Reducing Voice over WLAN Call Setup Delay under Limited Access Point Buffer

Abstract

Voice over WLAN (VoWLAN) service is regarded as one of the killer applications for public and private WLANs' but it suffers from a number of technical challenges, especially the power consumption problem. In order to conserve the energy of a VoWLAN device during idle, the power saving mode (PSM) defined in the IEEE 802.11 is widely employed. Experimental results indicate that 300ms to 1000ms listen intervals of PSM for a VoWLAN device could achieve better energy efficiencies. Unfortunately, low-end WLAN access points (APs) typically have limited memory spaces, and can buffer incoming packets for a small time window, say 100ms. For a VoWLAN device requesting long listen intervals, packets queued on the APs might be dropped or overwritten when the buffer overflows. This phenomenon introduces considerable call setup delays for session initiation protocol (SIP) that implements the exponential retransmission mechanism of lost messages. This study investigates the problem and proposes three approaches to reduce call setup delays for VoWLAN applications. Simulation results demonstrate that the proposed mechanisms reduce call setup delays significantly for low-power VoWLAN devices.