ownloaded 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 Activity at the BSS Data and signaling messages arrive at the BSS via the Gb interface and through the Network Service/Frame Relay layer. The frames arriving at the PCU pass through the BSS GPRS Protocol (BSSGP) where the information and signaling messages are separated into LLC frames, GPRS mobility management (GMM) information, and network management (NM) infor- mation. For data and signaling messages destined to the GPRS mobile sta- tion, the LLC frames pass through a relay entity (LLC relay) before being positioned into the RLC and MAC, respectively. The RLC/MAC layer pro- vides services for information transfer over the physical layer of the GPRS interface. These functions include backward error correction (BEC) proce- dures enabled by selective retransmission of blocks of data with errors. The MAC function arbitrates access to the shared medium between multiple mobile stations and the GPRS network. Medium Access Control (MAC) Layer The MAC layer provides capability for multiple mobile stations to share a common transmission medium. It interfaces directly with the physical layer. For the uplink (such as a mobile attempting to originate access), the MAC layer plays the role of an arbitrator managing the limited physical resources among many competing requestors. The reservation protocol used for contention resolution among the various mobile station devices is based on the Slotted Aloha protocol. In addition, many services may be competing for the same limited radio resource within a single mobile sta- tion. The MAC layer coordination function is responsible for resolving these contentions. For the downlink (such as a mobile termination), the MAC layer aids in the queuing and scheduling of the access attempts. Contention resolution 