y it merely delays the transit of that data. is being revolutionised by the availability of high speed fibre sys- The next sign of overloading is when users cannot make new calls tems. Because of the inherent low loss and error free transmission and are left with 'busy' messages. Normal network practice is to capabilities of fibre a complete rethink on the multiplexing require- design a network for 30% to 40% theoretical usage, which leaves ments has come about. Instead of relying on the multiplexer to enough capacity to cope with fluctuations in traffic density. Note provide an error free path it only need provide a basic routing path. that this not only includes trunk bandwidth capacity but also the This is the basis of frame relay. As can be seen form Figure 40.14, ability to set up and clear calls. Other factors which should be taken the routing path is part of the link layer and as such network into account for sizing the network is that the majority of traffic addressing only works on a link by link basis. Each node will have Frame relay connection Upper layers Transport End to end checking performed by the data ink layer Network Data link Physical Figure 40.14 Layer model of frame relay The future of multiplexing 40/17 Higher Specified Specified layers Q.922 Q.922 Q.922 Q.922 Data link layer CORE LAP-D + CORE LAP-D + CORE LAP-D + CORE LAP-D + Physical Physical Physical Physical Figure 40.15 Frame relay standards a routing table indicating where the frame should go but, having cross-connect equipment. The underlying rationale for SDH is to transmitted it does not have any mechanism for checking whether it provide a transparent 'bit pipe' for synchronous data. arrived. SDH works on the principle of Virtual Containers (VCs) which Figure 40.14 shows the layer model of frame relay. Note that it contain the various services. These are assembled into Administra- only operates in the lower half of the data link layer. Figure 40.15 tive Units (AUs) which can then be assembled as frames into shows the standards asso