Thursday, November 16, 2006

Talkin' 'bout my generation

As an engineer, I often assume an idea is self-evident. But clearly that is not true for the concept of third-generation wireless mesh, if the emails I have received about the last post are any indication.

So today, a brief non-technical discussion of the generations of wireless mesh that may make the preceding post more intelligible.

In all of this discussion, I believe it is important to keep in mind two real-world problems. First, in the unlicensed frequency bands (such as 802.11), RF spectrum (and especially use and re-use of available channels) is always a major limiting factor. Interfering radio sources may spring up at any time. Channel and routing agility is the only answer for this unavoidable problem.

In addition, the RF spectrum contention in 802.11-based systems creates throughput problems that can only be effectively dealt with by creating separate contention domains. This in turn, is only possible through the use of multiple logical- or physical radios.

Third-generation wireless mesh solutions incorporate these two elements. And what about the first two generations? Glad you asked.

The first solutions for outdoor wireless networking created access points that could not only connect to users, but also could create links from node-to-node. First developed for military applications, this first-generation product is sometimes called "ad hoc" or "single radio" mesh. This was the first wireless technology to allow some extended distance between wired or fiber connections.

A welcome initial step, users rapidly discovered challenges with first-generation technology. Because a single radio provides both service (connection to individual user devices) and backhaul (links across the mesh to the wired or fiber connection), 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 Voice and video applications because of excessive and varying delay across the network. This is precisely what Google is discovering with the first-generation mesh technology deployed in Mountain View.

In an effort to solve the contention and congestion issue, second-generation mesh was developed by placing two radios in each node. This technology separated user traffic from backhaul traffic by creating a separate network for each. This was often achieved by combining an 802.11b/g service radio with an 802.11a backhaul radio in each node.

While this offers significant performance improvement over first-generation mesh, problems remain. With heavy user demand, there is still contention and congestion on the backhaul links, which of necessity still share a single radio channel. This limits the number of radio hops before another costly wired or fiber Ethernet connection is needed. The performance of voice and video applications also suffers because of the congestion in the backhaul stream.

Third-generation solutions add more logical- or physical radios. To overcome the problems of congestion and contention, one radio is used to create a link to its upstream (nearer the wired source or "root") node. Another radio creates a link downstream to the next neighbor node. Unlike the second-generation solution, these two radios make use of different channels. This increases the bandwidth of the network in two ways.

Firstly, each node may be sending and receiving simultaneously to its upstream and downstream neighbors, unlike the backhaul radio of the second-generation wireless mesh, which must continually "turn around" between sending and receiving upstream and downstream. (It's even worse for the first-generation single-radio mesh network used in Mountain View; the node must manage users and backhaul on the same radio!) Secondly, 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- to 1000 times or more compared to first- and second-generation solutions.

Third-generation wireless mesh solutions also dynamically detect and avoid interference. Each individual node contains the equivalent of a radio spectrum robot, monitoring other radio traffic, tracking its neighbor third-generation mesh nodes, and adjusting the topology and channel mapping of the network instantaneously, automatically, and without disturbing users' sessions.

This third-generation technology is what is needed for major metro deployments, and the problems in Mountain View provide ample proof of that fact.

Francis daCosta


Anonymous Steve Kimberley said...

Thanks, Francis, for this clear outline of the topic! I was looking for information on the range of mesh nodes and found my way herre from Wikipedia. 3rd generation sounds like the right solution to the bandwidth contention issue - should have been this way fromt he start!

3:26 AM  
Anonymous Anonymous said...

Thanks for this VERY easy to understand and helpful information.

Scott, NZ.

7:13 PM  
Blogger David Fawcett said...

Great article Francis.

Thanks for taking the time to write it.

4:28 AM  

Post a Comment

<< Home