Calculating Relative Clock Drifts Using IEEE 802.11 Beacons

This paper proposes an approach to calculate the relative clock drift occurring between two communicating nodes in an WLAN system using IEEE 802.11 beacon frames. The IEEE 802.11 beacon frames contain a timestamp field that tells the time at which the packet was sent from the transmitting node. If a IEEE 802.11 packet is received, the WLAN card stores a timestamp that precisely depicts the time at which the packet was received. Using these timestamps, we developed an algorithm that calculates the relative clock drift between two nodes and conducted many experimental measurements using off-the-shelf WLAN cards showing the clock drifts under different circumstances. Knowing the relative clock drifts is important for two-way time-of-arrival (TOA) location tracking.