. Crossbar Switches

A crossbar switch connects n inputs to m outputs in a grid, using electronic micro-switches (transistors) at each crosspoint (see Figure 9.1-5). The major limitation of this design is the number of switches required. Connecting n inputs by m outputs using a crossbar switch requires nxm crosspoints. For example, to connect 1000 inputs to 1000 outputs requires a crossbar with l,000,000 crosspoints. This factor makes the crossbar impractical because it makes the size of the crossbar huge, and inefficient because statistics show that, in practice, fewer than 25 percent of the crosspoints are in use at a given time. The rest are idle.

Fig. 9.1-5 Crossbar switch

Multistage Switches

The solution to the limitations of the crossbar switch is multistage switching, in which crossbar switches are combined in several stages. In multistage switching, devices are linked to switches that, in turn, are linked to a hierarchy of other switches (see Figure 9.1-6).

The design of a multistage switch depends on the number of stages and the number of switches required (or desired) in each stage. Normally, the middle stages have fewer switches than do the first and last stages. For example, imagine that we want a multistage switch as in Figure 9.1-6 to do the job of a single 15-by-15 crossbar switch. Assume that we have decided on a three-stage design that uses three switches in the first and final stages and two switches in the middle stage. Because there are three of them, each of the first-stage switches has inputs from one-third of the input devices, giving them five inputs each (5 X 3=15).

Fig. 9.1-6 Multistage switch

Next, each of the first-stage switches must have an output to each of the intermediate switches. There are two intermediate switches; therefore, each first-stage switch has two outputs. Each third-stage switch must have inputs from each of the intermediate switches; two intermediate switches means two inputs. The intermediate switches must connect to all three first-stage switches and all three last-stage switches, and so must have three inputs and three outputs each. Multiple Paths Multistage switches provide several options for connecting each pair of linked devices. Figure 9.1-7 shows two ways traffic can move from an input to an output using the switch designed in the example above.

In Figure 9.1-7a, a pathway is established between input line 4 and output line 9. In this instance, the path uses the lower intermediate switch and that switch's center output line to reach the last-stage switch connected to line 9. It could have used the upper intermediate switch just as easily.

Figure 9.1-7b shows a pathway between input line 13 and output line 2. What other paths could have been used to make this connection?

Blocking Let us compare the number of crosspoints in a 15-by-15 single-stage crossbar switch with the 15-by-15 multistage switch that we described above. In the single-stage switch, we need 225 crosspoints (15 X 15). In the multistage switch, we need

• Three first-stage switches, each with 10 crosspoints (5X2) , for a total of 30 crosspoints at the first stage.

• Two second-stage switches, each with 9 crosspoints (3X3), for a total of 18 crosspoints at the second stage.

• Three third-stage switches, each with 10 crosspoints (5X2), for a total of 30 crosspoints at the last stage.

4 connected to 9 4 connected to 9

Fig. 9.1-7 Switching path

4 connected to 9 4 connected to 9

Fig. 9.1-7 Switching path

The total number of crosspoints required by our multistage switch is 79. In this example, the multistage switch requires only 35 percent as many crosspoints as the single-stage switch.

This savings comes with a cost, however. The reduction in the number of crosspoints results in a phenomenon called blocking during periods of heavy traffic. Blocking refers to times when one input cannot be connected to an output because there is no path available between them---all of the possible intermediate switches are occupied.

In a single-sage switch blocking does not occur. Because every combination of inputs and outputs has its own switch, there is always a path. (Cases where two inputs are trying to contact the same output don't count. That path is not blocked; the output is merely busy.) In the multistage switch described in the example above, however, only two of the first five inputs can use the switch at a time; only two of the second five inputs can use the switch at a time, and so on. The small number of outputs at the middle stage further increases the restriction on the number of available links.

In large systems, such as those having 10,000 inputs and outputs, the number of stages can be increased to cut down the number of crosspoints required. As the number of stages increases, however, possible blocking increases as well. Many people have experienced blocking on public telephone systems in the wake of a natural disaster when calls being made to check on or reassure relatives far outnumber the ordinary load of the system. In those cases, it is often impossible to get a connection. Under normal circumstances, however, blocking is not usually a problem. In countries that can afford it, the number of switches between lines is calculated to make blocking unlikely. The formula for finding this number is based on statistical analysis, which is beyond the scope of this book.

9.1.2 Time-division Switches

In a time-division switch, the slots are divided by time instead of space. Switching is accomplished using time-division multiplexing (TDM). TDM alone, however, is not sufficient; a device known as a time-slot interchange (TSI) must be used.

Figure 9.1-8 shows a system connecting four input lines (A, B, C, and D, in that order) with four output lines (W, X, Y, and Z, in that order). Imagine that each input wants to send data to one of the outputs according to the following pattern:

A^Y B^Z C ^ W D ^ X Figure 9.1-8a shows the results of ordinary time-division multiplexing. As you can see, the desired switching is not accomplished. Data are output in the same order as they are input. Data from A go to W, from B go to X, from C go to Y, and from D go to Z.

In Figure 9.1-8b, however, we insert a device called a time-slot interchange (TSI) into the link. A TSI changes, the ordering of the slots based on the desired connections. In this case, it changes the input order of A, B, C, D to C, D, A, B. Now, when the demultiplexer separates the slots, it passes them to the proper outputs.


Fig. 9.1-8 Time-division multiplexing, without and with a time-slot interchange (TSI)

How a TSI works is shown in Figure 9.1-9. A TSI consists of random access memory (RAM) with several memory locations. The size of each location is the same as the size of a single time slot. The number of locations is the same as the number of inputs (in most cases, the number of inputs and outputs are equal). The RAM fills up with incoming data from time slots in the order received. Slots are then sent out in an order based on the decisions of a control unit.

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  • eero
    How are circuit switched connections multiplexed?
    8 years ago
  • maximilian
    Is folded switch a circuit switch?
    4 years ago
    How no of crosspoint requried are reduced in the multistage seitches?
    4 years ago
  • Neftalem
    Which switch combines crossbar switches in several stages?
    4 years ago
  • Olli
    How cross bar switch multiplexer works?
    3 years ago
  • bobbi campbell
    How to design switch using multi stage space division swith technology?
    3 years ago
  • millard
    How time division switching works?
    3 years ago
  • amina zula
    What is a crossbar circuit switching?
    3 years ago
  • arlo
    What is limiting factor in a crossbar switch?
    2 years ago

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