| The increasingly mature of WLAN is attracting more and more users to use it,especially in some hot areas, a number of mobile users obtain network services such asvoice, video and the transmission of files through APs.Users can really access WLANanytime and anywhere.Meanwhile, WLAN is also easy to install, extend its functions andmanage. IEEE 802.11 is the first WLAN standard of IEEE, which is mainly used to solvethe wireless access of campus LAN and office LAN. It plays an important role in today'snetwork technology.However, the study found the traditional 802.11b WLAN using CSMA/CA as its MAClayer protocol can only provide best-effort service, but not differentiated service. Therefore,the transmission way can not completely meet the service form of the real-time multimediaservices, and it will even cause the decline of the QoS of users when the network isoverloaded. With the rapid development of multimedia services these years, the mobileusers more and more intensely hope that they can be provided with the multimedia serviceswith QoS guarantees, and smoothly send and receive datas of real-time voice, real-timevideo and other general datas. Therefore, how to provide QoS support in 802.11 WLAN hasbecome one of the important research directions.At the end of 2005, the IEEE 802.11e was used to solve the problem of WLAN. Thelatest edition of 802.11 revision which was issued in Jane, 2007, uses 802.11e as a standardfeature of series equipment of IEEE 802.11. IEEE 802.11e is aimed to provide QoS featureson 802.11 MAC layer. It is the expansion of IEEE 802.11. By setting priorities, IEEE802.11e not only ensures the service quality of the multimedia applications, but alsodecentralizes the ordinary 802.11 terminals, in order to provide QoS guarantee of WLANbased on 802.11, to meet the QoS requirements of the real-time multimedia applications ofWLAN, and to ensure the smoothness of the multimedia applications. The HCF defined inIEEE 802.11e mainly includes two kinds of different channel access mechanism,respectively including EDCA and HCCA. IEEE 802.11e has done beneficial exploration onhow to provide QoS guarantee on the MAC layer, and promoted the rapid development ofthe standardization and the relevant industries. Therefore, it has further promoted thedevelopment of WLAN, and provided better service to users. Currently, much work has been done in the various research and improvement of IEEE802.11e. But most are mainly concentrated in EDCA mechanism with remarkableachievements. There are so few HCCA simulation studies either at home or abroad. Andonly the foreign organisations have independently developed and realized the HCCAframework. Some relatively representative work is the HCCA framework implemented bythe computer network architecture of Pisa University in Italy, and the FHCF frameworkimplemented by INRIA, Sophia Antipolis. Although these HCCA schemes are effective,most details and parameters considerd by them are too limited, less functional, and highcomplexity implementation.According to the definition of ITU-T (ITU-T for ITU TelecommunicationStandardization Sector), admission control is used by QAP to judge whether to accept thisconnection or not according to the service characteristics, QoS requestment and the currentsituation of network resources when the new connection applications are established.Admission control plays an important role in avoiding overloaded network andguaranteeing the stability of the system, especially be essential for the real-time multimediaservices. Research shows that IEEE 802.11e has the better distinguishing quality of servicewhen the network is not overloaded. But it will increase the possibility of conflicts ofservices when the network is overloaded, which could lead to the decreased networkperformance, and the doubtful service quality of services with higher priorities. Therefore,we cannot ignore the role admission control mechanism plays in the IEEE 802.11e. IEEE802.11e allows researchers and major equipment manufacturers to design the HCCAadmission control mechanism according to IEEE 802.11e. Threrfore, it has left a certainroom for the research work of this paper to think and to design.This paper makes a comprehensive analysis and research of IEEE 802.11e. A HCCAsoftware architecture based on IEEE 802.11e standard is put forward to.It mainly includesfive modules, channels scanning, association/reassociation, disassociation, QAP admissioncontrol for new service and polling scheduling. Then based on this framework, afull-functional HCCA platform has been implemented by using NS2. Finally, the platform'svalidity and rationality is validated by simulating a resource reservation scheme based onHCCA successfully. Simulation results show that the resource reservation scheme hashigher success rate of admission control than the scheme without resource reservation in thescenes of two different degrees of network load, that is, the resource reservation schemecan effectively reduce the failure rate of admission control and greatly improve the QoS ofHCCA services. Moreover, success rate of admission control in the overloaded network were significantly lower than that in the lighter load network. It accords with the theoreticalresult.Based on the resource reservation scheme, this paper further considers the monitoringof network load condition, network revenue and so on. This paper tries to coordinate thevarious needs of users and the income of Network Operator. An improved HCCA admissioncontrol algorithm based on the reward and punishment mechanism is put forward to in it.The algorithm ascendes the level of access rules, designs a new flexible framework ofreward and punishment, and allows operator appropriately adjust adjustable parameters ofthis framework so as to meet different needs. For example, to adjusts the access probabilityof different levels of services. The experiments make the plan that the operator requiresservices with higher priorities surely have higher success rate of admission control as anexample, and selects some datas to do the parameter settings of this framework according tothe scheme. This paper has done the simulation on the new algorithm. Results show that theimproved HCCA admission control algorithm has the higher success rate of admissioncontrol than that of both the resource reservation scheme based on HCCA and the schemewithout resource reservation when the network is overloaded. And it can effectively reducethe failure rate of admission control and greatly improve the QoS of HCCAservices.Moreover, the services with higher priority have higher success rate of admissioncontrol than that with lower priority when the network is overloaded by using the improvedHCCA admission control algorithm. The malicious resources reservers are refused toacccess network by QAP in the algorithm when they apply for the system to reserveresources constantly. Finally, the average revenues of the improved HCCA admissioncontrol algorithm are higher than that of the resource reservation scheme based on HCCAwhen the network load is becoming heavier and heavier.It can bring more network benefitsfor the system. |
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