LLE: A timer extension mechanism for alarm-triggered traffic in IEEE 802.11ah WLANs

摘要

Traffic generated by sensors can be classified into three types, periodic, on-demand, and alarm-triggered. In an IEEE 802.11ah WLAN, AP needs to support up to 8,000 sensors. To alleviate the channel contention, two new mechanisms, target wake time (TWT) and restricted access window (RAW), are introduced to facilitate channel reservation. While TWT is tailor-made for periodic traffic, a RAW-based paging scheme is designed in this paper for contention-free scheduling of on-demand traffic. Notably, alarm events (e.g., fire) are unpredictable and yet alarm reporting is time-critical. It is thus inefficient to handle alarm-triggered traffic by a channel reservation scheme. To this end, we propose to enhance the contention-based DCF by using a standard-compliant timer mechanism called limited local extension (LLE). With LLE, when a sensor with a pending alarm-report detects an alarm-report sent by another sensor, it extends its own backoff timer by a random duration. The random duration is chosen such that the total amount of extension is bounded by the current contention window size. Based on an existing alarm event propagation model, simulation results show that LLE can greatly shorten the alarm-reporting time.