Wireless Mesh Networking
Subject:   Why Conventional Mesh Networks Dont Scale
Date:   2004-05-29 14:36:50
From:   meshdynamics
As founder and CTO of a Wireless Mesh networking company, I have pondered long and hard about whether or not I should write this article.

The buzz on mesh networking certainly works in our favor. However, there is more hype than reality around mesh networking.

In the end, that can cause the bubble around mesh to bust - rapidly. Its time for a reality check on what mesh can and cannot do.

First, mesh networking is not a new concept. In some ways, the internet is a mesh network. It scales because because it does not suffer from the limitations of conventional wireless mesh networks which are:

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 25 client stations at each node of a 4 hop mesh network.

4- The clients at the last rung will receive -at best- less than a *millionth* of the total bandwidth at the root. The reason: each of rhe 25 clients at each hop are sharing the bandwidth available at that level equally, in a true ad hoc mesh network. And with each hop the radio that provides access to the next level has its bandwidth reduced hy half. So the poor guys that the 4th rung get 1/25 * 1/2 * 1/25 * 1/2 .. you get the message.

5- None of the above takes into account the problesm of contention. To avoid collisions, data transmissions in radio follow a CSMA protocol that while very effective, is not particularly efficient. With contention, the bandwidth available is reduced even further.

6- Why has this not been noticed as yet? Because - despite the hype - there are not a lot of mesh networks around and they are usually low bandwidth applications: Browsing and email for example. 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.

In my next article I shall delve into some challenges for high perfomance - enterprise class - wireless mesh networking.

Francis daCosta
Founder and CTO

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