Why OLPC mesh wireless networking won't work

Why OLPC mesh wireless networking won't work

Summary: 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.

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Why OLPC mesh wireless networking won't workOne 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: 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): AP and bridged extension wireless LAN

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.

Topics: Intel, Laptops, Networking, Wi-Fi

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137 comments
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  • One remark

    In most cases these will be used in a classroom, where in most cases there won't be wireless network or internet available. It enables the pupils to exchange files etc. etc.

    AFAIK these where never meant to bring internet to these rural areas. (just share it where it could be available).

    You're solution would work in a dense environment where lots of people would be living in a confined area. Unfortunately most of these kids live in rural area's and sometimes have to walk several miles before they get to school.

    So in theory, you're solution would work (if enough money available). But in practice it would only make the solution more expensive and depending on the availability of electricity (which might be more easily available in the future, but currently solar panels are still expensive).
    tombalablomba
    • You're mistaken

      "You're solution would work in a dense environment where lots of people would be living in a confined area. Unfortunately most of these kids live in rural area's and sometimes have to walk several miles before they get to school"

      You're mistaken. The centralized model works in a confined or open area. The mesh model falls flat on its face as soon as the students spread out. Did you even look at the illustrations?

      Changing files wirelessly using peer-to-peer is still not as efficient as a centralized AP. How many people use adhoc wireless LAN networking or their IRDA links to transfer files?
      georgeou
      • re

        [i]You're mistaken. The centralized model works in a confined or open area. The mesh model falls flat on its face as soon as the students spread out. Did you even look at the illustrations?[/i]

        Yes i did look at your illustrations. But you forget that this is meant for in a classroom. I understand that when the student is @ home he won't have any real value of the mesh, but in sparsely populated environments where parts of these will be used, you're solution is overly expensive unless you want to provide everyone with internet access and PC's. (Then wimax or even G3.5 would be a much better and cheaper solution) Of course it would be great if we could provide everybody with internet access, but considering the cost and the amount of money most of these people have, it's rather utopian.

        [i]Changing files wirelessly using peer-to-peer is still not as efficient as a centralized AP. How many people use adhoc wireless LAN networking or their IRDA links to transfer files?[/i]

        Though inefficient, it beats having no network whatsoever around. Consider two brothers living remotely which will be able to exchange files etc.
        tombalablomba
        • we're also forgetting this is meant for the third world

          i agree with george, mesh works in theory but rarely in practice, HOWEVER, when you're talking about networking in the jungle, or sahara, or slums, you kinda get what you can.

          AP > mesh > nothing
          Valis Keogh
        • Uh oh - something Ou doesn't want to see

          a reaction showing he is wrong.. = PAIN!
          nizuse
    • Directional antennas are cheap!

      Highly directional antennas are extremely cheap to fabricate (e.g. a Pringles can will do) using commercially avaiable/proven adapters as opposed to an expensive, unproven and proprietary solution.
      sf_boston
      • I know

        But getting energy in remote areas can be a lot more expensive.

        What George forgets to mention is that his solution for linking two locations requires line of sight... (So you will rarely get the 10 km's). Installing these solutions is specialized work and unless you can fix them very solid need a lot of maintenance. (An elephant scratching his hiney would upset it...).

        So one part of the solution isn't expensive, to make everything work and keep it working will cost you a lot.
        tombalablomba
        • Cheap USB directional adapter

          I carry a Hawking USB directional wireless adapter that I purchased 3 years ago for $15. It requires no power and provides 6db gain which gives me several times the range of the built in laptop antenna. I often can pick up a good signal from houses/buildings that are more than 1,000 feet away simply by pointing the antenna out the window. The antenna has a built in stand so pointing it in different directions is not that difficult. The software provides a site monitor so you can easily see the signal strength of the various access points in range. It wouldn't be that hard to imagine a similar device working with the OLPC. For an additional 50 cents you could easily build a more directional yagi that would offer 14db gain that would provide an order of magnitude increase in range at a cost of being slight more difficult to adjust. Unlike what people say here these are extremely easy to adjust given a decent site monitor utility. Finally 802.11n greatly increases the distance by taking advantage of the multi-path noise that's created from reflected objects to boost the signal.
          sf_boston
          • RE

            your antenna will work both ways, regardless of you're using an AP or a mesh network.

            My main problem is that the mesh is designed to work in the class room, and the solution provided by George has to much drawbacks in the area's where the OLPC will be deployed.

            He's creating a single point of failure. Is ignoring the power requirments (there's a reason why these OLPC's have some sort of energy generator build insite......) and is using first world logic to a third world problem.
            tombalablomba
    • throwing this in

      I was looking for a wireless repeater and came across this interesting debate and did some research.

      From wikipedia.org
      http://en.wikipedia.org/wiki/Mobile_ad-hoc_network
      http://en.wikipedia.org/wiki/Wireless_mesh_network

      Seems there is 3 ways to use the OLPC network:
      -access point for internet
      -school mesh seems like a WLAN intranet
      -simple mesh WLAN

      School and simple don't require connecting to the internet. For us, a WLAN w/o internet might be lame, but for them...jeez, i don't think they even have phones.

