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

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

Silvio Sampaio (IDMEC, University of Porto, Portugal) and Francisco Vasques (University of Porto, Portugal)
DOI: 10.4018/978-1-4666-5888-2.ch609
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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:

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

  • They can provide an additional layer of indirection/virtualization;

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

  • 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.

Key Terms in this Chapter

Cross-Layer Design: Here, information from one layer in the network stack is made available to another layer(s). For instance, the MAC layer may exchange information to the network layer or vice versa in order to improve the routing metric calculation, or the overall routing performance.

Wireless Mesh Network (WMN): It is a generic term that refers to a communication network composed of wireless nodes in a mesh topology. In general terms, a mesh topology each node acts as a router, forwarding packets on behalf of other nodes.

Peer-to-Peer (P2P): It is network structure, in which every machine plays the role of client and server at the same time. Although commonly used in distributed media file sharing systems, the P2P technology is also used for other applications, such as Voice over IP (VoIP).

IEEE 802.11: Family of standards which defines a set of Media Access Control (MAC) and PHYsical layer (PHY) specifications for implementing Wireless Local Area Network (WLAN) computer communication. The standard provides the basis for current wireless network products using the Wi-Fi brand.

Distributed Hash Table (DHT): It is a scalable structure that allows finding the host of desired content rather quickly. DHT is based on hash tables to store, lookup, and retrieve information in geographically distributed locations, providing a failsafe mechanism for distributed computing. Actually, an abstract overlay network is implement on top of the physical network topology.

Network Scalability: It is a network property that ensures that the overall network performance may not significantly degrade, regardless the size of the network increasing.

Mobile Ad-hoc NETwork (MANET): It is a self-configuring infrastructureless network of mobile devices connected by wireless links. The main constraint of this type of network is the mobility of the nodes.

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