Abstract— 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. Index Terms— PPC, 802.11 MAC, collision avoidance, mesh network I. INTRODUCTION Compared to single-hop access-point-based networks, i.e., wireless local area networks (wireless LANs), wireless mesh networks present greater complexity. Their features of arbitrary and diverse topologies and multi-hop connections lead to more complicated medium access and thus mutual interference. Therefore, wireless mesh networks need more sophisticated medium access control and interference management mechanism than conventional wireless LAN. The most dominant technology to realize wireless mesh networks is the IEEE 802.11 [10]. Its ubiquity has made it the first choice for this new generation of wireless access networks although it was originally designed for wireless LANs. Indeed, when directly employed in a wireless mesh network, IEEE 802.11 MAC does not utilize the full network capacity. Thus, improvements have been proposed to meet the needs of wireless mesh networks. In this paper, we identify a problem of the RTS/CTS/DATA/ACK 4-way handshake of 802.11 MAC, called persistent pseudo-clearance (PPC), which causes a large number of data frame losses and retransmissions. Consequently, the link layer error may propagate to upper layers of the network protocol stack, resulting in routing errors or TCP sending agent back-off, and thus triggering further performance degradation. Essentially, PPC occurs when two sending/receiving pairs interfere with each other so that repetitive frame losses at the MAC layer are experienced. We notice that, despite the random backoff behavior of the senders regulated by the 802.11 MAC, such a cycle of losses can well last long enough to exhaust the maximum number of retransmits. If a mesh Internet gateway [3] is one of the senders involved in PPC, its overall capacity can be significantly impaired since delivering PPC-related frames costs excessive resources and may block backlogged frames of other flows from being serviced. To fully utilize the relaying capacity of mesh gateways, we explore using a multicast-based RTS, i.e., MRTS [16][18][11]. MRTS is a powerful extension to the existing 802.11 MAC, originally proposed to transmit frames opportunistically and to overcome the head-of-line (HOL) blocking problem. Here, we show that MRTS can effectively cope with PPC. Further, we cycle-shift the receiver addresses contained in the MRTS frame to poll other available receivers alternatively. This solution is thus called Shift-MRTS, or SMRTS. In the rest of this paper, we first describe PPC and the conditions for it to occur in Section II. We then review related work that attempts to solve problems similar to PPC but from the aspect of network performance improvement in Section III. In Section IV, we provide our solutions to PPC, MRTS and SMRTS, where we elaborate the rationale of extending RTS with multicasting capabilities. To verify the effectiveness of our methods, we design experiments and show the results. Potential extensions of this work are described in Section V. Persistent Pseudo-Clearance Problem in IEEE802.11 Mesh Networks and its Multicast Based Solutions Jian Zhang 1 Yuanzhu Peter Chen 2 Ivan Marsic 1 1 Department of Electrical and Computer Engineering, Rutgers University 2 Department of Computer Science Memorial University of Newfoundland Proceedings of the 2007 15th IEEE Workshop on Local and Metropolitan Area Networks 1-4244-1100-9/07/$25.00 ©2007 IEEE 1