| Wireless Local Area Network (WLAN) system is the combination of computer network and wireless communication technology, which can be used to construct the communication links between computers. WLAN system is viewed as one of the potential radio access sub-systems in the personal communication networks of future. IEEE 802.11 is a widely accepted standard for WLAN system which has been supported by most international manufacturers and gained biggish quotient in the world market.IEEE 802.11 can provide data transmission scheme with good performance. In recent years the support of the real-time multimedia service (including speech, image and video etc.) in the wireless channel is the trend of the communication system. Thus the requirement of QoS (Quality of Service) supporting schemes in WLAN is critical.This dissertation focuses mainly on the MAC (Media Access Control) layer QoS supporting technologies in IEEE 802.11 WLAN:The architecture of IEEE 802.11 WLAN MAC layer and its inner modules are discussed. The comparison of EDCA (Enhanced Distributed Channel Access) function and DCF (Distributed Coordination Function) defined in IEEE 802.11e is made by computer simulations. The performance of EDCA is analyzed extensively based on the construction of its Markov analytical model.Since the parameter configuration of EDCA is performed in static mode, the change in the amount of services load and error rate performance are not considered in the setup processes of each station. Thus it may cause frequently collision, back-off and even the drop of packets in the system and finally result in the degradation of throughput and delay performance. In order to avoid such effects, a new algorithm is proposed which can adaptively adjust the size of contention windows according to the increasing of service flows in the BSS (Basic Service Set) .Simulation results show that by using the new algorithm the system can get remarkly improvement in the throughput and delay performance in heavy load conditions. |
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