 sends a request DNS query in which it provides the APN Network Identifier and APN Operator Identifier. The DNS responds with a list of the available GGSNs to use, in a preferred order. After that, the SGSN1 Figure 5-12 tcic.com.mnc.mcc.gprs The mobile-initiated =APN =Operator ID PDP context GGSN2 activation continued. GGSN2 PDP Type: IP APN: Internet SGSN2 PDP Type: IP APN: tcic.com GGSN2 Intranet tcic.com Tunnel Creation PDP Type: X.25 APN: Concert PDP Type: IP PDF Type: IP @GGSN2 APN: tcic.com APN: tcic.com Charging PDP@ II II Quality of Service: Gateway @ MS:" *Precedence class Function *Delay classes Quality of GGSN1 Service *Reliability class *Peak throughput class *Mean throughput class 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 Chapter 5 SGSN knows which GGSN to select and sends the activated PDP context to the correct serving gateway and opens a GTP tunnel if it has not already done SO. A tunnel between the SGSN and the GGSN is identified by the TID. Upon receipt of the Create PDP Context Request message, with the help of the APN, the GGSN needs to determine the following: The access mode used on the external network (in this example, we consider an intranet access with a secure tunnel) The IP address allocation type (in this example, we use the Dynamic Host Configuration Protocol [DHCP] server) Finally, the GGSN starts the billing records for this PDP context. It also returns an accept message to the SGSN, including the IP address for the mobile station. Network-Initiated Packet Data Protocol Context Activation The network may also initiate a PDP context if data arrives for a mobile user who has not established an address already. Figure 5-13 shows this network-initiated context activation. When receiving a PDP PDU, the GGSN determines if the network-initiated PDP context activation proce- dure has been in