ssage transport layers, which means that the BICC-CS2 can be carried on top of the TDM (SS7), ATM (SAAL) or IP (SIGTRAN). A default implementation is to carry the BICC-CS2 messages on top of IP, using the SIGTRAN protocol stack. As SIGTRAN uses permanent 'association' between signalling units in the counterpart element to deliver signalling messages through an IP network, in a full mesh topology all the signalling links need to be configured and maintained, making it challenging (but not impossible) to implement a full mesh topology in large networks. One solution is to split large networks (i.e. with more than 10/20 MSSs/GCSs) into regions (on a geographical base or administrative base), with all the MSS/GCS in one region fully meshed for signalling, and then add some more elements (GCSs) dedicated to STP functionality between regions. Figures are not currently available for estimating the signalling transfer capability of the GCS used as the STP and the value of 1M BHCA for GCS can be used even if conservative (as the GCS used as the STP will not have GMSC or SCP functionalities, the signalling processing capacity will probably be higher). When the backbone network is based on IP transport, the signalling for call control between MSS/GCS elements in the release 4 core circuit network can be based on SIP protocol. SIP messages are carried on top of UDP/IP (or TCP/IP) and an MSS/GCS uses DNS functionalities to get IP addresses of the counterpart MSS/GCS when performing call control related signalling. Therefore, the use of IP routing for the messages and of DNS to get those IP addresses makes the configuration of signalling networks fairly easy and not limited by the number of MSSs/GCSs in the network. When using SIP, there is no need for a transit signalling layer. Core Network Planning and Optimisation Ethernet Figure 4.22 Alternative protocol stacks available in MAP interfaces C, D, E, F and G in the MSS/GCS The MAP-D interface between the visited MSC server and HLR is used for keeping the mobile subscriber lo