DUCHA: A NEW DUAL-CHANNEL MAC PROTOCOL FOR MULTIHOP AD HOC NETWORKS

Abstract—IEEE 802.11 MAC protocol has been the standard for Wireless LANs and is also implemented in many simulation software for mobile ad hoc networks. However, IEEE 802.11 MAC has been shown to be quite inefficient in the multihop mobile environments. Besides the well-known hidden terminal problem and the exposed terminal problem, there also exists the receiver blocking problem which may result in link/routing failures and unfairness among multiple flows. Moreover, the contention and interference from the upstream and downstream nodes seriously decrease the packet delivery ratio of mulitihop flows. All these problems could lead to the “explosion” of control packets and poor throughput performance. In this project, we first analyze these anomaly phenomena in multihop mobile ad hoc networks. Then, we present a novel effective random medium access control (MAC) protocol based on IEEE 802.11 MAC protocol. The new MAC protocol uses an out-of-band busy tone and two communication channels, one for control frames and the other for data frames. The newly designed message exchange sequence provides a comprehensive solution to all the aforementioned problems. Extended simulations demonstrate that our scheme provides a much more stable link layer, greatly improves the spatial reuse, and works well in reducing the packet collisions. It improves the throughput by 7% to 20% for one-hop flows and by 2 5 times for multihop flows under heavy traffic comparing to the IEEE 802.11 MAC. 

INTRODUCTION
Contention based medium access control (MAC) protocols have been widely studied for wireless networks due to the low cost and easy implementation. IEEE 802.11 MAC is such a protocol that has been the standard of wireless LANs and has also been incorporated in many wireless testbeds and simulation packages for mobile ad hoc networks. It adopts four-way handshake procedures, i.e., RTS/ CTS/ DATA/ ACK. Short packets, RTS and CTS, are used to avoid collisions between long data packets. The NAV (Network Allocation Vector) value carried by RTS/ CTS/ DATA/ ACK is used to reserve the medium to avoid potential collisions (i.e., virtual carrier sensing) and hence mitigate the hidden terminal problem. The ACK is used as a confirmation of the successful transmission without errors. 

However, the effectiveness of IEEE 802.11 MAC in multihop mobile ad hoc networks has been widely recognized as a serious problem. The packet collision over the air is much more severe in the multihop environments than that in the wireless LANs. The packet losses due to such kind of MAC layer contentions will definitely affect the performance of the high layer networking schemes such as the TCP congestion control and routing maintenance because a node does not know whether an error is due to the collision or the unreachable address.

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