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Data rate question

Unanswered Question
Jan 7th, 2006
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I upgraded a radio from "B" to "G" the other day and began immediately to have a loss of connectivity when the client got further away from the AP. This client had no problems when the radio was a "B" radio. The only changes I made after installing the radio was to change the data rates in IOS to allow connectivity from 1 - 54 megs.


I then read that with "G" speeds you loose distance in terms of how far the signal will travel. I changed the config in the AP to allow 1 - 36 megs and the client began to work fine.


Therefore I have two questions:

1. Was this coincidental? I was under the impression that the client would automatically stepdown to a speed that it could "talk" to the AP with.


2. I also read that broadcast and multicast traffic is sent at the lowest speed allowed on that AP. If so, is it best to have only 1 speed defined on the AP and that speed being the fastest speed that will still allow connectivity?


Thanks,

Brian

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scottmac Sun, 01/08/2006 - 07:05
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What kind of distance are we talkin' about here?


The system "g" has two basic characteristics:


1. ) It uses the same frequency and channels as "b," so from an RF transmission point of view, the transmission distances should be the same in the same environment.


2. ) Since there is more data encoded into the "g" signal (higher data density), any interference of X duration that occurs during the transmission of a data frame will affect more data.


I suspect the second is what's happening to your signal. 802.11 transmissions automatically encode redundant infirmation in the frame to offset most common noise and still be able to reconstitute the frame at the receiving end.


If the noise or interference is of sufficient duration, you'll kill enough data elements that the frame cannot be properly reconstituted, and the errored frame is dropped.


So, from the sounds of it, the threshold point is ~36Mbps ... above that, the noise or interference is corrupting enough of the data that the frame cannot be rebuilt.


I believe the reason your AP is not "downshifting" is that the type of frame used to detect the stable link speed is short enough that the noise or interference is not affecting it (or not affecting it often enough) to cause the AP do downspeed.


So, one of the first things you can try to relieve this problem is to reduce your frame size and / or your RTS threshold.


Both of these parameters are available in the Web GUI under the interface screen.


Also try other channels (if you're in North America, stick with 1, 6, and 11).


If you use a utility like "Netstumbler" (it's free), it can show you what kind of channel utilization and noise in your enviromnet.


Good Luck


Scott


vhashrjacksb2 Sun, 01/08/2006 - 08:34
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Thanks for your time and the answer. I will try your suggestions next week at work. I am still curious about the the speeds you should have defined on the AP. Is it best to have only the "threshold" speed or is it best to have multiple speeds defined? Again, this is based on my reading that an AP will send broadcast and multicast at the lowest speed defined on the AP.


Thanks,

Brian

scottmac Sun, 01/08/2006 - 11:15
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It's best to define a range. If your client, for whatever reason, can't connect at the one speed you've defined on the AP, it won't connect at all.


If the range of speeds defined as mandatory on the AP, and the client can't match the config, the client won't connect.


If you had no problems with broadcast / multicast traffic under 802.11b, then you shouldn't have any additional problems with 802.11g.


Also, if you're using "short" preambles, the lowest data rate (IIRC) is two meg, versus one meg for "long" preambles.


Good Luck


Scott


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