Wireless Scan and Frequency Usage

What it is

The scan and the frequency monitor are two built in tools that show what is on the air around you.

The scan lists the wireless networks that a radio can hear. The frequency usage tool measures how busy each channel is and how much noise it carries.

You use them before you choose a frequency, and again when a link slows down. Measuring first beats guessing.

How it works

Both tools make the radio listen on the frequencies in its scan list and report what it hears.

The scan

Run the scan on a wireless interface:

/interface wireless scan wlan1

The result is a list of access points that the radio heard. For each one you see the address, the SSID, the channel, the signal strength and the noise. Flags show extra facts, such as whether the network uses encryption or runs RouterOS.

Use it to find out which networks are near, which channels they use and how strong they are.

The interface stops while scanning

A normal scan takes over the radio. A running interface drops its clients while the scan runs, so do not scan on a busy access point at a bad moment.

There is a background mode that lets the interface keep working while it scans:

/interface wireless scan wlan1 background=yes

Background scanning is the gentler option. On a link that matters, scan from a spare radio, or during a maintenance window.

Narrowing the scan

The scan visits the frequencies in the scan list of the interface. A shorter list finishes sooner.

/interface wireless set wlan1 scan-list=2412,2437,2462

Set the list to the channels you care about. Reset it afterwards if the interface is a station that must find an access point on any channel.

Frequency usage

The frequency monitor measures activity on each frequency:

/interface wireless frequency-monitor wlan1

For each frequency it shows how much of the time the channel was in use, and the noise floor. The noise floor is the background level of energy with no signal. A good channel has low usage and a low noise floor.

Usage tells you how busy the channel is with Wi-Fi traffic and other energy. Two channels can look equally quiet in a scan list, but one may carry much more traffic. The monitor shows that difference.

Comparing the two tools

The scan sees only Wi-Fi networks that announce themselves. A hidden network or a non Wi-Fi interferer may not appear. That is why you also look at usage and, where the card supports it, the spectral tools in the next page.

A simple lab routine

A good practice routine on a lab device:

• Set the scan list to the channels of the band you will use.
• Run the scan and write down the neighbouring networks and their channels.
• Run the frequency monitor and note the usage and noise on each frequency.
• Pick the frequency with the lowest usage and noise, away from strong neighbours.
• Set the frequency on the access point and check again after clients connect.

Reading the results

A strong neighbour on your channel is a problem. Its signal is above the noise by a large amount, so it takes airtime from you. A weak neighbour far above the noise floor is a smaller problem.

High noise on a frequency with no networks points to a non Wi-Fi source, such as another radio system or electrical interference. In that case change frequency, or find the source.