ceiving the strongest and selects it. At this point, the mobile listens to the base station for news of incoming data packets. Periodically, the mobile station uses its idle time to listen to transmis- sions from neighboring base stations and evaluates the signal quality of their transmissions. If the mobile determines that a different base station signal is received stronger (better) than the current base, then the mobile may begin to listen to the new base station instead. This means that the mobile will listen to a different signal. The process of moving from one base station to another is called cell reselection. In some cases, the mobile station informs the network that it has changed cells by performing a location update procedure. When data arrives for an idle mobile station, the network broadcasts a notice that it wants to establish communications with that mobile. This is called paging and is very similar to the paging process in wireless voice networks. Routing Area Cells Routing Area 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. GPRS Introduction GPRS Introduction A group of neighboring cells can be grouped together to form a routing area. Network engineers use routing areas to strike a balance between location-updating traffic and paging traffic. Mobile stations that have been actively sending or receiving data are tracked at the cell level. (The network keeps track of the cell that they are currently using.) Mobile stations that have been inactive (idle) are tracked at the routing area level (the network keeps track of the routing area). Attaching to the Serving GPRS Support Node When a GPRS mobile station wants to use the wireless packet data net- work services, it must first attach to a Serving GPRS Support Node (SGSN), as shown in Figure 2-15. When the SGSN receives a request from a mobile station, it makes sure that it wan