rmally, the mobile station stays in the cell selection and reselection mode. The cell update process occurs when the mobile station is in the ready state and in packet idle mode. The mobile station continually moni- tors the signal quality and strength from the adjacent cells and determines the power control of the received signal. If the mobile determines that a different cell has a better quality signal, then a cell reselection occurs. This occurs in the sequence that follows as a preferred way of performing the cell update: 1. The mobile station has selected a new cell in the current RA and is camping on it. No handover occurs in GPRS, but the network knows precisely where the mobile station is (if it is the ready state). 2. The mobile station performs the cell update procedure by sending an uplink LLC frame of any type containing the mobile station's identity to the SGSN. In the direction toward the SGSN, the BSS shall add the Cell Global Identity (CGI), including the routing area code (RAC) and location area code (LAC) to all BSSGP frames. 3. The SGSN records this mobile station's change of cell, and further traffic directed toward the mobile station is conveyed over the new cell (SGSN considers the new CGI). 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 Cell Update in the Packet Transfer Mode Again, the mobile station normally stays in the cell selection and reselection mode. The mobile station continually monitors the signal quality and strength from the adjacent cells and determines the power control of the received signal. If the mobile determines that a different cell has a better quality signal, then a cell reselection occurs. The cell reselection process occurs with the mobile station in the ready state and in packet transfer mode as follows: 1. The mobile statio