itiated. The GGSN may send a Send Routing Information for GPRS (IMSI) message to the HLR (via the SGSN). The HLR returns a Send Routing Information for GPRS ACK (the information contained includes the IMSI, SGSN address, and cause) message to the GGSN: If a request can be served, the HLR includes the IP address of the serving SGSN. If a request cannot be served, the HLR only includes cause to indicate the reason for the negative response (cannot find network address and SO on). If the SGSN address is present and cause is not present or equal to a No Paging Response, the GGSN sends a PDU Notification Request message to the SGSN indicated by the HLR, which acknowledges it by sending a PDU Notification Response message. 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. Main GPRS Procedures Main GPRS Procedures Figure 5-13 The network-initiated PDP context activation. Send Routing info for GPRS (IMSI) Send Routing info for GPRS Ack (IMSI,SGSN,Address,Cause) Create PDP Context Request Request PDP Context Activation (PDP type, PDP address) PDP Notification Response PDP Context Activation Procedure The SGSN sends a Request PDP Context Activation (PDP type, PDP address) message to request that the mobile station activate the indicated PDP context, using the PDP context activation procedure (same as the mobile station-initiated). GPRS Data Transfer from the Mobile Station After attaching to the SGSN and activating a PDP context, the mobile sta- tion is now known to the external packet data network (PDN) and can send and receive information to and from the networks. Now a user application at the mobile station is going to generate IP or X.25 packets. The packets contain a source address, a destination address, and information. The flow of the packets is listed in the following steps and shown in Figure 5-14: 1. A logical link exists between the SGSN and