An experimental performance analysis of MAC multicast in 802.11b networks for VoIP traffic

In this paper, we present experimental studies on the use of MAC multicast in IEEE 802.11b wireless networks. Our experiments show that the maximal throughput achievable in such networks is 1.76 Mbps for an 802.11 Data Rate of 2 Mbps. We study the behavior of VoIP traffic over MAC multicast in such networks in multihop scenarios with and without hidden stations. The maximal number of hops for VoIP traffic over 802.11 MAC multicast is four as critical loss ratios are approached for more than four hops. While loss turns out to be problematic, round trip time and delay characteristics of the VoIP streams remain acceptable in all scenarios. With hidden stations, VoIP traffic cannot be relayed with acceptable loss ratios at all. Thus, although VoIP traffic can tolerate some frame loss, MAC multicast can in general only be used if additional higher-layer mechanisms are in place to mitigate MAC frame loss. We also study the effect of an additional random backoff collision avoidance mechanism in hidden station situations.