J. Parallel Distrib. Comput. 68 (2008) 361 –371 www.elsevier.com/locate/jpdc Improving routing in long-distance wireless mesh networks via a distributed embedded router O. Mirabella ∗ , Lo Bello, A. Raucea Department of Computer Engineering and Telecommunications, University of Catania, V.le A. Doria 6, 95125 Catania, Italy Received 10 November 2006; accepted 7 May 2007 Available online 23 May 2007 Abstract The paper addresses some issues concerning routers for long-distance wireless mesh networks (LDWMNs) which are becoming increasingly popular in the last few years, above all to provide Internet access for small extra-urban communities. The paper discusses some main features of LDWMNs, the design challenges and the reasons behind the implementation choices. A router is described, based on a modular architecture which provides fault tolerance and guarantees gracefully degrading performances in case a fault occurs. One important feature of the router is the full-duplex capability of its wireless links which provides high throughput. Finally, the paper describes the implementation of the router using low-cost embedded components and addresses some issues concerning the management of the wireless links in the wireless mesh backbone. © 2007 Elsevier Inc. All rights reserved. Keywords: Long-distance wireless mesh networks; Routers; Embedded architecture; Fault tolerance; Modular architecture; Performance evaluation 1. Introduction Wireless networks are becoming increasingly popular due to the possibility of simple, flexible connection of a number of users who can exchange data or access the Internet via an access point. There is, however, another interesting area of application which has up to now been the domain of wired networks: long-distance mesh networks (LDMNs). These net- works are traditionally realized using fiber optic backbones, but this is extremely expensive, above all when connections are long (stretching to tens of kilometers) and outside built-up ar- eas. In the last few years long-distance wireless networks have begun to appear, above all to provide Internet access for small extra-urban communities. A wireless implementation is eco- nomical and satisfactory, especially when the number of users to be connected is not high and a single hop (or at most a few hops) is sufficient for the users to be reached. The standards used for this purpose are those of the IEEE 802.11x [9] fam- ily, especially developed for wireless applications, supported by a large number of manufacturers, and based on well-tested ∗ Corresponding author. Fax: +39 95 738 2397. E-mail addresses: omirabel@diit.unict.it (O. Mirabella), lucia.lobello@diit.unict.it (L. Bello), araucea@diit.unict.it (A. Raucea). 0743-7315/$ - see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jpdc.2007.05.005 low-cost technologies.. The most common network topologies are linear, with only a few hops, but more complex mesh topolo- gies are beginning to replace the equivalent wired networks [4]. However, the CSMA/CA protocol used in the IEEE802.11x standards was designed for indoor, short-distance applications and is not efficient in long-distance mesh applications. The CSMA/CA protocol is, in fact, burdened down with functions needed to solve medium access contention, which are not nec- essary in a long-distance wireless mesh network (LDWMN) in which the links are point to point. Another critical feature of the protocol is the handling of acks, as in LDWMNs the distances between two nodes add delays that may lead to a time-out, thus causing unnecessary retransmissions. In the realization of a LDWMN, it is therefore necessary to adapt the behavior of the protocols and implement devices that will take into account the particular features of this kind of network. These include: • A lack of mobility, linked to the fact that high-gain an- tennas need to be used in order to achieve long-distance connections. These antennas are directional and so have to be accurately oriented, thus excluding the possibility of mobile nodes. This lack of mobility simplifies data rout- ing, which can be achieved in the same way as in wired networks.