Mobile ad hoc networks (MANETs) represent complex distributed systems that comprise wireless mobile nodes that can freely and dynamically self-organize into arbitrary and temporary, ‘‘ad-hoc’’ network topologies, allowing people and devices to seamlessly internetwork in areas with no pre-existing communication infrastructure, e.g., disaster recovery environments. Ad hoc networking concept is not a new one, having been around in various forms for over 20 years. Traditionally, tactical networks have been the only communication networking application that followed the ad hoc paradigm. Recently, the introduction of new technologies such as the Bluetooth, IEEE 802.11 and Hyperlan are helping enable eventual commercial MANET deployments outside the military domain. These recent evolutions have been generating a renewed and growing interest in the research and development of MANET. This paper attempts to provide a comprehensive overview of this dynamic field. It first explains the important role that mobile ad hoc networks play in the evolution of future wireless technologies. Then, it reviews the latest research activities in these areas, including a summary of MANETs characteristics, capabilities, applications, and design constraints. The paper concludes by presenting a set of challenges and problems requiring further research in the future. � 2003 Elsevier B.V. All rights reserved.

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Mobile ad hoc networking: imperatives and challenges

This paper evaluates the ability of a wireless mesh architecture to provide high performance Internet access while demanding little deployment planning or operational management. The architecture considered in this paper has unplanned node placement (rather than planned topology), omni-directional antennas (rather than directional links), and multi-hop routing (rather than single-hop base stations). These design decisions contribute to ease of deployment, an important requirement for community wireless networks. However, this architecture carries the risk that lack of planning might render the network's performance unusably low. For example, it might be necessary to place nodes carefully to ensure connectivity; the omni-directional antennas might provide uselessly short radio ranges; or the inefficiency of multi-hop forwarding might leave some users effectively disconnected.The paper evaluates this unplanned mesh architecture with a case study of the Roofnet 802.11b mesh network. Roofnet consists of 37 nodes spread over four square kilometers of an urban area. The network provides users with usable performance despite lack of planning: the average inter-node throughput is 627 kbits/second, even though the average route has three hops.The paper evaluates multiple aspects of the architecture: the effect of node density on connectivity and throughput; the characteristics of the links that the routing protocol elects to use; the usefulness of the highly connected mesh afforded by omni-directional antennas for robustness and throughput; and the potential performance of a single-hop network using the same nodes as Roofnet.

/pdf/architecture-and-evaluation-of-an-unplanned-802-11b-mesh-2cbadubcmd.pdf

Architecture and evaluation of an unplanned 802.11b mesh network

This paper presents ZigZag, an 802.11 receiver design that combats hidden terminals. ZigZag's core contribution is a new form of interference cancellation that exploits asynchrony across successive collisions. Specifically, 802.11 retransmissions, in the case of hidden terminals, cause successive collisions. These collisions have different interference-free stretches at their start, which ZigZag exploits to bootstrap its decoding.ZigZag makes no changes to the 802.11 MAC and introduces no overhead when there are no collisions. But, when senders collide, ZigZag attains the same throughput as if the colliding packets were a priori scheduled in separate time slots. We build a prototype of ZigZag in GNU Radio. In a testbed of 14 USRP nodes, ZigZag reduces the average packet loss rate at hidden terminals from 72.6% to about 0.7%.

/pdf/zigzag-decoding-combating-hidden-terminals-in-wireless-282gpsraz4.pdf

Zigzag decoding: combating hidden terminals in wireless networks

This paper proposes FlashLinQ--a synchronous peer-to-peer wireless PHY/MAC network architecture. FlashLinQ leverages the fine-grained parallel channel access offered by OFDM and incorporates an analog energy-level-based signaling scheme that enables signal-to-interference ratio (SIR)-based distributed scheduling. This new signaling mechanism, and the concomitant scheduling algorithm, enables efficient channel-aware spatial resource allocation, leading to significant gains over a CSMA/CA system using RTS/CTS. FlashLinQ is a complete system architecture including: 1) timing and frequency synchronization derived from cellular spectrum; 2) peer discovery; 3) link management; and 4) channel-aware distributed power, data rate, and link scheduling. FlashLinQ has been implemented for operation over licensed spectrum on a digital signal processor/ field-programmable gate array (DSP/FPGA) platform. In this paper, we present FlashLinQ performance results derived from both measurements and simulations.

https://users.ece.utexas.edu/~shakkott/Pubs/FlashLinQ-long.pdf

FlashLinQ: a synchronous distributed scheduler for peer-to-peer ad hoc networks

Wireless IEEE 802.11 networks in residences, small businesses, and public "hot spots" typically encounter the wireline access link (DSL, cable modem, T1, etc.) as the slowest and most expensive part of the end-to-end path. Consequently, network architectures have been proposed that employ multiple wireless hops in route to and from the wired Internet. Unfortunately, use of current media access and transport protocols for such systems can result in severe unfairness and even starvation for flows that are an increasing number of hops away from a wired Internet entry point. Our objective is to study fairness and end-to-end performance in multihop wireless backhaul networks via the following methodology. First, we develop a formal reference model that characterizes objectives such as removing spatial bias (i.e., providing performance that is independent of the number of wireless hops to a wire) and maximizing spatial reuse. Second, we perform an extensive set of simulation experiments to quantify the impact of the key performance factors towards achieving these goals. For example, we study the roles of the MAC protocol, end-to-end congestion control, antenna technology, and traffic types. Next, we develop and study a distributed layer 2 fairness algorithm which targets to achieve the fairness of the reference model without modification to TCP. Finally, we study the critical relationship between fairness and aggregate throughput and in particular study the fairness-constrained system capacity of multihop wireless backhaul networks.

