The Impact of Head of Line Blocking in Highly Dynamic WLANs

Wireless LAN (WLAN) based networks can rely on the concept of automatic repeat request to overcome temporary channel outages, such as those due to interference. However, the used acknowledgement scheme for unicast transmissions blocks other transmissions until a message has either been successfully transmitted or until the retry limit has been reached. We investigate this head of line blocking problem, its impact, and first steps toward a final solution. As a research methodology, we build upon an analytical model, computer simulations, and also experimentation in a lab environment. We consider the effects for a single node in a small and static network as well as studying the application layer performance in a highly dynamic network—we investigate Vehicular Ad Hoc Networks (VANETs) as a prime example—spanning several kilometers. Aside from considering the case of malicious users conducting easy active and passive attacks, we show that in highly dynamic networks this blocking behavior can (massively) be observed as local topologies change. Here, the discussed effects can lead to massive message loss and increased latencies in the order of seconds.