Several studies have shown that multi-hop networks using IEEE 802.11 wireless LANs, based on a RTS/CTS MAC, exhibit significant throughput degradation. A reason for this degradation is that a RTS/CTS based MAC does not allow simultaneous transmissions, even if these are ideally feasible. This paper presents the design of MACA-P, a RTS/CTS based MAC protocol that supports simultaneous transmissions in ad-hoc wireless networks. When no transmitter (tx) node is a neighbor of a receiver (rx) node, MACA-P strives to support parallel transmissions even when some of the rx nodes are neighbors or some of the tx nodes are neighbors. The paper first specifies the conditions under which such parallel transmissions are possible. MACA-P contains a contention-based reservation phase to reserve the channel for a subsequent data transmission interval. A key feature of MACA-P is that the data transmission interval does not occur immediately after the reservation phase but is delayed by a control phase interval. This phase allows multiple sender-receiver pairs to synchronize their data transfer and ensures that neighboring nodes coordinate their reception or transmission times in a distributed manner such that multiple transmission do not cause a collision at a receiver. In addition to the RTS and CTS messages, MACA-P uses a new RTS' control message that is used to either alert neighboring nodes of a re-alignment of the data transmission interval or turn down a proposed re-alignment by a data receiver.
By: Arup Acharya, Archan Misra, Sorav Bansal
Published in: RC22325 in 2002
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