Deafness and Virtual Carrier Sensing with Directional Antennas in 802.11 Networks ; CU-CS-971-04

Inexpensive analog phase array antennas are on the verge of becoming widely available. These versatile antennas are capable of very rapidly altering their gain pattern to form complex patterns. However, previous research has shown that problems arise when using the stock 802.11 MAC protocol with directional antennas, and new MAC protocols have been designed to address these issues as well as exploit some of their new capabilites. Unfortunately, most of these protocols sacrifice interoperability with existing 802.11 equipment, making incremental deployment difficult. Even when directionally-aware protocols are interoperable with existing equipment, unanticipated problems may arise. In particular, we have found that the problem of deafness [1] to be of high importance when directional antennas are used in community networking scenarios. In this work we present a taxonomy of simple directional enhancements to the 802.11 MAC which maintain interoperability with existing equipment. We also evaluate two of these schemes in various community networking scenarios. In addition, we enhance their resistance to excessive RTS/CTS messages caused by deafness using two different schemes: RTS Validation and RTS/CTS Filtering. Furthermore, we introduce an antenna steering heuristic which trades off some spatial reuse for a decrease in deafness.