e uplink transfer from the mobile station to the network. In fixed allocation mode, the network communicates to the base station a set of physical charnels that the mobile is to use for the transfer of data. The network can specify either a consecutive or nonconsecutive range of physical channels using an allocation bitmap. In dynamic allocation mode, the mobile reads the USF from the header of each RLC/MAC data block. When the mobile station detects its assigned USF, it can transmit either a single RLC/MAC block or a set of four RLC/MAC blocks. Because the mobile station is constantly monitoring the USF, the allocation scheme can change dynamically. Extended dynamic allocation works much in the same way as the dynamic allocation. The main difference is that the system may specify a range of physical channels for the mobile to transmit data on. This provides a higher throughput in the uplink direction. In each case, the network controls the mode that is to be used. Not all GPRS systems are capable of supporting each of these modes. All mobile stations that support GPRS services support the fixed allocation and dynamic allocation modes. The network may either support fixed allocation or dynamic allocation. The extended dynamic allocation mode is optional. The MAC Header The main function of the MAC layer is the control of multiple mobile sta- tions sharing a common resource on the GPRS air interface. The RLC data block is passed down to the MAC layer where a MAC header is added. The 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 format of the MAC header is dependent upon the direction of data transfer. The fields in the header are shown in Figure 6-27 and lay out as follows in Table 6-1. Figure 6-27 The MAC header. Payload Countdown Uplink Value RLC/MAC Block MAC Header RLC Header R