A cross-layer approach to increase spatial reuse and throughput for ad hoc networks

Ad hoc networks employing adaptive-transmission protocols can alter transmission parameters to suit the channel environment. Channel-access mechanisms are used to govern temporal use of the transmission medium amongst nodes. Effective operation of a channel-access mechanism can improve the ability of an adaptive-transmission protocol to accommodate changing channel conditions. The interoperability of these two mechanisms motivates cross-layer design of adaptive-transmission protocols. In this thesis we examine the integration of a new channel-access mechanism with a physical-layer adaptive-transmission protocol to create a cross-layer protocol with enhanced capabilities. We derive specific physical-layer measurements which are used to control channel-access behavior in a distributed manner. We propose a distributed heuristic using cross-layer information to drive a channel-access protocol which works in conjunction with an adaptive-transmission protocol. We show that the new protocol outperforms statically configured transmission protocols as well as protocols which act independently of cross-layer enhancements

Measurement and analysis of real-world 802.11 mesh networks

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2010.Includes bibliographical references (p. 63-65).Despite many years of work in wireless mesh networks built using 802.11 radios, the performance and behavior of these networks in the wild is not well understood. This is primarily due to a lack of access to data from a wide range of these networks; most researchers have access to only one or two testbeds at any time. In recent years, however, these networks have been deployed commercially and have real users who use the networks in a wide range of conditions. This thesis analyzes data collected from 1407 access points in 110 different commercially deployed Meraki wireless mesh networks, constituting perhaps the largest study of real-world 802.11 mesh networks to date. After analyzing a 24-hour snapshot of data collected from these networks, we answer questions from a variety of active research topics, including the accuracy of SNR-based bit rate adaptation, the impact of opportunistic routing, and the prevalence of hidden terminals. The size and diversity of our data set allow us to analyze claims previously only made in small-scale studies. In particular, we find that the SNR of a link is a good indicator of the optimal bit rate for that link, but that one could not make an SNR-to-bit-rate look-up table that was accurate for an entire network. We also find that an ideal opportunistic routing protocol provides little to no benefit on most paths, and that "hidden triples"-network topologies that can lead to hidden terminals-are more common than suggested in previous work, and increase in proportion as the bit rate increases.by Katrina L. LaCurts.S.M

Channel-Access and Routing Protocols for Wireless Ad Hoc Networks with Directional Antennas

Medium-access control (MAC) and multiple-hop routing protocols are presented that exploit the presence of directional antennas at nodes in a wireless ad hoc network. The protocols are designed for heterogeneous networks in which an arbitrary subset use directional antennas. It is shown that the new protocols improvement the network`s performance substantially in a wide range of scenarios. A new MAC protocol is presented that employs the RTS/CTS mechanism. It accounts for the constraints imposed by a directional antenna system, and it is designed to exploit the capabilities of a directional antenna. It is shown that the receiver blocking problem is especially detrimental to the performance if the network includes nodes with directional antennas, and a simple solution is presented. A further improvement to the MAC protocol is presented which results in more efficient spatial reuse of traffic channels in the heterogeneous network. The protocol includes a mechanism by which a negotiating node pair dynamically determines if a traffic channel that is in use in the local area can be used concurrently to support additional traffic. It is shown that the new protocol yields significantly better performance than two existing approaches to the reuse of traffic channels. It is also shown that the improvements are achieved over a wide range of network conditions, including different network densities and different spread-spectrum processing gains. A new distributed routing protocol is also presented for use in heterogeneous wireless ad hoc networks. Two components of the routing protocol are jointly designed: a congestion-based link metric that identifies multiple routes with low levels of congestion, and a forwarding protocol that dynamically splits traffic among the multiple routes based on the relative capabilities of the routes. It is shown that the new routing protocol is able to exploit the decoupling of paths in the network resulting from the presence of nodes with directional antennas. Furthermore, it is shown that the protocol adapts effectively to the presence of advantaged nodes in the network. This approach to joint routing and forwarding is shown to result in a much better and more robust network performance than minimum-hop routing

Adaptive protocols for mobile ad hoc networks

Recent advances in low-power technologies have resulted in the proliferation of inexpensive handheld mobile computing devices. Soon, just like the Internet empow- ered a whole new world of applications for personal computers, the development and deployment of robust ubiquitous wireless networks will enable many new and exciting futuristic applications. Certain to be an important part of this future is a class of networks known as "mobile ad hoc networks." Mobile ad hoc networks (or simply "ad hoc networks") are local-area networks formed "on the spot" between collocated wireless devices. These devices self-organize by sharing information with their neigh- bors to establish communication pathways whenever and wherever they are. For ad hoc networks to succeed, however, new protocols must be developed that are capable of adapting to their dynamic nature. In this dissertation, we present a number of adaptive protocols that are designed for this purpose. We investigate new link layer mechanisms that dynamically monitor and adapt to changes in link quality, including a protocol that uses common control messages to form a tight feedback control loop for adaptation of the link data rate to best match the channel conditions perceived by the receiver. We also investigate routing protocols that adapt route selection according to network characteristics. In particular, we present two on-demand routing protocols that are designed to take advantage of the presence of multirate links. We then investigate the performance of TCP, showing how communication outages caused by link failures and routing delays can be very detrimental to its performance. In response, we present a solution to this problem that uses explicit feedback messages from the link layer about link failures to adapt TCP's behavior. Finally, we show how link failures in heterogeneous networks containing links with widely varying bandwidth and delay can cause repeated "modal" changes in capacity that TCP is slow to detect. We then present a modifed version of TCP that is capable of more rapidly detecting and adapting to these changes