Analysis And Research Of Quality Of Service For VoIP Over WLAN

As a real-time application,the VoIP phone technology based on IEEE 802.11 wireless LAN has its special needs.Delay,jitter and drop rate are also the main respects influencing the MOS of VoIP on WLAN.The IEEE 802.11 MAC layer protocol adopts the mechanism of Distributed Coordination Function(DCF),which is based on CSMA/CA(Carrier Sense Multiple Access with Collision Avoidance) to perform wireless medium access control.This protocol doesn't consider the transmission of real-time traffic,such as VoIP over WLAN,thus the target of this thesis is to insure the VoIP traffic to be transmitted in time by optimizing DCF scheme.Under the infrastructure operation mode of IEEE 802.11,for N simultaneous voice calls,downlink traffic from AP is much heavier than uplink traffic from each station. However, the normal DCF mode of WLAN requires that the AP contends (as an 'equal') with the other stations to obtain access to the channel. This means that AP becomes the bottleneck of providing good quality of voice call. Especially when data streams exist in the downlink of AP, voice quality gets worse.In this thesis, a proposed scheme named DQ-DCF insures the voice traffic to be transported in time by using two queues with different priorities in AP ,and also setting the voice traffic queue to have the higher priority than the data traffic queue.The simulation results using NS2 facility prove that the capacity of voice call is increased under the premise of assuring the perfect voice quality.We analyze the throughput and medium access delay performance of DCF scheme by Markov model. The analysis result shows that,for the normal DCF protocol lacks the measurement scheme for the number of active nodes ,the packet transmission probability namedτcan't adjust to the wireless network load,so the wireless network performance isn't good.This thesis proposes a fuzzy self-adaptive DCF scheme monitoring the number of active nodes by AP. AP measures the number of active nodes in time, and adjusts CWmin of nodes .The simulation result presents that theτcan maintain around the optimal value,and the throughput and medium access delay are optimized.