Limiting Wifi Transmit Power in Busy Places

Sometimes I am quite amazed what one can learn from glancing over log files every now and then. Did you know for example that Wifi Access points can instruct mobile devices to limit their transmission power?

This makes a lot of sense in places such as for example university libraries, where a lot of devices are present and Wifi access points are very close together to ensure good performance of the network. When I was recently at a university library, I noticed the following entry in my syslog:

Limiting TX power to 4 dBm as advertised by ...

And indeed, when querying the status of my wireless interface, it has limited itself to 4 dBm:


wlp3s0 IEEE 802.11 ESSID:"eduroam" 
Mode:Managed Frequency:5.500 GHz Access Point: 00:00:00:00:00:00 
Bit Rate=115.6 Mb/s Tx-Power=4 dBm 
Retry short limit:7 RTS thr:off Fragment thr:off
Power Management:on
Link Quality=46/70 Signal level=-64 dBm 
Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:2 Invalid misc:460 Missed beacon:0

The full transmit power of a Wifi device is 23 dBm, or 200 milliwatts. 4 dBm on the other hand is just 2.5 milliwatts. So how did that impact my performance? Not much, iperf3 showed uplink and downlink datarates in excess of 100 Mbit/s.

And here is part the Information Element from a Wifi Beacon Frame where the transmit power can be advertised:

Tag: Country Information: Country Code DE, Environment Any
Tag Number: Country Information (7)
Tag length: 60
Code: DE
Environment: Any (0x20)
Country Info: First Channel Number: 36, Number of Channels: 1, Maximum Transmit Power Level: 5 dBm
First Channel Number: 36
Number of Channels: 1
Maximum Transmit Power Level: 5dBm