Abstract-In multi-hop ad hoc networks, stations may pump more traffic into the networks than can be supported, resulting in high packet-loss rate, re-routing instability and unfairness problems. This paper shows that controlling the offered load at the sources can eliminate these problems. To verify the simulation results, we set up a real 6-node multi -hop network. The experimental measurements confirm the existence of the optimal offered load. In addition, we provide an analysis to estimate the optimal offered load that maximizes the throughput of a multi-hop traffic flow. We believe this is a first paper in the literature to provide a quantitative analysis (as opposed to simulation) for the impact of hidden nodes and signal capture on sustainable throughput. The analysis is based on the observation that a large-scale 802.11 network with hidden nodes is a network in which the carrier-sensing capability breaks down partially. Its performance is therefore somewhere between a carrier-sensing network and an Aloha network. Indeed, our analytical closed-form solution has the appearance of the throughput equation of the Aloha network. Our approach allows one to identify whether the performance of an 802.11 network is hidden-node limited or spatial-reuse limited.

This study attempts to exploit the potential of link directionality to increase the achievable capacities of ad hoc networks. When an IEEE 802.11 ad hoc network achieves capacity C by using a single channel, the targeted capacity by using two channels should be 2C. However, most of the dual-channel 802.11 protocols proposed in the literature appear only to be able to achieve less than 60% of the 2C targeted capacity. The authors thus propose a link-directionality-based dual-channel medium access control protocol in an attempt to double the capacities of networks using the single-channel IEEE 802.11 protocol. The main idea is to assign channels according to link directionality to allow a link to transmit simultaneously within the carrier-sensing region of another link provided that these transmissions do not interfere with each other. Simulations show that our proposed scheme can achieve more than 85% of our targeted capacities, 0.85 Â 2C ¼ 1.7C, in large-scale random topologies. In lattice and irregular topologies, the throughput is boosted up to 2.83C and 2.13C, respectively. An approach for capacity analysis is also introduced to determine the throughput improvements that can be achieved by our proposed protocol. We believe using link directionality for channel allocations is a key step that yields significant potential for multiplying the capacity of ad hoc networks.

Interferences in an ad-hoc network can be defined as a set of constraints that specify which groups of nodes cannot transmit simultaneously, and they have significant implications for the network capacity and other performance measures. This paper expounds the difference between two types of interferences: 1) physical interferences due to the receiver's inability to decode a signal when the powers received from other signals are large; and 2) protocol interferences imposed by the specific multi-access protocol being used to coordinate transmissions of nodes. We model interference types 1 and 2 in terms of graphs. Based on the insights obtained, we devise a scheme that modifies 802.11 slightly to make its capacity scale with the number of nodes.

In this paper, we propose a link-directionality-based dual channel MAC protocol in an attempt to double the capacities of networks using the single-channel IEEE 802.11 protocol. When an IEEE 802.11 ad-hoc network achieves capacity C by using a single channel, the targeted capacity by using two channels should be C ⋅ 2 . However, most of the multichannel 802.11 protocols proposed in the literature only appear to be able to achieve less than 60% of the C ⋅ 2 targeted capacity. Simulations show that our proposed scheme can achieve more than 106% of our targeted capacities, 1.06* C ⋅. We believe this is a first paper in the literature to propose a MAC protocol to transmit RTS/DATA and CTS/ACK of a link on different channels, a key step that yields significant potential for multiplying the network capacities of ad-hoc networks.