Piggybacked-Ack-aided Concurrent Transmissions in Wireless Networks

It is well-known that MAC protocols for multihop wireless networks face the problems of hidden and exposed nodes due to the limited transmission range of wireless nodes. The Distributed Coordination Function (DCF) mechanism of the IEEE 802.11 standard [2] addresses the issue of hidden node problem by employing a four-way handshake of RTS-CTSDATA-ACK frames. In this exchange, the transmitter emits RTS and DATA frames and the receiver responds with CTS and ACK frames. During this whole period, all nodes that are in the range of either the transmitter or the receiver has to remain silent so as to not interfere with the reception of DATA frame at the receiver and CTS/ACK frames at the transmitter. While this approach alleviates the hidden node problem, it does not address the exposed node problem. During a transmission A→B, it does not permit a feasible simultaneous transmission C→D by a node C which is in the range of A, even if B and D are outside the range of C and A respectively. For example, in Fig. 1, when 2→1 transmission is on-going, no other transmission is allowed even though three transmissions 2→1, 5→6, and 3→4 can happen concurrently. There have been some proposals [1,4] for enabling concurrent transmissions and thus increasing the overall throughput of a wireless network. We briefly mention two such proposals that are quite relevant to our work in the following. The MACA-P scheme proposed in [1] uses an extended control phase to synchronize the DATA and ACK phases of all concurrent transmissions to avoid the problem of DATA of a transmission and ACK of another transmission interfering with each other. To achieve this alignment, RTS and CTS frames carry the times of the scheduled DATA and ACK phases. Another scheme proposed in [4] tries to squeeze in a secondary transmission (without RTS and CTS) concurrently with the primary transmission (with RTS and CTS). This scheme requires that the size of DATA of secondary transmission is smaller than that of primary transmission,