 the mobile station. The link is identified by the TLLI specific to the mobile station. A table exists in the mobile station that holds the mapping information of the mobile to the TLLI and the associated NSAPI. The SNDCP layer takes 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 Figure 5-14 GPRS Data transfer Src=1.2.3.4 from mobile. Dest=a.b.c.d. a.b.c.d. IP Payload 1.2.3.4 Step 2. NSAPI=2 TLLI+NSAPI Original IP Src=1.2.3.4 Packet Step 1. Dest=a.b.c.d. IP Payload GPRS PLMN SGSN IP Address Step 3. Intra- TID+GGSN IP Address Step 4. PLMN IP TLLI=1, NSAPI=2 TID 1 GGSN IP Original IP Backbone Packet TLL|=2, NSAPI=2 TID 2 GGSN IP TLLI=3, NSAPI=2 TID 3 GGSN IP the original IP packet and adds header information containing the TLLI and the NSAPI information. These packets are then sent to the SGSN. 2. The table at the SGSN also holds mapping information of the TLLI and NSAPI to the corresponding TID and GGSN IP addresses. At the SGSN, the header that contains the TLLI and NSAPI is removed and a GTP header containing the TID and the GGSN IP address is put in its place. 3. The packets are sent to the GGSN in the IP format with the IP address of the SGSN as the source address and the GGSN IP address as the destination. The TID is also part of the IP datagram (packet). 4. At the destination (GGSN), the header is stripped off and the original IP or X.25 packet is obtained. This packet can now be routed to its destination from the destination address field of the packet. GPRS Data Transfer to the Mobile Data transfer to the mobile station is similar to the data transfer from the mobile station. Figure 5-15 shows the steps, which follow this sequence: Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subj