In general, admission control is a network’s QoS mechanism that determines whether a new session (or connection), with given bandwidth and delay requirements, can be established or not. For providing QoS, this procedure has been applied to both wireline and wireless networks. In the case of WiMAX, whenever a new session wants to make use of the wireless network, an admission control request is sent by the SS to the BS. This admission control request will be accepted by the BS if there is enough available bandwidth, QoS guarantees for bandwidth and delay can be met and the QoS of existing connections is not disturbed. An admission control scheme for WiMAX has been proposed together with the derivation of rules for each of the four classes of WiMAX. In addition, a token bucket based admission control for rtPS flows has been proposed. Omitting any further discussion involving admission control, we now present a brief overview of bandwidth allocation mechanisms in WiMAX.
The BS allocates bandwidth on a per SS basis, known as the grant per subscriber station (GPSS); further, each SS distributes this bandwidth among all of its active connections. The SS can efficiently distribute the allocated bandwidth as it has up-to-date information about the queue status of each connection. Thus, GPSS requires a packet scheduler at each SS, which may increase the complexity and the cost of an SS. However, GPSS is scalable to a large number of SSs and is, therefore, the only bandwidth allocation mechanism being employed in the current WiMAX.[*]
With GPSS, each SS treats various connections separately at its own level and these are then pooled together as one entity for bandwidth allocation at the BS. Thus, the scheduler at the BS will only need a small amount of information about the overall bandwidth required by a particular SS. This approach has the additional advantage of avoiding the time lag in receiving updated information about individual connections at the SS. Once a lump of bandwidth has been granted to a particular SS, then it is responsible for the appropriate scheduling, according to priorities and the QoS for each active connection. This process greatly reduces the workload on the BS. For instance, suppose that an urgent packet arrives at the SS, then the BS does not need to have information about it and it is the duty of the scheduler at the SS to provide the required throughput and delay.
BS and SS communicate with each other by using a bidirectional path, viz., Uplink (UL: SS to BS) and Downlink (DL: BS to SS); whereas the bandwidth requirements are made by the UL and grants are made by the DL. WiMAX supports both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes as shown in Figures 1 and 2, respectively.
Figure 1: An example of Burst FDD bandwidth allocation. (From IEEE-802.16-2004, IEEE standard for local and metropolitan area networks—Part 16: An interface for fixed and mobile broadband wireless access systems, October 2004. With permission.)
Figure 2: Frame structure of TDD. (From IEEE-802.16-2004, IEEE standard for local and metropolitan area networks—Part 16: An interface for fixed and mobile broadband wireless access systems, October 2004. With permission.)
In FDD mode, both the UL and DL are operating at separate frequencies and DL data can be sent in bursts. To facilitate various types of modulation, a fixed duration frame is used for both the DL and UL transmissions. Also, it allows the simultaneous use of both full and half duplex SSs; a full duplex SS can transmit and receive data at the same time whereas a half duplex SS can either transmit or receive data at any given time. If half duplex SSs are used, then bandwidth controller will not allocate UL bandwidth at the same time that it is expecting to receive data on DL channel. It should also take into account the allowance for propagation delay, SS transmit/receive transition gap, and SS receive/transmit transition gap. In FDD mode, the use of a fixed duration frame, for both the DL and UL channels, also helps in simplifying the design of algorithms for bandwidth allocation. It can be noted that a full duplex SS can listen to a DL channel continuously, whereas the half duplex SS can only listen to a DL when it is not transmitting on the UL channel.
In the case of TDD, the UL and DL transmissions occur at different time intervals, while usually employing the same frequency. A TDD frame is also of fixed duration and is composed of one DL and one UL subframe. For easy partitioning of bandwidth, a TDD frame is divided into an integer number of physical slots. Also, TDD framing is adaptive and the bandwidth allocated to the UL and DL parts can vary and is controlled by the higher layers. The DL-MAP and UL-MAP messages define the usage of the corresponding transmission intervals. The BS also regularly transmits DL and UL channel descriptors, DCD and UCD, for the physical description of the corresponding channel. The complete list of MAC management messages. It should be noted that a WiMAX network can be planned with either FDD or TDD, but the former mode has been discussed more frequently in the literature.
