Potential pitfalls of the message in message mechanism in modern 802.11 networks

We study the performance impact of the Message in Message (MIM) mechanism in modern 802.11 networks. The MIM mechanism refers to the capability of receiver to abandon an ongoing reception of an 802.11 MAC frame and shift to decode another frame with a higher signal strength. MIM is a common feature in modern 802.11 adapters and it has been shown to improve spatial concurrency. However, our measurement study in a campus WLAN shows that under certain conditions, MIM could cause a throughput degradation of more than 30% when enabled, instead of improving it as expected. With comprehensive experiments using commercial 802.11n adapters, we characterize the impact of MIM for a range of parameters and for different scenarios. We show that a simple adaptive MIM scheme can potentially achieve throughput that is close to the optimal. Our method is practical because it can be easily implemented in existing commodity 802.11 adapters.