Towards The Quality Of Service For VoIP Traffic In Wireless Networks

With the wide deployment of IEEE 802.11 wireless network,more and more wireless applications and services are provided to users.As one of interesting wireless services,wireless Voice over Internet Protocol(VoIP)can provide low-cost real time voice service to mobile users,having the greater commercial potential.Although VoIP in wired networks is already a very mature techonology,there are new challenges for VoIP in wireless networks.Towards the Quality of Service(QoS)for VoIP traffic in wireless networks,the QoS problems can be divided into three main categories,namely,handoff,capacity,and call admission control.This paper focuses on these three issues.Handoff is caused by the mobility of users in WLANs.When the user moves from one wireless access point to another wireless acess point,the handoff will occur.And the handoff delay will be produced with the voice data disconnection caused by the handoff.If the handoff delay is too large,it will affect the QoS of the VoIP service.This paper analyzed the causes and factors that influence the handoff delay.In order to reduce the handoff delay,this paper proposed two methods,the pre-scanning Fast Handoff Scheme(PSFH)and the behavior-prediction-based fast handoff scheme(BPFH).Stations with BPFH can record the handoff sequence information in the cashe,and predict the next AP using Markov process model and exponential weighted moving average model.In simulations,BPFH can effectively reduce the handover delay,while reducing unnecessary short handoffs.Especially for mobile stations with regular routing,BPFH can accurately predict next APs on its behavior path.Voice capacity in wireless networks is important for VoIP over wireless network.This paper analyzed voice capacity in constant bit rate,variable bit rate,RTS/CTS and retransmission cases respectively,and proposed capacity model.Simulations are done to analyze the factors of VoIP capacitly.The voice capacity is limited by the imbalance of wireless down and up traffic.Although IEEE 802.11e protocol provides support for the QoS of real-time service,it does not optimize the parameters to improve the voice capacity.So this paper proposed an optimization model based on the delay and packet loss rate constraints.This model distinguishes the AP from stations by CWmin,AIFS and TXOPLimit to relieve bottlenecks in the AP and increase the voice capacity.Call admission control(CAC)is an effective way to protect QoS.This paper proposed a call admission conctol scheme by dynamically adjusting resources.When wireless resources are tight,VoIP clients downgrade QoS level by increasing the packetization interval to provide resoueces for new calls and handoff calls.When the resources are idle,VoIP clients upgrade QoS level by decreasing the packetization interval to improve bandwidth utilization.In simulations,the proposed CAC scheme can protect the QoS of the current calls,minimize the wasted bandwidth,increase capacity and support handoff calls.Finally,this paper proposed a VoIP-based railway freight dispatching telephone system and analysed its performace on the testbed.