WiFiTodd

 

“Nothing good is easy, or quick”. – me

A good wireless design is created with the characteristics, and measurements from the type of access point that will be used in production. One of the challenges is how to determine what the capabilities are of a particular access point. Cisco doesn’t make this easy by using a power level map to dBm.

Cisco maps AP transmit power to numbers 1 through 8*. I had referenced the AP Datasheet for the values of each particular AP. The data sheet shows the values from the low end of 2 dBm to 23dBm for a Cisco 3700 series access point.

*Mostly. But not always. Read on fellow wireless person.

I would cross reference a map inside the WLC Cli to map the Tx power to the Cisco power setting (1 – 8). This is a multi-step process since you have to do it for every access point type in your wireless design and for each frequency.

The reality is much more complex. Mind Blown. *Ka-pow!* The AP Tx power and corresponding number is also dependent on the channel it is assigned.

So you have to check the AP Type, Frequency, Channel, and then look at the power map inside the Cisco WLC to get the Tx power setting to configure and document your design. (OR you could use Radio Resource Management but that is another blog post!).

Summary of variables: AP Type ->Frequency->Channel->Power setting map

Here is a Cisco 3700 AP set to channel 157. Notice the type of AP, the channel, and the power mapping.

Another little piece of good knowledge on this output is the Channel Assigned By field and the Tx Power Assigned By field. If the AP is set to a static channel or power it will be listed as User. If it is done by the controller through the RRM process, it will say DCA or DTPC.

Here is a Cisco 3700 AP set to channel 44. Notice the type of AP, the channel, and the power mapping.

Here is a Cisco 1142 set to channel 44. Notice the type of AP, the channel, and the power mapping.

So even with the same type of AP, power level 4 doesn’t always mean Txpower of 14dBm. It might mean 6dBm or some other value. That is a huge difference since the signal power doubles every 3 dBm difference (or is divided by half every 3dBm).

You can see some of this information in the GUI, but not all. You can’t find the power map in the GUI. So you do not know what the Cisco power level corresponds to without using the CLI.

Wouldn’t it be great if there was one command or screen to show you all the infromation in one place? Rather than looking at multiple sources? Well there is.

There is an advanced CLI command that shows you everything you need in one place.

show advanced 802.11a txpower

When you look at the output you can find all the information you need to plan your wireless design and/or configure the access point.

! the command for 2.4GHz is show advanced 802.11b txpower.

Now you know, and knowing is half the battle!

! Andrew von Nagy (www.revolutionwifi.net) mentioned that other variables can also play a part in the Txpower. ex. Reg domain, TxBF on/off, Antennas.