Wireless Advanced Settings · Topic 5 of 6
TX Power Settings
Learn how to set and test transmit power on a RouterOS wireless interface, what dBm means, how tx-power-mode works and why power must work both ways.
- 4 min
- TX power is set with
tx-power-modeandtx-power, in dBm. - +3 dB doubles power, and +10 dB is ten times.
- Cards send less power at higher data rates.
What it is
TX power is how much radio power the interface sends into the antenna cable.
It is measured in dBm, a logarithmic unit. More power is not always better, because a link has two directions and both must work.
The right setting depends on the distance, the antenna, the rules of your country and the noise that you create for others.
How it works
You choose a power mode and a value, check it against the rules, and test the effect on both directions.
Units
Power is given in dBm, which compares the power with one milliwatt. A few reference points help:
| Change | Effect on power |
|---|---|
| +3 dB | Double |
| -3 dB | Half |
| +10 dB | Ten times |
| 0 dBm | One milliwatt |
| 20 dBm | One hundred milliwatts |
Because the scale is logarithmic, small numeric changes are large changes in power.
The settings
Two settings control the power:
/interface wireless set wlan1 tx-power-mode=all-rates-fixed tx-power=17tx-power-modedecides how the value is used.tx-poweris the value in dBm.
| Mode | Meaning |
|---|---|
default | The interface uses the power that the card and driver choose |
card-rates | The card's own table of power per data rate is used |
all-rates-fixed | The tx-power value is used for all data rates |
manual-table | The power for each data rate comes from a table that you define yourself |
These descriptions are simplified, and the exact behaviour of the modes depends on the card and the driver. When you need an exact figure, test it on the real device and read the result, instead of assuming.
Power depends on the rate
A card cannot send the same power at every data rate. Higher rates need a cleaner signal, so the card usually sends less power at high rates than at low ones. This is why a link can reach a distance at a low rate but not at a high one.
Power and the legal limit
The legal limit is on EIRP, which includes the antenna gain. With frequency-mode=regulatory-domain, RouterOS reduces the power to stay inside the limit. In manual-txpower mode you must do the sum yourself.
More antenna gain means less allowed card power. Check the sum before you raise the power.
Power works in both directions
A link needs the signal to be heard at both ends. If you raise only your own power, the far station hears you well, but you may still not hear it. The registration table makes this visible:
/interface wireless registration-table print statsCompare signal-strength and tx-signal-strength. If you transmit strongly but the far side is weak, raising your power does not help the return path.
Why more power can hurt
- It raises interference for neighbouring links.
- It can push a card into lower quality output at high levels.
- It can make clients stay connected to a distant access point instead of roaming.
- It can exceed the legal limit.
A smaller cell with lower power can give better overall performance when many access points share an area.
A lab routine
- Note the signal at the far end with the present power.
- Lower
tx-powerby a known step, such as 3 dB. - Read the new
signal-strengthon the far end and compare. - Check
tx-ccqand the rates, to see if quality changed. - Choose the lowest power that keeps good rates and a safe margin.
The change in the received signal should roughly follow the change in power. If it does not, look at the cabling, the antenna or noise.
Start from a lower power and raise it only as needed. It is easier to add power than to explain interference.
What to remember for the exam
Six facts cover this topic.
- TX power is set with
tx-power-modeandtx-power, in dBm. - +3 dB doubles power, and +10 dB is ten times.
- Cards send less power at higher data rates.
- The legal limit is on EIRP, which includes antenna gain.
- Power must work in both directions, so compare both signal values.
- More power can raise interference and does not always improve the link.
Common mistakes
Four errors are typical.
Maximum power by default. It adds interference and may break the legal limit.
Forgetting antenna gain. The radiated power is higher than the card setting.
Fixing only one side. A strong access point cannot cure a weak client.
Assuming equal power at all rates. The card sends less at high rates.
Check yourself
Work through these three questions.
What does +3 dB mean for power?
It doubles the power. A change of +10 dB is ten times the power.
Why can raising only the access point power fail to improve a link?
The client must also be heard by the access point. The return path depends on the client power and antenna.
What does the legal limit apply to?
It applies to EIRP, which is the transmit power minus cable loss plus antenna gain. A higher gain antenna means less allowed card power.
Topic 5 of 6 in Wireless Advanced Settings
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