s. For example, a GPRS mobile is communicating with a computer system con- nected to an X.25 or IP network. The other computer may be unaware that the mobile station is, in fact, mobile. It may only know the mobile station's PDP address. The packets, as shown in Figure 2-17, need to be routed as follows: Assume that the mobile station has attached to an SGSN and activated its PDP address. Packets sent from the other computer to the mobile station 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 Chapter 2 Radio Figure 2-17 The data transfer. IP/X.25 GPRS PLMN Intra- PLMN IP Backbone first travel across the public packet data network to reach the GGSN that anchors the PDP address. From here, the GGSN must forward the packets to the SGSN to which the mobile station is currently attached. Obviously, packets flowing in the reverse direction must be first routed through the SGSN and GGSN before being passed to the public packet data network. Communication between the GPRS Serving Nodes (GSNs) makes use of a technique known as tunneling. Tunneling is the process of wrapping the network layer packets into another header SO that they can be routed through the GPRS PLMN IP backbone network. Inside the network, pack- ets are routed based on the new header alone and the original packet is car- ried as the payload. Once they reach the far side of the GPRS network, they are unwrapped and continue along their way through the external network. From this point onward, they are routed based on their original (internal) header. Using tunneling within GPRS solves the mobility problem for the packet networks and helps to eliminate the complex task of protocol inter- working. Mobile IP also makes use of tunneling to route packets to mobile nodes. In mobile IP, packets are only tunneled from the fixed network to the mobile station. Packets flowing