Analysis And Improvement Of The HCCA

With the rapid development of communications. People have more increasing need for communications networks. They would like to communicate with anyone at any time, anywhere including data, voice, image and so on. But when the 802.11e was applied, people find that there is a great weakness in 802.11e. no guarantee for transmission of the multimedia flows quality. The 802.11e is proposed to provide a solution.The 802.11e has proposed the Hybrid Coordination Function(HCF).HCF, providing improved access bandwidth and reduce the high-priority communications delay, replaces DCF and PCF in a new Access Control Mode. The HCF is made of a contention-based channel access, known as Enhanced Distributed Coordination Access(EDCA),and of a HCF Controlled Channel Access(HCCA).But it's show that the 802.11e can not guarantee the Quality of Service under high loads.Now the research of 802.11e standard focuses on the EDCA. This article first introduces the background of the 802.11 standards, performance and evaluation of the capability, then researches the simple scheduler of the HCCA and proposes two new algorithms based on the simple scheduling algorithm:(1) This paper proposes PI-HCCA algorithm, because Simple scheduling algorithm does not reflect the conditions of network load when it assigns TXOPs. PI-HCCA algorithm uses feedback control principle to set up a PI controller with the relationship of the length of the queue and the opportunity to be sent. PI-HCCA algorithm assigns TXOPs with the length of the queue.(2) Although the PI controller can avoid the residuals, it uses too long time to achieve the stability of the system. It is difficult to set parameters. This paper proposed API-HCCA algorithm to overcome the lack of the PI controller: API-HCCA algorithm adjusts PI parameters online using the theory of self-adaptive (least-squares algorithms).The proposed algorithms have been implemented in NS. Simulation results have shown that, the proposed algorithms guarantee the QoS of multimedia flows and improved the performance of the system.