Cisco 1242 AP Preamble Mode

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Sep 30th, 2010
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I would like to know if configuring only 802.11g bit rate force the AP to use the the 20usec ERP-OFDM preamble. My understanding of the protection mode where b&g clients coexists force the AP to used the long 192 usec preamble so b client can understand that the air channel is utilized for the duration defined in the length field.

In other words, can we assume that as soon as we configured bit rate like 2,5,5,11 Mb/sec the AP is using the long 192 usec preamble



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Nicolas Darchis Thu, 10/07/2010 - 08:57
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Hi Stephane,

This is independant. Short preamble is activated with command "preamble-short" under the radio.

The protection mode is activated as soon as a .11b client is heard in the vicinity. What it does is simply send a CTS (clear to send) packet to itself before transmitting. This CTS is coded in .11b rates and declare for how long the next transmission will keep the network busy. It's a way of telling .11b clients "hey, you will hear something you don't understand during x milliseconds, just wait".


Steph1963 Thu, 10/07/2010 - 10:46
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Hi Nicolas,

Thanks for the answer, did not know that preamble-short command exists. Do you know if this command can only be applied to b rate?

If b clients can connect to an AP that supports b and g rate, I was under the impression that the AP was using by default the long b preamble so b client understand. I am still not sure when b and g rate are supported by the AP if the AP is only using b preamble for every client or only for b client and OFDM (g) preamble for g client.

Thanks for your help


Nicolas Darchis Thu, 10/07/2010 - 22:36
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Hi Stephane,

I think you're mixing 2 concepts.

Concept 1 : Long or short preamble. Short preamble were already available at the time of 802.11b. Cisco APs still support long preambles for the sake of compatibility. You can restrict to short preambles only (more efficient). This is regardless of the rate used.

Concept 2 : Protection mode when a .11b client is present. A Clear-To-Send frame is sent before every .11g frames. The CTS frame is sent in .11b data rate so that .11b clients can understand.

I hope it clarifies ?

So for your questions, when b and g clients are presents, EVERYONE (except .11b clients obviously) sends a CTS frame in .11b rate to warn of upcoming .11g frames.



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