ing; billing record generation; operational and maintenance functions; and collection of performance statistics. The CN comprises of circuit and packet switching systems, trunk transmission, signal- ling systems, the access network and service platforms. Figure 3.4 highlights the 3G side represented in layers to point out some of the new elements incorporated to the legacy GSM network. Each layer contains a distinct net- work element based on the CN infrastructure evolution. However, it is not restricted to the CN layers, it includes, e.g. the radio layer and others as follows: the radio network layer illustrating new WCDMA base stations (BSs) and the RNC to be described in Chapter 4; the mobile switching layer, which regroups the 3G SGSN, 3G MSCs with their upgraded associated components, such as HLR, VLR, AuC, EIR, and their new CPS unit enabling IP telephony; the transit-IP layer, which not only serves as the backbone layer for transiting traf- fic between nodes, but also incorporates the IP bypass mediation device for signal- ling and user data between CS and PS. The GGSN may is also part of this layer. the signalling layer, comprising mainly of STPs connected to the other elements; the management layer composed of the integrated network management systems and network mediation systems as illustrated in Figure 3.3; the service layer will comprise all value-added service platforms, such as SMC, VMS, intelligent network platform and customer care centres, ISP, billing platform, GSM 1800 MHz evolving elements and new UMTS or 3G specific elements co-existing seamlessly. The UMTS Network and Radio Access Technology Radio Network Layer Mobile Switch Layer Transit - IP Layer PSTN/GSM Network 3G-BTS 3G MSC UTRA BSs IP Function Figure 3.4 Representation of the CN and radio access layers. For background completeness on the CN side, the main GPRS elements and terminal connections to a GPRS network are described next from the functional level. 3.2.1.1 Main Packet Switched Network Elements The Serving GPRS Support Node (S