 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 very important and critical issue, the routing of emergency calls can be mentioned. Calls to the police and fire brigade must be routed to the nearest geographical place and must contain the correct calling line identification (CLI) of the calling user. Interconnection 15.11.3 INTERCONNECT IN INTERNET See Figure 15.16. In the hierarchical Internet the most important interconnection points are called global interconnect points 