Wireless Mesh Networks

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Wireless Mesh Networks,


"A wireless mesh network (WMN) is a communications network made up of radio nodes organized in a mesh topology. Wireless mesh networks often consist of mesh clients, mesh routers and gateways.The mesh clients are often laptops, cell phones and other wireless devices while the mesh routers forward traffic to and from the gateways which may but need not connect to the Internet. The coverage area of the radio nodes working as a single network is sometimes called a mesh cloud. Access to this mesh cloud is dependent on the radio nodes working in harmony with each other 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. The animation below illustrates how wireless mesh networks can self form and self heal. Wireless mesh networks can be implemented with various wireless technology including 802.11, 802.16, cellular technologies or combinations of more than one type.

A wireless mesh network can be seen as a special type of Wireless Ad-Hoc Network. A wireless mesh network often has a more planned configuration, and may be deployed to provide dynamic and cost effective connectivity over a certain geographic area. An ad-hoc network, on the other hand, is formed ad hoc when wireless devices come within communication range of each other. The mesh routers may be mobile, and be moved according to specific demands arising in the network. Often the mesh routers are not limited in terms of resources compared to other nodes in the network and thus can be exploited to perform more resource intensive functions. In this way, the wireless mesh network differs from an ad-hoc network, since these nodes are often constrained by resources." (http://en.wikipedia.org/wiki/Wireless_mesh)


1. Explained by the Wikipedia at http://en.wikipedia.org/wiki/Wireless_mesh_network

Wireless mesh networking is mesh networking implemented over a Wireless LAN.

"This type of Internet infrastructure is decentralized, 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. 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 best mobile networks are those that provide a seamless handover between the mobile device and the fixed infrastructure points." (http://en.wikipedia.org/wiki/Wireless_mesh_network)


"In basic terms, the mesh provides an alternative to established methods of linking computers together and connecting them to the internet. In practice, it can be used to build large networks far more quickly and cheaply than has previously been possible. As a result, wireless networks are viable in unexpected places. New Orleans, still without a phone service after Hurricane Katrina, recently began building a free, citywide network using mesh technology, while the whole of Macedonia is now one big wireless hotspot. Networks are also providing web connections to people in the parts of the UK untouched by phone-based broadband, as well as in developing countries that have never had effective telephone networks". (from Times Online, quoted by http://www.smartmobs.com/archive/2006/01/28/wireless_mesh_n.html)

3. Jason Tashea:

"A mesh network creates reliable and redundant wireless internet access. Instead of relying on a wired access point to the internet like a traditional network, a mesh network uses wireless radio nodes that speak to each other, thus creating decentralized wireless access points. Because a mesh network does not have to communicate through a central organization (like an ISP), if one node goes down the network will self heal — allowing service to continue without interruption.

You are probably wondering, how is this different than your WiFi at home? For one, mesh networks are actually wireless. If you think of your at-home wireless router, it is wired directly to the internet. Within a mesh network, only one node needs to be hardwired. All the other nodes, of which there could be hundreds, do not require direct access to the internet, just access to the mesh network itself. This allows a mesh network to operate without laying new cable, or as a local network during a service outage." (http://technical.ly/2015/04/06/12-communities-experimenting-mesh-networks/)


From http://www.worldchanging.com/archives/003133.html :


"CUWiN -- the Champaign-Urbana community WIreless Network -- brings together a bunch of worldchanging ideas into one useful package: Free/Open Source software to create ad-hoc municipal wireless networks using recycled old PCs. The software -- which can be downloaded from cuwireless.net -- just needs to be burned onto a CD, which can then be used to boot a PC (even something as old as a 486) with a wireless card. Once the system boots, the software configures itself, looking for other nodes to connect to; the CUWiN system uses "ad hoc networking" principles to link machines together to reach the computer that's actually connected to the Internet." (http://www.worldchanging.com/archives/003133.html)

From Magnus Lawrie:

* DjurslandS.net

DjurslandS.net in Denmark is among the largest CWNs in the world).

