Praise for Fundamentals of WiMAX

9.2 MAC PDU Construction and Transmission 315Table 9.3 Bandwidth Request MAC Header FieldsFieldLength(bits)DescriptionHT 1 Header type (set to 1 for such header)EC 1 Encryption control (set to 0 for such header)Type 3 TypeBR 19Bandwidth request (the number of bytes of uplink bandwidth requested bythe SS for the given CID)CID 16 Connection indentifierHCS 8 Header check sequenceApart from the generic and bandwidth request headers, WiMAX also defines five subheadersthat can be used in a generic MAC PDU:1. Mesh subheader. Follows generic header when mesh networking is used.2. Fragmentation subheader. Follows the generic MAC header and indicates that the SDUis fragmented over multiple MAC PDUs.3. Packing subheader. Indicates that multiple SDUs or SDU fragments are packed into asingle MAC PDU and are placed at the beginning of each SDU or SDU fragment.4. Fast-feedback allocation subheader. Indicates that the PDU contains feedback from theMS about the DL channel state information. This subheader provides the functionality forchannel state information feedback for MIMO and non-MIMO implementations.5. Grant-management subheader. Used by the MS, conveys various messages related tobandwidth management, such as polling request and additional-bandwidth request. Usingthis subheader is more efficient than the bandwidth-request PDU for additional bandwidthduring an ongoing session, since it is more compact and does not require the transmissionof a new PDU. The bandwidth-request PDU is generally used for the initial bandwidthrequest.Once a MAC PDU is constructed, it is handed over to the scheduler, which schedules theMAC PDU over the PHY resources available. The scheduler checks the service flow ID and theCID of the MAC PDU, which allows it to gauge its QoS requirements. Based on the QoS requirementsof the MAC PDUs belonging to different CIDs and service flow IDs, the scheduler determinesthe optimum PHY resource allocation for all the MAC PDUs, on a frame-by-frame basis.(Scheduling algorithms and their various pros and cons in the context of an OFDMA system arediscussed in Chapter 6). The scheduling procedure is outside the scope of the WiMAX standardand has been left to the equipment manufacturers to implement. Since the scheduling algorithmhas a profound impact on the overall capacity and performance of the system, it can be a key featuredistinguishing among implementations of various equipment manufacturers.