WMNs are low-cost access networks built on cooperative routing over a backbone composed of stationary wireless routers. WMNs must deal with the highly unstable wireless medium. Therefore, the design of algorithms that consider link quality to choose the best routes are enabling routing metrics and protocols to evolve. In this work, we analyze the state of the art in WMN metrics and propose a taxonomy for WMN routing protocols. Performance measurements for a WMN, deployed using various routing metrics, are presented and corroborate our analysis.

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Routing Metrics and Protocols for Wireless Mesh Networks

Multihop wireless mesh networks are becoming a new attractive communication paradigm owing to their low cost and ease of deployment. Routing protocols are critical to the performance and reliability of wireless mesh networks. Traditional routing protocols send traffic along predetermined paths and face difficulties in coping with unreliable and unpredictable wireless medium. In this paper, we propose a simple opportunistic adaptive routing protocol (SOAR) to explicitly support multiple simultaneous flows in wireless mesh networks. SOAR incorporates the following four major components to achieve high throughput and fairness: 1) adaptive forwarding path selection to leverage path diversity while minimizing duplicate transmissions, 2) priority timer-based forwarding to let only the best forwarding node forward the packet, 3) local loss recovery to efficiently detect and retransmit lost packets, and 4) adaptive rate control to determine an appropriate sending rate according to the current network conditions. We implement SOAR in both NS-2 simulation and an 18-node wireless mesh testbed. Our extensive evaluation shows that SOAR significantly outperforms traditional routing and a seminal opportunistic routing protocol, ExOR, under a wide range of scenarios.

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SOAR: Simple Opportunistic Adaptive Routing Protocol for Wireless Mesh Networks

The great advances made in the wireless technology have enabled the deployment of wireless communication networks in some of the harshest environments such as volcanoes, hurricane-affected regions, and underground mines. In such challenging environments suffering from the lack of infrastructure, traditional routing is not efficient and sometimes not even feasible. Moreover, the exponential growth of the number of wireless connected devices has created the need for a new routing paradigm that could benefit from the potentials offered by these heterogeneous wireless devices. Hence, in order to overcome the traditional routing limitations, and to increase the capacity of current dynamic heterogeneous wireless networks, the opportunistic routing paradigm has been proposed and developed in recent research works. Motivated by the great interest that has been attributed to this new paradigm within the last decade, we provide a comprehensive survey of the existing literature related to opportunistic routing. We first study the main design building blocks of opportunistic routing. Then, we provide a taxonomy for opportunistic routing proposals, based on their routing objectives as well as the optimization tools and approaches used in the routing design. Hence, five opportunistic routing classes are defined and studied in this paper, namely, geographic opportunistic routing, link-state-aware opportunistic routing, probabilistic opportunistic routing, optimization-based opportunistic routing, and cross-layer opportunistic routing. We also review the main protocols proposed in the literature for each class. Finally, we identify and discuss the main future research directions related to the opportunistic routing design, optimization, and deployment.

A Survey on Opportunistic Routing in Wireless Communication Networks

Recently, there has been a growing interest of using network coding to improve the performance of wireless networks, for example, authors of proposed the practical wireless network coding system called COPE, which demonstrated the throughput gain achieved by network coding. However, COPE has two fundamental limitations: (1) the coding opportunity is crucially dependent on the established routes and (2) the coding structure in COPE is limited within a two-hop region only. The aim of this paper is to overcome these limitations. In particular, we propose DCAR, the distributed coding-aware routing mechanism which enables: (1) the discovery for available paths between a given source and destination and (2) the detection for potential network coding opportunities over much wider network region. One interesting result is that DCAR has the capability to discover high throughput paths with coding opportunities, while conventional wireless network routing protocols fail to do so. In addition, DCAR can detect coding opportunities on the entire path, thus eliminating the ?two-hop? coding limitation in COPE. We also propose a novel routing metric called coding-aware routing metric (CRM) which facilitates the performance comparison between ?coding-possible? and "coding-impossible? paths. We implement the DCAR system in ns-2 and carry out extensive evaluation. We show that when comparing to the coding mechanism in, DCAR can achieve much higher throughput gain.

DCAR: Distributed Coding-Aware Routing in Wireless Networks

In the problem of routing in multi-hop wireless networks, to achieve high end-to-end throughput, it is crucial to find the “best” path from the source node to the destination node. Although a large number of routing protocols have been proposed to find the path with minimum total transmission count/time for delivering a single packet, such transmission count/time minimizing protocols cannot be guaranteed to achieve maximum end-to-end throughput. In this paper, we argue that by carefully considering spatial reusability of the wireless communication media, we can tremendously improve the end-to-end throughput in multi-hop wireless networks. To support our argument, we propose spatial reusability-aware single-path routing (SASR) and anypath routing (SAAR) protocols, and compare them with existing single-path routing and anypath routing protocols, respectively. Our evaluation results show that our protocols significantly improve the end-to-end throughput compared with existing protocols. Specifically, for single-path routing, the median throughput gain is up to 60 percent, and for each source-destination pair, the throughput gain is as high as $5.3\times$ ; for anypath routing, the maximum per-flow throughput gain is 71.6 percent, while the median gain is up to 13.2 percent.

