JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN ELECTRONICS AND COMMUNICATION ENGINEERING

A Survey of Wireless Mesh Networks: Last-Mile Internet Connectivity

1M. Arif Siddiqui, 2Qazi SHoeb Ahmad, 3M.H. Khan

1Faculty of Applied Sciences,Integral University, Lucknow, India

2Faculty of Applied Sciences,Integral University, Lucknow, India

3Department of Computer Science & Engineering

Institute of Engineering & Technology,

UP Technical University, Lucknow, India

ABSTRACT: Wireless Mesh Networks (WMNs) consists of mesh routers and mesh clients has emerged recently to provide high speed last-mile internet connectivity anytime anywhere. The gateway and bridging functionalities in mesh routers enable the WMNs to be integrated with other networks such as IEEE 802.11, IEEE 802.15, IEEE 802.16 etc. WMN is going under rapid progress which would lead to provide better wireless services in near future. This paper presents a detailed overview of architecture of WMN and its applications in wireless local area networks (WLAN), wireless metropolitan area networks (WMAN), wireless personal area networks (WPAN) and wireless sensor networks (WSNs). Finally, the advantages, constraints and possible applications of WMNs have been discussed.

Keywords :Wireless Mesh Networks, Mobile Ad-Hoc Networks, Wireless Local Area Networks, Wireless Metropolitan Area Networks, Wireless Personal Area Networks, Wireless Sensor Networks.

ISSN: 0975 –6779| NOV 09 TO OCT 10| Volume 1, Issue 1 Page 1

JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN ELECTRONICS AND COMMUNICATION ENGINEERING

  1. INTRODUCTION

In recent years, a cost effective multi-hop WMNs has emerged to provide better services for high speed last-mile connectivity anytime any where. WMNs consistsad hoc distribution of mesh routers and mesh clients [1]. Mesh routers are either stationary or almost stationary wireless nodes and forms the backbone of WMNs to provide the network access to both the mesh and conventional clients. While, the mesh clients are either stationary or mobile, and form a client mesh network among themselves and with other mesh routers.

In WMN, each node operates not only as host but also as a router, forwarding packets on behalf of other nodes that may not be within direct wireless transmission range of their destination. Multi-hop relaying helps to extend the radio coverage without using costly base stations, improve the traffic performance in given scenarios [2], reduce the transmitting power of mesh clients and promote the robustness of a network. A mesh router can be a user terminal, performing the required computing tasks, like a computer. However, it can also be a very small device and put into a place simply for increasing mesh connectivity or relaying traffic. In addition to mesh networking among mesh routers and mesh clients, the gateway/bridge functionalities in mesh routers enable the integration of WMNs with various other networks. Conventional nodes equipped with wireless network interface cards (NICs) can connect directly to WMNs through wireless mesh routers. Customers without wireless NICs can access WMNs by connecting to wireless mesh routers through, for example, Ethernet. Thus, WMNs will greatly help users to be always-on-line anywhere, anytime.Therefore, instead of being another type of ad-hoc networking, WMNs diversify the capabilities of ad-hoc networks. This feature brings many advantages to WMNs, such as low up-front cost, easy network maintenance, robustness, reliable service coverage, etc.

WMNs can be applied in WLAN, WMAN, WPAN and WSNs. Next generation WLANs, WMANs and high speed WPANs are intended to offer high quality multimedia communication services at low cost in different scenarios. The development of standards such as IEEE 802.11 [3], IEEE 802.15 (WPAN) [4], IEEE 802.16 (WMAN) [5] are actively working on introducing multi-hop mesh elements in their next generation standards.

Several research labs companies have already realized the potential of this technology and started building the test beds and offering wireless mesh networking products. Recently, a number of test beds [6], [7], [8],[9], [10] and industrial implementations [11], [12], [13], [14], [15], [16], [17], [18] have been established to carry out research and development for WMNs.. A number of pilot mesh projects across the world (Freifunk OLSR Experiment in Berlin, Germany [19], the Dharamsala mesh in India [20] and Peebles Valley in South Africa [21]) have also demonstrated that a community can establish and maintain a wireless mesh network and have access to a range of modern information and communication services. However, for a WMN to be all it can be, considerable research efforts are still needed. For example, the available medium access control (MAC) and routing protocols are not scalable; throughput drops extensively as the number of nodes or hops in WMNs increases. The remaining paper has been organized as follows:

In section 1.2 the architecture of WMNs has been presented. In section 1.3, we have compared the WMNs with MANETs. In section 1.4, we have discussed the mesh networking concepts in IEEE 802.11, IEEE 802.15, and IEEE 802.16. Finally, in section 1.5 and 1.6 the advantages, constraints and applications have been discussed respectively.

