Robust Floor Acquisition in the Presence of Multiple Fading Channels

Abstract

In this paper, we investigate the impact of multipath fading on the floor acquisition mechanism of IEEE 802.11, and exploit multi-channel diversity to combat its adverse effects. We first provide experimental data to show that multi-path fading effects observed at the MAC layer appear at timescales that can interact negatively with the RTS-CTS-DATA-ACK handshake. These multi-path fading effects often result in control packets being destroyed, and cause the handshaking mechanism to malfunction. We identify two problems that result from this negative interaction, viz., the fading exposed and fading hidden terminal problems. Testbed measurements indicate that these multipath fading effects are statistically uncorrelated across different frequency channels. The presence of such multiple independent channels provides an additional degree of freedom to mitigate the negative impact of multipath fading. We exploit this multi channel diversity to design SIX, a state information exchange protocol that distributes MAC state information among neighbors. This information can be used to undo mistakes made in setting the IEEE 802.11 network allocation vector (NAV), a procedure we term NAV overriding. Furthermore, the transmitter can use this information to do receiver selection to the neighbor with the best link condition. These mechanisms combine to provide a robust floor acquisition mechanism that exploits MAC layer diversity in lossy environments. We evaluate the effectiveness of the SIX scheme with ns-2 simulations, and show that it can successfully harness multi-channel diversity to achieve reliable floor acquisition and improve performance as compared to the baseline IEEE 802.11 MAC.