QQDCA: Adapting IEEE 802.11 EDCA for unicast transmissions at high topology dynamics

We present Multi-Queue Distributed Channel Access (QQDCA), an improved MAC scheme to mitigate the head-of-line (HOL) blocking problem observed in highly dynamic networks when Wireless LAN (WLAN) unicast (re-)transmissions are used. Focusing on Vehicular Ad Hoc Networks (VANETs) as a prime example, our novel MAC protocol can be used for all IEEE 802.11 based wireless networks having very dynamic topologies. In brief, failing unicast messages create a problem by blocking all remaining frames in the transmit queue until they are successfully acknowledged or the maximum retransmission counter expires. This HOL blocking has been identified as a rare or temporary phenomenon in the late 1990s already. However, it turned out that the problem is both much more frequent and persistent in VANETs due to their very dynamic nature. Here, it may take a substantial amount of time in which no other message can be sent. We present and discuss QQDCA to address this problem, analytically show its benefits, and evaluate the protocol in an extensive simulation study. QQDCA can reduce the delay induced by HOL blocking by up to 95 %.