In the WiMAX standard (802.16), four scheduling classes have been defined: Unsolicited grant service (UGS), real-time polling service (rtPS), nonreal-time polling service (nrtPS), and best effort (BE). As illustrated in Figure 1, each traffic connection is associated with one of the four scheduling services, and the SS scheduler selects packets to be transmitted from each queue based on the scheduling policy employed. Usually, the scheduler selects packets to be transmitted from the highest priority queue that is not empty. Transmission of packets from lower priority queues are postponed until there is no packet available to send from a higher priority queue. Since UL traffic is generated at SS, the SS scheduler is able to arrange the transmission based on the up-to-date information on the current numbers/status of UL connections, which help to improve QoS performance. In the following, we review the various scheduling algorithms provided in the standard for handling the transmissions of packets belonging to these various services.
