We explore the use of directional antennas to improve the performance of broadcasting in ad hoc networks. We investigate both the performance of unicast traffic in the presence of broadcast traffic and the performance of broadcast traffic when mixed with unicast traffic, which is different from previous investigations reported in the literature in which broadcast traffic is investigated in isolation. Through extensive simulation experiments with three MAC schemes, we show that throughput and delay can vary widely even in networks in which nodes are uniformly distributed. We also show that the use of a MAC protocol that utilizes directional antennas can help to improve the performance of broadcast traffic in ad hoc networks, in terms of both throughput and delay, through a more aggressive channel access scheme that maximizes spatial reuse.

Three collision-avoidance protocols are analyzed that use omni-directional packet reception together with omni-directional transmissions, directional transmissions, or a combination of both. A simple model is introduced to analyze the performance of these collision avoidance protocols in multi-hop networks with arbitrary topologies. The numerical results of this analysis show that collision avoidance using a narrow antenna beamwidth for the transmission of all control and data packets achieves the highest throughput among the three collision avoidance schemes considered. Simulation experiments of the popular IEEE 802.11 MAC protocol and its variants based on directional transmissions and omni-directional packet reception validate the results predicted in the analysis. The results further show that narrow-beamwidth transmissions can also reduce the average delay experienced by nodes. It is concluded that the advantage of spatial reuse achieved by narrow-beamwidth transmissions outweighs that of conservative collision avoidance schemes featured by the omnidirectional transmission of some control packets. This is due to the fact that the latter requires far more stringent coordination of nodes with their neighbors and hidden terminals, which can lead to much more channel resource wasted due to nodes' excessive waiting time.

This paper studies the fairness with which competing flows share the channel in ad hoc networks using collision avoidance protocols. It is shown that the required multihop coordination makes the backoff-based distributed fair queueing schemes less effective. Using extensive simulations of two competing flows with different underlying network configuration, it is shown that the commonly used flow contention graph is insufficient to model the contention among nodes and that various degrees of unfairness can take place. The fairness problem is more severe in TCP-based flows due to the required acknowledgment traffic, and TCP throughput is also negatively affected. A measurement based fair scheme is analyzed in which nodes estimate their fair share of the channel from overheard traffic and adjust their backoff window accordingly (voluntarily); it is shown that such a scheme achieves much better fairness but sacrifices too much throughput. These results indicate that more explicit information exchange among contending nodes is mandatory to solve the fairness problem conclusively while maintaining reasonable throughput.

A novel hybrid channel access scheme that combines sender-initiated collision-avoidance handshakes is proposed for multi-hop ad hoc networks. The new scheme is compatible with the popular IEEE 802.11 MAC protocol and involves adding very simple queue management and bookkeeping work mechanisms. Simulation experiments show that the new scheme can alleviate the fairness problems existing in applications running on either UDP or TCP with almost no degradation in throughput. More importantly, it is also shown that without explicit information exchange among nodes, the fairness problem cannot be solved conclusively.

The quest for efficient medium access control (MAC) protocols for multi-hop ad hoc networks has aroused great interest in using directional antennas. Some MAC protocols using directional antennas have been proposed in the past; they trade off spatial reuse and collision avoidance via a combination of omnidirectional and directional transmission modes. It is argued that the benefit of spatial reuse achieved by a MAC protocol that uses a directional mode in all transmissions can outweigh the benefit of a conservative collision avoidance MAC protocol that sends some omni-directional control packets to silence potential interfering nodes. We present detailed simulation experiments of the popular IEEE 802.11 MAC protocol and its variants that make use of a directional transmission mode in sufficiently random networks. It is concluded that, in contention-based MAC protocols for multi-hop networks infested with hidden terminals, the aggressive channel access scheme featured by all-directional transmissions indeed outperforms other conservative schemes in terms of enhanced throughput and reduced delay.

The paper presents the first analytical model to derive the saturation throughput of collision avoidance protocols in multi-hop ad hoc networks with nodes randomly placed according to a two-dimensional Poisson distribution. We show that the sender-initiated collision-avoidance scheme performs much better than the ideal CSMA scheme with a separate channel for acknowledgments. More importantly, we show that the collision-avoidance scheme can accommodate much fewer competing nodes within a region in a network infested with hidden terminals than in those cases without hidden terminals or with just a few, if reasonable throughput is to be maintained. Simulations of the popular IEEE 802.11 MAC protocol show that it cannot ensure collision-free transmission of data packets and thus throughput can degrade well below what is predicted by the analysis of a correct collision avoidance protocol. Based on these results, a number of improvements are proposed for the IEEE 802.11 MAC protocol.

Due to hidden terminals and a dynamic topology, contention among stations in an ad-hoc network is not homogeneous. Some stations are at a disadvantage in opportunity of access to the shared channel and can suffer severe throughput degradation when the traffic load is high. This is the so-called "fairness problem" encountered mainly in existing MAC protocols such as IEEE 802.11 Distributed Foundation Wireless Medium Access Control (DFWMAC) that is used as a basis for most performance evaluation of routing protocols in mobile ad-hoc networks (MANET). The binary exponential backoff (BEB) used in DFWMAC protocol to resolve contention is known to always favor the last station that succeeds its transmission, which exacerbates the fairness problem in heavy traffic. This paper reviews a measurement-based backoff algorithm for the DFWMAC protocol that achieves statistical fair access to the shared medium, and then models analytically the resultant system as a feedback control system. The analytical model confirms the fairness property of the algorithm, as well as its convergence and show the impact of different parameters of the algorithm on the performance of the system.

The medium access control (MAC) protocol through which mobile stations can share a common broadcast channel is essential in an ad-hoc network. Due to the existence of the hidden terminal problem, partially-connected network topology and lack of central administration, existing popular MAC protocols like IEEE 802.11 Distributed Foundation Wireless Medium Access Control (DFWMAC) may lead to "capture" effects which means that some stations grab the shared channel and other stations suffer from starvation. This is also known as the "fairness problem". This paper reviews some related work in the literature and proposes a general approach to address the problem. This paper borrows the idea of fair queueing from wireline networks and defines the "fairness index" for ad-hoc network to quantify the fairness, so that the goal of achieving fairness becomes equivalent to minimizing the fairness index. Then this paper proposes a different backoff scheme for IEEE 802.11 DFWMAC, instead of the original binary exponential backoff scheme. Simulation results show that the new backoff scheme can achieve far better fairness without loss of simplicity.