ding Unit This unit is responsible for modulating the signal in the downlink direction and demodulating in the uplink direction. It is responsible for summing and multiplexing the signals and also processing the signals. This unit contains the digital signal processors that are responsible for coding and decoding signals. RADIO INTERFACE PROTOCOL ARCHITECTURE Network Interface Unit This unit acts as an interface between the BS and the transmission network or any other network element, such as co-sited cross-connect equipment. Radio Network Controller (RNC) The radio network controller (RNC) is similar to the BSC in GSM/GPRS networks, but is rather more complicated and has more interfaces to handle. The RNC performs radio resource and mobility management functions such as handovers, admission control, power control, load control, etc. In fact the RNC plays a dual role in a WCDMA radio network, which should be un- derstood from a network planning perspective. A radio network controller can be SRNC (serving RNC) or DRNC (drifting RNC). From one mobile, if the RNC terminates both the data and related signalling then it is called the serving RNC. If the cell that is used by this UE is controlled by an RNC other than the SRNC, then it is called the DRNC. More details on the structure of the RNC are given in Chapter 8, in the discussion of access transmission in UMTS networks. 7.2 RADIO INTERFACE PROTOCOL ARCHITECTURE 7.2.1 Introduction Based on the OSI reference model, Figure 7.3(a) shows the first three layers of the WCDMA radio interface protocols. These three layers are needed for the functioning (set-up, release, configuration) of the radio network bearer services. Radio Resource Control (RRC) Radio Link Control (RLC) Medium Access Control (MAC) Physcial layer Figure 7.3(a) Basic radio interface protocol architecture Layer 1 is the physical layer, the actual medium of transfer. Planning engineers should note that this layer is not just a 'physical medium' but should also be able to perform certain functions. The ma