Friday, 23 May 2014

Cisco 3700 access point power levels

If you've been familiar with Cisco wireless controllers over the years you will know that when setting the transmit power level of the access points you get an option of 1-7, rather than anything meaningful, like dBm or mW.

We have just installed WLC 7.6 and some 3702 access points in our building to test for a wider deployment, and despite the data sheet (pictured below) listing these as the power levels power available on 2.4GHz, the options you get are only 1-5:




I was not sure where on this scale 1, 2, 3, 4 and 5 sat, and whether 5 was as low as 7 was on the older access points, or whether we now can only turn the radios down to 8 dBm. I connected a console cable to one of the AP's and did a sh run int dot11radio0’ command after changing to each power level (1-5) on the controller.

Under 'power local', the following numbers were displayed:



Power level 1 = There was no entry for power local
Power level 2 = Power local 13
Power level 3 = Power local 10
Power level 4 = Power local 7
Power level 5 = Power local 4

To my mind it made sense for these to be in dB, as the numbers go up and down by 3 with each power level (which would double or halve the power). However these don't tally with the table above, unless one is rounding up and one is rounding down, and you can only go as high as 40 mW.


On the older AP's, in this case a 3502, the power local output appeared to match exactly with what we believed them to be:




So I was left scratching my head and decided to have a look what I could find from the controller side.


I telnetted into the controller on software version 7.6 and found there are two options a lot simpler than the method I used above to find the power levels. 


First there is the 'show advanced 802.11b txpower' command, which outputs as below:



This shows you all the channel and power settings currently used by your access points with all the options at then end in brackets. The 'b' can be substituted for 'a' to view the 5Ghz.

The other command I tried was a 'show ap config 802.11b [ap name]'. This will probably be of more use when you have many more AP's and varying models on a controller than I did during our test. You can then see all the power levels supported by that access point, as in this screenshot:



...and the 5GHz (UNII-1, again substituting 802.11b for 802.11a):




While I understand Cisco kit is shipped and used all over the world, meaning a 1-5 or 1-7 scale would be easier to display than all the allowed levels in each domain, the documentation needs to be clearer and more complete. It also needs to be clear for people on the helpdesk and the people carrying out installations, who may not have access to the controllers. To a lot of people the Cisco AP's all look the same (a square white box) and those not so au fait with wireless assume they all act the same and behave in the same way. It would be great if Cisco could provide a chart for each regulatory domain for each access point/radio. Surely that wouldn't be too difficult?

Since my investigations detailed above, I have found that Will Jones started making a chart similar to what we need on his blog at http://www.wjcomms.co.uk/2013/11/27/cisco-ap-power-to-dbm-tables/. It's a good idea.

What this seems to suggest to us is that the 3702 will have less transmit power on the 2.4GHz interface than our other AP's, so when replacements, upgrades and swap-outs are required, people need to know what they are installing and that it will do the same job with regards to coverage.

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