n overview of what SDH is designed to achieve, and the general Synchronous Containers comprises only a few different types, each of which has been sized to accommodate one or more of the way it goes about this task. common plesiochronous transmission rates, without wasting too As with PDH, SDH is designed to transport isochronous traffic much bandwidth. The operation of mapping a plesiochronous cir- channels and is focused very much on layer one of the well known cuit into such a Synchronous Container is very similar to the normal ISO seven layer OSI protocol hierarchy. Again, like PDH, SDH is stuffing operation performed in a conventional PDH multiplexer. based on a hierarchy of continuously repeating, fixed length frames. However, in this case, the plesiochronous channel is being syn- This contrasts with the majority of competing standards, almost all chronised not to the master oscillator in a PDH multiplexer, but to of which attempt to deliver more efficient use of both transmission the frequency of the Synchronous Container, which is, in turn, and switching plant by employing some form of packetisation of the synchronous to the basic SDH frame structure. (See Figure 42.5.) transported information. Usually, these packets are launched into the network asynchronously and arrive at their destinations with a Before examining the question of exactly what the SDH frame non-deterministic delay. Consequently, they present something of a structure is synchronous with, it is worth digressing to discuss a problem to delay sensitive voice circuits, which still constitute 80% further operation specified by SDH. This is the attachment of an of all traffic, in even the world's most advanced networks. SDH, on Overhead, known as the Path Overhead (POH), to each Syn- the other hand, was formulated on the premise that despite the chronous Container. (See Figure 42.4.) The idea is that once a pressure to support novel forms of business oriented services, it is plesiochronous circuit has been loaded into a Synchronous Con- of par