  1. NETWORK ARCHITECTURE

There are two types of nodes in a WMN called mesh routers and mesh clients. Mesh clients can bedifferent kinds of user devices with wireless NIC, such asPCs, laptops, PDAs, and mobile phones. They have limited resources and capabilitiesin terms of energy supply, processing ability, radio coverage range, etc. Wirelessmesh routers can be access points (AP) of WLAN,sink nodes of wireless sensor network, base stations (BS) of cellular network. Mesh routers areusually much more powerful than clients in terms of computation and communicationcapabilities, and have continuous power supply. They usually stay static andsupply connections and services for mesh clients i.e. compared to conventional wireless routers that perform only routing, mesh routers have additional functionalities to enable mesh networking. The architecture of WMNs can be classified into three major categories based on network topology [1]:

Infrastructure/backbone WMNs:

Figure 1.1 [1] shows an example of infrastructure/backbone wireless mesh networks. In the figure, mesh routers form a wireless mesh topology that has self-configuration and self-healing functions built into them. Some mesh routers are designated as gateways which have wired connectivity to the Internet. The integration of other networking technologies is provided by connecting the BS of the network that connects to WMNs to the mesh routers. Here, the clients communicate to the BS of its own network and the BS in turn communicates to the mesh router to access the WMN.Such a wireless multi-hop backbone network provides the flexibility tointegrate WMNs with the existing wireless communication systems. More than one gateway can be added by simply connecting more base stations to the Internet via wire line. Deploying more gateways in the WMNs can improve not only the network capacity but also the reliability. That is, if one gateway fails, the traffic can be delivered by alternative routes and gateways.

Client WMNs:

Figure 1.2 [1] shows an example of client wireless mesh networks. In the figure, mesh clients form the mesh topology to perform routing and configuration functionalities. In client WMNs the nodesgenerally uses one type of radio technology on the devices to coordinate among themselves to provide routing, network configuration, service provisioning, and other application provisioning. This architecture is very similar to wireless ad hoc networks in whichdata packets gets transmitted through multiple nodes to reach the destination.

Hybrid WMNs:

Figure 1.3 [1] shows an example of hybrid wireless mesh networks which is combination of infrastructure and client WMN. In hybrid WMNs, mesh clients can perform mesh functions with other mesh clients as well as accessing the network and infrastructure/backbone provides the connectivity to the other networks such as Wi-Fi, WorldwideInteroperability for Microwave Access (WiMAX) and sensor networks etc. These hybrid WMNs may use multiple technologies for both backbone and back haul. The hybrid WMN would be the most useful in the future [1].

ISSN: 0975 –6779| NOV 09 TO OCT 10| Volume 1, Issue 1 Page 1

JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN ELECTRONICS AND COMMUNICATION ENGINEERING

ISSN: 0975 –6779| NOV 09 TO OCT 10| Volume 1, Issue 1 Page 1

JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN ELECTRONICS AND COMMUNICATION ENGINEERING

ISSN: 0975 –6779| NOV 09 TO OCT 10| Volume 1, Issue 1 Page 1

JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN ELECTRONICS AND COMMUNICATION ENGINEERING

Figure 1.1: Infrastructure/backbone WMNs

Figure 1.2: Client WMNs

Figure 1.3: Hybrid WMNs

  1. WIRELESS AD-HOC V/S MESH NETWORKS

In ad hoc networks, all the nodes are assumed to be mobile and there isno fixed infrastructure for the network. These networks find applicationswhere fixed infrastructure is not possible, such as military operations inthe battlefield, emergency operations, and networks of handheld devices.Because of lack of infrastructure the nodes have to cooperate among themselvesto form a network. Due to mobility of the nodes in the network, thenetwork topology changes frequently. So the protocols for ad hoc networkshave to handle frequent changes in the topology. In most of the applicationsof ad hoc networks, the mobile devices are energy constrained as they are operating on battery. This requires energy-efficient networking solutions for ad hoc networks.

