Performance of VoIP over IEEE 802.11G DSSS-OFDM mode with IEEE 802.11E QOS support

This paper examines, via simulation, the performance of an 802.11e MAC over an 802.11g PHY operating in DSSS-OFDM mode. The DSSS-OFDM scheme provides data rates of up to 54Mb/s as well as interoperability with 802.11b nodes. Due to the widespread use of 802.11b nodes, such interoperability is an important consideration. This paper involves a study of the number of simultaneous bidirectional G.711 VoIP calls that can be supported by such a WLAN. The results show that this mode of operation introduces a very significant overhead. The actual number of calls that can be carried is limited to 12 when using the 24Mb/s data rate and 13 when using either the 36Mb/s or 54Mb/s rates. These results demonstrate the well-known disparity between uplink and downlink performance, with the downlink imposing the limit on the number of calls that can be carried by the system in the cases studied. The results also show that when when a significant amount of lower priority traffic is introduced into the system, it can have a significant impact on VoIP call capacity despite the use of 802.11e.