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Four Simultaneous Access Points OK for 802.11b

Posted by ryuzaki0 on from the slashdot-needs-a-wireless-topic dept.

jlouderb writes "ExtremeTech is reporting on a new analysis that shows that four of the 11 802.11b channels can actually be used simultaneously, rather than the just the three used today. This has big ramifications for multi-access point installations, especially in taller buildings. The analysis was done by the CTO of an 802.11b startup called Cirond and a white paper with all the details should be posted to their site later today."

11 of 73 comments (clear)

  1. Why four is better than three by ites · · Score: 3, Interesting

    Ask any map maker. You cannot tile a surface of arbitrary shapes with three colours. You need four colors. Since a WiFi channel acts as a map-filling color, the ability to use four channels gives real advantages.
    Well, it made sense when I thought of it. :-)

    --
    Sig for sale or rent. One previous user. Inquire within.
    1. Re:Why four is better than three by BlueUnderwear · · Score: 4, Interesting
      Yes, but they are speaking about tall buildings here? So wouldn't they need to solve the 3 dimensional equivalent of the map coloring problem (a map is only 2D), and thus need more than "colors"?

      However I agree, 4 is still better than 3.

      --
      Say no to software patents.
    2. Re:Why four is better than three by topham · · Score: 3, Interesting

      Sure signals would leak from floor to floor, but floors are made of more significant material thanmost walls in buildings as well. Which means then atenuate the signal more. So, inherently less interference from the floors below (assuming all else is equal... it isn't...)

      According to one article I read on the antenas used for a particular access point it mostly radiated on the plane. Meaning there would be less signal from above or below, combined with the added issue of a floor/ceiling between them and your almost home free...

      YMMV...

    3. Re:Why four is better than three by rossifer · · Score: 2, Interesting

      Start with the simplest manifold, which is a sphere. That only needs four colors. Which should make sense because maps are 2d projections of a mostly spherical surface and maps only need four colors.

      Now add a topological "handle" to the sphere (poke a hole in it and make a torus or donut). Now you need six colors, though there's no proof of that, only a counter example for five.

      It turns out that for each "handle" or tunnel in the manifold you need zero, one, or two more colors. If the next handle doesn't interact (stick two donuts together at the edge), then you don't need more colors. If you poke another hole in the donut from the edge into the middle, you do need another color.

      Another way to think about it. Take a map that requires four colors to draw. Now add a tunnel in place between two areas of the map that have the same color and say that the border between the two endpoints is halfway through the tunnel (so that two areas with the same color touch). You'll have to add another color to print that map now. Repeat for five colors, then six, etc. You can always add another tunnel that will force two regions of the same color to be in contact, so for an arbitrarily complex manifold, you can't specify how many colors it will take.

      Wait, that actually is a proof. :)

      Regards,
      Ross

  2. Why the frequency spread? by Big+Mark · · Score: 3, Interesting

    According to the article, the channel centres are seperated by 5MHz, yet the channels themselves span 27MHz.

    Now, I'm probably missing something really, really obvious, but why don't they just limit the spread to, say, 2.4MHz from the centre? Surely that way all the channels (11 or 13, depending on where you are) could be used and you would still have a small buffer zone between channels?

    Is is just that they can't make the frequency generators precise enough, or something?

  3. In other news... by ihowson · · Score: 5, Interesting

    Using seven of the available channels was ranked as "sort of OK", nine described as "highly dangerous" and eleven of the available channels ranked as "marginally suicidal", by a team of highly trained hamster analysts.

    If channel overlap is an issue where you are, you probably have too many damn AP's. Witness, rooms 424 and 417 of the EE building at Sydney University - two access points PER ROOM. Admittedly, they're large rooms (labs). I'm told that one is meant to be taken out of each and used elsewhere.

  4. Ramifications not that big by mindstrm · · Score: 5, Interesting

    and the hundreds of RF engineers who have already spoken and said that only 3 channels do not overlap are still correct.

    802.11b Channels are 22Mhz wide, and spaced 5Mhz apart.... grab a pencil and paper and figure it out. You can't get more than 3 channels without overlap.