250 volunteer built radio stations share fast internet connections with over 5000 remote rural households in an area of 3360 sq/km (Neilsen, 2007, p.3).

The Djursland peninsula, extending East from Jutland, is encircled by a fibre-optic backbone. At the end of the 1990s, despite the proximity of this communications ’trunk road’, most Djursland homes fell into the ’last mile’ of connectivity, where broadband was technically inoperable. For incumbent telecom providers, extending coverage was financially unjustifiable - market failure had occurred. By now Djursland had been in economic decline for some years. The closure in 1998 of Djursland’s only regional newspaper was the motivation to set up an internet information portal. Boevl, the computer enthusiasts’ group who took up this challenge had engaged in socially-active projects since 1992. With grant aid they had established a free access computer workshop, undertaken computer recycling and instigated fifteen similar projects in Lithuania and Kazakhstan. By practical steps of widening access to information and services, Boevl developed their vision of the ICT-Society in Djursland and from January 2001, worked with a decision making board towards the ideal of ”lightning fast internet access for a fixed price all over Djursland” (Neilsen, 2007, p.7). A phased development programme followed, with the pilot project expected to receive EU support. However, an obscure funding process resulted in the ruling that (even in the absence of competition) government intervention in the market was not allowed. As the options for external support dwindled, by increments Djursland’s network - built on DIY pragmatism more than Free Software idealism - became self-sustaining.

Through economies of scale and hobbyist-led technical innovations costs reduced, whilst individuals worked to greatly expand the network. Members of DjurslandS.net provided themselves fast internet access, at one-third of the subscriber fees in more ’economically viable’ urban areas.

* Freifunk:

Freifunk was established in Berlin in 2002 to create independent, community-based, non-commercial, open and uncensored data networks (Freifunk, 2004). Freifunk has done this using wireless technology, operating on unlicensed segments of the radio spectrum. Technical efforts have centred on adapting OpenWRT – Free Software run on consumer wireless routers.

The resulting ’firmware’, developed by a small team of volunteers, manages bandwidth allocation in order to maintain an equal flow of data throughout a wireless network; Regardless of network size, traffic, or the relative locations of two communicating points, data will be exchanged with equal priority.

A flexible ’mesh’ networking arrangement (many nodes connecting to many neighbour nodes) invests control of the network in the end devices, not an additional layer. By sharing technology and experiences, several Freifunk networks were established in Berlin. These were connected through wireless links known as the Berlin Backbone (BBB). Although the linked networks continued to operate independently, the BBB could be seen as an administrative layer which takes control away from end devices, whilst enhanced possibilities for local audio and video applications such as free radio, internet telephony and webTV advance the case for uncensored data and net neutrality. Freifunk’s social aim has been to strengthen existing organization and to develop e-democracy and grass roots structures appropriate to the digital era. Balances between social and technical participation in Freifunk networks is achieved by the absence of privileged nodes; horizontalism remains built in the network, whilst the hobbyist aspect of the organization keeps the network free to use and free of commercial interests. A diversity of participants and agendas is maintained through router firmware designed to put control with users." (http://ditch.org.uk/download/commons_wireless_autonomy.pdf)


Magnus Lawrie:

"Two types of wireless mesh network exist. These we have seen in two distinctive CWNs: infrastructure (with a separate administrative layer) and ad-hoc (where control is ’in’ the network). These principles extend more or less easily to describe user participation in the network. An organizational hierarchy is more discernable in Djursland, whereas such distinctions are not so clearly deliniated in the Freifunk community. The case studies show cultures where organizational questions (technical and social) are always informed by a view where ”architecture is politics” (Kapor, 2006). Contrasting legal and technical systems for Free Content distribution present alternative models for dealing in scarce natural resources and cheap-to-copy data. Keeping Boyle’s objections in mind, we might consider the fate of software and public good in the Global Commons to be intertwined. Certainly, in this context, much thinking is being redefined. To this extent such hitherto arcane fields as economics and law are being opened to scrutiny, and even celebrity. In 1989 political rhetoric concentrated on the grand themes of Left versus Right. Today opposing sides are drawn less by political lines, more by pragmatic aims. The diversity of CWN membership, as well as recent Intelectual Property case law, suggests it is now far more a matter of incumbents versus newcomers. Participants in CWNs may be recognized as empiricists, not ideologues (Lessig, 2001, p.69), driven by rough consenus and running code (Clark, 1992). The ’viral’ and self-organizing characteristics of CWNs however suggest both a culture and economics of autonomy, which values the Free Software Definition." (http://ditch.org.uk/download/commons_wireless_autonomy.pdf)


