2) and (n + 3). If this message is not correct, then the mobile station listens to the next three TA messages that also contain the updated TA value. Therefore, the updated TA value for up to four mobile stations that sent an access burst on PTCCH of the multiframes n and n + 1 is contained in multiframes n + 2 to n + 7. 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. Radio and MS-PCUSN Interfaces Radio and MS-PCUSN Interfaces Identifiers Limitations Several different identifiers were used during the development of the GPRS. Many retain some or all of their values from GSM. Others are spe- cific to GPRS. Regardless of the origin, the limitations concern the maxi- mum number of mobile stations that the MAC layer can multiplex onto the same time slot (PDCH). The Temporary Flow Identifier The TFI differentiates two TBFs, which have a common PDCH (uplink or downlink) allocated. For any direction of the packet transfer, the opposite direction may have some acknowledgments, SO collisions may occur for com- munications on the same time slot even though the packet transfer direc- tions are different. In other words, the TFI differentiates the TBFs corresponding to different mobile stations that have been allocated the same PDCH. This may be the case to control only the TBFs of the same TDMA. The TFI is coded on 5 bits, SO 32 (25) different TFI values are possible. Consequently, the TFI limitation is, at most, 32 mobile stations: TDMA 32 UL + 32 DL. The Timing Advance Index The TAI differentiates two mobile stations communicating on the same time slot for the TA continuous update procedure. The mobile stations com- municating on the same time slot may transfer or receive data packets because mobile stations receiving data packets also need to update their TA for acknowledgment transmission. The TAI is coded with four bits, SO 16 (24) different TAI va