on the frame format: The address field is fixed at 1 octet. The FCS field is fixed at 3 octets. The control field depends on transfer mode being used as follows: In unacknowledged mode, The Information frame (format UI) has a 2-octet control field length. The Control frame (format U) has a 1-octet control field length. Address Figure 8-5 Octet Field The information PD C/R mapping on the LLC frame. Address Field (1 octet) Description Control Field GPRS Mobility Management (variable length, max. 36 octets) User Data 1 Information Field User Data 2 (variable length, max. N-201 octets) Frame Check User Data 3 Sequence (FCS) Field User Data 4 (3 octets) 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. Mobile Station to SGSN Interface Mobile Station to SGSN Interface In acknowledged mode, The Information frame (format I) has a 3-octet control field length plus a possibility of an acknowledgment bitmap (max of 36 octets) The Supervisory frame (format S) has a 2-octet control field length plus a possibility of an acknowledgment bitmap (max of 34 octets). GPRS Ciphering Environment In normal operation, the use of the radio link is considered both hostile (noisy and error prone) and a security risk in that it is open to active inter- ception techniques. To prevent the unauthorized reception in the radio link, security in GSM networks is based on two primary techniques: Authentication Ciphering (encryption) Authentication The Authentication Center (AuC) is responsible for generating a set of para- meters known as triplets. A triplet consists of a Cipher Key (K) Random Number (RAND) Signed Response (SRES) The RAND is a randomly generated number from a number pool con- taining 2128 numbers. The RAND, coupled with the Identification Key (Ki), is used to calculate K and SRES. Kj is a secret number allocated on a per-sub- scriber basis and is only held at th