Radio and MS-PCUSN Interfaces Chapter 6 Frame Figure 6-25 Transmissions at the MAC layer. 123456781234567 Physical Transmission Channel Medium =MS User 1 =MS User 2 When multiple physical channels are allocated for the transmission medium, the mobile station essentially sends data in parallel. This provides the capability to increase the data rate between the mobile station and the network. In Figure 6-25, two mobile users are actively transmitting streams of data. Time slots 5 and 6 are allocated as the shared transmission medium. The MAC layer on the network side schedules the mobiles in order to stagger the transmission of data. So, user 1 may send data on the one TDMA frame and user 2 may send data on the next TDMA frame. The stan- dards enable the network to schedule a maximum of eight mobiles to use the same transmission medium. Key Identifiers for the MAC Layer The MAC layer uses several identifiers to transfer data. A brief description of two of them is listed in the following section. Temporary Block Flow Used to identify a series of RLC/MAC blocks to/from a specific mobile station. The TBF is unique for a given direction (uplink/downlink). Each mobile station occupying a radio resource is assigned a TBF for the duration of the data transfer. Because data transfers are typically bursts of data followed by idle time, the TBF is temporary; it only lasts until all RLC/MAC blocks have been transferred and acknowledged. Temporary Flow Identity Uniquely identifies each TBF for a given direction. The TBF, TFI, and direction uniquely identify a RLC data 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 block. The message type together with the TBF, TFI, and the direction designates the RLC/MAC control message. The following concepts are depicted in Figure 6-26. Data Burst 1 The MAC la