itioning the payload bandwidth of a VC4. The poten- (SOH). The combination of the SOH plus the VC4 is termed a tially complicated nature of this partitioning can be better under- Synchronous Transport Module (STM), but another way of looking stood by replacing the two dimensional representation of the VC4 at this structure is to regard the VC4 as fitting neatly into the payload payload structure by a three dimensional one. (See Figure 42.15.) area of the STM. (See Figure 42.12.) Like the VC4, the STM repeats For this representation, the order of transmission is left to right, front every 125s. In order to appreciate the structure of an STM, and to back, top to bottom. Figure 42.15 not only shows TU12s and particularly the SOH, it is best to revert to using the two dimensional TU3s, as we have already discussed, but also, examples of other representation of a block of bytes. Figure 42.12 shows the standard types of TUs, notably the TU11, which is the TU used to accommo- representation of the smallest STM, known as an STM-1, which has date a 1.5Mbit/s (DS1) signal in North America. This payload 9 rows of 270 bytes each. This produces a transmission rate of flexibility extends to alternative mechanisms for constructing a 155.520Mbit/s. VC4 payload in which, for instance, a normal 140Mbit/s signal is The significance of the STM SOH is that, unlike the VC4 POH, mapped directly into the C4, without any need for recourse to the is it generated afresh by every network element that handles the notion of TUGs. Because of the variety of possible VC4 payloads, VC4. This handling of the VC4 includes the operations of creating, part of the VC4 overhead (i.e. the H4 byte) is reserved for indicating multiplexing or routing within the network, even if such multiplex- the exact structure of this payload. This ability of a single VC4 to ing or routing happens to be completely inflexible (e.g. hardwired). carry a mixture of different sized VCs within its payload is con- When a network element (i.e. line terminal, multiplexer or cros