Why Open Mesh Networks are beneficial

Mark Cooper:

"The technologies at the heart of the digital revolution are also at the heart of the deployment of open wireless networks in the spectrum commons. The potential spectrum carrying capacity has been the direct beneficiary of the convergence of progress in digital technology and the institutional development of networks. When users add radios that help by cooperating in receiving and forwarding signals, i.e. act as repeaters, carrying capacity of the network increases. Smart nodes get their expanding brainpower from decentralized computational capacity to communicate seamlessly, utilizing embedded coordination protocols.

Smart technologies in mesh networks cooperating to deliver messages also show the beginning of anti-rivalry characteristics. The ability of each node to receive and transmit messages, even when they are neither the origin nor the destination, expands the capacity of the network. This intelligence is the key to mesh networks’ immense capacity.

The Spectrum Commons in which these networks exist exhibits the characteristic of inclusiveness, since the more nodes on the net-work, the greater the value to users. The denser the nodes in the commons, the greater is the commons’ communications capacity. The combination of digital technology and network organization has turned the old logic on its head; adding users on a mesh network improves performance. Mesh Networks allow devices to share their resources dynamically, allowing more communications to take place with less power.

However, even with new technology, there is still the challenge of how to ensure cooperation among users. Since cooperation is the key to the capacity gain, if users chose not to cooperate, the mesh network will not work. Therefore, more devices are transitioning to “embed coordination” to ensure cooperation. For example, radios become smart by embedding intelligence – algorithms – that take on the functions necessary to transmit a signal after listening to the spectrum and finding available frequencies to use and determining the power necessary." (http://cyberlaw.stanford.edu/system/files/From+Wifi+to+Wikis+and+Open+Source.pdf)

Why Wireless Meshworks don't work at scale


" After a couple years I developed a pretty good understanding that wireless mesh networks aren’t actually a good way to build a real network. These are a few of those reasons.

  • Reason 1: Management is hard and expensive. The biggest cost for the networks I ran was actually maintaining them once they were built. It’s not just replacing hardware either: you haven’t lived until you’ve hunted down transient connectivity problems resulting from RF weirdness in urban areas. It’s hard, and even if you’re relying on volunteers to do the work to keep costs down, you’re going to spend all your time just maintaining basic connectivity. Then there are network-level issues, like traffic shaping/throttling: the wireless channel is hella bandwidth constrained, so you must do extensive shaping to ensure everyone gets fair access to your limited resources.

There is a reason the only large scale mesh networks (i.e., Freifunk, Athens Community Wireless, etc) are run by a relatively tight-knit group of smart, motivated people — it’s a significant undertaking even without doing things in a decentralized fashion. My colleague used to run the largest mesh network in the world (not joking!), and eventually his group switched to a carefully-planned, point-to-point wireless network due to mesh’s management overhead, from both an RF and network perspective. When you’re building large systems you want as little unpredictability as possible, and unfortunately unplanned mesh networks just don’t deliver there.