      According to the integrated peripherals
      http://laptop.org/en/laptop/hardware/specs.shtml
      "supports diversity reception; capable of mesh operation when CPU is powered down;"

      OLPC claims 2-3 times the range of regular laptops.
      This guy seems to be doing various tests
      http://dev.laptop.org/~quozl/
      2006-12-12: AP to OLPC, 200m?
      radio range test: between two OLPC, connect at 1.3 km?
      also tried a 6 km test but failed "possibly due to fresnel effect and terrain obstruction, or due to excessive 2.4GHz noise from nearby cities"

      This review
      http://www.pcworld.com/article/id,140931-page,1/article.html
      mentions that "Using the XO even on a broadband wireless network is as slow as thawing ice outdoors in a Boston winter".
      It seems it was tweaked for range and handling traffic at the cost of speed to create a better mesh network. No sense in having it fast if you it's out of range.

      So the repeater I am checking out is the Meraki Mini.
      They made a successful mesh network in San Francisco. Some also worked on the OLPC. They also have a solar-powered repeater.
      I forgot the reason why but you cannot have a lot of repeaters in one area. it was a reason why you don't see them around but Meraki has a system where you need to register the location so they know where they all are.
      zee_oh_six@...
  • RE: Why OLPC mesh wireless networking won't work

    George raises valid points, although his "won't work" title is rather harsh! My(ahem- extensive!)2.4GHz wireless experiences certainly confirm desk level mesh nodes will have very limited ranges,no doubt further crippled by kids working sprawled out on the floor! A OLPC news picture some months back showed a OLPC/XO wastefully ceiling mounted to ensure good mesh coverage in fact... YIKES!

    Although "smart",the OLPC IEEE802.11n is still subject to wireless limits -almost ANYTHING in the propagation path of microwaves blocks such signals!Inbuilt WiFi adapters are especially hindered by PC metal work,office partitions,solid walls & even the "water filled" user themselves. LOS (line of sight) links of 100s of metres may drop to just 10s of m when a user moves behind a tree or building.

    Raising the antenna even to window height typically gives ENORMOUS improvements,& to avoid having to work standing up this most readily can be down using USB WiFi adapters. Given the OLPC target market I'm hence of the opinion that the preferred mesh technique should be USB based,as it further allows unskilled user "signal sweet spot" positioning of the adapter via dirt cheap lossless USB cables & extenders. Range boosting DIY antenna can be rustled up as well - see the celebrated "WokFi" site => www.usbwifi.orcon.net.nz,which has empowered numerous "poor men" since it's 2004 roll out.
    Manukanz
  • I think the people at MIT

    are a damn sight smarter than you or any of us posting here. I also think they have thought about this and there are solutions to assist as well. You also are applying the conventional American way of thinking to the problem and applying American needs to the solution. The recipients are not American nor are they at a technology level where they need hi speed networking. These are the foundation children for building a more technology oriented society for themselves. These kids are not going to know they are not getting the fastest internet or networking experience, and it won't matter. The fact they will have it at all is a pure joy in of itself.

    And since this will be a first for many, unlike the spoiled American's, the recipients of this technology will be happy to have it, even if it is slow and not always reliable. It's a damn sight better than what they have now when it comes to technology.

    So stop your bashing on the program. I don't see you doing anything positive to in this vein. The idea is to start building at the child level, a technology base. And to do it as cost effective as possible. So that means there are going to be some short comings, the same levels of performance we see here are not going to be initially available there. This is there baby step into the world of technology. ]:)
    Linux User 147560
    • Even if

      Even if the "people at MIT" are smarter then the rest of us that doesn't make them infallible. In fact, if they think as highly of themselves as you seem they are probably well on their way to disaster by hubris.

      That said it seems to me that this shouldn't be an either or proposition. Why wouldn't the same radio be able to partake in MESH network or a traditional AP based network? Would that have increased the cost?
      Levi Miller
      • they can

        the radio's can take part in a traditional ap based network
        tombalablomba
        • nice MIT guys

          The MIT guys have already tested it. I worked in that project at MIT a few years back (moved west). And it works. You really think they would release it if it wasn't?
          anplymoth
          • That wasn't my point.

            The mesh stack may "work", but being practical and useful is a whole different ballgame. Client-side mesh solves a problem that was already solved much better by a cheaper technology.
            georgeou
  • RE: Why OPLC mesh wireless networking won't work

    I wonder, George, have you ACTUALLY put together a network with these laptops, and ACTUALLY run tests to back up your statements?
    Have you had one in your hands to ACTUALLY use it and know what the capabilities are?
    Just some questions.....
    TTGIT Guy
    • LOL, Guttman Anyone? Anyone? NT

      .
      bka1959
    • Range and power is something that's fundamental

      Range and power is something that's fundamental. No one will ever suggest that you can push a small low-power radio and a small lid antenna more than 50 meters effectively. One big antenna and one relatively powerful 300mW radio will easily have a 10:1 advantage in range and the area covered is 100 times larger. There's no getting around that.
      georgeou
      • Read my post again

        George, read my post again so you understand the questions; they're really quite simple.
        Once again, you dance around and avoid responding in a meaningful manner.
        TTGIT Guy