September 25th, 2007
Why OLPC mesh wireless networking won't work
One of the touted features of the $200 OLPC laptop is the peer-to-peer mesh topology networking feature that can theoretically bring an Internet infrastructure where there is no network infrastructure. The problem is that peer-to-peer wireless LAN mesh topology sounds better than it actually works and there’s a good reason it isn’t used commercially.
[UPDATE 9/27/2007 - I should clarify that OLPC mesh technology applies to the XO laptop shown on the left or to the Intel Classmate [current version of Classmate doesn't support mesh]. Intel is also on the board of OLPC so it’s not OLPC versus Intel. Intel is also providing some help on technology based on the centralized Access Point and Bridge model. OLPCs can also work with centralized wireless LAN infrastructures and that is the point of this blog; that the two technologies work best together and that they’re not mutually exclusive. A $60 Linksys router running modified Linux and a $20 antenna can provide fast and reliable infrastructure for the entire school.]
The word “mesh” is traditionally highly regarded in the networking world because every IT student is taught in Computer Networking 101 that “mesh topology” is the most advanced form of networking. Mesh topology traditionally conjures up the image of multiple redundant links with high-performance distributed loads but that only applies to the wired networking world when multiple physical links are used to build the network. High-performance and load-distribution does not apply to wireless mesh topology especially when we’re talking about typical implementations that use a single radio and a single radio frequency. In fact, every wireless relay adds another hop and the relay action doubles the radio contention because the same data has to be retransmitted on the same radio frequency.
Even if we ignore the delay and contention problems of mesh topology wireless LANs, there’s an even more fundamental problem facing the peer-to-peer mesh technology being implemented in projects like the OLPC. The radios and antennas are so small that it would take hundreds of OLPC devices with perfect spacing to replace a single high-powered Access Point with high-gain antennas. Consider the illustration below where I compare OLPC laptops that are capable of transmitting up to 50 meters with their small 30mW radios and small antennas versus a centralized AP that’s capable of 400 meters range.
Mesh versus Access Point topology:
Note that I’m being very conservative with the 400 meter range with a 300mW Access Point because those things can easily go twice as far. But even with a mere 8:1 advantage in range, it would take more than a hundred OLPC laptops to cover the same area. If we’re talking about a more realistic 16:1 advantage in range, then it would take more than 400 OLPC laptops to cover the same area and they would all have to be spaced out perfectly. We also have the possibility of using 500mW radios and 16 dBi antennas for even longer range in rural areas. When we consider the fact that a single failure in one of the mesh nodes due to battery drainage, moving out of range, software hang will cause the entire mesh scheme to break, there simply is no way to get around the centralized architecture.
Last week at Intel’s IDF convention in San Francisco, Intel’s “World Ahead Program” was showing off some cheap commodity technology and blueprints that would empower schools with wireless networking and Internet access. These blueprints and part lists allow the schools to build their own wireless infrastructure with cheap off-the-shelf components. The all-in-one Wireless Access Point and Wireless Bridge box (dual radio) allows remote locations that lack wired Internet uplinks to bridge wirelessly to the central uplink. I came up a slightly modified version shown in the illustration below to show the flexibility of this architecture.
AP and bridged extension wireless LAN (full size):
With a few of these “towers” with sufficient transmit power and high-gain omni-directional antennas for client access and directional antennas for the backhaul; we can reliably cover a very large campus.
George Ou is Technical Director of ZDNet. See his full profile and disclosure of his industry affiliations.
