 Channel Codec Unit (CCU) inside the BTS. Sometimes the reference materials show this layer divided into two sublayers, including the Physical RF layer The physical RF layer corresponds to the modulation and demodulation tasks, similar to GSM current techniques (GMSK modulation and Viterbi demodulation), but plans call for this to change in further evolutions of GPRS with the introduction of a more spectrum-efficient modulation (Enhanced Data rates for GSM Evolution [EDGE]). Physical link layer The physical link layer provides information transfer over a physical channel on the radio interface. It provides channel-coding functions (forward error correction [FEC]), interleaving, Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Radio and MS-PCUSN Interfaces Radio and MS-PCUSN Interfaces Figure 6-1 PCUSN The protocol stacks for the RLC and MAC layers. Network Application IP/X.25 IP/X.25 Relay SNDCP SNDCP LLC Relay BSSGP BSSGP GSM RF GSM RF L1bis 1 bis Abis, Agprs radio channel measurement functions (received quality and signal levels, timing advance measurement, physical link congestion detection), and radio management procedures (cell selection/reselection, power control, and discontinuous reception [DRX]). It does not perform ciphering (which is handled by the Logical Link Control [LLC] layer). The MS-to-PCU interface corresponds to the supervision level of the radio communications vis-a-vis RLC/MAC layer. This layer manages the logical link between the BSS and the MS. Packet Logical Channels GPRS uses some GSM broadcast channels for frequency tuning (Frequency Control Channel [FCCH]) and synchronization (Synchronization Channel [SCH]). However, for other purposes, specific new packet logical channels are defined, which are carried by a packet-switched channel Packet Data Channel (PDCH). PDCH is the generic name for the physical channel Downlo