through the gateway (GGSN). The public packet data network is only concerned that the address belongs to a specific GGSN. The GGSN hides the mobility of the station from the rest of the packet data network and from computers connected to the public packet data network. Statically assigned PDP addresses are usually anchored at a GGSN in the subscriber's home network. Conversely, dynamically assigned PDP addresses can be anchored either in the subscriber's home network or the network that the user is visiting. When a mobile station is already attached to a SGSN and wants to begin transferring data, it must activate a PDP address. Activating a PDP address establishes an association between the mobile's current SGSN and the GGSN that anchors the PDP address. The record kept by the SGSN and the GGSN regarding this association is called the PDP context. 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 Radio Figure 2-16 Obtaining a PDP context from the IP/X.25 GGSN. GPRS PLMN Intra- PLMN IP Backbone It is important to understand the difference between a mobile station attaching to a SGSN and a mobile station activating a PDP address. A sin- gle mobile station attaches to only one SGSN; however, it may have multi- ple PDP addresses that are all active at the same time. Each of the addresses may be anchored to a different GGSN. If packets arrive from the public packet data network at a GGSN for a specific PDP address and the GGSN does not have an active PDP context corresponding to that address, it may simply discard the packets. Con- versely, the GGSN may attempt to activate a PDP context with a mobile station if the address is statically assigned to a particular mobile. Data Transfer Once the mobile station has attached to a SGSN and activated a PDP address, it is now ready to begin communicating with other device