kets with external packet data networks (PDNs) after a successful GPRS attach, a mobile station must apply for one or more addresses used in the PDN: for example, for an IP address in case the PDN is an IP network. This address is called the PDP address. For each session, a so-called PDP context is created, which describes the characteristics of the session. It contains the PDP type (for example, IPv4), the PDP address assigned to the mobile station (10.0.0.1), the requested QoS, and the address of a GGSN that serves as the access point to the PDN. This context is stored in the MS, the SGSN, and the GGSN. With an active PDP context, the mobile station is visible for the external PDN and is able to send and receive data packets. The mapping between the two addresses, PDP and IMSI, enables the GGSN to transfer data packets between PDN and MS. A user may have several simultaneous PDP contexts active at a given time. The allocation of the PDP address can be static or dynamic. In the first case, the network operator of the user's home-PLMN permanently assigns a PDP address to the user. In the second case, a PDP address is assigned to the user upon activation of a PDP context. The PDP address can be assigned by 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 Function of GPRS Elements the operator of the user's home-PLMN (dynamic home-PLMN PDP address) or by the operator of the visited network (dynamic visited-PLMN PDP address). The home network operator decides which of the possible alternatives may be used. In case of dynamic PDP address assignment, the GGSN is responsible for the allocation and the activation/deactivation of the PDP addresses. It will use a combination of DNS and DHCP protocols to facilitate these interactions. The Domain Name System (DNS) and DHCP server is a multiadminis- trator database application fo