There are few terms such as dB, dBi, dBm are used to describe the amount of change in power measured at points in a system to a reference power level.
The term dBi is used to describe the power gain rating of antennas. The real antennas are compared to an isotropic antenna (a theoretical or imaginary antenna) that sends the same power density in all directions, thus the use of dBi.
Antennas are compared to this ideal measurement, and all FCC calculations use this measurement (dBi). For example, a Cisco omni-directional AIR-ANT4941 has a gain of 2.2 dBi, meaning that the maximum energy density of the antenna is 2.2 dB greater than an isotropic antenna.
The term decibel (dB) is mainly used for attenuation or amplification of the power level. dB is a logarithmic ratio of a signal to another standardized value.For example dBm is where the value is being compared to 1 mW (milli Watt)
Power (in dB) = 10 * log10 [signal/reference]
For example 100mW in dBm equal to 10* log10[100/1] resulted in 20dBm. In the same way 1000mW is 30dBm. As you can see dB is not a linear scale & increase of 10dB mean signal power level up by 10 times. Decrease of 10dB means signal went down by 1/10th of the original value. Therefore 10mW = 10 dBm.
Also increase or decrease of 3dB means power level up or down by factor of 2. Since you know 100mW is 20 dBm. Then reduce 3 dB (ie 17 dBm) equal to 50 mW. In similar logic 25mW = 14dBm, 12.5 mW = 11 dBm and so on.
If you go on the upwards, 200mW= 23 dBm, 400 mW= 26 dBm.
These calculations become useful if you have given a task to assign custom power level to a Access Point.
In Unified Wireless deployment power level represent by a numerical value rather actual dBm as shown in the below example for a given AP. Value 1 represent the max power and reduce by half ( or decrease of 3 dBm) in each subsequent level.
You can find the AP Tx power by “show ap config 802.11a|802.11b <ap_name> ” CLI command.
Here are examples of Tx Power of few different type of AP models. This is Tx Power for the 3602I model AP.
(WLC1) > show ap config 802.11a AP01 Tx Power Num Of Supported Power Levels ............. 5 Tx Power Level 1 .......................... 14 dBm Tx Power Level 2 .......................... 11 dBm Tx Power Level 3 .......................... 8 dBm Tx Power Level 4 .......................... 5 dBm Tx Power Level 5 .......................... 2 dBm Tx Power Configuration .................... AUTOMATIC Current Tx Power Level .................... 1 Tx Power Assigned By ...................... DTPC (WLC1) > show ap config 802.11b AP01 Tx Power Num Of Supported Power Levels ............. 7 Tx Power Level 1 .......................... 22 dBm Tx Power Level 2 .......................... 19 dBm Tx Power Level 3 .......................... 16 dBm Tx Power Level 4 .......................... 13 dBm Tx Power Level 5 .......................... 10 dBm Tx Power Level 6 .......................... 7 dBm Tx Power Level 7 .......................... 4 dBm Tx Power Configuration .................... AUTOMATIC Current Tx Power Level .................... 1 Tx Power Assigned By ...................... DTPC
Here is same output for 3502I AP
(4402-a) > show ap config 802.11a 3502-d 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 .................... CUSTOMIZED Current Tx Power Level .................... 3 (4402-a) >show ap config 802.11b 3502-d Tx Power Num Of Supported Power Levels ............. 8 Tx Power Level 1 .......................... 20 dBm Tx Power Level 2 .......................... 17 dBm Tx Power Level 3 .......................... 14 dBm Tx Power Level 4 .......................... 11 dBm Tx Power Level 5 .......................... 8 dBm Tx Power Level 6 .......................... 5 dBm Tx Power Level 7 .......................... 2 dBm Tx Power Level 8 .......................... -1 dBm Tx Power Configuration .................... CUSTOMIZED Current Tx Power Level .................... 3
Here is the output for 1242 AP model
(4402-a) > show ap config 802.11a 1252-c Tx Power Num Of Supported Power Levels ............. 6 Tx Power Level 1 .......................... 14 dBm Tx Power Level 2 .......................... 11 dBm Tx Power Level 3 .......................... 8 dBm Tx Power Level 4 .......................... 5 dBm Tx Power Level 5 .......................... 2 dBm Tx Power Level 6 .......................... -1 dBm Tx Power Configuration .................... AUTOMATIC Current Tx Power Level .................... 1 (4402-a) > show ap config 802.11b 1252-c Tx Power Num Of Supported Power Levels ............. 8 Tx Power Level 1 .......................... 20 dBm Tx Power Level 2 .......................... 17 dBm Tx Power Level 3 .......................... 14 dBm Tx Power Level 4 .......................... 11 dBm Tx Power Level 5 .......................... 8 dBm Tx Power Level 6 .......................... 5 dBm Tx Power Level 7 .......................... 2 dBm Tx Power Level 8 .......................... -1 dBm Tx Power Configuration .................... AUTOMATIC Current Tx Power Level .................... 4
It looks like even if you set a certain power level to a AP, Tx power may not exactly the value display in the above command output. It will determine by the regulatory restrictions in each domains.
Below blog post from Revolution Wi-Fi explain this in detail. Please read this to get proper understanding of determining Tx power of a light weight AP.
mrn-cciew 