But in the case of WMNs, mesh routers are assumed to be fixed (or have limited mobility) and form a fixed mesh infrastructure. The clients are mobile or fixed and utilize the mesh routers to communicate to the backhaul network through the gateway routers and to other clients by using mesh routers as relaying nodes. These networks find applications where networks of fixed wireless nodes are necessary. There are several architectures for mesh networks, depending on their applications. In the case of infrastructure backbone networking, the edge routers are used to connect different networks to the mesh backbone and the intermediate routers are used as multi-hop relaying nodes to the gateway router, as shown in Figure 1.1. But in the case of community networking, every router provides access to clients and also acts as a relaying node between mesh routers. When comparing ad hoc wireless networks and WMN, we can summarize important differences as follow:

  • have a constant power source, while WMN nodes have better energy storage and power source due to the static topology, formed by fixed relay nodes.
  • Deployment may be easy in ad-hoc networks, while in WMN we may require planning.
  • For the application scenario, most ad-hoc wireless networks are temporary, and WMN are mostly semi-permanent or permanent. In addition, WMNs can be used for both military and civilian applications; an example is the provision of low cost Internet services in public places.
  1. MESH NETWORKING IN IEEE 802.11, 802.15, 802.16

The current and increasing interest in wireless mesh technologies has led tothe creation of working groups (WG) as part of the standardization activities. Inthese WGs, the study groups (SG) or task groups (TG) are in charge of developmesh functionality for wireless local area networks (IEEE 802.11s) [3], wirelesspersonal area networks (IEEE 802.15.5)[4], and wireless metropolitan area networks (IEEE 802.16)[5].

In this section, we describe these standards and their goals as clear objectivesthat remark the current needs and trends of wireless mesh networks

IEEE 802.11s

In September 2003, IEEE formed the 802.11s SG which, in July 2004, becamethe “extended service set (ESS) Mesh Networking” or 802.11s Task Group (TGs),and it is the most advanced group of the 802.11 WG.The current objective of this TG is to apply mesh technology to WLANs bydefining a Wireless Distribution System (WDS) used to build a wireless infrastructurewith MAC-layer broadcast/multicast support in addition to the unicasttransmissions. The TG should produce a protocol that specifies the installation,configuration, and operation of WLAN mesh. Moreover, the specification should includethe extensions in topology formation to make the WLAN mesh self-configure and self-organized, and support for multi-channel, and multi-radio devices. At theMAC layer, a selection path protocol should be incorporated, instead of assigningthe routing task to the network layer [22][23].

The WLAN Mesh architecture comprises the following IEEE 802.11 based elements:

  • Mesh points (MP) which supports (fully or partially) mesh relay functions, and implementoperations such as channel selection, neighbor discovery, and forming andassociation with neighbors. Additionally, MPs communicate with their neighborsand forward traffic on behalf of other MPs.
  • Wireless distribution system (WDS) which is a collection of MPs, and servessimilarly to a Distribution System (DS) in a WLAN.
  • Mesh access point (MAP) which is a MP but acts as an AP as well. Therefore,
  • MAPs can operate in a WLAN Mesh or as part of legacy IEEE 802.11 modes.
  • Mesh point portals (MPP) are another kind of MP that allows the interconnection ofmultiple WLAN meshes to form a network of mesh networks. Moreover, MPP canfunction as bridges or gateways to connect to other wired or wireless networks inthe DS.