End-to-end performance and fairness in multihop wireless backhaul networks

IEEE 802.11 MAC mainly relies on two techniques to combat interference: physical carrier sensing and RTS/CTS handshake (also known as "virtual carrier sensing"). Ideally, the RTS/CTS handshake can eliminate most interference. However, the effectiveness of RTS/CTS handshake is based on the assumption that hidden nodes are within transmission range of receivers. In this paper, we prove using analytic models that in ad hoc networks, such an assumption cannot hold due to the fact that power needed for interrupting a packet reception is much lower than that of delivering a packet successfully. Thus, the "virtual carrier sensing" implemented by RTS/CTS handshake cannot prevent all interference. Physical carrier sensing can complement this in some degree. However, since interference happens at receivers, while physical carrier sensing is detecting transmitters (the same problem causing the hidden terminal situation), physical carrier sensing cannot help much, unless a very large carrier sensing range is adopted, which is limited by the antenna sensitivity. We investigate how effective is the RTS/CTS handshake in terms of reducing interference. We show that in some situations, the interference range is much larger than transmission range, where RTS/CTS cannot function well. Then, a simple MAC layer scheme is proposed to solve this problem. Simulation results verify that our scheme can help IEEE 802.11 resolve most interference caused by large interference range.

/pdf/how-effective-is-the-ieee-802-11-rts-cts-handshake-in-ad-hoc-qdp08cpt71.pdf

How effective is the IEEE 802.11 RTS/CTS handshake in ad hoc networks

IEEE 802.11 MAC mainly relies on two techniques to combat interference: physical carrier sensing and RTS/CTS handshake (also known as “virtual carrier sensing”). Ideally, the RTS/CTS handshake can eliminate most interference. However, the effectiveness of RTS/CTS handshake is based on the assumption that hidden nodes are within transmission range of receivers. In this paper, we prove using analytic models that in ad hoc networks, such an assumption cannot hold due to the fact that power needed for interrupting a packet reception is much lower than that of delivering a packet successfully. Thus, the “virtual carrier sensing” implemented by RTS/CTS handshake cannot prevent all interference as we expect in theory. Physical carrier sensing can complement this in some degree. However, since interference happens at receivers, while physical carrier sensing is detecting transmitters (the same problem causing the hidden terminal situation), physical carrier sensing cannot help much, unless a very large carrier sensing range is adopted, which is limited by the antenna sensitivity. In this paper, we investigate how effective is the RTS/CTS handshake in terms of reducing interference. We show that in some situations, the interference range is much larger than transmission range, where RTS/CTS cannot function well. Two independent solutions are proposed in this paper. One is a simple enhancement to the IEEE 802.11 MAC protocol. The other is to utilize directional antennas. Simulation results verify that the proposed schemes indeed can help IEEE 802.11 resolve most interference caused by large interference range.

Effectiveness of RTS/CTS handshake in IEEE 802.11 based ad hoc networks

Emerging wireless ad hoc networks find their most important applications in untethered, mobile, multihop scenarios where there is no wired infrastructure. Yet, when the wired infrastructure (say, the Internet) is within reach, opportunistic connections to Internet sites may be established across the multihop network to transfer files and update databases. These file transfers use TCP for reliability and congestion control. However, recent experiments with ad hoc, multihop 802.11 networks have exposed serious instabilities when TCP connections span both wired and wireless domains. In particular, some TCP connections capture the wireless channel and drive the throughput on other connections virtually to zero. This is most surprising in view of the fact that connections between 802.11 (single hop) wireless LAN stations and the Internet are well behaved. In fact they are routinely used in most Campuses, Businesses and Research Labs. This paper is an experimental study of the unstable behavior of TCP across 802.11 ad hoc networks and the wired Internet. We investigate the fairness issues of multiple TCP flows as well as the coexistence of TCP flows and video streams in the wired/wireless scenario. Detailed analysis of the measurement results is also presented. The paper will prove very valuable to future commercial and military ad hoc networks.

/pdf/tcp-behavior-across-multihop-wireless-networks-and-the-wired-1d2hy89tc3.pdf

TCP behavior across multihop wireless networks and the wired internet

Emerging wireless ad-hoc networks find their most important applications in untethered, mobile, multihop scenarios where there is no wired infrastructure. Yet, when the wired infrastructure (say, the Internet) is within reach, opportunistic connections to Internet sites may be established across the multihop network to transfer files and update databases. These file transfers use TCP for reliability and congestion control. Many believe that TCP should not be used in ad-hoc network due to it's inability to adapt to the high loss environment, but we believe that TCP will always have a part to play in wireless network in one form or another; studying the behavior of TCP will provide the direction for that adaptation. Recent experiments with ad-hoc, multihop 802.11 networks have exposed serious instabilities when TCP connections span both wired and wireless domains. In particular, some TCP connections capture the wireless channel and drive the throughput on other connections virtually to zero. This is most surprising in view of the fact that connections between 802.11 (single hop) wireless LAN stations and the Internet are well behaved, stable and fair. The problem of the unfairness compounds further when TCP connections have to share the bandwidth with in multihop ad-hoc network. In this paper, we present the issues regarding the wireless transport protocols by experimentally analyzing TCP performance.

Sang Bae

Papers

How effective is the IEEE 802.11 RTS/CTS handshake in ad hoc networks

Effectiveness of RTS/CTS handshake in IEEE 802.11 based ad hoc networks

TCP behavior across multihop wireless networks and the wired internet

Measured analysis of TCP behavior across multihop wireless and wired networks