A study of Skype over IEEE 802.16 networks: voice quality and bandwidth usage

Skype, one of the popular VoIP applications, has its own redundancy mechanism to mitigate the impact of packet loss at the expense of additional bandwidth usage. However, the benefit of voice quality improvement through redundancy is reduced greatly in the case of consecutive loss of Skype packets. When Skype is running over wireless networks, the high likelihood of having consecutive packet loss in a noisy wireless environment could lead to an interesting scenario: using more bandwidth for a lower voice quality. In this thesis, we study the impact of the HARQ retransmission mechanism of WiMAX to the voice quality of Skype. Two key parameters for the HARQ mechanism of WiMAX (i.e. Maximum numbers of retransmission and the delay of sending ACK) and their interplay with voice quality are investigated in this research. While HARQ retransmission may reduce the redundancy usage in Skype, it introduces additional bandwidth usage during the retransmission. The overhead of WiMAX retransmission with different HARQ parameters and overall bandwidth usage of Skype are also discussed in this thesis. Numerical analysis and simulation of voice quality and bandwidth usage of Skype over WIMAX are presented. Results show that HARQ retransmission can reduce the impact of bursty noise and improve the voice quality of skype up to 22%. The overall bandwidth consumption can be reduced by 76% when HARQ is engaged. Combining our studies on voice quality and bandwidth usage, we attempt to obtain the optimal HARQ parameters that can lead to the highest voice quality over bandwidth usage ratio for Skype over WiMAX.