n is in packet transfer mode. As it gets near the limits of the cell, the radio quality decreases; therefore, the RLC/MAC layer asks for many retransmissions. As the radio quality decreases, the actual data reception also decreases. The LLC retransmissions increase. 2. The LLC layer begins to receive fewer packets. Now the mobile station, which is constantly in reselection mode, realizes that a new cell has a stronger C2+ cell_reselect_hysteresis. 3. Therefore, it releases its Temporary Block Flow (TBF) in the current cell, selects the new cell, and reestablishes a TBF in this new cell. 4. Now the LLC starts receiving packets again. If some of them were lost, LLC will request retransmissions. The previous process assumes that the RLC and LLC layers are in acknowledged mode. Routing Area Updates (Intra-SGSN) The RA update on an intra-SGSN arrangement is fairly simple and easy to accommodate. An RA update procedure occurs when the mobile station detects that it has entered a new RA or the periodic RA update timer has expired. The mobile station performs the RA update procedure as follows: 1. The mobile station sends an RA Update Request (old RAI and the update type) to the SGSN. The update type can be a periodic RA update or normal RA update. 2. This is preceded by an access request (one- or two-phase access with an MM procedure). 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 Chapter 8 3. The SGSN allocates a new P_TMSI and sends it in the RA UPDATE ACCEPT message. RA Updates (Inter-SGSN) The need to perform an RA update when the mobile station enters a new SGSN area creates a more complex update procedure. This is not to imply that the update procedure is at risk or constantly fails, but just that the procedure requires more steps and takes on more complex decisions. Fig- ure 8-10 shows the sequence. 1. The 