he McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Function of GPRS Elements Function of GPRS Elements Authentication Encryption Compression MM performs the following tasks as a part of the overall scheme: Session management State control; mobile station state Data packet routing on the downlink, including location tracking The SGSN performs authentication and cipher-setting procedures based on the same algorithms, keys, and criteria as in existing GSM; however, the ciphering algorithm is optimized for packet data transmission. Additional functions of the SGSN, shown in Figure 4-7, include the following: Temporary storage capability Tracks the mobile station location by routing areas or cell location Keeps track of the connected Gateway GPRS Support Node (GGSN) SO that it can determine what GGSNs can be used for connections to the IP networks, intranets, the Internet, or X.25 networks Maintains a log of the active Packet Data Protocol (PDP) contexts (this is a VLR functional equivalent) Figure 4-7 The additional functions of the SGSN. GPRS PLMN 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. Function of GPRS Elements Chapter 4 SGPRS attach Receives the MS's subscription info from the Home Location Register (HLR) Notifies the old SGSN of MS attach request Verifies response to identity request Initiates a detach request; notifies MSC/VLR of detach procedure Authentication User's subscription info (International Mobile Subscriber Identity [IMSI] or SGSN # attached to) Gateway GPRS Support Node (GGSN) The GGSN (Figure 4-8) serves as the interconnect point between the SGSN and the external packet data network, requiring features to provide secure communications between GPRS users and IP. Moreover, the GGSN pro- vides tunneling capabilities within the GPRS network system itself. The Figur