hing, routing and transit for user traffic. The CN also contains the databases and network management functions. The basic CN architecture for UMTS is based on the GSM network with GPRS. All equipment has to be modified for UMTS operation and services. The CN is divided into the CS and PS domains. Circuit switched elements are the Mobile Services Switching Centre (MSC), Visitor Location Register (VLR) and Gateway MSC. Packet switched elements are the Serving GPRS Support Node (SGSN) and the Gateway GPRS Support Node (GGSN). Network elements like EIR, HLR, VLR and AUC are shared by both domains. The Asynchronous Transfer Mode (ATM) is defined for UMTS core transmission. The ATM Adaptation Layer type 2 (AAL2) handles the circuit switched connection and the packet connection protocol AAL5 is designed for data delivery. A typical 3G network is shown in Figure 1.2. UMTS QoS Classes UMTS network services have different quality of service (QoS) classes for four types of traffic: conversational class (e.g. voice, video telephony, video gaming); streaming class (e.g. multimedia, video on demand); interactive class (e.g. web browsing, network gaming, database access); background class (e.g. email, SMS, downloading). Conversational Class The best examples of this class are voice traffic and real time data traffic such as video telephony, video gaming, etc. This traffic runs over CS bearers. The quality of this class is dependent totally on subscriber Cellular Networks PSTN, ISDN, PLMNs 3G-SGSN 3G-GGSN Radio Access Network Core Network Figure 1.2 UMTS network perception. The key aspect of this class is low end-to-end delays (e.g. less than 400 ms). For speech coding/decoding the adaptive multirate (AMR) technique will be used. Upon request, AMR coders can switch the bit rates every 20 ms of speech frame. Thus, during busy hours, bit rates can be lowered to offer higher capacity by sacrificing quality. Also, the coverage area of the cell can be increased by decreasing bit rates. Thus, this technique helps in balancing coverag