Most of the existing medium access control (MAC) protocols for wireless local area networks (WLANs) provide prioritized access by adjusting the contention window sizes or interframe spaces for different traffic classes. Those MAC protocols can only provide statistical priority access and limited service differentiation. In this project, a novel token-based scheduling scheme is proposed for a fully-connected WLAN that supports both voice and data traffic. The proposed scheme can provide guaranteed priority access to voice traffic and, at the same time, provide more precise and quantitative service differentiation for data traffic, which provides great flexibility and facility to the network service provider for service class management. Simulation results demonstrate that the proposed scheme can guarantee a small delay for voice traffic. For data traffic, it can effectively achieve proportional differentiation among different classes, while achieving fair resource sharing within the same class. In addition, compared with a contention based scheme and a centralized polling scheme, the proposed scheme significantly improves the channel utilization by avoiding collisions (in the contention based scheme) and the polling overhead (in the polling scheme). The performance analysis of the proposed scheme is also presented. The accuracy of the analytical results is verified by computer simulations.
INTRODUCTION
IN recent years, with the growth of multimedia applications and the advances of wireless communications technology, quality-of-service (QoS) provisioning over wireless networks has attracted extensive attention in both academia and industry. One most promising wireless network is the wireless local area networks (WLANs), which have been widely deployed to provide high-rate data services at low cost over local area coverage. Most of the existing medium access control (MAC) schemes for WLANs are contention window based schemes. One common feature of those schemes is that they provide priority channel access to different traffic classes by assigning different inter-frame space (IFS) intervals and/or contention window (CW) sizes to different classes. High priority traffic (e.g., real-time voice) is assigned smaller IFS, CWmin and CWmax values, thus has a larger chance than low priority traffic to access the channel. However, those schemes provide only statistical rather than guaranteed priority access to high priority traffic. High priority traffic may suffer performance degradation due to a heavy load of low priority traffic. Such statistical priority access is difficult to satisfy the delay requirement of real-time traffic. On the other hand, although these schemes can provide a certain degree of service differentiation, it is difficult to quantify the degree of service differentiation, and even more difficult to adjust the degree flexibly among different classes based on some specific requirements of customers or network service providers. For example, when customers are charged differently for different services, it is desired that the received services (or resources) are proportional to what they are charged. Such kind of service model is referred to as proportional differentiation model [6], which assures that the performance of a class is proportional to that of another class according to a ratio preset by the network service provider. Although the actual service performance (e.g., throughput or delay) of each class may vary with the traffic load, the performance ratios among classes remain constant. Such a feature provides great flexibility and facility to network service providers for service management. Most of the existing MAC schemes for WLANs are contention window based schemes without support for the proportional service differentiation.
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