Design Issues In Wireless Mesh Networks
There are many issues that need consideration when a WMN is designed for a particular application. These design issues can be broadly classified into architectural issues and protocol issues. The architectural design issues and protocol design issues are described in Section 1.4.1 and Section 1.4.2.
1.4.1 Network Architectural Design Issues
A WMN can be designed in three different network architectures based on the network topology: flat WMN, hierarchical WMN, and hybrid WMN. These categories are briefly discussed below.
1.4.1.1 Flat Wireless Mesh Network
In a flat WMN, the network is formed by client machines that act as both hosts and routers. Here, each node is at the same level as that of its peers. The wireless client nodes coordinate among themselves to provide routing, network configuration, service provisioning, and other application provisioning. This architecture is closest to an ad hoc wireless network and it is the simplest case among the three WMN architectures. The primary advantage of this architecture is its simplicity, and its disadvantages include lack of network scalability and high resource constraints. The primary issues in designing a flat WMN are the addressing scheme, routing, and service discovery schemes. In a flat network, the addressing is one of the issues that might become a bottleneck against scalability.
1.4.1.2 Hierarchical Wireless Mesh Network
In a hierarchical WMN, the network has multiple tiers or hierarchical levels in which the WMN client nodes form the lowest in the hierarchy. These client nodes can communicate with a WMN backbone network formed by WMN routers. In most cases, the WMN nodes are dedicated nodes that form a WMN backbone network. This means that the backbone nodes may not originate or terminate data traffic like the WMN client nodes. The responsibility to self-organize and maintain the backbone network is provided to the WMN routers, some of which in the backbone network may have external interface to the Internet and such nodes are called gateway nodes.
1.4.1.3 Hybrid Wireless Mesh Network
This is a special case of hierarchical WMNs where the WMN utilizes other wireless networks for communication. For example, the use of other infrastructure-based WMNs such as cellular networks, WiLL networks, WiMAX networks, or satellite networks. Examples of such hybrid WMNs include multihop cellular networks [2], throughput enhanced wireless in local loop (TWiLL) networks [3], and unified cellular ad hoc networks [4]. A practical solution for such a hybrid WMN for emergency response applications is the CalMesh platform [5]. These hybrid WMNs may use multiple technologies for both WMN
backbone and back haul. Since the growth of WMNs depend heavily on how it works with other existing wireless networking solutions, this architecture becomes very important in the development of WMNs.
1.4.2 Network Protocol Design Issues
The design issues for the protocols can be described in a layer-wise manner starting from the physical layer to the application layer. Some of these protocol design issues are presented below.
1.4.2.1 Physical Layer Design Issues
At the physical layer, the main design issue is the choice of an appropriate radio technology. The choice of a radio technology can be based on: (i) technological considerations and (ii) economic considerations. The main technological considerations include the spectral efficiency, physical layer data rate, and the ability to operate in the presence of interference. For example, the choice of technologies such as code division multiple access (CDMA), ultra wide band (UWB), and multiple input multiple output (MIMO) are more suitable for WMN physical layer than the most popular physical layer technology, orthogonal frequency division multiplexing (OFDM) used in today's WMNs. For example, today's physical layer technology, primarily based on OFDM provides a maximum physical layer data rate of 54 Mbps. In a highly dense network with high interference, this capacity may not be sufficient. Therefore, development of new and high data rate physical layer such as UWB is a physical layer challenge. In addition to the choice of a particular physical layer technology, programable radios or cognitive radios add another dimension to the WMN physical layer design. This is emphasized by some of the applications of WMNs such as emergency response and military applications where the spectrum used for communication depends on the unused spectrum in a given locality. In such applications, a software-defined radio with cognitive capabilities would be an ideal choice. In addition to the technological considerations mentioned above, the second most important requirement is economical or social where the simplicity of the physical layer technology will lead to inexpensive devices and hence better social affordability of WMNs. An example of this is evident in the success of today's IEEE 802.11b-based WMNs where the inexpensive network interface cards contributed to the success of the proliferation of WMNs. Therefore, while choosing the physical layer technology, a network designer should look at the application and user scenario as well.
