ference between the STM SOH and a VC POH is that the SOH can be divided into two parts, known as the Multiplexer Section OverHead (MSOH) and the Regenerator Section OverHead (RSOH) (see Figure 10.11). The reason for this is that SDH Standards STM-4 MSOH Created and Destroyed at These Two Points STM-1 STM-1 REGEN REGEN STM-4 STM-4 STM-4 STM-4 RSOH Created and Destroyed at Each of These Pairs of Points Fig. 10.16 Line transmission system showing the MSOH operating between LTEs, while only the RSOH is recalculated between each pair of regenerators. on long line transmission systems, the regenerators do not need to perform the rather costly, and in this case unnecessary, operation of generating and destroying the complete SOH. Instead, only a subset of an SOH is processed, leading to a reduction in gate count, power consumption, and SO on, but still preserving the ability to detect traffic errors and access some management channels. On the other hand, the line terminal equipment processes both the RSOH and MSOH (see Figure 10.16). Finally, as with VCs, STMs come in various sizes. As mentioned earlier, the smallest size is termed an STM-1, and it can accommodate a single VC-4. However, larger sizes exist whose bitrates are integer multiples of the basic STM-1 rate. ITU-T G.707 currently recog- nizes the STM-4, STM-16, STM-64 and STM-256, and it is likely that in the future STM-1024 will be used. For all these higher-rate STMs, the construction mechanism is the same: The payload is produced by straight byte interleaving of the tributary VC-4s, while the SOH is constructed in a more complicated way, particularly in relation to the way the error- checking bytes are calculated. Other Sizes of VCs and Payloads The preceding description of the loading of a 2-Mbit/s circuit into a VC-12, a VC-4, and thence an STM-1 mentioned the existence of other sizes of VC. The complete family of VCs, together with their allowed multiplexing routes up to STM-1, are shown in Figure 10.13. So far mappings into these synchronous containers h