IEEE 802.15.5

In May 2004, the IEEE 802.15 WG formed the Task Group 5 (TG5) with theobjective of developing a standard for WPANfurther divided into low rate mesh WPAN and high rate mesh WPAN [4]. MeshWPANs are small closed wireless networks with a flat hierarchy which generally donot have an Internet gateway, or APs.For TG5, there are two types of applications: a) low-rate applications suchas sensor networks, and b) high-rate applications such as control and maintenanceapplications, and multimedia applications. TG5 develops a single “WPAN Mesh”document for both types of applications, but separate them when necessary TG5 shall provide specifications for an architectural framework for interoperable,stable, and scalable wireless mesh topologies for WPAN devices [22]. Thecurrent TG5 proposal defines a tree based approach named adaptive robust tree(ART) which considers a mesh tree routing approach, multicasting, and key redistribution.In ART, each node maintains an ART Table (ARTT) to track the treebranches through assigned addresses. ART also defines three phases:

a)Initialization or configuration:In initialization phase, the ART tree is established while nodes jointthe network. This phase also considers two additional stages association and address assigning. In association, nodes gradually joint the network starting in the root node ofthe tree. While in address assigning,nodes indicate the number of addresses they need built when the tree is complete, and finally all nodes have an ARTT.

b)Operation:In operation phase, nodes start sending and receiving data. Reconfigurationmay also take place if more nodes join the network.

c)Recovery:The recovery phase starts when network is out of order for link failure or routing nodefailure. In recovery phase only parts of the tree can be reconstructed while otherparts are still operating properly. In this manner, the repair and recovery may notneed to reassign addresses to nodes.

On top of the ART, a meshed ART (MART) is formed in such a way that rootnodes are connected one to another. Therefore, root nodes treat each other as childor leaf node in the local routing tree. In this way, these nodes decide on whichneighbor to choose as next hop by using the tree previously formed.

IEEE 802.16

In 1999, the IEEE established the IEEE 802.16 WG on Broadband WirelessAccess Standards which aims to develop recommended practices and standards tosupport the development and deployment of broadband wireless metropolitan areanetworks (WMAN)[5]. The official name for the IEEE 802.16 family of standards is Wireless MAN, but the industry named it as WiMAX which basically, standardizes the physical (PHY) layerand MAC sub layer.Currently, various amendments are in progress, such as the case of IEEE 802.16jamendment for Multi-hop Relay Specification [24]. Then, the TG was formed inMarch 2006 to work on wireless relay networks (WRN) which bases on master-slavearchitecture.

In IEEE 802.16j, a BS functions as an AP, and controls theWMAN. Subscriber stations (SS) are wireless nodes that synchronize to the BSand other neighboring SSs in order to exchange frames via the central BS, andforward data on request of its BS. Also, IEEE 802.16j improves previous IEEE 802.16standards by introducing relay stations (RSs) which incorporates BS functionalitiesto extend the coverage area by relaying traffic between a SS and a BS or betweenother RSs or between an RS and a BS. There are three types of RSs:

1.Fixed relay station (FRS) which does not have mobility.

2.Nomadic relay station (NRS) which has fixed location for periods comparable toa user session.

3.Mobile relay station (MRS) that forwards data even when being in motion.

Due to the increased interests, and low cost of WMN, 802.16j is more likely tobe successfully deployed in the market. The FRS concept offers the possibility tocheaply increase the range of BS.

  1. ADVANTAGES AND CONSTRAINTS OF WMNS

Advantages

There are many attractive features of WMN that make them a viable networking solution in a tactical environment. The multi-hop feature of WMN enable all nodes to serve as routers or access points, gives them several advantages over other networking schemes. If the nearest AP or neighbor is congested, a new route is formed to next closest node with the least amount of traffic. This method, known as hopping, is repeated until the data reaches its destination. WMNs work on the same principle as the Internet, which is just a wired multi-hop network. When email is sent via the Internet, the journey to the recipient involves hops to many servers. The routes are mainly dependent on network traffic density. The email may hop from west to east and then back to west before reaching its final destination in the Midwest. The journey is much longer but more efficient and faster.

One WMN advantage resulting from its multi-hopping phenomena is redundant continuous communication links. Redundancy, in turn, brings priceless reliability and availability that is required in a tactical operation environment. The fact that a WMN gets stronger when more and more nodes are added results in the additional advantages of scalability and robustness. By scalability, we mean the WMNs ability to expand the number of nodes without making major changes to the system or application software. Because a WMN is not dependent on the performance of any one node, it is naturally robust. In WMN architecture, if a node is unable to detect its routing neighbor, data will be routed along an alternative path and WMN will continue to function.