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15.4.4 ATM switching
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Keep in mind that the octets in the header form a serial bit stream when being transmitted, as shown in Fig. 15.3d. A cell is switched through a network on a path determined by the VPI and VCI fields. At each node in the network a cell is switched from one physical link to another. Two basic types of ATM switches are encountered. In one type, known as an ATM digital cross connect switch (DCS) the VPI field in a cell is overwritten by a new number on switching, while the VCI is left unchanged. This enables a number of virtual channels to be switched together, thus speeding up the switching process. The functioning of the DCS is illustrated in Fig. 15.4a, where for simplicity only the VPIs and VCIs are shown. These have been given hypothetical numbers for illustration purposes, for example 200/12 indicates a VCI of 200 and a VPI of 12. The look-up table in the DCS determines the new VPI for the virtual paths, and also the physical output link to which the paths must
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U T S R Q P 150/5 190/5 180/8 170/8 210/10 200/10
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ATM switch Lookup table In Out 200 10 12 a 210 10 12 a 170 8 9 a 180 8 9 a 190 5 11 b 150 5 11 b
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S R Q P 180/9 170/9 210/12 200/12
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U T 150/11 190/11
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Z Y X W V 180/11 145/7 140/7 135/3 130/3
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ATM cross connect Lookup table In Out VCI VPI VCI VPI 130 3 135 11 135 3 125 10 140 7 132 9 145 7 145 9 180 11 140 7 (b)
Z Y X 140/7 145/9 132/9
b c a a a
V 135/11
W 125/10
Virtual path Virtual circuits (c)
(a) ATM switch; (b) ATM cross connect; (c) virtual circuits and path.
be switched. Different suppliers offer different types of switching stages (the switch fabric). For ease of identification the cells have been labeled P, Q, R, S, T, U. The second basic type of switch is referred to as an ATM switch. This differs from the DCS in that both the VPI and VCI fields are overwritten with new numbers on switching. Figure 15.4b illustrates the functioning of an ATM switch, where the VPI and the VCI of cells are both overwritten by new numbers based on the look-up table. Again, for ease of identification the cells have been labeled V, W, X, Y, Z. Although the virtual channels and virtual paths are formed over a single physical link, it is common practice to show the virtual channels bundled into a virtual path as in Fig. 15.4c.
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From Fig. 15.3 it is seen that the VPI field at the UNI has 8 bits, and therefore 28 or 256 virtual paths can be supported. At the NNI the VPI field is increased to 12 bits, allowing 212 or 4096 virtual paths to be supported. The VCI has 16 bits and therefore in theory the number of virtual channels is 216 or 65536. This in practice is limited to 64000 (see Russell, 2000).
15.4.5 Permanent and switched virtual circuits
A virtual path may be set up on a permanent basis. The switching identifiers are preset so that the path does not change, and is available for use as required. This is referred to as a permanent virtual circuit, abbreviated PVC (note the use of the word circuit to describe this). A path may also be established anew each time a connection is required. The users are disconnected when the transfer is finished, and a new connection will be required if another session is asked for. This is described as a switched virtual circuit (SVC) the term switch being used in analogy with telephone usage. The terms PVC and SVC can also be applied to virtual channels.
15.4.6 ATM bandwidth
A major advantage claimed for ATM is that it can provide bandwidth on demand, also known as flexible bandwidth allocation. The concept of bandwidth and its relationship to bit rate is described in Sec. 15.2. With the transmission of digital signals over networks the significant parameter is the bit rate. It is common practice, therefore, to state bandwidth as a bit rate. The maximum bit rate that a physical link can support is referred to as the speed of the link. Denoting the speed by S, the maximum bandwidth available for the payload can be calculated simply as S multiplied by the ratio of payload bits per cell to total bits per cell. The payload has 48 octets and hence the payload bits per cell is 8 48 384. An ATM cell has a total of 53 octets and therefore there is a total of 8 53 424 bits per cell. (The situation is somewhat more complicated as in certain cases the 48 octets contain some overhead bits, but this will be ignored for the present calculation). If a single user makes use of all the cells and these are transmitted without a break at speed S, then the payload bandwidth would be: BW S 384 424 (15.1)
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