Wireless Mesh Networking is a method to route data, voice and instructions between
wireless nodes or wireless Wi-Fi access points. It allows for continuous connections and reconfiguration
around broken or blocked paths by "hopping" from node to node until the destination is reached. A mesh
network whose nodes are all connected to each other is a fully connected network. Wireless mesh networks
differ from other networks in that the component parts can all connect to each other via multiple hops,
and they generally are not mobile. Mesh networks can be seen as one type of ad hoc network.
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Wireless Mesh Networks Contents
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Wireless mesh networking is a wireless network that is implemented over a series
of outdoor Wi-Fi access points that that all meshed together into a single,
unified wireless network.
Whereas the Internet is mostly a wire-based, co-operative electronic
communication infrastructure similar to the international postal agreement, in
that messages are mutually delivered and relayed within their separate domains
free of charge (i.e. if you relay my messages within your domain I'll relay
yours within mine), Mesh is a wireless co-operative communication infrastructure
between a massive amount of individual wireless transceivers (i.e. a wireless
mesh) that have Ethernet type capabilities.
This type of infrastructure can be decentralized (with no central server) for
less scalable applications or centrialized controlled for high scalable
applications (with a central server),
both are relatively inexpensive, and very reliable and resilient, as each node need only
transmit as far as the next node. Nodes act as repeaters to transmit data from nearby
nodes to peers that are too far away to
reach, resulting in a network that can span large distances, especially over
rough or difficult terrain.
Wireless mesh networks are also extremely reliable, as each
node is connected to several other nodes. If one node drops out of the network,
due to hardware failure or any other reason, its neighbours simply find another
route. Extra capacity can be installed by simply adding more nodes. Mesh
networks may involve either fixed or mobile devices. The solutions are as
diverse as communications in difficult environments such as emergency
situations, tunnels and oil rigs to battlefield surveillance and high speed
mobile video applications on board public transport or real time racing car
telemetry.
The principle is similar to the way packets travel around the wired Internet
— data will hop from one device to another
until it reaches a given destination. Dynamic routing capabilities included in each device allow this
to happen. To implement such dynamic routing capabilities, each device needs to
communicate its routing information to every device it connects with, "almost in
real time". Each device then determines what to do with the data it receives —
either pass it on to the next device or keep it. The routing algorithm used should attempt to always ensure that
the data takes the most appropriate (fastest) route to its destination.
The choice of wireless router technology for
wireless mesh networks is crucial. In a traditional wireless network where laptops connect to a single
access point, each laptop has to share a fixed
pool of bandwidth. With mesh
technology and adaptive radio, devices in a mesh network will only connect with
other devices that are in a set range. The advantage is that, like a natural
load balancing system, the more devices the more bandwidth becomes available,
provided that the number of hops in the average communications path is kept
low.
There are three distinct generations of wireless mesh products today. In the
first generation one radio provides both backhaul (packet relaying) and client
services (access to a laptop). In the second generation, one radio relays
packets over multiple hops while another provided client access. This
significantly improved backhaul bandwidth and latency. Third generation wireless
mesh products use two or more radios for the backhaul for higher bandwidth and
low latency. Third generation mesh products
are replacing previous generation products as more demanding applications like
voice and video need to be relayed wirelessly over many hops of the mesh
network.
One-radio "Ad Hoc" Wireless Mesh. In first-generation mesh products, a single
radio provides both service (connection to individual user devices) and backhaul (links across the
mesh to the wired or fiber connection), so wireless congestion and contention takes place at every
node. Users soon discovered that only one or two radio "hops" were possible
between connections to the wired or fiber Ethernet. Support is also very poor
for Video and Voice applications because of excessive and varying delay (latency) across the network.
Two-radio Wireless Mesh, shared backhaul. To solve these contention and
congestion issues, second-generation mesh was developed by placing two radios in
each node, combining an 802.11b/g service radio with an 802.11a backhaul
radio.While this offered a performance improvement in terms of bandwidth over first-generation mesh,
problems remain. With heavy user demand, there is still significant contention
and congestion on the backhaul links. This limits the number of radio hops
before another costly wired or fiber Ethernet connection is needed.
Three Radio with Multi-radio Wireless Backhaul. Third-generation mesh
networking products add at least two physical radios for the backhaul. One
backhaul radio is used to create a link to its upstream (nearer the wired source
or "root") node. Another backhaul radio creates a link downstream to the next
neighbor node. Unlike second-generation solution, these two radios may make use
of different channels.
This increases the performance of the network in three ways:
- Each node may be sending and receiving simultaneously to its upstream and
downstream neighbors, unlike first-or second-generation nodes, which must
continually "turn around" between sending and receiving upstream and downstream.
- Because each link is managed independently, the available channels may be
re-used across the network. This expands the available spectrum, increasing
performance of the network 50 times or more compared to first- and
second-generation solutions.
- Radio is a shared medium. Nearby radios not part of the mesh network are
constantly competing for air space. In first and second generation technologies,
the backhaul channel is effectively "locked" once selected. They have only one
backhaul radio on each node and if the channel has to be changed it affects all
nodes.
In contrast, third generation mesh nodes have two backhaul radios.
Those
Mobile ad-hoc networking(MANet),
and mesh networking are therefore closely related, but mobile ad hoc networks
also has to deal with the problems introduced by the mobility of the nodes.
Mesh networks are self-healing: the network can still operate even when a
node breaks down or a connection goes bad. As a result, a very reliable network
is formed. This concept is applicable to wireless networks, wired networks, and
software interaction.
Devices in an 802.11s wireless mesh network are labelled as Mesh Points (MP). They form mesh
links with one another, over which mesh paths can be established using a routing protocol. 802.11s
defines a default mandatory routing protocol (Hybrid Wireless Mesh Protocol, or HWMP), yet allows
vendors to operate using alternate protocols, one of which is described in the draft (Radio Aware
Optimized Link State Routing). HWMP is inspired by a combination of AODV (RFC 3561)and tree-based
routing, while RA-OLSR is based on OLSR (RFC 3626).
MPs can be individual devices using mesh services to communicate with other devices in the network.
They can also be 802.11 Wi-Fi Access Points (APs) and provide access to the mesh network to mobile
clients, which have broad market availability. Also, MPs can take the role of a gateway and provide
access to one or more 802.3 networks through a Mesh Portal. In both cases, 802.11s provides a proxy
mechanism to provide addressing support for non-mesh 802 devices, allowing for end-points to be
cognizant of external addresses.
802.11s also includes mechanisms to provide deterministic network access, congestion control and power save.
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