The GSM architecture consists of three major interconnected subsystems that interact with themselves and with users through certain network interface. The subsystems are Base Station Subsystem (BSS), Network Switching Subsystem (NSS) and Operational Support Subsystem (OSS In this section some fundamental views of the WCDMA Radio Access Network will be presented. This includes the WCDMA RAN architecture itself, the radio interface protocol architecture, the Radio Access Bearer concept and the role of the transport network in a WCDMA RAN. 4.1 Radio Access Network (RAN) Architecture The main purpose of the WCDMA Radio Access Network is to provide a connection between the hand�set and the core network and to isolate all the radio issues from the core network. The advantage is one core network supporting multiple access technologies. The WCDMA Radio Access Network consists of two types of nodes: Radio Base Station (Node B) The Radio Base Station handles the radio transmis�sion and reception to/from the handset over the radio interface (Uu). It is controlled from the Radio Network Controller via the Iub interface. One Radio Base Station can handle one or more cells. Radio Network Controller (RNC) The Radio Network Controller is the node that con�trols all WCDMA Radio Access Network functions. It connects the WCDMA Radio Access Network to the core network via the Iu interface. There are two distinct roles for the RNC, to serve and to control. The Serving RNC has overall control of the handset that is con�nected to WCDMA Radio Access Network. It controls the connection on the Iu interface for the handset and it terminates several protocols in the contact between the handset and the WCDMA Radio Access Network. The Controlling RNC has the overall control of a par�ticular set of cells, and their associated base stations. When a handset must use resources in a cell not con�trolled by its Serving RNC, the Serving RNC must ask the Controlling RNC for those resources. This request is made via the Iur interface, which connects the RNCs with each other. In this case, the Controlling RNC is also said to be a Drift RNC for this particular handset. This kind of operation is primarily needed to be able to provide soft handover throughout the network. Radio Access Bearers The main service offered by WCDMA RAN is the Radio Access Bearer (RAB). To establish a call con�nection between the handset and the base station a RAB is needed. Its characteristics are different depending on what kind of service/information to be transported. The RAB carries the subscriber data between the handset and the core network. It is composed of one or more Radio Access Bearers between the handset and the Serving RNC, and one Iu bearer between the Serving RNC and the core network. 3GPP has defined four different quality classes of Radio Access Bearers: • Conversational (used for e.g. voice telephony) – low delay, strict ordering • Streaming (used for e.g. watching a video clip) – moderate delay, strict ordering • Interactive (used for e.g. web surfing)