802.11a network throughput/range and channel selection

Unanswered Question
Jun 12th, 2009

Hi All,

I seem to remember reading somewhere (although I can't find it now) that with an 802.11a wireless network, network throughput and range will differ depending on which channel in the frequency range you select.

I think I read something along the lines of (I might have this the wrong way round) lower channels will give you good throughput but will not travel through thick walls well whereas higher channels can pass through thick walls but offer lower throughput.

Does this sound right to anyone or did i just dream this up? If it is right, is there a middle ground in terms of channel selection that offers a good compromise? I.e. reasonable throughput and signal penetration

Many thanks, n3ilb

I have this problem too.
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rob.huffman Fri, 06/12/2009 - 06:44

Hi n3ilb,

I don't believe that to be the case here :) The Channels still all work in the 5Ghz range. The power settings would have some bearing but not the actual "A" Channel.



dancampb Fri, 06/12/2009 - 08:42

To some degree there is some truth to this. Different channels in the 802.11a band have different maximum tx power levels. Basically channels 36-48 only have a tx power of 15dBm. The other channels have a mx tx power of 17dBm.

George Stefanick Fri, 06/12/2009 - 09:24

So if i have a 1131 and put it on an upper band you are saying power level 1 is 17 dBm and if i use a lower channel power level 1 will be 15 dBm?

Is this docuemnted anywhere? I would really appreciate any further info you have on this subject ...

I am aware of the standard and the allowance the upper bands have, but i never thought gave it much though on the cisco radios...

dancampb Sun, 06/14/2009 - 08:04

Correct. Depending on the channel you've selected power level 1 could be a different value. This is due to the regulatory requirements, not something Cisco came up with.

George Stefanick Sun, 06/14/2009 - 13:10

understood. But is there a cisco document that discusses this? What power for what channel?

Matthew Fowler Sun, 06/14/2009 - 17:12

You can see the power level to actual values by running "show ap config 802.11a ".


Tx Power

Num Of Supported Power Levels ............. 7

Tx Power Level 1 .......................... 17 dBm

Tx Power Level 2 .......................... 14 dBm

Tx Power Level 3 .......................... 11 dBm

Tx Power Level 4 .......................... 8 dBm

Tx Power Level 5 .......................... 5 dBm

Tx Power Level 6 .......................... 2 dBm

Tx Power Level 7 .......................... -1 dBm

Tx Power Configuration .................... AUTOMATIC

Current Tx Power Level .................... 1

Otherwise, the best document to look at is the 802.11 standard (http://standards.ieee.org/getieee802/download/802.11-2007.pdf). I expect you already know this, but just posting for any interested lurkers.

Particularly Annex I (I.2.2 Transmit power levels) which I have attached as a screenshot.

George Stefanick Sun, 06/14/2009 - 19:04

Lets keep this convo going ... i appreciate your response...

I just put my AP on channels 36 and 161 ... both stated TX 1 @ (17 dBm)

Unless im missing something the max is 50mW across all bands ... ? Am I wrong? It doesnt appear to me the TX power is changing ...

Matthew Fowler Mon, 06/15/2009 - 17:28

Hi George,

Yeah it looks like the output from that command doesn't change. Sorry about that.

If I do this though:

(WiSM-slot1-2) >debug ap enable MattDeskAP1142

(WiSM-slot1-2) >debug ap command "show controllers do 1" MattDeskAP1142

Channel (40,36)

*Jun 16 11:20:01.578: MattDeskAP1142: Configured Power: 14 dBm (level 1)

Channel (149,153)

*Jun 16 11:15:28.398: MattDeskAP1142: Configured Power: 17 dBm (level 1)

So, it does lower it's power (to 14dBm) in UNII-1. (This is a -N AP)


George Stefanick Thu, 06/18/2009 - 14:16

thanks Matt ... i cant say i ever read this in a cisco document anywhere. in fact if you select a lower band channel with an autonomous ap it allows you to select 17 dBm (50mW). You would think that it would be 'grayed out as an option'.

Thanks again ...

rob.huffman Tue, 06/16/2009 - 04:31

Hey Guys,

Well, you really do learn something new every day! +5 points to all of you for this excellent, albeit, surprising, update :)



jackmanshawn Tue, 06/16/2009 - 10:02

Indeed, great response, guys. Also, keep in mind that at 802.11a we really should avoid the DFS bands for critical applications. If any of you have ever used a network analyzer on a traditional spectrum analyzer (Anritsu, et al.) and have done a return loss measurement across the HUGE 5GHz band you will find that antennas themselves aren't tuned the same across all channels. I've seen greater than 6dB with some antennas. In fact, be REALLY cautious in your antenna selection (if using external antennas) because some are clearly stated UNII-1 and UNII-2 and don't mention UNII-3 (vice-versa).


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