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Spatial Reusability-Aware Routing in Multi-Hop Wireless Networks

Traditional routing schemes select the best path for each destination and forward a packet to the corresponding next hop. While such best-path routing schemes are considered well-suited for networks with reliable point-to-point links, they are not necessarily ideal for wireless networks with lossy broadcast links. Consequently, opportunistic routing schemes that exploit the broadcast nature of wireless transmissions and dynamically select a next-hop per-packet based on loss conditions at that instant are being actively explored. It is generally accepted that opportunistic routing performs substantially better than best-path routing for wireless mesh networks. In this paper, we analyze the efficacy of opportunistic routing. We define a new metric EAX that captures the expected number of any-path transmissions needed to successfully deliver a packet between two nodes under opportunistic routing. Based on EAX, we develop a candidate selection and prioritization method corresponding to an ideal opportunistic routing scheme. We then conduct an off-line comparison of best-path routing and opportunistic routing using our EAX metric and MIT Roofnet trace. We observe that while opportunistic routing offers better performance than best- path routing, the gain is not as high as commonly believed.

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On the Efficacy of Opportunistic Routing

Network coding is known to improve network throughput by mixing information from different flows and conveying more information in each transmission. Recently there have been some proposals for applying network coding to wireless mesh networks leveraging the broadcast nature of wireless transmissions. These approaches exploit coding opportunities passively while forwarding packets but they do not proactively change routing of flows to create more coding opportunities. In this paper, we attempt to investigate the extent of performance gain possible when routing decisions are made with the awareness of coding. We first define the expected number of coded transmissions for a successful exchange of packets between two nodes through an intermediate node. We then formulate optimal routing with coding, given the topology and traffic, as a linear programming problem. We conduct a preliminary evaluation of coding-aware routing and show that it offers significant gain particularly when there are many long distance flows.

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Routing with opportunistically coded exchanges in wireless mesh networks

Opportunistic routing schemes that exploit the broadcast nature of wireless transmissions for selecting the best next-hop at that instant among a set of candidates are being actively explored [1-3]. These schemes which we refer to as opportunistic any-path forwarding (OAPF), reduce the number of transmissions needed for reliable packet delivery.

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On selection of candidates for opportunistic anypath forwarding

Static broadband wireless networks, due to their ease of deployment, are likely to proliferate in the near future. The major stumbling block, however, is that wireless links are prone to external interference, channel fading, inclement weather, etc. Therefore scalable and reliable routing despite frequent link quality fluctuations is needed for accelerating the growth of these networks. Most of the wireless routing schemes proposed in the literature are less suitable for these networks, as they are designed primarily for mobile ad hoc networks with dynamic and unpredictable topologies. In this paper, we propose a novel link-state-based blacklist-aided forwarding (BAF) approach, that takes advantage of the fact that the nodes and therefore their adjacencies are relatively static, for scalable packet delivery in static wireless networks. Under BAF, each packet carries a blacklist, a minimal set of degraded-quality links encountered along its path, and the next hop is determined based on both its destination and blacklist. BAF provides loop-free delivery of packets to reachable destinations regardless of the number of degraded links in the network. We evaluate the performance of BAF and show that it is not only reliable but also scalable.

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Blacklist-aided forwarding in static multihop wireless networks

Under link-state routing protocols such as OSPF and IS-IS, when there is a change in the topology, propagation of link-state announcements, path recomputation, and updating of forwarding tables (FIBs) will all incur some delay before traffic forwarding can resume on alternate paths. During this convergence period, routers may have inconsistent views of the network, resulting in transient forwarding loops. Previous remedies proposed to address this issue enforce a certain order among the nodes in which they update their FIBs. While such approaches succeed in avoiding transient loops, they incur additional message overhead and increased convergence delay. We propose an alternate approach, loopless interface-specific forwarding (LISF), that averts transient loops by forwarding a packet based on both its incoming interface and destination. LISF requires no modifications to the existing link-state routing mechanisms. It is easily deployable with current routers since they already maintain a FIB at each interface for lookup efficiency. This paper presents the LISF approach, proves its correctness, discusses three alternative implementations of it and evaluates their performance.

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Zifei Zhong

Papers

On the Efficacy of Opportunistic Routing

Routing with opportunistically coded exchanges in wireless mesh networks

On selection of candidates for opportunistic anypath forwarding

Blacklist-aided forwarding in static multihop wireless networks

Avoiding transient loops through interface-specific forwarding