er occurrences of the PACCH. The PACCH shares resources with the PDTCH currently assigned to one mobile station. A mobile station currently involved in packet transfer can be paged for circuit- switched services on this PACCH. PTCCH UL and DL is a dedicated control channel for timing advance (TA) updates. The uplink portion of the Timing Advance Common Control Channel uses random access bursts to provide an estimation of timing advance. The downlink portion of the timing advance transmits timing advance information to several mobile stations. Typically, one PTCCH downlink is paired with several PTCCH uplinks. In timing advance for GSM, the receiving nodes estimate the appropriate time for the reception of the bursts from the mobile station. The cell sizes tend to be limited to a maximum radius of 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 35 km. The maximum timing advance is 63 bits (0 to 63). The duration of a single bit is 3.69 ms. Because the path to be equalized is a two-way service, the maximum physical distance between the BTS and a mobile station is half the maximum delay, or 70 km / 2 = 35 km. The random access burst can accommodate a maximum delay over a distance of 75.5 km; that way the bursts can appear at the BTS receiver with a high possibility that another mobile's normal burst will not cover them. With larger cells, this cannot be assured. Clearly, the overall system performance is partially based on distances from the BTS. The variability that creeps into the network includes the mobility of the users, SO that they are at different locations and distances from the BTS. As a result, the propagation delay is different. Moreover, the moving target in a mobile station creates a very changing reception from the mobile. Each of these conditions must be met in order to