    The article lacks any real detail, other than a brief but accurate (typo aside, channel 1 goes to 2423MHz, not 2433Mhz) description of the 802.11b channel scheme.

    One of the benefits of DSSS is that you can deal with interference to a good degree. If you use four channels, as widely spaced as possible, instead of three, you narrow your bandwidth, but not by too much. I imagine the overlap could be reduced to between 2Mhz for the end channels, and 4Mhz for those in the middle, possibly only 2 for those in Europe where the spectrum allocated is wider. Given how DSSS works, this may not affect data rates noticeably... this is what they are probably going to talk about in the alleged whitepaper.

    Not sure why it's an article yet... there's no info yet.

  5. ?? Leave it to the pros by greensquare · · Score: 2, Interesting
    1) Why do you assume that since it's covering the whole building there will be a lot of people? I like the the network where most people use wired ethernet when they can, but they can fall back wireless when out of their office, say visting a coworkers office, or working in a LAB, or checking system status on a PDA.

    2) The number of people using the network is only 1 factor in a larger equation for determining the viability of WLAN. Depending on usage, a single WLAN Access Point may not be fast enough for 1 person. Or it could be fast enough for 500 people. All depends what they are using it for. Are servers used extensively?, for Files? Applications? Do people surf the net? Do processing jobs suck large files over the network?

    3) This article does not state that adjacent channels won't interfere. They are saying that they won't interfere VERY MUCH. If you are setting up a 3D grid of WLAN AP's, full coverage with a little interference using 4 channels is better then what can be done using 3 channels. With only 3 channels with 2 AP's per floor Some adjacent AP's will need to be on the same channel. That would be bad. With 4 channels, the overall network capacity is a little lower, but you have better coverage.

    4) I Agree, people like you should definitely pay somebody for a site survey. Probably you should hire someone for the entire network design, not just wireless. I'd be glad to do the job, I'll sub the work out for $10,000, and put the other $190,000 in my wallet.

  6. Re:hah by mindstrm · · Score: 4, Interesting

    Where did he say frequencies don't overlap?

    Where did you read that this is about spatial separation?

    What kind of real business are you in where you think 802.11b can only handle 10 users "doing work". Network intensive work, perhaps, but many, many businesses use their network for email and surfing only.

    The article says they are talking about acceptable levels of interefernece that do not degrade performance... which is entirely possible with some channel overlap.. that's one of the benefits of direct-sequence spread spectrum, it's inherently redundant, many times over.

  7. Re:802.11b - learn the truth by GoRK · · Score: 5, Interesting

    It's not illegal the way they do it. They use a DSSS frequency that is 33mhz wide on the upper half and 33mhz wide on the lower half. This is the same type of usage that you'd get colocating 802.11b dsss ap's on channels 1, 6 and 11. The same laws are not true in canada, though. You are technically only supposed to use a maximum of 50% of the band for your entire setup.

    most FHSS radios also use 100% of the spectrum, but they break it up into one of 74 or so chunks that transmit one at a time. The illegal thing to do is to synchronize let's say 74 fhss radios so that they transmit without accidentally hopping on top of one another. These laws are not the same everywhere, though, and in some parts of the world, it's indeed legal and a very good thing to run synchronized fhss radios. Incedentally, the alvarion fhss radios actually support this operation: but you actually have to use them in africa and such other places.

    Anyway, kudos to the parent poster. I have been arguing his points for years here on /. (ever since the editors and seemingly everyone else creamed their panties over the bastardized 802.11b). Really, deploying 802.11b for anything more complicated than a single or dual AP installation so you can walk around your house/office with your laptop is probably a bad move. There are a lot of better wireless technologies out there and a lot of them aren't even that much more expensive than 802.11b crap.

    ~GoRK

  8. Re:802.11b - learn the truth by OverCode@work · · Score: 3, Interesting

    Time synchronizing FHSS transmitters to use the entire spectrum also happens to defeat one of the main purposes of FHSS, which is to reduce multipath problems by changing frequencies before a second path reaches the receiver. I would expect that technique to result in a lot of interference.

    -John