Due to its simplicity and robustness, the IEEE 802.11 wireless LAN (WLAN) standard has been accepted widely for many application areas. However, the 802.11 standard was originally proposed with minimal quality of service (QoS) support, and this limitation has drawn increasing concerns as the WLAN technologies are being recommended to offer high-quality and real-time services across the network. While new mechanisms for QoS support have been proposed by the IEEE 802.11e group, further investigations on the new standard in conjunction with the legacy 802.11 configuration are important for having a sustainable improvement on QoS in various working conditions. Within the 802.11 protocol architecture, the medium access control (MAC) sub-layer is responsible for supporting QoS. The legacy 802.11 MAC has two operation modes characterised by the coordination functions: the Distributed Coordination Function (DCF) and the Point Coordination Function (PCF). These two operation modes face vigorous challenges for maintaining QoS in the wireless environment featuring high packet loss rates, large latency, jitter and time varying. In the DCF mode, all the stations in one Basic Service Set (BSS) or all the data flows in one station compete for the resources and channel assignment with the same priorities. There is no differentiation mechanism in DCF to guarantee bandwidth, packet delay and jitter for high-priority stations. Therefore it can only support best-effort services but without assurance, and suffers from the significant throughput degradation when the number of stations involved increases. The PCF is defined to allow stations to have priority access to the wireless medium, coordinated by a station called Point Coordinator (PC), thus it can support time-bounded services such as voice IP, audio and video conferencing. However, the QoS performance of PCF is limited  due to several problems such as unpredictable beacon delay, unknown transmission time of the polled stations and inefficient central polling schemes. As a result, the performance of PCF high-priority traffic will be deteriorated, especially when the traffic loads are high. The QoS performance of the legacy 802.11 can be enhanced by introducing service differentiation (SF) in both the DCF and PCF modes -. Two examples for the DCF based SF schemes are the Blackburst scheme  and the Distributed Fair Schedule scheme . In Blackburst, low priority stations use the ordinary CSMA/CS access method, while all the high priority stations access the medium with equal and constant intervals. This scheme aims to minimise the delay of real-time traffic, but when the constant access intervals for the high-priority traffic cannot be guaranteed its performance depredates considerably. The DPS scheme introduces 'fair' scheduling for medium access by including the packet size in the calculation of the backoff interval, resulting in the traffic more often containing the packets of smaller size. The QoS enhancements described in the upcoming 802.11e standard are realised through both the DCF and PCF based SF schemes, such as the Enhanced Distributed Coordination Function (EDCH) and the Hybrid Coordination Function (HCF). EDCF achieves significant improvements for high-priority QoS traffic, but at the cost of worse performance for lower priority traffic. HCF provides much more efficient use of the medium when it is heavily loaded, and compensate the lower priority traffic with reasonable bandwidth. However, HCF is centralised, thus less robust. To evaluate the QoS performances of the different MAC functions, we generate the simulation results in throughput and average access delay versus the number of stations with priority for EDCF, PCF, Blackbusrt and DPS schemes, as shown in Fig.1. Tables presented The simulation model includes 30 stations, with 4 of them being low priority stations, and one access point. As it can be seen from Fig.1, there is no single scheme that would produces optimal performances, in terms of throughput as well as delay, under different traffic conditions. When the number of stations is between 10 to 30, EDCF can provide better QoS support than the other schemes, but its throughput is lower than the other's when the number of stations is below 10. Also, EDCF and HCF can only work well when admission control is applied at the same time . To support QoS in such a complex environment it is required that the MAC mechanism proposed should be able to adapt to the changes in traffic loads and channel conditions, e.g., combining EDCF and Blackburst or DFS, and to optimise the trade-offs between channel efficiency, priority and fairness. Also importantly, the integration of the service differentiation and efficient error control is the key to achieving sustainable QoS in the wireless network.
|Title of host publication||2004 International Workshop on Wireless Ad-Hoc Networks|
|Number of pages||3|
|Publication status||Published - 31 Oct 2005|
|Event||2004 International Workshop on Wireless Ad-Hoc Networks - Oulu, United Kingdom|
Duration: 31 May 2004 → 3 Jun 2004
|Conference||2004 International Workshop on Wireless Ad-Hoc Networks|
|Period||31/05/04 → 3/06/04|