On clock synchronization over wireless LAN using timing advertisement mechanism and TSF timers

摘要

The most recent version of the IEEE 802.11 standard, 802.11-2012 has extended the means of clock synchronization for wireless LANs. One of the mechanisms in the standard is called timing advertisement (TA). It involves calculating and maintaining the time offset with respect to the timing synchronization function (TSF) timer of the access point (AP). In the wireless stations, the TSF-based synchronization mechanism and the offset is combined to establish synchronization to an external timebase. However, such a method provides only loose synchronization unless the skew estimate between the external time source and the TSF timer of the AP is also conveyed to the nodes in the network, for which the standard provides no support. This study, in turn, provides another method, which is called SyncTSF, in which the external clocks at the AP and the nodes are synchronized to their respective TSF timers. Through analysis and measurements, it is shown that both SyncTSF and TA-based synchronization are limited by the frequency skews between the TSF timers of the AP and the nodes. Hence, for these methods to offer good synchronization accuracy, a mechanism to distribute frequency skews from the AP to the nodes should be provided.