QoS Research Of Improved Algorithm In VoWLAN

Wireless LAN (WLAN, Wireless Local Area Networks) with its access toflexible, low cost, fast and efficient advantages soon got people’s attention andapplication since it appears the outset, with the improvement of user demand forreal-time voice, video and other multimedia services in WLAN, voicecommunication technology based on wireless local area network (VoWLAN) cameinto being. Although VoWLAN has broad market prospects, but the quality of serviceQoS (Quality of Service) issues have severely constrained its development, it can notprovide excellent QoS service for VoIP (Voice over IP) services which have highreal-time requirements. In order to provide support for QoS in WLAN, MAC protocolin which play a key role must ensure provide differentiated services for differentbusiness categories, while the original IEEE802.11protocol uses a best-efforttransport service is not able to achieve a good service differentiation to meet the QoSrequirements of real-time multimedia services.To this end, IEEE802.11workinggroup proposed a new protocol named IEEE802.11e which supports QoS in2005.For the original802.11MAC protocol on QoS mechanism is insufficient, theIEEE802.11e corresponding enhances the MAC layer, it presents an enhancedchannel access mechanism HCF (Hybrid Coordination Function), which defines themechanisms of enhanced distributed channel access (EDCA) and HCF controlledchannel access mechanism (HCCA),they respectively corresponding to provide leveland parameterized QoS service, to enable WLAN to support higher QoS requirementsfor real-time business, ensure that all the business a more rational and efficient use ofnetwork resources. Wherein, EDCA introduces AC (Access Category) to prioritizetraffic on the distinction, EDCA defines4AC in total, which respectively correspondto8user priority in the802.11e standard. Different AC has different EDCAparameters, which are the maximum contention window (CW), the minimumcontention window (CW), arbitration interframe space (AIFS) and transmissionopportunity (TXOP). Based on these parameters, ACs which has different prioritieswill get different channel access opportunities through the competition, thus achievingservice differentiation.Although EDCA has realized the support for QoS to a certain extent compared with DCF which provides fairness in competition, however there are still somelimitations of its own. Through the analysis of both protocol theory and mathematicalmodels of EDCA, summed up its deficiencies in terms of QoS guarantees mainly havefollowing several points: static channel access parameter settings so as not to be ableto adapt to changes in the network in real time, EDCA only provides servicedifferentiation among different ACs while between different sites for the same AC hasnot been achieved service differentiation, when the network at a low load or high load,the performance of part of the AC traffic is not optimistic: at low load low-priority ACperformance is sacrificed too much, at high load high-priority AC performanceremains to be further optimized.For several issues summed up in EDCA mechanism, this thesis proposed animproved MAC parameters (CW) adaptive algorithm (BCC-EDCA) on the basis ofexisting improved algorithm, it mainly adjusts the CW to an adaptive value dynamically,based on the current network status in real time. Use comprehensive collision rate tojudge the congestion status of the network, algorithm BCC-EDCA will no longerdirectly set the CW to the minimum value after sending data successfully, but make theCW adjust adaptively based on the size of comprehensive collision rate. In addition,simulation experiments verify the effectiveness of the improved algorithm. Accordingto the simulation results, BCC-EDCA algorithm increases the system throughput whilealso reduces the packet loss rate of sites to some extent, making the networkperformance is improved. The adaptive adjustment of parameter CW makesBCC-EDCA outperforms the standard EDCA.