Virtual Utilization and VoIP Capacity of WLANs Supporting a Mix of Data Rates

Today’s WLANs offer high data rates of up to 11 and 54 Mbps for IEEE 802.11b respective 802.11a/g making this technology very attractive for real-time applications like VoIP calls. However, only a certain part of communication can take place at high data rates. Additionally, a certain amount of overhead is introduced by the CSMA/CA-based medium access scheme. Thus, challenging questions remain how the load situation of a WLAN cell can be evaluated adequately and which costs arise if some users transmit only at low rates. In this work, we motivate a ”virtual” utilization metric for the evaluation of the current situation in a WLAN cell and investigate simulatively the influence of low-rate transmissions in a WLAN network used for pure VoIP traffic . The virtual utilization is based on the physical and virtual occupancy duration for each transmission. Since the occupancy duration is measured at the AP, which is the bottleneck of a WLAN cell, we are able to identify the resources that every user consumes for his transmissions—including the effects of transmission rates as well as technology dependent overheads. For the VoIP simulation studies, our metric is the number of calls that is servable at acceptable quality for every single user. ”Acceptable” thereby means that certain VoIP QoS constraints are not violated. For each single data rate, the VoIP capacity, i.e., the maximum number of calls, is determined at first. Afterwards, the influence of a single low-rate user is investigated and set in relation to the capacity results, which leads to the identification of costs invoked by the low-rate transmissions.