Exploiting DHT's Properties to Improve the Scalability of Mesh Networks

Introduction

Over the past few years, the IEEE 802.11 family of standards has become a dominant solution for Wireless Local Area Networks (WLANs) due to its performance, low cost and fast deployment characteristics (Hartmann & Meister, 2008; Sgora, Vergados, & Chatzimisios, 2009). The rapid growth of both Internet and wireless communications, create an increasing demand for wireless broadband access and higher data rates (Jiang et al., 2006).

In this context, Wireless Mesh Network (WMN) concept appears as a promising solution for wireless environments, due to its characteristics, and fields of application (Farkas & Plattner, 2005). Akyildiz et al. (2005) gives numerous application examples for WMNs, which goes from simple communication environments as broadband home networking, community and neighborhood networks, enterprise networking, building automation, to more complex environments as industrial networks, disaster recovery networks, public safety and many others. In order to cope with all requirements from these applications, one of the most important characteristics is the scalability. A WMN that does not scale well has limited usefulness.

This is the case of traditional IEEE 802.11-based mesh networks, which offers limited scalability, caused mostly by the underlying IEEE 802.11 mechanisms and multi-hop communication issues. This characteristic makes it a questionable candidate for large-scale network deployments. In fact, scalability is a well-known issue in multi-hop networking, not only WMNs. Studying the scalability of this type of network is important as WMNs tend to be deployed over large areas and hence the self-organization property may gather a large number of stations whitin the same mesh network. Basically, the lack of network scalability means that when the size of the network increases, its performance behavior may significantly degrade¹. In such a case, traditional routing protocols may not be able to find paths and MAC protocols may manifest high delays.

Seeking for more scalable solutions, researchers have looked to scalable mechanisms created for Peer-to-Peer (P2P) systems, such as the Distributed Hash Table (DHT). Originally designed to create an overlay network that allow more scalable and faster to search and to recover information over the P2P systems, now its use is proposed to increase the network scalability and speed up the overall network performance.

Galán-Jiménez & Gazo-Cervero (2011) presented an interesting survey on the use of overlay networks. The authors emphasize the following properties of overlay networks:

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  They can be built on top of one or more existing networks;

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  They can provide an additional layer of indirection/virtualization;

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  They can change properties in one or more areas of the underlying networks;

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  They can change an existing network layer.

From these definitions it becomes obvious that WMNs may potentially take advantage from the scalable structure provided by DHTs. However, if in one hand, deploying DHTs structures over WMNs represents promising possibilities, in other hand several challenges need to be investigated to make clear the tradeoff resulting from this combination. Therefore, this chapter aims to points out the benefits as well as the issues related to the deployment of IEEE 802-11-based Mesh Networks by using the distributed characteristics of the DHTs. Moreover, we emphatisize the challenges that must be overcome to create more scalable IEEE 802.11-based WMNs, using DHTs.