 A circuit mode user traffic interface. A packet mode user traffic interface. Access Network Service layer servers Service layer Interworking networks Access networks Signalling HSS /HLR Signalling gatewav gateway Internet intranets GSM/EDGE Control Control Common server Control server PSTN/ISDN functions server Control layer MGW/router Router Media gateway MGW/router Transport network Media/ (limited inter- MGW/rout gateway Signalling working) User plane Connectivity layer Figure 13.11 Access connections to the core network What is described could be logically illustrated by means of Figure 13.11, which, however, does not separate the two user traffic cases. In the long run a unification to an all-IP interface is foreseen, such as 3GPP, release 5 and 6. Some mobile systems are briefly described in Section 13.11. Access independency is a goal. It also becomes a unification factor, since a common access-core type interface is required. The signalling is key to access independence. As mentioned, a future SIP solution is expected. Instead of only describing the access fragmentation (which would require a book in itself if done thoroughly) this chapter also looks at some common access features. Some unification trends will be covered under the heading 'Fundamental plans'. The more commonality, the better are the possibilities to utilize common resources in the expensive access part, with ROCE impact. 13.6 THE DISTRIBUTED DIMENSION 13.6.1 STRUCTURE OF THE ACCESS NETWORK See Figures 13.2 and 13.12. A customer access unit (CAU) can for example be located at the business or residential premises or at a pole or mast. The CAU serves as a termination point for the public network. The unit can provide access to a number of services (voice, data, video). The unit can be composed of several sub-units, such as network termination, a set-top box and devices connected to a local area network. The CAU can also refer to a device shared by several end users, for example, a hub located in the basement of a multiple tenant building, a