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WiFi 2.4 Ghz Freq Channel Allocation

Discussion in 'GENERAL Wireless Discussion' started by RadioRaiders, Jun 12, 2007.

  1. RadioRaiders

    RadioRaiders RF Black-Belt Junior Member Senior Member

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    Does anyone know the reasoning behind why the 2.4 Ghz 802.11 freq spectrum jams 12 freq channels into a band better suited for 3?

    Is there a reason why all but 3 channels are overlapping? To me, it seems it would be better if only 3 non-overlapping freqs were assigned, to avoid interference. :confused:

    [​IMG]
     
  2. bobolito

    bobolito Diamond Senior Member Senior Member

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    I always thought about that. I can understand maybe the FCC didn't have enough spectrum available, but if that was the case, why not designate just 3 channels? Why would someone switch from channel 6 to channel 7 when it virtually makes no difference?

    Maybe the whole issue was for users to be able to "fine tune" the frequency used since that band is shared by many other devices. Another thing is that since WiFi is similar to CDMA in the sense that it spreads spectrum, you can effectively have two WiFi access points on the same channel next to each other and they'll both work fine because the technology is very tolerant to interference.
     
  3. RadioRaiders

    RadioRaiders RF Black-Belt Junior Member Senior Member

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    WiFi is different than CDMA in terms of interference, in that WiFi uses CSMA to block users from transmitting on the same channel at the same time. This makes interference from other useres in the same band impossible and causes data rates to slow for users because they have to "wait in line".

    CDMA however will always allow everyone to transmit at the same time. Only each user will cause the overall SIR to increase, and the added interference will cause a lower modulation rate to compensate for the noise, and reduce data rates to users.

    I was thinking something along the same lines, but I still don't see it yet. Maybe the extra bit of freq that is not overlapping is meant to allow a gauranteed connection on each AP in case of congestion? :confused:
     
  4. bobolito

    bobolito Diamond Senior Member Senior Member

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    But then again, if there is WiFi congestion, moving up one channel isn't going to help because WiFi channels are 20 Mhz wide and switching up one channel only moves you up 4Mhz. But let's say your microwave oven or your cordless phone is causing interference in the upper edge of channel 3. Since that portion overlaps with the lower edge of channel 6 (the most common channel used for WiFi in the US), moving up to channel 7, or even better 8, can avoid the interference.
     
  5. RadioRaiders

    RadioRaiders RF Black-Belt Junior Member Senior Member

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    After some Google-ing, the best answers i came up with was:

    -To switch channels to avoid external interference (cordless phones, microwave ovens, etc.)
    -To fully utilize the whole band (including the gaps between channels 1,6,11)

    It seems most large scale WiFi networks (ie: office buildings, etc.) use just the 3 non-overlapping channels in a 1/3 re-use pattern. It looks like all the other channels are just to give users some extra flexablity if they are trying to operate in a highly congested area, or an area with alot of interference.

    If anyone else has any other ideas or theories, please post them :dunno:
     
  6. Dogma

    Dogma Senior Member Senior Member

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    On these Wireless routers (802.11g) how many users can be on an AP to actually use the router at the same time? And does getting an AP established much different than actually being allocated channels for dwnlding & uplding?
     
  7. bobolito

    bobolito Diamond Senior Member Senior Member

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    I run computer lab with about 30 computers all wireless using the same AP with no problem.
     
  8. RadioRaiders

    RadioRaiders RF Black-Belt Junior Member Senior Member

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    I'm not sure what the theoretical amount would be, or what it would be based on (is there any limitation based on signaling?), but here's a practical example:

    Sometimes manufacturers list the max number of users possible on their devices (usually around 60-100), but I'm not sure what this is referring to. I think it may be referring to the number of unique MAC addresses the AP can handle (ie: if you want to grant access based on MAC addresses for additional security)
     

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