mobile station sends an RA Update Request (old RAI and the update type) to the SGSN. The update type can be either a periodic RA update or normal RA update. The PCU adds the CGI before passing the message to the SGSN. 2. The new SGSN sends an SGSN Context Request message (old RAI, TLLI, New SGSN Address) to the old SGSN to get the MM and PDP contexts for the mobile station. The old SGSN responds with an SGSN Context Response message (MM Context, PDP Contexts, LLC ACK). The LLC acknowledge contains the acknowledgments for each LLC connection used by the mobile station. Figure 8-10 The RA update inter- SGSN. Routing area update request SGSN Context Request SGSN Context Response SGSN Context Ack. Forward Packet Update PDP Context Request Update PDP Context Response Update Location Cancel Location Cancel Location Ack. Insert Subscriber Data Insert Subsoriber Data Ack. Routing Area Update Accept Update Location Ack. Routing Area Update Complete 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. Mobile Station to SGSN Interface Mobile Station to SGSN Interface 3. The new SGSN sends an SGSN Context Acknowledge message to the old SGSN. The new SGSN is ready to receive data packets belonging to the activated PDP contexts. 4. The old SGSN duplicates the buffered network layer protocol data units (N-PDUs) and starts tunneling them to the new SGSN. 5. The new SGSN sends an Update PDP Context Request message (new SGSN Address, tunnel identifier [TID], QoS Negotiated) to the Gateway GPRS Support Node (GGSN) concerned. The GGSNs update their PDP context fields and return an Update PDP Context Response message (TID). The new SGSN informs the Home Location Register (HLR) of the change of SGSN by sending an Update Location message (SGSN Number, SGSN Address, IMSI) to the HLR. 7. The HLR sends a Cancel Location message (IMSI, Cancellation Type) to the old SGSN wi