Chapter Preview
TopIntroduction
Wireless Mesh Networks (WMN) is a current attractive and “hot” topic due to its potential capability to support a myriad of wireless radio and access technologies such as IEEE 802.16 (WiMAX), IEEE 802.11 (WLAN) and IEEE 802.15 (WPAN), thus providing the required flexibility to integrate different radio access networks (Akyildiz & Wang, 2009). It allows creating large-scale hybrid networks, extending the connectivity and network services. Moreover, with the pervasive use of standard IEEE 802.11 devices, this standard is the most common wireless technology used in current WMN’s deployments. For this reason this article is focused on the research challenges and new trends of IEEE 802.11-based WMNs only.
Traditionally, mesh networking research works has mostly been carried out using routing protocols for ad-hoc networks which do not scale well on wireless mesh settings (Baumann et al., 2008). In other words, the use of just current 802.11 ad-hoc technologies is not sufficient for multi-hop wireless mesh networks, as it is the case of IEEE 802.11s wireless mesh networks. The lack of specific routing protocols designed for wireless mesh networks, that take the mesh properties into consideration is one of the major concerns.
Although the hop count is still the traditional routing metric used in most of the common routing protocols designed for multi-hop wireless networks, the minimum hop-count metric may lead to poor performance making it unsuitable for WMNs (Draves, Padhye, & Zill, 2005; Liu et al., 2008; Borges, Curado, & Monteiro, 2011). Thus, much research work has been conducted to propose more efficient routing metrics for wireless multi-hop networks, in order to replace the traditional hop-count as the routing metric. At the same time, new routing protocols have been proposed to make use of these new metrics in order to improve the overall routing performance in WMNs. This article also aims to give an overview of the most representative works related to routing metrics and protocols that have been proposed for WMNs and new trends in this field.
The IEEE 802.11s standard is relatively new, released in September 2011, and now part of the IEEE 802.11 set of standards (LAN/MAN standards Committee, 2012). Briefly, the IEEE 802.11s aims to create a framework to enable wireless mesh networking for standard IEEE 802.11 devices. However, there are still few proposals/implementations in full compliance with the specifications of the standard. Therefore, this article covers a broad set of IEEE 802-11 based routing solutions instead of being constrained to the IEEE 802.11s WMNs.
Key Terms in this Chapter
IEEE 802.11: The 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.
IEEE 802.11s: An amendment to the recently released IEEE 802.11-2012 standard that aims to allow IEEE 802.11devices to form mesh topology.
Exposed Node Problem: Another basic issue in DCF, which basically occurs when a node, in the transmission range of another node, senses the medium busy and defer its transmission even though that transmission from them would not cause a collision. The main effect of this issue is to limit the spatial reuse, also affecting the overall network throughput.
Network Scalability: 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): 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.
Hidden Node Problem: A typical issue of collision free operation in DCF. The problem arises when stations located outside the transmission range of each other are unable to detect the ongoing transmission and becomes “hidden” to each other. Thus, in such a case the hidden node transmit packets simultaneously, which potentially leads to collision. In order to minimize collision, RTS/CTS control frames to control the transmission and reception. However, this approach is not a complete solution and may decrease the overall network throughput.
Network Planning: In general terms, the Network Planning approach address the problem of given a location to be provided connectivity using a set of nodes, determine the minimum cost network topology, considering the hardware, the antenna type and orientation, and radio transmission powers, such that the balance between power, interference, and performance (e.g. in term of throughput or delay) are satisfied.
Wireless Mesh Network (WMN): 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.