High-speed transmission, fast deployment, and cost saving have made Broadband Wireless Access (BWA) systems a rapidly emerging field of activity in computer networking, attracting significant interests in the communities of academia and industry. In the mean time, the IEEE standard for BWA systems, IEEE 802.16 [1–3], has gained global acceptance and popularity in wireless computer networking markets and is also anticipated to take place of broadband access solutions like digital subscriber line (DSL) and cable.
The IEEE 802.16 standard specifies two modes for sharing the wireless medium: point-to-multipoint (PMP) and mesh (optional) modes. In the PMP mode, the nodes are organized into a cellular-like structure, where a base station (BS) serves a set of subscriber stations (SSs) within the same antenna sector in a broadcast manner, as shown in Figure 1.
The communication path between SSs and BS has two directions: uplink channel (from SSs to BS) and downlink channel (from BS to SSs). The downlink channel is a broadcast channel, while the bandwidth of uplink channel is shared by the SSs. The subframe in uplink channel includes three periods: Initial Maintenance period, Request Connection Opportunities period, and Scheduled Data grants period. The BS announces these periods and associates burst classes in the preceding downlink subframe's uplink map (UL-MAP).
Initial ranging and bandwidth request are two primary parts of the Call Admission Control (CAC) procedure. The BS periodically reserves bandwidth in the uplink channel for SSs to register or send their bandwidth request. When a SS needs registration or bandwidth, it has to go through the contention resolution procedure to send its requests.
The IEEE 802.16 contention resolution mechanism is controlled by two sets of parameters: the number of the contention slots and the backoff initial/maximum window values. These parameters are set at the BS and transmitted to SSs in the UL-MAP. When an SS has information to send and wants to enter the contention resolution process, it sets its internal backoff window size equal to the request size of initial backoff window defined in the uplink channel descriptor (UCD) message. The SS randomly selects a number within its initial backoff window. This random value indicates the number of contention transmission opportunities that the SS defers before transmitting. However, collisions might still occur if two or more SSs select the same backoff value. When this happens, the SS increases its backoff window by a factor of two, as long as it is less than the maximum backoff window. The SS randomly selects a number within its new backoff window and repeats the deferring process described above. This retry process continues until the maximum number of retries has been reached.
According to the IEEE 802.16 standard, the backoff parameters of its collision resolution mechanism are far from optimal setting since it selects a small initial value of backoff window by a naive assumption of a low level of congestion in the system. Hence, the problem of choosing the right set of backoff parameters for the current network level remains unsolved and left as an open issue since this strategy might incur a high collision probability and the channel utilization could be degraded in congested scenario.
Although in literatures there have been excellent discussions on the issues on contention resolution mechanism and its performance analysis [4, 5]. However, these studies do not propose any mechanisms to force the SSs to adopt an adaptive backoff window size that maximizes the channel capacity for current channel status. In [6], Yao et al. analyzed the impact of contention slots allocation on system throughput and thus proposed an algorithm to optimize the utilization of uplink bandwidth by dynamically adjust the number of contention slots. In [7], Sayenko et al. presented analytical calculations to determine optimal values for the backoff initial/maximum values and an optimal number of the request transmission opportunities. In [8], Lin et al. proposed an efficient performance improvement method by using dynamic window adjustment for initial ranging. However, none of the above studies is satisfactory since they did not tell us how to run-time estimate the channel status. The algorithm proposed in [9] automatically adjusts the initial contention window to a near optimal point according to the traffic activity, thus avoiding bandwidth wastage due to improper contention window setting. However, this scheme was designed for WLANs, and we did not know whether the proposed algorithm can be applied to IEEE 802.16 standard.
Based on above observations, we propose that a proper choice of the size of backoff window in accordance with current channel status, which has a great influence on overall network performance. Hence, in this paper, two pragmatic adaptive algorithms, namely semi-dynamic and quasi-dynamic contention resolution scheme, that allow the base station to adjust its backoff window size dynamically are proposed. Both schemes can be implemented in the present IEEE 802.16 standard with only relatively minor modifications and use very simple feedback signals. In addition to the analytical analysis, we have also carried out comprehensive simulations implemented by network simulator NS2 [10] to evaluate the performance of the proposed schemes. The results show that both schemes are able to achieve higher performance in comparison with the legacy IEEE 802.16 standard.
The remainder of this paper is organized as follows. Sections 2 and 3 introduce the proposed semi-dynamic and quasi-dynamic contention resolution schemes, respectively. Simulation and experimental results are given in Section 4, followed by Section 5 which concludes this paper.