tection, but rely on duplication avoidance functions in the CN. Control plane signalling User plane information Duplication avoidance UuS boundary control L2/PDCP L2/BMC L2/RLC Logical Channels L2/MAC Transport Channels Figure 3.13 Radio interface protocol architecture (ovals are service access points) (after [9]). 3.8.1.1 Service Access Points and Service Primitives At SAPs each layer provides services where a set of primitives or operations defines every service that a layer provides to the upper layer(s). There exists control services Control SAPs (C-SAP) enabling the RRC layer to control lower layers locally (i.e. not requiring peer-to-peer communication) See primitives in [9]. C-SAP primitives can bypass one or more sub-layers. The UMTS Network and Radio Access Technology 3.8.2 Services and Functions in Layer 1 L1 or the physical layer provides information transfer services to MAC and upper lay- ers. We characterize these services by how and with what features information gets transferred over the air interface, and we denote them transport channels 8. 3.8.2.1 Services 3.8.2.1.1 Transport channels In principle we classify transport channels in two groups, i.e. common and dedicated channels. The 1st group has in-band identification of UEs when addressing particular UEs. The 2nd group has identification of UEs through the physical channel, i.e. code and frequency for FDD and code, time slot and frequency for TDD. Table 3.3 Transport Channel Summary [8] Transport channels description Common Random Access Channel A contention based uplink channel used for transmission of (RACH) relatively small amounts of data, e.g. for initial access or non- real-time dedicated control or traffic data. Common Packet Channel A contention based channel used for transmission of bursty (CPCH) data traffic. This channel only exists in FDD mode and only in the uplink direction. The common packet channel is shared by the UEs in a cell and therefore, it is a common resource. The CPCH is fast power controlled. Forward Access Channel Com