A modified time synchronization function in IEEE 802.11 using differentiated contention window

Time synchronization is an important design issue of wireless communication for time critical applications, in which the order or the simultaneously launching of events is of great interest. The IEEE 802.11 standard for wireless LAN specified a distributed coordinated function (DCF) mode, which is designed for the ad-hoc communication. Time synchronization function (TSF) has been defined as an essential component to support the frequency hopping spread spectrum (FHSS) physical layer and the power saving (PS) mode. In the PS mode specified for DCF, it requires that all the participating stations switch to active state and use contention based backoff mechanism to transmit the time synchronization beacon during each target beacon transmission time (TBTT). This paper presents a modification to the existing TSF for DCF by using differentiated backoff counter ranges according to the clock accuracy of stations. Using the proposed mechanism, the probability of collision among beacons can be significantly reduced. So less time is required for the successful transmission of beacon frames, which, in term of energy efficiency, is equivalent to less energy consumption. Furthermore, the clocks can be converged quickly and a smaller clock deviation can also be achieved compared with previous approaches.