In this section some design guidelines are given for a “channel-aware” and “QoS-aware” scheduler to be implemented at the BS of a WiMAX PMP network for the delivery of DL traffic to a set of distributed SSs with active connections of different traffic nature. The scheduling algorithm, which is running at the WiMAX base station, needs to fulfill the following requirements:
§ Efficient link utilization: The scheduler shall take opportunistic decision and not assign a transmission opportunity to a flow with a currently low-quality link, because the transmission will be wasted.
§ Delay bound: The algorithm shall be able to provide delay bound guarantees for individual flows, to support delay-sensitive applications; besides, it shall prevent too late packet transmissions from wasting bandwidth.
§ Fairness: The algorithm shall redistribute available resources fairly among flows; thus providing short-term fairness to error-free flows and long-term fairness to error-prone flows.
§ Throughput: The algorithm shall provide guaranteed short-term throughput to error-free flows and guaranteed long-term throughput to all flows.
§ Implementation complexity: A low-complexity algorithm is necessary to take quick scheduling decisions.
§ Scalability: The algorithm shall operate efficiently when the number of flows sharing the channel increases.
To match all the above-mentioned needs, (1) a class-based wireless scheduling is recommended to fulfill the WiMAX service class differentiation needs, and also to simplify interworking with the Internet (supporting class-based differentiated services); (2) “per class” service differentiation must be achieved, and simple “per flow” mechanisms (e.g., lead/lag counters per each flow) must be provided to guarantee fair service to traffics within a class; (3) channel awareness must be exploited to make efficient use of wireless resources, and a compensation technique for missed transmission opportunities must be provided to guarantee proportional fairness in sharing the bandwidth; (4) useless compensation must be avoided through simple measures, e.g., periodic buffer cleaning of over-delayed packets could be used; and (5) simple expedients must be provided to avoid monopolization of the scheduler by a lagging flow after the recovery of its channel, and to guarantee graceful throughput degradation of leading flows; this could come at a very low cost by using combination of lead/lag counters and queue parameters.
In Figure 1, the reference channel- and QoS-aware scheduling architecture is illustrated. It is considered to be implemented at the MAC layer of a WiMAX BS, which manages local traffic queues for packet delivery over the DL channel. The illustrated framework must be able to provide per-class differentiated QoS and fair service to flows in the same class through the operation of the following QoS-support modules:
§ An error-free service scheduler that decides how to provide service to traffic flows based on an error-free channel assumption.
§ A lead/lag counter for each traffic flow that indicates whether the flow is leading, in sync with, or lagging its error-free service model and in which extent.
§ A compensation technique that is used to improve fairness among flows. A lagging flow is compensated at the expense of a leading flow when its link becomes error free again. The system maintains credit/debit counters for each lagging/leading flow.
§ Separate per-class packet queues used to support rtPS, nrtPS, and BE traffic flows.
§ A means for monitoring and predicting the channel state of each backlogged flow.
Figure 1: Channel-aware QoS scheduling architecture in the BS.
