Wireless Tools · Topic 2 of 3
Spectral Scan, Spectral History and Snooper
Learn how spectral scan, spectral history and the snooper show energy on each frequency, non Wi-Fi interference and per network traffic on a channel.
- 5 min
- The spectral scan shows energy per frequency, including non Wi-Fi sources.
- The spectral history shows the energy over time, so it shows bursts.
- The ordinary scan lists only Wi-Fi networks.
What it is
Spectral scan, spectral history and the snooper show what the ordinary scan cannot.
The ordinary scan lists Wi-Fi networks. The spectral tools show raw energy on each frequency, including energy from devices that are not Wi-Fi. The snooper shows traffic and networks on the channels as seen by the radio.
Together they help you find interference, and they help you decide which channel is truly the best.
How it works
The spectral tools measure signal energy per frequency, and the snooper summarises traffic per channel.
Spectral scan
The spectral scan shows the energy the radio receives on each frequency in the range you ask for:
/interface wireless spectral-scan wlan1It gives a live, text based display. A strong energy peak means something is transmitting on that frequency, whether it is Wi-Fi or not. A flat low floor means the frequency is quiet.
This is the tool to use when a channel looks empty in the scan but the link is still poor. Microwave ovens, cordless phones, video senders and other radio devices all show up as energy. The ordinary scan cannot see them.
Spectral scanning needs a card and driver that support it, so not every wireless interface offers it.
Spectral history
The spectral history keeps a record of the energy over time:
/interface wireless spectral-history wlan1Instead of one live picture, it builds a history, where each new line is a new moment and colour shows the energy. You can see whether interference is steady, or comes and goes.
| Tool | Shows | Best for |
|---|---|---|
| Spectral scan | Energy now per frequency | Finding a source at this moment |
| Spectral history | Energy over time per frequency | Finding bursts and patterns |
A steady stripe on one frequency means a constant source. Short bursts that repeat suggest a device that turns on and off, such as an oven or a nearby radio link.
The interface stops while measuring
Like the scan, the spectral tools use the radio for measuring. Clients may lose service while they run. Use a spare radio or a quiet period, and stop the tool when you finish.
Snooper
The snooper listens to the channels and summarises the traffic it sees:
/interface wireless snooper snoop wlan1It shows how busy each channel is, how many networks are on it, and which networks and stations are active. This helps when you want to know who is using a channel and how much traffic they send.
The ordinary scan shows that a neighbouring network exists. The snooper shows how much that network is actually using.
What the snooper tells you
| Information | Meaning |
|---|---|
| Channel usage | How much of the time the channel is busy |
| Networks on the channel | Which SSIDs share the frequency with you |
| Stations | Which clients are active, and how much they send |
| Traffic | Rates for each network and station |
A channel with many networks but little traffic may still be a good choice. A channel with one network sending constant traffic may be a bad one.
Using the tools together
A useful lab routine:
- Run the ordinary scan to list networks.
- Run the spectral scan to look for energy on frequencies that show no network.
- Run the spectral history for a while to catch bursts.
- Run the snooper to see which networks carry real traffic.
- Choose the frequency with the least interference and the lowest load.
Limits
These tools describe one place and one time. A site survey at the access point position is more useful than one done from a desk. Repeat the check at busy hours, because a quiet channel at night can be busy during the day.
What to remember for the exam
Six facts cover this topic.
- The spectral scan shows energy per frequency, including non Wi-Fi sources.
- The spectral history shows the energy over time, so it shows bursts.
- The ordinary scan lists only Wi-Fi networks.
- The snooper shows channel usage, networks and stations.
- These tools use the radio, so they can disturb normal operation.
- Spectral tools need a card and driver that support them.
When a quiet looking channel still gives a poor link, look for non Wi-Fi interference with the spectral history.
Common mistakes
Four mistakes are common.
Expecting the scan to show interference. It lists Wi-Fi networks and misses other energy.
Using one short measurement. Bursty sources need the history, or a longer time.
Measuring from the wrong place. The result must come from the position of the antenna you will use.
Running the tools on a live access point. The clients may lose service while the radio is measuring.
Check yourself
Work through these three questions.
What can the spectral scan find that the ordinary scan cannot?
It shows energy on each frequency, including energy from non Wi-Fi devices. The ordinary scan only lists Wi-Fi networks.
Why use spectral history instead of one spectral scan?
It shows the energy over time, so you can see bursts and repeating patterns. A single scan only shows one moment.
What does the snooper add to a list of neighbouring networks?
It shows how busy each channel is and which networks and stations are active. This tells you how much traffic they actually carry.
Topic 2 of 3 in Wireless Tools
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