and the virtual channel identifier (VCI, 9 Octets 261 Octets STM-1 AU-4 PTR VC-4 POH ATM Cell 53 Octets Figure 41.4 Mapping ATM cells into the SDH STM-1 41/6 Asynchronous Transfer Mode particular address field code on a particular incoming multiplex, to a particular outgoing multiplex (space switching) whilst placing the VCI a appropriate code in the cells' address field for use at the next switching node (asynchronous time switching). This function is VCI b performed according to a translation stored at the switch control and maintained for the duration of the 'virtual connection' (Figure 41.7). Before cell switching can take place, cell delineation is performed VCI b on the incoming multiplex cell streams, to permit extraction of cells VCI c and analysis of their headers. The 'idle cells', inserted into the multiplex at times of low source activity, are now removed and it is VPI y an important feature of ATM switching that the matrix is not burdened with handling these cells: the switch handles only the real traffic from the links Figure 41.6 Virtual paths and virtual channels. VCla and VCIb represent two of the possible values of VCI within the The transit time of an ATM switch will depend in the main on the VP link with the value VPI X. Similarly VPI X and VPI y are traffic levels on the queues providing access to the outgoing links values within the physical layer connection. (Based on and may vary considerably with time: the queues have to be dimen- Fig. 1/1.150) sioned such that the probability of cell loss due to overflow is acceptably small. as in Figure 41.5). The virtual path identifier field offers the possi- bility of establishing semi-permanent virtual paths. These would be established in cross-connects controlled via network management 41.3.3.3 Switch implementation and may be used for private circuits, for access network paths to particular service providers or for semi-permanent virtual trunks. There are many possible implementation architectures for the inter- Each of these could carry a number 