Save-Energy VoIP over Wireless LANs

The key idea of saving energy of the wireless interface is to allow it to sleep as much as possible reducing the time spent in idle state. There is typically about one order of magnitude difference between the power consumption in the idle and sleep states. This is a difficult problem since the radio may not know when exactly it has to wake up to receive incoming packets and will lose them if it stays in the sleep state. A standardized solution to this issue is the Power Save Mode (PSM) which was introduced in the IEEE 802.11 standard for infrastructure WLANs. Emerging dual-mode phones incorporate a Wireless LAN (WLAN) interface along with the traditional cellular interface. The additional benefits of the WLAN interface are, however, likely to be outweighed by its greater rate of energy consumption. This is especially of concern when real-time applications that result in continuous traffic, are involved. WLAN radios typically conserve energy by staying in sleep mode. With real-time applications like Voice over Internet Protocol (VoIP), this can be challenging since packets delayed above a threshold are lost. Moreover, the continuous nature of traffic makes it difficult for the radio to stay in the lower power sleep mode enough to reduce energy consumption significantly. In this work, we propose the Green Call algorithm to derive sleep/ wake-up schedules for the WLAN radio to save energy during VoIP calls while ensuring that application quality is preserved within acceptable levels of users. We evaluate Green Call on commodity hardware and study its performance over diverse network paths and describe our experiences in the process. We further extensively investigate the effect of different application parameters on possible energy savings through trace-based simulations. We show that, in spite of the interactive, real-time nature of voice, energy consumption during calls can be reduced by close to 80 percent in most instances. Index Terms—Voice over IP (VoIP), wireless LANs, energy consumption, portable communication devices, Internet.