n of the mobile station is known only as to which routing area it is in. (The routing area can consist of one or more cells within a GSM location area.) When the SGSN sends a packet to a mobile station that is in the standby state, the mobile station must be paged. Because the SGSN Figure 3-6 Application The GPRS traffic protocol stack. IP/X.25 IP/X.25 Relay SNDCP SNDCP UDP/TCP Relay BSSGP BSSGP GSM RF GSM RF L1bis L1bis 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. System Architecture System Architecture knows the routing area in which the mobile station is located, a packet- paging message is sent to that routing area. After receiving the packet-pag- ing message, the mobile station gives its cell location to the SGSN to establish the active state. Packet transmission to an active mobile station is initiated by packet paging to notify the mobile station of an incoming data packet. The data transmission proceeds immediately after packet paging through the chan- nel indicated by the paging message. The purpose of the packet-paging mes- sage is to simplify the process of receiving packets. The mobile station has to listen to only the packet-paging messages, instead of all the data packets in the downlink channels, reducing battery use significantly. When a mobile station has a packet to be transmitted, access to the uplink channel is needed. The uplink channel is shared by a number of mobile stations, and its use is allocated by a BSS. The mobile station requests use of the channel in a packet random access message. The trans- mission of the packet random access message follows Slotted Aloha proce- dures. The BSS allocates an unused channel to the mobile station and sends a packet access grant message in reply to the packet random access mes- sage. The description of the channel (one or multiple time slots) is included in the packet access g