Effect of Transmission Opportunity and Frame Aggregation on VoIP Capacity over IEEE 802.11n WLANs

Wireless Fidelity (WiFi) networks have gained unprecedented growth in recent years and have played an important role in the popularity of wireless VoIP. IEEE 802.11n is the most widely used WiFi standard today. Transmission Opportunity (TXOP) and Frame Aggregation (FA) are two important Medium Access Control (MAC) Layer enhancements provided by IEEE 802.11n standard. In this work our focus is on the determination of optimal values of TXOP and FA that maximize VoIP capacity. We first determine the optimal value of FA that maximizes VoIP capacity. Our simulation results show that optimal value of FA that maximizes VoIP capacity is 14. At 10 ms packetization interval this value of FA provides a gain of 26% in VoIP capacity as compared to the VoIP capacity with no FA. Secondly, we find that the optimal value of TXOP that maximizes VoIP capacity is 13. At 10ms packetization interval this value of TXOP gives a gain of 32% in VoIP capacity as compared to VoIP capacity with default value of TXOP. We then determine the VoIP capacity when optimal values of TXOP and FA are simultaneously used. Our study reveals that simultaneous use of optimal values of both FA and TXOP further increase VoIP capacity. A gain of 44% in VoIP capacity is achieved when optimal values of TXOP and FA are used simultaneously as compared to the VoIP capacity with default values of both FA and TXOP. We further determine VoIP capacity over User Datagram Protocol (UDP) and TCP Friendly Rate Control (TFRC) protocol in the presence of Transmission Control Protocol (TCP) traffic. We note that in the presence of TCP traffic, TFRC with optimal values of TXOP and FA provides an average gain of 37% as compared to TFRC with default values of FA and TXOP.