Research On VoIP Capacity And Call Admission Control In Wireless Local Area Networks

With the development of wireless communication technology, the traditional data servicesupported by Wireless Local Area Networks (WLANs) cannot meet the requirement of people. It isstrongly demanded that the real-time service, such as voice, video etc, can be supported by WLANs.At the same time, as a kind of technology for voice transmission in IP networks, Voice over InternetProtocol (VoIP) achieves the integration of voice and data transmission and becomes an importantinternet application. Therefore, it is a promising tendency to deploy VoIP over WLANs, which canbring the convenient and low-cost voice service to people. IEEE802.11standard is the widelyaccepted technology specification for WLANs, where the multiple random Medium Access Control(MAC) protocol is developed based on the well known Carrier Sense Multiple Access/CollisionAvoidance)(CSMA/CA) mechanism. Nowadays, the large numbers of WLANs-related studies areconducted on the basis of IEEE802.11MAC. In this dissertation, the IEEE802.11-based WLANsare also chosen as the main object of studies, focusing on the problem to effectively deploy VoIPover WLANs. Specifically, this problem involves three aspects, namely:1) performance analysis ofIEEE802.11MAC,2) VoIP capacity analysis of IEEE802.11WLANs and3) admission control inIEEE802.11WLANs. The main works are summarized as follows:(1) As the beginning of studies, this dissertation elaborates the significance of the studies onperformance analysis, VoIP capacity analysis and admission control in WLANs, and points out thethe relations among the three aspects and the main object of studies. Moreover, the surveys of thestudies on the three aspects are given and the existing problems be pointed out.(2) In this dissertation, modeling and analysis for asynchronous backoff phenomena of IEEE802.11Distributed Coordination Function (DCF) are studied. In single-hop WLANs based on IEEE802.11DCF protocol, packet collisions can produce asynchronous backoff phenomena amongnodes. To analyze the effect of asynchronous backoff phenomena on throughput of IEEE802.11DCF, this dissertation proposes a new analytic model of three-dimensional Markov chain, in whichthe unit-time of state transitions is of a fixed length equal to physical-layer slot and the satetransitions of freezing duration are considered. Furthermore, according to the proposed model, thesaturated throughput is analyzed. As proven by comparison with simulation, the analytic model isaccurate for both the packet transmission mechanisms employed by IEEE802.11DCF, and leads tomore correctness than referenced model.(3) In this dissertation, VoIP capacity analysis and optimization for IEEE802.11e single-rate WLANs are studied by using the node differentiation. This dissertation proposes a new analyticalmodel for the VoIP capacity over a WLAN, in which the three parameters of IEEE802.11e EDCA(contention window (CW), arbitration inter-frame space (AIFS), transmission opportunity (TXOP))are adjusted to provide a differentiation service (DS) for the access point (AP) and the terminalnodes. Moreover, this model also reflects the ON-OFF model of voice flows and the delay bound ofvoice uplink and downlink flows. Based on the analytical model, two differentiation schemes, ascalled separate differentiation (SD) and joint differentiation (JD), are analyzed in which the SDutilizes one arbitrary parameter and the JD uses two arbitrary parameters to provide differentiationservice. As a result, with aid of the analytical model, the optimal values of CW, AIFS and TXOP oftwo differentiation schemes can be obtained for improving the VoIP capacity. Finally, simulationresults confirm that the analytical model is accurate and effective for the optimization of VoIPcapacity.(4) In this dissertation, VoIP capacity improvements for IEEE802.11multi-rate WLANs arestudied. In IEEE802.11WLANs, a node can adopt different transmission rate from the othersaccording to the channel conditions, and this can result in multiple transmission rates to existsimultaneously. However, the presence of low transmission rates can affect the performance andVoIP capacity of a WLAN. To handle this problem, this dissertation elaborates the improvementschemes for voice capacity from two aspects. On the one hand, the indirect method, whichstrugglingly keeps a high rate node from unnecessarily lowering its data rate, is proposed toimprove VoIP capacity. In the context of this method, this dissertation proposes a novel rateadaptation scheme called Auto Rate Fallback with Adaptive Threshold Control (ARF-ATC). Inmultiple contending nodes environment, ARF-ATC can effectively mitigate the effect of framecollision on rate adaptation decision by adaptively adjusting rate-up and rate-down thresholdaccording to the current collision level. Simulation results show that ARF-ATC can achieve higherthroughput and VoIP capacity than the other existing rate adaptation schemes. Furthermore, thesimulation results also demonstrate that ARF-ATC can response effectively to the varying channelcondition. On the other hand, the direct method, which provides a differentiation service for theaccess point and different rate nodes, is proposed to improve VoIP capacity. In the context of thismethod, this dissertation proposes a mathematical analysis model for the voice capacity of IEEE802.11DCF multi-rate WLAN. With the analytical model, the optimal value of CW and voicecodec can be obtained for differentiating the access point, the low rate nodes and the high ratenodes. Finally, the analysis and simulation results are compared to confirm the accuracy andeffectiveness of the proposed schemes.(5) In this dissertation, a model-based admission control in IEEE802.11e WLANs is studied. Admission control is an important measure which can guarantee the quality of service (QoS)requirements of voice flows and improve the utilization efficiency of network resources.Considering the circumstance of heterogeneous voice flows, this dissertation proposes amodel-based and flow-oriented admission control scheme. Firstly, by applying Markov chain, thisdissertation proposes a unsaturated analytical model for the IEEE802.11e EDCA protocol, whichconsiders the condition of non ideal transmission channel and the character of the occurrence ofbackoff countdown at the beginning of time slot in EDCA protocol. Furthermore, according to theproposed model, the media access delay and throughput of a flow are analyzed, and the admissioncontrol scheme is proposed. Finally, the simulation results are shown to confirm that the proposedadmission control scheme can guarantee the delay and throughput requirements of voice flows, andcan admit more voice flows to improve the utilization efficency of network resources by choosingthe appropriate values of the minimum contention window or the appropriate varieties of voiceflows.