Wireless Lan Mac Technology And Its Qos Enhancement

Wireless local area network (WLAN) is becoming more and more important in our daily life and is expected to support all kinds of new applications which require different quality of service (QoS). Network layer and data link layer are mainly used for data transmission. Hence it is significant to study WLAN MAC and its enhancement of QoS management and control.Our research work bases on IEEE 802.11 standard and its QoS enhancement IEEE 802.1 le standard draft. In this thesis, 802.11 MAC and 802.1 le MAC are investigated widely, based on the analysis of architecture and functions, using the mathematical model for theoretical analysis and OPNET for validation of function and investigation of performance.Two coordination functions for access control and packet delivery of 802.11 MAC are analyzed. One is DCF (Distributed Coordination Function), the other is PCF (Point Coordination Function).A Markov model is used for calculation of DCF throughput. It is disclosed that the key parameters are the number of contending stations and the length of contention window. The maximum throughput and the best parameters are offered by theoretical analysis. The influence of parameters is studied, based on performance simulations. Basic DCF and RTS/CTS mechanism are compared and suitable application situations are suggested.The investigation of PCF emphasizes on the polling of PC. The polling efficiency is calculated and the polling scheme is considered as key of the PCF implementations. Moreover, the PCF functions of WLAN reference model provided by OPNET are validated and the polling scheme is simulated for research.Additional coordination function HCF (Hybrid Coordination Function) and bursting transmission of 802.11 e are analyzed.The throughput formula of bursting transmission is derived. It is indicated that the number of frame exchanges during a transmission opportunity (TXOP) is the key parameter. The equivalent station method is proposed to analyze contention access part of HCF (named as EDCF) which provides prioritized access, and the improved Markov model is used to study the performance. According to the "Minimum Performance Requirement", a singlepoll scheme and a multipoll scheme are put forward for the realization of controlled access part of HCF (named as Polled TXOP). The polling efficiencies of the two schemes are calculated.Based on the theoretical analysis, C codes are added to expand the functions ofsimulation model. The prioritized QoS and parametered QoS performances of 802.1 le MAC are studied through simulations.The simulation results validate that bursting transmission is helpful for increasing throughput. With and without TCP, the throughput performances impacted by the number of frame exchanges in a TXOP are investigated. The results of throughput as well as media access delay show that EDCF can provide obvious prioritized access. The singlepoll scheme and the multipoll scheme are applied to realize Polled TXOP. The simulations validated that Polled TXOP can offer good parametered QoS service. Moreover, the advantage of multipoll scheme is proved by comparison of polling efficiency.