Nattavut Smavatkul

Power management for VoIP over IEEE 802.11 WLAN

Power consumption is a critical issue for voice-over-IP (VoIP) over wireless LAN applications, especially when handheld 802.11 voice terminals are used as the end-user device. This paper provides an overview of power save procedures defined in the IEEE 802.11 wireless LAN standard, and proposes a novel power management technique for stations and services that use contention-based channel access, which we call unscheduled power save delivery (UPSD). We further investigate the performance of UPSD in combination with the eDCA mechanism defined in the 802.11e draft standard via an OPNET-based simulator. The results indicate that UPSD permits a lower duty cycle and delivers better VoIP capacity than legacy techniques.

Voice capacity evaluation of IEEE 802.11a with automatic rate selection

This paper analyzes the transmission of voice and isochronous traffic in general over an 802.11a wireless local area network. In particular, we provide a simple analytic technique for estimating the capacity of an 802.11a access point under the contention-based access method when all stations can select an optimal transmission rate using automatic rate selection. Capacity is additionally estimated using a wireless LAN (WLAN) system simulator that models contention-based access in a noise limited channel, in which packet erasures are modeled using quasi-static link simulation techniques. The impact of automatic rate selection on capacity is evaluated. Results from the analytic and simulation based methods of estimating capacity are compared. In addition, impact on voice capacity from data traffic is studied.

Optimization of a link adaptation algorithm for voice over wireless LAN applications

Auto rate fallback (ARF) is a link adaptation algorithm that uses layer-2 acknowledgement messages to make decisions to increase or decrease the transmission rate. The algorithm is configured by choosing thresholds to define its behavior. The ARF algorithm is applied to voice-over-IP users in an IEEE 802.11a wireless LAN system, and system simulation is used to determine the optimal ARF configuration. Configuration performance is judged using a unique voice quality metric, which is used to establish a measure of system capacity for a single access point with several voice users. Results indicate that the ARF configuration can significantly impact capacity, and one configuration of ARF provides a capacity within 13% of the available capacity.

Link adaptation evaluation for WLAN using a voice quality metric

Link adaptation, the process of adapting a mobile users link speed as channel conditions change, is investigated for IEEE 802.11a wireless local area networks (WLANs). An ideal algorithm using channel signal-to-noise ratio and a practical algorithm based on acknowledgement message (ACK) reception are discussed, and their performance is compared to cases without link adaptation. The analysis is performed by simulating a combination voice and data WLAN system, and voice call quality is judged using a unique quality metric. The metric contains two scores: a packet loss percentage and an estimate of the number of audible imperfections based on the pattern of packet loss. System capacity estimates result from applying the metric, and are used to compare the link adaptation schemes.