anisms for constructing a VC-4 payload in which, for instance, an unstructured 140-Mbit/s signal is mapped directly into the C4, without any need for recourse to the notion of TUGs. Because of the variety of possible VC-4 payloads, part of the VC-4 overhead SDH Standards (i.e., the H4 byte) is reserved for indicating the exact structure of this payload. This ability of a single VC-4 to carry a mixture of smaller VCs within its payload is considered to be especially useful in the access portion of the network, where a PTO usually will not have the freedom to dedicate particular VC-4s to carrying a single type of lower order VC. Fitting a VC-4 into a Synchronous Transport Module Although a VC-4 can have its payload constructed in a variety of ways, its POH conforms to the same principles as those of a VC-12; that is, it is generated and attached at the point where the C4 is loaded and remains unchanged until the C4 is unloaded at its destination. As with the VC-12, the VC-4 POH is capable of indicating that errors have been introduced into the VC-4 payload during its journey, but it is not capable of identifying which network element was responsible. This problem is solved by the addition of yet another set of over- head bytes to the VC-4, known as Section Overhead (SOH) bytes. The combination of the SOH plus the VC-4 is termed a Synchronous Transport Module (STM), but another way of looking at this structure is to regard the VC-4 as fitting neatly into the payload area of the STM (see Figure 10.11). Like the VC-4, the STM repeats every 125 us. To appreciate the structure of an STM, and particularly the SOH, it is best to revert to using the two-dimensional representation of a block of bytes. Figure 10.11 shows the standard representation of the smallest STM, known as an STM-1, which has nine rows of 270 bytes each. This produces a transmission rate of 155.520 Mbit/s. The significance of the STM SOH is that, unlike the VC-4 POH, it is generated afresh by every network element that handles the VC-4. This handling of