1.4.2.2 Medium Access Control Layer
The design of MAC layer protocol assumes significance in a WMN because achievable capacity depends heavily on the performance of MAC protocol. In addition to a fully distributed operation, the major issues faced by the popular CSMA/CA-based IEEE 802.11 distributed coordination function (DCF) are: (i) hidden terminal problem, (ii) exposed terminal problem, (iii) location-dependent contention, and (iv) high error probability on the channel. In order to increase the network capacity, multiple radios operating in multiple channels are used. Therefore, new MAC protocols are to be designed for operating in multichannel MR-WMN systems. MAC protocols are also to be adapted to operate in different physical layer technologies such as UWB and MIMO physical layers. Another popular research issue for better MAC performance is the use of cross-layer interaction mechanisms that enable the MAC protocol to make use of information from other layers. In traditional wireless or wired networks, each layer works with its own information making it unable to make the best use of the network-centric properties. In general, the MAC layer protocol design should include methods and solutions to provide better network scalability and throughput capacity.
1.4.2.3 Network Layer
Unlike the routing protocols for ad hoc wireless networks, the routing protocols, depending on its network scenario, face different design issues in a WMN. Since WMN is relatively a static network, the routing can make use of table-driven routing approaches such as that used in wired networks or in ad hoc wireless networks [12]. The main issues faced by routing protocol in a WMN are: (i) design of routing metric, (ii) minimal routing overhead, (iii) route robustness, (iv) effective use of support infrastructure, (v) load balancing, and (vi) route adaptability. The routing metric design plays a crucial role in achieving good performance. The best routing metric may also differ in its performance. For example, in WMNs, the routing metric design has to take the link level signal quality into account for better end-to-end performance. Routing protocols for WMNs, while providing a good end-to-end performance, should also consume minimum bandwidth for setting up paths. In addition, the use of wireless medium demands quick path reconfiguration capability in order to maintain the robustness of the path. Another important aspect is the load-balancing capability that needs to be incorporated with the routing protocol. Finally, a routing protocol for WMN must be adaptable to the network dynamics. The routing protocol can be classified into either flat routing protocol or hierarchical routing protocol based on the type of network where the routing protocol is applied.
1.4.2.4 Transport Layer
At the transport layer, the biggest challenge is the performance of transport protocols over the WMN. Since a WMN has large round-trip time (RTT) variations and these RTT variations are dependent on the number of hops in the path, the end-to-end TCP throughput degrades rapidly with throughput. The packet loss, collision, network asymmetry, and link failures can also contribute to the degradation in transport layer protocol performance. The popular transport layer for the Internet, TCP, performs very poorly in its original form over a WMN. The transport layer needs to be refined or rewritten for making it more efficient on a WMN. Some of the design issues for a transport layer protocol for WMN are: (i) end-to-end reliability, (ii) throughput, (iii) capability to handle network asymmetry, and (iv) capability to handle network dynamism.
1.4.2.5 Application Layer
The most popular application for WMNs is the Internet access service. Essentially, a WMN needs to provide Internet services for residential areas or businesses. In such a situation, though data services make primary service over a WMN, voice services such as voice over Internet protocol (VoIP) are also important. Therefore, it is very essential to provide support for both the time-sensitive and the best-effort traffics. In addition to the basic data and voice traffic support, the network provides service discovery mechanisms. Since most of the network services are in fully distributed form, static service discovery mechanisms may not be effective in a WMN. Another important requirement for the application layer protocol design is to handle the heterogeneity of networks as the data may pass through a variety of networks before being delivered to the end application.
1.4.2.6 System-Level Design Issues
The above-mentioned issues are generic to a WMN and these issues are revisited in detail for a MR-WMN system in Section 1.5. In addition to the protocol design issues, a WMN requires system-level solutions. Some examples for system-level issues are: (i) cross-layer system design, (ii) design for security and trust, (iii) network management systems, and (iv) network survivability issues.
Some of the primary challenges faced by a WMN can be alleviated by the use of an MR-WMN and therefore, subsequent sections focus on this.
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