he HSS, the I-CSCF server forwards the SIP message to the S-CSCF server, which acts as host to the call-control logic. Because the destination address is located in the fixed ISDN network, the SIP message is forwarded to the ISDN gateway network, which may be physically located in either the home network or the visited network. Within the ISDN gateway network, a media gateway control function (MGCF) converts the SIP message into appropriate ISDN messages and forwards them to the external ISDN network. While the control signalling messages travel from the visited network via the home network to the ISDN network, the user-plane stream is sent directly from the GGSN in the visited network to the media gateway in the ISDN gateway network, taking the shortest optimal path to the destination. The media gateway at the edge facing the external ISDN network is controlled by the MGCF. 15.11 ROUTING 15.11.1 ROUTING OF CONTROL TRAFFIC IN FIXED NETWORKS When the control layer gets physically disconnected from the connectivity layer there is a need to organize pure server networks. The example below is based on a possible network for a very large fixed operator. The operator runs two distinct administrative areas and interconnects with a number of other operators. Figure 5.15 shows transit or interconnect telephony server locations between areas or operators. There are at least two Understanding Changing Telecommunications - Building a Successful Telecom Business BICC network structure, example Interconnect to other operators Interconnect More than 1 More than 1 More than 1 Area 1 Area 2 Local Say 10-20 Transit Say 10-20 Figure 15.15 Routing of interconnect control traffic Internet Tokyo Sydney Stockton Washington Pennsauken London EBONE EU net Stockholm Swip Net Telia Net Global one Telenordia Telegate Sunet Figure 15.16 Interworking GIX points in the Internet telephony server gateways for reliability reasons. A transit/interconnect telephony server has no underlying MGWs. 15.11.2 ROUTING OF EMERGENCY CALLS As an example of a