A signalling connection control part ((SCCP) user adaptation) layer, M3UA (MTP3 user adaptation) layer and M2UA (MTP2 user adaptation) layer. SIGTRAN is mandatory in 3G release 4 networks (see Figure 4.16). The overhead introduced by SIGTARN depends on the application part data, user adaptation layer used and OH introduced by underlying layers. Typically ISUP messages are relatively short and in the order of a few tens of bytes, whereas CAP and MAP messages can be significantly larger. The overhead also depends on the number of data units included in one SIGTRAN message. Figure 4.17 presents an example of one of SIGTARN's implementations. The SCTP is standardised by RFC 2960 in IETF and may be carried on an IPv4 or IPv6 layer. The SCTP is carried directly over IP. The SCTP common header is 28 bytes and each chunk has a 4 bytes header. The SCTP header should contain a common header, chunk description and payload identifier. Core Network Planning and Optimisation MSC Server MSC Server BSSAP RANAP SCCPb PSTN/ RAN/BSS MGWs act as Signalling GWs Figure 4.16 SIGTRAN implementation in a release 4 network (TCAP, transactions capability applica- tion part; GW, gateway) The length of the common header is 3 dword. The length of the chunk description is 1 dword. The length of the payload identifier is 4 dword. The total SCTP header size is 28 bytes. The size of the common message header of the M3UA is 2 dword. The common message header exists in every M3UA message. The main part of the bit rate is generated from the payload data messages. Therefore, an example describes only the structure of the payload data. The optional network appearance parameter is used in M3UA; its size is 2 dword. The routing label of the MTP3 message is encoded in separate fields. The OPC and DPC fields are both 1 dword long and the rest of the routing label parameters, like SI (a transaction capability application part), NI (a gateway), message priority and a signalling link set (SLS), are encoded in 1 dword. The total length of the M3UA message hea