An experimental study on the capture effect in 802.11a networks

TLDR

In wireless networks, collisions can still allow one frame to be received if its signal power and timing relative to others are favorable, a phenomenon known as capture effect, which prior studies have shown occurs only when the stronger frame arrives earlier or within the weaker frame’s preamble. The study aims to identify the timing, power, and bit‑rate conditions under which the capture effect occurs in IEEE 802.11a networks. The authors use an IEEE 802.11a testbed to measure capture‑effect conditions across varying timing, power differences, and bit rates. The measurements reveal that the stronger frame can be decoded regardless of timing, and when it arrives after the weaker frame, capture occurs in two distinct patterns depending on prior synchronization, with success determined by preamble detection followed by frame‑body FCS verification.

Abstract

In wireless networks, a frame collision does not necessarily result in all the simultaneously transmitted frames being lost. Depending on the relative signal power and the arrival timing of the involved frames, one frame can survive the collision and be successfully received by the receiver. Using our IEEE 802.11a wireless network testbed, we carry out a measurement study that shows the terms and conditions (timing, power difference, bit rate) under which this capture effect takes place. Recent measurement work on the capture effect in 802.11 networks [10] argues that the stronger frame can be successfully decoded only in two cases: (1) The stronger frame arrives earlier than the weaker frame, or (2) the stronger frame arrives later than the weaker frame but within the preamble time of the weaker frame. However, our measurement shows that the stronger frame can be decoded correctly regardless of the timing relation with the weaker frame. In addition, when the stronger frame arrives later than the weaker frame's arrival, the physical layer capture exhibits two very distinct patterns based on whether the receiver has been successfully synchronized to the previous weak frame or not. In explaining the distinct cases we observe that the successful capture of a frame involved in a collision is determined through two stages: preamble detection and the frame body FCS check.