at the website. Function of GPRS Elements Function of GPRS Elements Conveys all the messages sent by the mobile station over the stand- alone dedicated control channel (SDCCH) for GPRS purposes (addressing GSM/GPRS capabilities for class B mobile stations) GPRS suspend/resume from the BSC to the PCU in order to activate/deactivate service in GSM transfer Function of the PCUSN The main role of the PCU, as shown in the architecture in Figure 4-5, is to provide the interworking function between the two interfaces as follows: The packetized radio interface Agprs (synchronous connection-oriented link) The packet network interface Gb (asynchronous and connectionless) The PCU is responsible for the GPRS RLC/MAC layer function and GSM radio frequency L1bis PCUSN Figure 4-5 The PCUSN. RLC/MAC Block BSSGP Flow Control Management Frame Relay Links Agprs asynchronous synchronous 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. Function of GPRS Elements Chapter 4 Network services BSS GPRS Protocol (BSSGP) Relay function The PCUSN is introduced as a separate node in the BSS in order to pro- vide the PCU functionality. Serving GPRS Support Node (SGSN) Functions The SGSN, serving as the packet version of the Mobile Switching Center/Visitor Location Register (MSC/VLR), requires the services of a packet switch in order to properly perform its role in the GPRS network. Consequently, the SGSN is hosted in a high-end switching system. Fig- ure 4-6 shows the relationship of the SGSN in a GPRS network and the functions performed. The SGSN performs the following functions: Mobility management (MM) Routing of packet data using the Internet Protocol (IP) at Layer 3 Figure 4-6 The Serving GPRS Support Node Routing (SGSN). Resolution Mobility Management PCUSN Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 T