s from the mobile station an Activate PDP context with the optional values PDP type, PDP address, and APN. Figure 10-13 shows the information passed along the link. The SGSN compares the received values with its local subscribed values (this information exists locally because of the GPRS attach). If the PDP con- text is valid, the SGSN has to perform some operation like the following: Create a tunnel TID by combining the IMSI and the NSAPI. Send a query to the Domain Name System (DNS) server to obtain the IP address of the GGSN using the complete APN (with the Operator Identifier). The DNS server answers by giving it a list of IP addresses. Send a Create PDP Context Request message to the GGSN with the PDP type, PDP address, APN Network Identifier, TID, selection mode, and SO on. The APN is made of two parts: APN Network Identifier APN Operator Identifier The structure is MNCyyyMCCzzz.gprs (depending on the IMSI). GGSN A Figure 10-13 GTP Tunnel (PDP Context) 192.168.4.3 Using the PDP context SGSN role. PDP Context Activation IP Backbone GGSN B 192.168.4.6 DNS Server 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. SGSN-to-GGSN (Gn) and GGSN-to-PDN (Gi) Interface Chapter 10 Create PDP Context GGSN Role When receiving the Create PDP Context Request message coming from the SGSN, the GGSN uses the selection mode and the APN Network Identifier to validate the PDP Context. Figure 10-14 shows the role of the GGSN. According to the network configuration mode, the GGSN has to perform the following: DNS query to find the IP address of the remote switch Mobile IP @ assignment from the local Dynamic Host Configuration Protocol (DHCP) server or the remote DHCP or RADIUS server Tunnel creation toward the remote switch with a security protocol in case of public external network Authentication and authorization checks When these operations are complete, the 