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Four Simultaneous Access Points OK for 802.11b
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."
Besides, if you want that many people on your network, 802.11b is not appropriate given it's slow link speed. I don't think 802.11b was ever meant to be the only netfrastructure in a commercial setting. My company uses it for conference rooms so the "important people" can bring their laptops in to waste everyones' time with presentations about schedules and upcoming meetings. :)
Putting any more than about 10 people on a wireless network in a "we're trying to do actual work" setting is suicide... oh wait, this guy was with a startup, they probably just use it to play Quake...
That's why I said it's a modified 2D problem. It is a stack of 2D color problems, along with dealing with an adjacent 2D color problem. The "leakage" is always from one plane to an adjacent plane, each tiled, with no interaction across a plane. It is not truly 3D, as each plane simply interacts with one other plane, as far as a minimum answer goes, and while there is no limit to the answers for a 3D problem, there is for a modified 2D problem like this is. The maximum number of colors needed for such a modified 2D map is 8, with an alternating set of 4 colors every level.
Erik
YOU ARE SAYING IMPUDENCE TO ME! THAT IS IMPUDENCE!
Channels are 22MHz wide, not 27...
If they limit the spread to 2.4Mhz, almost 10 times less spectrum, then the data rates would drop and the succeptability to outside interference from other 2.4Ghz radiators would go way, way, way up.
It has nothing to do with how precise they can make the equipment, and everything to do with playing nice in the 2.4Ghz spectrum.
The reason there is channel overlap is because channel overlap is NOT as bad as everyone makes it seem.. poeple still tend to think in terms of normal radio bands.. where overlap destroys the signal. This is direct sequence spread spectrum, it can handle some level of noise.. hence the overlap.
There are so many nonsensical articles about what 802.11b can and can not do that I thought I'd set the record straight.
... well, sit down and draw yourself a map.
... I've seen DSSS signal quality go from excellent to unuseable just by flipping on a Cirronet AP in the same area.
There are eleven channels available in north america - 22MHz wide, spread from 2402 to 2483 MHz, with 5 MHz guard bands between them. Channels 1, 6, and 11 don't overlap, the others
There are many other things in 802.11b besides DSSS 802.11b cells that you 31337 kids can h4x0r - I've got Western Mux Tsunami and Adtran Tracer T1 bridges. T1s are full duplex - these types of radios split the ISM band 50/50 - one end sends with the bottom half and listens at the top, the other side is opposite, and they use 100% of the spectrum.
The other thing you'll find are FHSS systems in the ISM band. The most common is the Alvarion (previously Breezecom) Breeze Access II three meg access radios, but Cirronet's lower speed ISP products is starting to appear in rural areas.
If you're working inside a building with full duplex T1 bridges or a hot FHSS somewhere outside its definitely going to make a difference, and that goes double if you're running an 802.11b system outside. Putting one of these things near an 802.11b AP is basically like sand blasting a soup cracker
Assuming you've got no problems to deal with other than your 802.11b, other's 802.11b, and building layout you've still go trouble.
The 802.11b MAC layer is *broken*. If I pull up and start listening on a channel you're using, even if you've got WEP enabled, I can see your mac addresses and I can *issue disconnect requests* after forging your MAC and the AP *will honor the disconnect*. WEP is the equivalent of an ESP (encapsulated security payload) in IPsec and it protects your data, but the MAC layer needs something like the IPsec AH(authentication header) so that an intruder can't manipulate the MAC layer.
Building systems always have dead spots. Always. 2.4 gig bounces like crazy when there is sheet metal (HVAC duct work) is in the area. You get reflected signals (multipath) which causes corrupt frames, you get dead spots due to the signal being blocked, etc, etc. You can add further misery by trying to use an AP with 'diversity'. Drop the word from your vocabulary - its 'perversity' mode - just take the time to monitor FCS errors on an AP with this enabled and you'll know what I mean - turn that stuff on in a situation where both antennas can see the same signal and you'll toast 50% of incoming frames *every time*.
The 802.11b MAC layer is *broken*. If you want detailed knowledge I'd strongly suggest a read of the OReilly's 802.11 Wireless Networks book, but the game goes something like this. The channel you're in is a *shared* resource - that means you share with the rest of the world. 802.11b stations gain exclusive access to the channel they're on by settings a NAV (network allocation vector) in certain frames. Even if you have a WEP protected network the exposed MAC layer you're using will honor NAVs *from devices not on your network*. So when the same intruder who was disconnecting individual stations a few paragraphs back gets tired of that he can start issuing bent control frames that plug up the spectrum and bring your network to a crawl.
I've barely scratched the surface here. If you see a pretty diagram and a lot of marketing buzzwords, understand that the reality is much, much more grim.
I am very easy to get along with, but I don't have time to waste being nice to people who are being stupid. -Theo