Jack Unger wrote:

You raise some good points... and here are some more differences between Matt's fully-meshed WIRED network example and the real-world conditions under which WIRELESS mesh networks are so often deployed today.

My example actually used wireless P2P links, although it was meant to apply just as well to other mediums.

1) REROUTING - Only a node failure or a high peak traffic load would normally force a routing path change on a fiber/copper network. On a wireless mesh, routing path changes will also result from interference caused by other same-network nodes, interference from other networks, and interference from other wireless non-network sources. Routing path changes will also be caused by the movement of obstructions and other rf-reflective objects such as trees and vehicles.

Rerouting occurs regularly with a mesh wired network due to load balancing and QoS concerns. Although, I would agree with you that in a more traditional ring topology, rerouting would only occur on a node failure or overload with a wired network. I would also like to point out that someone of the issues you raise generally only occur on street-level networks. Using a wireless mesh network on rooftops or towers avoids many of the issues you raised. At the same time, many mesh vendors now use layer2 metrics such as rssi, signal to noise ratio, and RF frame errors in addition to layer3 metrics to select the best path. I believe these layer2 metrics are required for a proper street-level network.

2. CAPACITY - Fiber/copper networks typically start out with high-capacity (compared to wireless) full-duplex links. Wireless mesh networks start out with low-capacity half-duplex links.

What capacity any network starts with is up to folks deploying it and is not a function of medium.

3. CONNECTIVITY - Fiber/copper mesh network nodes have two or more paths to other nodes. "Real-world" wireless mesh networks may contain nodes that, in some cases (the traditional "mesh" definition not withstanding) only have a path to one other node. For example, obstructions may block paths to all but one (or even no) other nodes.

I believe almost any real-world network is going to have paths that aren't protected. For example, we have a building that served by a single fiber path because it is not economically to have diversity at that particular building. Every other building we have on fiber is connected via diverse paths.

4. ENGINEERING - Fiber/copper mesh networks are typically properly engineered for traffic-carrying capacity, QoS, latency, etc. "Real-world" wireless mesh networks are typically deployed in near-total ignorance of the Layer 1 (wireless layer) conditions. That's the great attraction (IMHO) of muni-mesh networking today. These networks are thrown up in the belief that they don't need any Layer 1 design or engineering expertise and that this will allow for quick, widespread deployment. Last time I looked however, there was still "no free lunch". I predict that the muni mesh networks that are "thrown up" today (Philadelphia will be a prime example, unless it's re-engineered correctly) will fail and fail miserably to meet the high expectations that have been raised like free or low-cost broadband for all. In addition, muni mesh networks today typically lack adequate traffic engineering and performance testing under load.

I believe you are entirely correct. However, that doesn't mean that a WISP can't properly engineer a mesh network. I would suggest the above is no different than folks stringing a bunch of Ethernet hubs together and expecting their LAN to work correctly.

I'm not saying that wireless mesh networks should never be used. There are certain (obstructed, short-link, low capacity) environments where they will be the best, most economical solution. I'm just saying that the false claims and marketing hype surrounding MOST (and let me repeat, MOST) of today's mesh networking claims, particularly mesh network nodes that contain just a single 2.4 GHz radio are going to come back to bite both the vendors and the cities that deploy these networks without sufficient wireless knowledge in the false belief that wireless mesh networks are just "plug-and-play".

Interestingly, Tropos the current poster child of wireless mesh networks is a single radio product, but all of their large networks have nodes backhauled by fixed wireless. In a sense, this means those networks are not based on a single radio. In fact, some market argue that using Canopy for backhauling Tropos --as is the case of Philadelphia-- is a better solution than to use an 802.11-based radio for backhaul like the multi-radio mesh network vendors do.

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