r section overshea (MSOH) operating between LTEs, while only the regenerator section overhead (RSOH) is recalculated between each pair of regenerators The SDH standards 42/15 TU-2 POINTERS (these all have the same value) Note that the 12 columns of each TU2 are actually dispersed throughout the VC4, rather than existing as the contiguous block shown here MAPPING FUNCTION 16 Mbit/s signal Figure 42.18 Loading of a hypothetical 16Mbit/s signal into 3 concatenated VC-2s (VC2 -3c) within a VC4 in an STM. The AU4 is used in all CEPT countries as the size of is constructed in a more complicated way, particularly in relation to traffic block on which networks are planned and operated. There is the way the error checking bytes are calculated. also an AU3, which is used mainly in North America. This refers to an alternative construction of an STM, whereby the payload con- 42.4.6 Other sizes of VCs and payloads sists not of a single VC4, but instead, of a group of three VC3s, together with their associated pointers. A further difference between The above description of the loading of a 2Mbit/s circuit into a the STM SOH and a VC POH is that the SOH can be divided into VC12, VC4 and thence into an STM-1, mentioned the existence of two parts, known as the Multiplexer Section OverHead (MS OH) other sizes of VC. The complete family of VCs, together with their and the Regenerator Section OverHead (RSOH). (See Figure allowed multiplexing routes up to STM-1 are shown in Figure 42.12.) The reason for this is that on long line transmission systems 42.14. So far mappings into these Synchronous Containers have the regenerators do not need to perform the rather costly, and in this been defined for all the common plesiochronous bit rates, together case, unnecessary operation of generating and destroying the com- with few others, notably the 125Mbit/s FDDI signal. This latter plete SOH. Instead, only a subset of a SOH is processed, leading to mapping is somewhat wasteful of transmission bandwidth, as it a reduction in gate count, power 