Identifying Channel Saturation in Wi-Fi Networks via Passive Monitoring of IEEE 802.11 Beacon Jitter

Every day large numbers of users connect to IEEE 802.11 networks in order to access the Internet and all sorts of services. However, due to their unplanned and unregulated nature, and the lack of admission control and Quality of Service Guarantees, these wireless networks can experience traffic demand that exceeds the network capacity. In this case, if a device tries to send more traffic, or if a new device joins the network, the aggregate throughput does not necessarily increase. In this paper we show that it is possible for IEEE 802.11 stations to detect a saturated channel by passively monitoring the beacon frames. Access points (AP) send beacon frames periodically and encode them using the strongest modulation and coding scheme, so that even stations far away from the sending APs can decode them correctly. When sending beacons, APs sense the channel first and, if it is busy, delay sending the frame, resulting in unevenly spaced beacon frames, whenever other transmitters are active. We present several experiments, under varying traffic loads, and analyze the distribution of the beacon jitter, whose variance increases as the offered load increases. We show that it is possible to determine, with an acceptable error rate, whether a channel is saturated by comparing the distribution of the beacon jitter with a reference distribution corresponding to a saturated channel.