  • Reason 2: Omni-directional antennas suck. The whole idea behind a mesh network is that each node in it can see multiple other nodes, so if one goes down, or if there is interference, the mesh routing protocol can find a new path through the network. In order to achieve this, you use omnidirectional antennas. Antennas are passive devices: they just focus RF energy. Omnidirectional antennas are very inefficient, since they throw your energy (i.e., signal) all about, when in reality you just want your signal to reach the handful of nodes nearby. This means your signal travels a shorter distance, and thus you need a higher density of nodes. In my experience, for a small apartment building this is at least one per floor to achieve a semblance of reliability. Even if all 15,000 people on the Darknet subreddit could install and maintain 10 devices, they wouldn’t cover all of Wichita, KS, not to mention the miles of farmland between it and the next town. And, to make matters worse, omnidirectional antennas also receive interference from every direction, making the mesh network less reliable.

  • Reason 3: Your RF tricks won’t help you here. You can get higher gain or directional antennas, but again this won’t help. Remember, antennas are passive, only focusing energy. Thus, a higher gain omnidirectional antenna has a radiation pattern more like a disc than a sphere, and the higher gain you go the thinner the disc gets: if your nodes are at different heights, they wouldn’t be able to “see” each other! Directional antennas allow you to focus your RF beam directly where you want it to go, but now your node can’t communicate with as many other nodes, eliminating a key property of the mesh network.

Amplifiers do nothing here, by the way. They only boost transmit power; the real limitation is receive sensitivity. Also, amplifiers are power-hungry and expensive (and there are legal limits to their power levels). Antennas are nice because by focusing both transmitted and received RF energy, they help with both (and they use no power and are relatively cheap to build).

  • Reason 4: Single-radio equipment doesn’t work; multi-radio equipment is very expensive. This is the biggest technical reason mesh networks don’t work for Internet access. If you’re using low-cost equipment, it will only have one radio transceiver. This means your node is half-duplex, meaning it can’t both send and receive at the same time. In addition, only one node in a given area can be transmitting at a time: if two nodes send at the same time, their signals “collide” and the receiver won’t be able to decode the message. This is even true if the senders are sending to different receivers: remember, omnidirectional antennas transmit in all directions, and pick up interference from all directions! And to make matters worse, every node is both a sender and a receiver, since every sent packet needs an acknowledgement. There are some tricks to mitigate these problems, but these problems are fundamental (see “hidden node problem”), especially when you have the density of nodes necessary to create a mesh network. Each of these challenges means that each node can only transmit for a small amount of time, and this reduces your effective bandwidth. In practice, a mesh network using single-radio equipment is unusable if a packet must travel more than three hops to its destination.

One solution would be to use multi-radio nodes. You would need two: one to transmit, one to receive. This solves the half-duplex problem, but you still have the interference issue, and if you use multiple channels to get around that problem you quickly will run out of RF spectrum, not to mention having the new problem of how to intelligently allocate spectrum to each node. This spectrum allocation task is an NP-hard scheduling problem, as is allocating non-interfering time-slots for single-radio equipment. There are also challenging practical considerations like how you efficiently implement a valid schedule once you compute it. And, because you still would need roughly the same node density as before, a network of multi-radio devices quickly becomes very expensive.

  • Reason 5: Unplanned mesh networks break routing. Once you have a mesh network, you have to figure out how to get packets across it. There are many protocols for mesh routing, like AODV, OLSR, and BATMAN. Fundamentally they require individual nodes to communicate with each other, which not only takes up further network resources, but also means that achieving a consistent routing state (i.e., one in which packets won’t get routed into black holes or loops) is extremely difficult for all the reasons distributed systems are hard to build. The unplanned nature of a grassroots mesh network exacerbates this problem, since poor RF-level connectivity means the connectivity state between nodes changes frequently, leading to more routing overhead in the network. It’s a bad cycle.

I’m not saying mesh networks don’t work ever; the people in the wireless mesh community I’ve met are all great people doing fantastic work. What I am saying is that unplanned wireless mesh networks never work at scale." (http://sha.ddih.org/2011/11/26/why-wireless-mesh-networks-wont-save-us-from-censorship/)

More Information

Essay: Wireless Networks as Techno-social Models. By Armin Medosch.

See also the entry on Hive Networks