Friday, March 23, 2012


WiMAX PHY is responsible for the transmission of data over the air interface (physical medium). The PHY receives MAC layer data packets through its interface with the lowest MAC sublayer, and transmits them according to the MAC layer QoS scheduling. WiMAX MAC layer comprises of three sublayers, which interact through service access points (SAP) to provide the MAC layer services, as shown in Figure 1. The convergence sublayer (CS) interfaces the WiMAX network with other networks by mapping external network data (from ATM, Ethernet, IP, etc.) to the WiMAX system. MAC common part sublayer (MAC CPS) provides majority of the MAC layer services. The MAC CPS receives data from the CS as MAC service data unit (MAC SDU) and efficiently packs them on to the payload of the MAC packet data unit (MAC PDU) through the process of fragmentation and aggregations. Fragmented parts of MAC SDU are used to fill (aggregate) remnant portions of MAC PDU payloads that cannot accommodate full MAC SDU during package. As WiMAX provides connection-oriented service, MAC CPS is also responsible for bandwidth request/reservation for a requested connection, connection establishment, and maintenance. In the WiMAX standard, bandwidth request/reservation is an adaptive process that takes place on a frame-by-frame basis. This allows more efficient resource utilization and optimized performance. Thus the MAC CPS is required to provide up-to-date data on bandwidth request/reservation for each connection, on a frame-by-frame basis. The MAC CPS also provides connection ID for each established connection and marks all MAC PDUs traversing the MAC interface to the PHY with the respective connection ID. This sublayer also performs QoS scheduling by deciding the orders of packet transmissions on the PHY, based on the service flow decided during connection establishments. Privacy sublayer provides authentication to prevent theft of services, and encryption to provide security of services.

Figure 1: WiMAX Protocol stack.
The ensemble of the activities of the three sublayers of the WiMAX MAC layer constitutes the MAC layer services. MAC layer services can broadly be categorized into two: periodic and aperiodic activities. Periodic activities are fast- or delay-sensitive types of activities and are carried out to support ongoing communications, thus they must be completed in one frame duration. Examples include QoS scheduling, packing, and fragmentation. Aperiodic activities are slow- or delay-insensitive types of activities. They are executed when and as required by the system, and are not bounded by frame durations. Examples include ranging and authentications for network entry.
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