A wireless mesh network (WMN) is a communications network made up of radio nodes organized in a mesh topology. It can also be a form of wireless ad hoc network.Chai Keong Toh Ad Hoc Mobile Wireless Networks, Prentice Hall Publishers, 2002.
A mesh refers to rich interconnection among devices or nodes. Wireless mesh networks often consist of mesh clients, mesh routers and gateways. Mobility of nodes is less frequent. If nodes constantly or frequently move, the mesh spends more time updating routes than delivering data. In a wireless mesh network, topology tends to be more static, so that routes computation can converge and delivery of data to their destinations can occur. Hence, this is a low-mobility centralized form of wireless ad hoc network. Also, because it sometimes relies on static nodes to act as gateways, it is not a truly all-wireless ad hoc network.
Mesh clients are often laptops, cell phones, and other wireless devices. Mesh routers forward traffic to and from the gateways, which may or may not be connected to the Internet. The coverage area of all radio nodes working as a single network is sometimes called a mesh cloud. Access to this mesh cloud depends on the radio nodes working together to create a radio network. A mesh network is reliable and offers redundancy. When one node can no longer operate, the rest of the nodes can still communicate with each other, directly or through one or more intermediate nodes. Wireless mesh networks can self form and self heal. Wireless mesh networks work with different wireless technologies including 802.11, 802.15, 802.16, cellular technologies and need not be restricted to any one technology or protocol.
History Wireless mesh radio networks were originally developed for military applications, such that every node could dynamically serve as a router for every other node. In that way, even in the event of a failure of some nodes, the remaining nodes could continue to communicate with each other, and, if necessary, serve as uplinks for the other nodes.
Early wireless mesh network nodes had a single half-duplex radio that, at any one instant, could either transmit or receive, but not both at the same time. This was accompanied by the development of shared mesh networks. This was subsequently superseded by more complex radio hardware that could receive packets from an upstream node and transmit packets to a downstream node simultaneously (on a different frequency or a different CDMA channel). This allowed the development of switched mesh networks. As the size, cost, and power requirements of radios declined further, nodes could be cost-effectively equipped with multiple radios. This, in turn, permitted each radio to handle a different function, for instance, one radio for client access, and another for backhaul services.
Work in this field has been aided by the use of game theory methods to analyze strategies for the allocation of resources and routing of packets.
Features
Architecture Wireless mesh architecture is a first step towards providing cost effective and low mobility over a specific coverage area. Wireless mesh infrastructure is, in effect, a network of routers minus the cabling between nodes. It is built of peer radio devices that do not have to be cabled to a wired port like traditional WLAN access points (AP) do. Mesh infrastructure carries data over large distances by splitting the distance into a series of short hops. Intermediate nodes not only boost the signal, but cooperatively pass data from point A to point B by making forwarding decisions based on their knowledge of the network, i.e. perform routing by first deriving the topology of the network.
Wireless mesh networks is a relatively "stable-topology" network except for the occasional failure of nodes or addition of new nodes. The path of traffic, being aggregated from a large number of end users, changes infrequently. Practically all the traffic in an infrastructure mesh network is either forwarded to or from a gateway, while in wireless ad hoc networks or client mesh networks the traffic flows between arbitrary pairs of nodes.J. Jun, M.L. Sichitiu, "The nominal capacity of wireless mesh networks" , in IEEE Wireless Communications, vol 10, 5 pp 8-14. October 2003
If rate of mobility among nodes are high, i.e., link breaks happen frequently, wireless mesh networks start to break down and have low communication performance.
Management This type of infrastructure can be decentralized (with no central server) or centrally managed (with a central server). Both are relatively inexpensive, and can be very reliable and resilient, as each node needs only transmit as far as the next node. Nodes act as routers to transmit data from nearby nodes to peers that are too far away to reach in a single hop, resulting in a network that can span larger distances. The topology of a mesh network must be relatively stable, i.e., not too much mobility. If one node drops out of the network, due to hardware failure or any other reason, its neighbors can quickly find another route using a routing protocol.
Applications Mesh networks may involve either fixed or mobile devices. The solutions are as diverse as communication needs, for example in difficult environments such as emergency situations, tunnels, oil rigs, battlefield surveillance, high-speed mobile-video applications on board public transport, real-time racing-car telemetry, or self-organizing Internet access for communities. An important possible application for wireless mesh networks is VoIP. By using a quality of service scheme, the wireless mesh may support routing local telephone calls through the mesh. Most applications in wireless mesh networks are similar to those in wireless ad hoc networks.
