itiate this procedure, the mobile station sends a Deactivate PDP Con- text Request (NSAPI) message to the SGSN and security functions may be executed, as shown in Figure 5-16. The SGSN sends a Delete PDP Context Request (TID) message to the GGSN, which removes the PDP context and returns a Delete PDP Context Response (TID) message to the SGSN. If a mobile station were using a dynamic PDP address, the GGSN would release this address and make it available for subsequent activation by other mobile stations. The SGSN returns a Deactivate PDP Context Accept (NSAPI) message to the mobile station. At GPRS detach, all PDP contexts for the mobile station are implicitly deactivated. Network-Initiated Packet Data Protocol Context Deactivation When the GGSN initiates the PDP context deactivate procedure as shown in Figure 5-17, it sends a Delete PDP Context Request (TID) message to the Figure 5-16 Mobile-initiated PDP context deactivation. Tunnel Deactivate PDP Context Request (NSAPI) Security Functions Delete PDP Context Request (TID) Delete PDP Context Response (TID) Deactivate PDP Context Accept (NSAPI) 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 5-17 Network-initiated PDP context deactivation. Tunnel Delete PDP Context Request (TID) Deactivate PDP Context Request (NSAPI) Deactivate PDP Context Accept (NSAPI) Delete PDP Context Response (TID) SGSN, which sends a Deactivate PDP Context Request message (NSAPI) to the mobile station. The mobile station removes the PDP context and returns a Deactivate PDP Context Accept (NSAPI) to the SGSN. The SGSN returns a Delete PDP Context Response (TID) message to the GGSN. The SGSN may not wait for a response from the mobile station before sending the Delete PDP Context Response message. Security Functions Security in GSM and GPRS networks is based on the follow