On the Scalability of IEEE 802.11 Ad Hoc Networks

The IEEE 802.11 standards support the peer-to-peer mode Independent Basic Service Set (IBSS), which is an ad hoc network with all its stations within each other's transmission range. In an IBSS, it is important that all stations are synchronized to a common clock. Synchronization is necessary for frequency hopping spread spectrum (FHSS) to ensure that all stations "hop" at the same time; it is also necessary for both FHSS and direct sequence spread spectrum (DSSS) to perform power management. This paper evaluates the synchronization mechanism, which is a distributed algorithm, specified in the IEEE 802.11 standards. By both analysis and simulation, it is shown that when the number of stations in an IBSS is not very small, there is a non-negligible probability that stations may get out of synchronization. The more stations, the higher probability of asynchronism. Thus, the current IEEE 802.11's synchronization mechanism does not scale; it cannot support a large-scale ad hoc network. To alleviate the asynchronism problem, this paper proposes a simple modification to the current synchronization algorithm. The modified algorithm is shown to work well for large ad hoc networks