Ofcom's radio carve-up could cut out mesh
Ofcom has published the first of its consultation documents looking at how Spectrum Usage Rights (SURs) are going to work in the real world, and the future looks bad for mesh networks with their lack of centralised control.
The document is a follow-up to the Ofcom statement on SURs, published last December, and represents a refinement of that proposal. It deals with specific instances where owners of neighbouring blocks of spectrum could experience interference, and how Ofcom intends to respond to any complaints on the matter.
While most of the document (pdf) is pretty technical, it does admit that control of spectrum usage is dependent on centralised control of transmitters, something impossible in mesh networks were every node is responsible for its own communications. Or, as the document puts it:
In a pure mesh there appears to be no easy way to control the mobile density or transmitted power. Therefore, it appears to us at the moment that an operator could not change from an existing licence to this type of deployment and still be able to demonstrate that they met their SUR requirements.
Mesh networks have demonstrated their ability to operate in unlicensed frequency bands,
My comment: Operate, yes, but not in a way that's both efficient and friendly to other users of that spectrum.
but it's hard to imagine how a company could make money from licensing spectrum and operating a mesh network, so the problem may be moot.
Ofcom states they'd be interested in hearing suggestions of how a mesh could be shown to conform to the SUR, if an operator applies for a change of use to deploy a mesh - but it's interesting to see how the open, flexible model can throw up its own technical limitations.
For more on mesh network problems, see
Ugly truths about mesh networks - they dont scale - for now. by Francis daCosta
As founder and CTO of a Wireless Mesh networking company, I have pondered long and hard about whether or not I should submit this.
The buzz on mesh networking certainly works in our favor. However, there is more hype than reality around mesh networking. Its time for a reality check on what mesh can and cannot do.
First, Mesh networks are not a new concept. In some ways, the internet is a mesh network. And it works, despite its size - because it does not suffer from the limitations of conventional wireless mesh networks:
1- Radio is a shared medium and forces everyone to stay silent while one person holds the stage. Wired networks, on the other hand, can and do hold multiple simultaneous conversations. 2- In a single radio ad hoc mesh network, the best you can do is (1/2)^^n at each hop. So in a multi hop mesh network, the Max available bandwidth available to you degrades at the rate of 1/2, 1/4, 1/8. By the time you are 4 hops away the max you can get is 1/16 of the total available bandwidth. 3- That does not sound too bad when you are putting together a wireless sensor network with limited bandwidth and latency considerations. It is DISASTROUS if you wish to provide the level of latency/throughput people are accustomed to with their wired networks. Consider the case of just 10 client stations at each node of a 4 hop mesh network. The clients at the last rung will receive -at best- 1/(16,0000) of the total bandwidth at the root. 4- Why has this not been noticed as yet? Because first there are not a lot of mesh networks around and second, they have not been tested under high usage situations. Browsing and email don t count. Try video - where both latency and bandwidth matter - or VOIP where the bandwidth is a measly 64Kbps but where latency matters. Even in a simple 4 hop ad hoc mesh network with 10 clients, VOIP phones wont work well beyond the first or second hop the latency and jitter caused by CSMA/CA contention windows (how wireless systems avoid collisions) will be unbearable.Mesh networks are a great concept. But the challenge lies in managing the dynamics of mesh networks so users receive an acceptable level of performance in terms of both latency and throughput.
Its time to focus on solving some real problems to make mesh networks scale and provide stable performance.