PoE-based access points Tx power level limitations

I recently noticed that Cisco AP 1142 Tx power level is not a constant value. It is dependent on a data rate in use.
Less words, here’s an example of “show controllers dot11Radio” taken from Cisco AIR-LAP1142N-E-K9:

interface Dot11Radio0
Radio AIR-AP1140G, Base Address b4a4.e3ca.f130, BBlock version 0.00, Software version 3.00.81
Serial number: <cut>
Number of supported simultaneous BSSID on Dot11Radio0: 16
Carrier Set: EMEA (EU) (-E)
Uniform Spreading Required: No
Configured Frequency: 2462 MHz Channel 11
Allowed Frequencies: 2412(1) 2417(2) 2422(3) 2427(4) 2432(5) 2437(6) 2442(7) 2447(8) 2452(9) 2457(10) 2462(11) 2467(12) 2472(13)
Listen Frequencies: 2412(1) 2417(2) 2422(3) 2427(4) 2432(5) 2437(6) 2442(7) 2447(8) 2452(9) 2457(10) 2462(11) 2467(12) 2472(13) 2484(14)
Beacon Flags: 0, Interface Flags 20105, Interface Events 0, Mode 9; Beacons are enabled; Probes are enabled
Configured Power: 17 dBm (level 1)
Active power levels by rate
1.0 to 11.0 , 16 dBm, changed due to regulatory maximum
6.0 to 48.0 , 13 dBm, changed due to regulatory maximum
54.0 to 54.0 , 11 dBm, changed due to regulatory maximum
6.0-bf to 54.0-b, 10 dBm, changed due to regulatory maximum
m0. to m5. , 13 dBm, changed due to regulatory maximum
m6. to m6. , 11 dBm, changed due to regulatory maximum
m7. to m7. , 10 dBm, changed due to regulatory maximum
m8. to m13. , 13 dBm, changed due to regulatory maximum
m14. to m14. , 11 dBm, changed due to regulatory maximum
m15. to m15. , 10 dBm, changed due to regulatory maximum
m0.-4 to m5.-4 , 13 dBm, changed due to regulatory maximum
m6.-4 to m6.-4 , 11 dBm, changed due to regulatory maximum
m7.-4 to m7.-4 , 10 dBm, changed due to regulatory maximum
m8.-4 to m13.-4, 13 dBm, changed due to regulatory maximum
m14.-4 to m14.-4, 11 dBm, changed due to regulatory maximum
m15.-4 to m15.-4, 10 dBm, changed due to regulatory maximum
6.0-d to 48.0-d, 13 dBm, changed due to regulatory maximum
54.0-d to 54.0-d, 11 dBm, changed due to regulatory maximum
OffChnl Power: 16, Rate 1.0
Allowed Power Levels: -1 2 5 8 11 14 17
Allowed Client Power Levels: 2 5 8 11 14 17

Why is it like this? Why higher data rates have lower Tx power levels? These were my questions too. Luckily, I came across Educause Archive, where a good explanation had been given by a Cisco TAC engineer. Here’s the most interesting part form that conversation:

Yes this power levels are real (don’t be amazed) it’s pretty much the same across the board with our competitors as well. What you are seeing here is not an FCC regulated limitation but rather one of PoE. When we design products, such as the 1140 we design to a power of approx 12.5 Watts (yes 802.3af is 15.4 Watts) but the device is designed less as there is loss in Ethernet cable etc. As the data rates go lower the transmitter power goes up since the transmitter EVM limit is relaxed.

EVM is the linear or distortion factor, the higher the data rate the less distortion is tolerated. Similar to receiver sensitivity gets better as the data rates go down (since it can decode better through the distortion).

If you have a need for higher transmitter power, take a look at the AP-1250 product which can accept a higher PoE rating (beyond that of 802.3af) using our power injector.

Want a higher Tx power levels (regulated by FCC/ETSI only)? Consider using a different AP models, like 1250 that requires a more powerful PoE injector to be installed for a full power capabilities.

By the way, if you take a careful look at Cisco 1140 Series Data Sheet on Cisco.com, you will find a small footnote, which tells the same but with no extra details:

Note: The maximum power setting will vary by channel and according to individual country regulations. Refer to the product documentation for specific details.

Good luck 🙂

Leave a Reply

%d bloggers like this: