The Research Of Voice-Over-Internet Protocal In IEEE 802.11 Wireless Lans

Voice over Internet Protocol (VoIP) has become more and more popular recently for its simplified infrastructure and significant cost reduction. With the diminishing cost of electronic hardware, IEEE 802.11-based Wireless Local Area Networks (WLANs) have been massively deployed in public and residential places such as classrooms, airports and apartments, and more and more devices and peripherals are integrated with WLAN access capability. Due to these important developments in recent years, there have been greatly increasing interests in VoIP in WLANs. It is well-known that widely deployed IEEE 802.11 WLANs employ a contention-based Medium Access Control (MAC) protocol as the DCF. Although DCF can well support the best effort traffic, it may introduce arbitrarily long delay and delay jitters. Thus, it is unsuitable for real-time applications with strict Quality of Services (QoS) requirements. This thesis focuses on VoIP in IEEE 802.11 Wireless Local Area Networks (WLANs).In this thesis, we first give a detail introduction about IEEE802.11 MAC protocol especially about DCF mode with is based CSMA/CA mechanism and how to using two-dimensional Markov chain to model the network in saturation mode. Then, we build a model for VoIP calls in IEEE802.11 WLANs and use this model to analyze the maximum number of VoIP calls supportable in a WLAN, and get the value in different transmitting interval by numerical method.Secondly, we induce E-model to evaluate the performance of VoIP calls. Base on this model, we using NS2(Network simulator 2) to simulate VoIP calls in IEEE 802.11 WLANs and then get the maximum number of VoIP calls supportable in the network which prove our analytical study. At the same time, we analyze the network status when VoIP calls existing and find the reason why the network can not support more VoIP calls.Moreover, base on the analytical and simulation study, we propose a new call admission control strategy, namely, Adaptive Transmission Interval Call Admission Control (ATICAC) to enhance VoIP calls in 802.11 WLANs. In ATICAC strategy, Base Station adaptively changes transmission interval of the active stations to prevent the network from saturation by controlling the average collision probability of the network. ATICAC can not only ensure the Quality of Service (QoS) of VoIP calls in 802.11 WLANs in the network, but also increase the number of VoIP calls in the network.Finally, we evaluate the performance of Adaptive Transmission Interval Call Admission Control scheme though analyses and simulation. We use two dimensional Markov chain to build the model of the network when ATICAC is used or not. Then, we get the close-form expression of blocking probability of voice and data traffic when ATICAC is used or not. In the NS2 simulation evaluation, the results indicate that ATICAC can support 50%-100% more VoIP calls and decrease the network blocking probability by 50%-80% according to different scenarios, which matches the analytical results very well.In real application, we implement voice data transmitting system base on RTP/RTCP protocol using JRTPLIB library on Linux and Windows platform. We implement and induce the method to calculate delay, jitter and loss rate of voice transmission through RTP/RTCP packet analysis. Base on this software, we can do VoIP emulations in larger network in future.