 stations can support four downlink and one uplink time slots (for example, Motorola). Many of the network resources (aside from the mobile stations) are limited to the two downlink and one uplink time slots due to timing advance situations regardless of the capability of the mobile station. The MAC layer manages this allocation and sharing, assigning the differ- ent blocks of the same PDCH to different users through static or dynamic allocation. Figure 6-12 shows an example of the allocation. In GPRS Phase I, the mobile station will be able to manage only up to three time slots simultaneously, and only the static allocation (bitmap) will be used. The PACCH carries all the dedicated signaling (it corresponds to SDCCH + SACCH + FACCH). It always carries RLC/MAC control blocks 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 Chapter 6 TDMA Frame 123456789 Figure 6-12 An example of PDTCH allocation. PDTCH PDTCH PDTCH X PDTCH PDTCH PACCH PACCH PDTCH PDTCH PDTCH|X PACCH PDTCH PTCCH Uplink PDTCH and downlink PDTCH on the same time slot are disassociated. (RLC/MAC layer signaling). It is used for paging, for PSI 1 and PSI 13 broadcast in packet transfer mode, ACK messages (answer to polling), RR request, assignment, and reassignment. On the uplink, the PTCCH carries an access burst in order for the BTS to update the TA value, and on the downlink, it carries the updated value of the TA to apply on the mobile station (even in the case of DL packet transfer because of the UL acknowledgments necessary). The PACCH and PDTCH positions (PDCH number) are provided to the mobile station in the Immediate Assignment or in the Resource Assignment message. It corresponds to one of the PDCHs assigned for PDTCH. One-Phase and Two-Phase Access The mobile station initiates a packet transfer by sending a Packet Channel Request message on 