Simulation Analysis of the IEEE 802.11e EDCA Protocol for an Industrially-Relevant Real-Time Communication Scenario

The IEEE 802.11e standard was published as an amendment to the original IEEE 802.11 standard. This amendment is intended to provide differentiated levels of QoS to the supported applications. The 802.11e amendment incorporates an additional coordination function called hybrid coordination function (HCF) that uses both a contention-based channel access method, called the enhanced distributed channel access (EDCA) and a controlled channel access, referred to as the HCF controlled channel access (HCCA). Under the EDCA mechanism, it is a common assumption to consider the highest access category (voice) adequate to support real-time communication. In this paper, we analyze the timing behavior of the EDCA function, when it is used to support real-time traffic. Basically, we assess the behavior of the voice category in open communication environments (i.e., a communication environment subject to external disturbances) when this access category is used to transfer small sized packets, generated in periodic intervals. We show that the transmission opportunity (TXOP) mechanism included in the IEEE 802.11e amendment improves the system throughput, for the case of message streams with small packet sizes. However, the impact of external disturbances upon the transfer of real-time messages is highly relevant. For instance, the average access delay for the real-time messages is more than one order of magnitude larger when the external disturbance increases the network load from just 10% to 30%. Furthermore, both the number of packet losses and the average size of the MAC queues forecast an unacceptable number of deadline losses for the real-time message streams, even for intermediate load cases.