Some current applications: U.S. military forces are now using wireless mesh networking to connect their computers, mainly ruggedized laptops, in field operations. They specifically use forms of mobile ad hoc network (MANET) using a range of military protocols. See . Electric smart meters now being deployed on residences, transfer their readings from one to another and eventually to the central office for billing, without the need for human meter readers or the need to connect the meters with cables. The laptops in the One Laptop per Child program use wireless mesh networking to enable students to exchange files and get on the Internet even though they lack wired or cell phone or other physical connections in their area. Smart home devices such as Google Wi-Fi, Google Nest Wi-Fi, and Google OnHub support Wi-Fi mesh (i.e., Wi-Fi ad hoc) networking. Several manufacturers of Wi-Fi routers began offering mesh routers for home use in the mid-2010s. Wireless mesh networks can be used to overcome gaps in cellular coverage for a vehicle tracking system. In applications such as fleet management, vehicles often travel through remote areas where a consistent GSM/cellular signal is not available, preventing telemetry data from being transmitted in real-time. A mesh network allows vehicles to act as mobile nodes. If one vehicle is out of cellular range, its GPS tracking unit can transmit its vehicle location data to a nearby vehicle that is in range. That second vehicle then acts as a relay, forwarding the data to the central server. This ensures that a continuous track and trace record is maintained, even in areas with poor connectivity. Some communications satellite constellations operate as a mesh network, with wireless links between adjacent satellites. Calls between two satellite phones are routed through the mesh, from one satellite to another across the constellation, without having to go through an earth station. This makes for a shorter travel distance for the signal, reducing latency, and also allows for the constellation to operate with far fewer earth stations than would be required for an equal number of traditional communications satellites. The Iridium satellite constellation, consists of 66 active satellites in a polar orbit and operates as a mesh network providing global coverage.
Operation The principle is similar to the way packets travel around the wired Internet – data hops from one device to another until it eventually reaches its destination. Dynamic routing algorithms implemented in each device allow this to happen. To implement such dynamic routing protocols, each device needs to communicate routing information to other devices in the network. Each device then determines what to do with the data it receives – either pass it on to the next device or keep it, depending on the protocol. The routing algorithm used should attempt to always ensure that the data takes the most appropriate (fastest) route to its destination.
Multi-radio mesh Multi-radio mesh refers to having different radios operating at different frequencies to interconnect nodes in a mesh. This means there is a unique frequency used for each wireless hop and thus a dedicated CSMA collision domain. With more radio bands, communication throughput is likely to increase as a result of more available communication channels. This is similar to providing dual or multiple radio paths to transmit and receive data.
Research topics One of the more often cited papers on wireless mesh networks identified the following areas as open research problems in 2005:
New modulation schemes To achieve higher transmission rate requires new wideband transmission schemes other than OFDM and UWB. Advanced antenna processing Advanced antenna processing including directional, smart and multiple antenna technologies is further investigated, since their complexity and cost are still too high for wide commercialization. Flexible spectrum management Tremendous efforts on research of frequency-agile techniques are being performed for increased efficiency. Cross-layer optimization Cross-layer research is a popular current research topic where information is shared between different communications layers to increase the knowledge and current state of the network. This could facilitate development of new and more efficient protocols. A joint protocol that addresses various design problems—routing, scheduling, channel assignment etc.—can achieve higher performance since these problems are strongly co-related. Note that careless cross-layer design can lead to code that is difficult to maintain and extend. Software-defined wireless networking Centralized, distributed, or hybrid? - In a new SDN architecture for WMNs is explored that eliminates the need for multi-hop flooding of route information and therefore enables WMNs to easily expand. The key idea is to split network control and data forwarding by using two separate frequency bands. The forwarding nodes and the SDN controller exchange link-state information and other network control signaling in one of the bands, while actual data forwarding takes place in the other band. Security A WMN can be seen as a group of nodes (clients or routers) that cooperate to provide connectivity. Such an open architecture, where clients serve as routers to forward data packets, is exposed to many types of attacks that can interrupt the whole network and cause denial of service (DoS) or Distributed Denial of Service (DDoS).
Examples A number of wireless community networks have been started as grassroots projects across the world at various points in time.
Other projects, often proprietary or tied to a single institution, are: ALOHAnet was first used in Hawaii in 1971 to connect the islands.