Persistent Pseudo-Clearance Problem in IEEE802.11 Mesh Networks and its Multicast Based Solutions

Wireless mesh networks are flexible solutions to extend services from wireless LANs. The current IEEE 802.11 Specification, however, needs to be modified in various ways to be a suitable technology for this purpose. In particular, in order to handle the well-known hidden node problem (HNP), the Specification adopts MACAW by employing an RTS/CTS/DATA/ACK 4-way handshake. Some flaws of this scheme have been noticed, e.g. the Masked Node Problem (MNP). In this work, we identify a critical problem of the Specification's 4-way handshake, called persistent pseudo-clearance (PPC). PPC occurs when for two sender/receiver pairs a CTS from one pair's receiver collides with the DATA frames of the other pair. This logjam can persist for a period of time despite of the random backoff the senders employ. The persistent frame losses in PPC can cause more serious problems. The effect of giving up a frame transfer after reaching the maximum number of retries can propagate to upper layers, causing routing errors or TCP sender backoff. Multicast RTS (or MRTS) provides a good solution framework to break the cycle of losses and retransmissions between such peers. With minimal modification to MRTS, we provide an effective and efficient solution to PPC. Our experiments show that MRTS breaks the logjam of PPC while fully utilizing the network capacity.