Wifi Unfairness Demonstration Site

Motivation and Objectives

Yigal Bejerano, Hyoung-Gyu Choi and Seung-Jae Han


The MAC (Medium Access Control) protocol of IEEE 802.11 Wireless Local Area Networks (also called WiFi Networks) is designed to provide long-term fairness to all stations also when using different transmission bit-rates. In reality, however, the long-term fairness of IEEE 802.11 Networks may be compromised, even if the stations transmit with the same bit rate. In the case of sufficient discrepancy between the strength of the received signals at the access point (AP) severe long-term unfairness may occur due to physical layer capture also known as capture effect.

Unfair service due to the capture effect occurs when several stations simultaneously attempt to send packets to a common access point (AP). In the event of a packet collision, if the received signal strength of one packet is sufficiently stronger than the strength of the other packets, then the AP can correctly decoded this packet despite the collision. As a result, a stronger signal station will experience higher throughput than weaker signal stations. After a packet loss, IEEE 802.11 MAC slows down the channel access attempts by increasing the size of contention window, which further aggravates the unfairness between a stronger signal stations and the weaker signal ones. Since the received signal strength is inversely proportional to the station distance from its servicing AP. we refer to this phenomenon as spatial unfairness.


This web-application demonstrates the unfair services that IEEE 802.11 networks offer to their users in the case of dominant upstream traffic. This spatial unfairness, which results from the interaction between the capture effects and IEEE 802.11 MAC protocol, is experimentally evaluated explained and analyzed in our paper, "Fairness Analysis of Physical Layer Capture Effects in IEEE 802.11 Networks" .

The web-site allows you to experiment with our analytical model and our realistic simulator, which take into account besides the IEEE 802.11 MAC protocol also the main wireless phenomena such as path-loss and shadowing. You may select various user locations as well as and wireless network characteristics and compare the results that are obtained from our simulation against various analytical model.