Wireless Standards · Topic 2 of 3
Bands and Channel Width
Understand the RouterOS band setting and channel width options, including 5, 10, 20 and 40 MHz, and the trade off between speed, range and noise.
- 4 min
- The band setting chooses the frequency range and the standards.
- The width setting is
channel-width, and the older standards use 20, 10 and 5 MHz. - A 10 MHz channel halves the rates, and a 5 MHz channel quarters them.
What it is
The band chooses the standards an interface uses, and the channel width sets how much spectrum one channel takes.
The two settings work together. The band decides which rates exist, and the width decides how fast those rates run and how many channels fit in the spectrum.
A wider channel carries more data, but it takes more space and picks up more noise. A narrower channel is slower, but it is more robust and leaves room for others.
How it works
The band and channel-width settings are chosen per interface, and both ends must agree.
The band setting
The band names tell you the frequency range and the standards. These are the common values:
| Band value | Meaning |
|---|---|
2ghz-b | 2.4 GHz, 802.11b only |
2ghz-b/g | 2.4 GHz, b and g |
2ghz-onlyg | 2.4 GHz, g only |
2ghz-b/g/n | 2.4 GHz, b, g and n |
2ghz-onlyn | 2.4 GHz, n only |
5ghz-a | 5 GHz, 802.11a only |
5ghz-a/n | 5 GHz, a and n |
5ghz-onlyn | 5 GHz, n only |
A station scans using its own band setting. If the band does not include the standard that the access point uses, the station never sees it. The same goes for a mismatch between 2.4 GHz and 5 GHz.
Channel width values
The width is set with channel-width. For the older standards, the values are 20 MHz, 10 MHz and 5 MHz. For 802.11n, additional values bond two 20 MHz channels into 40 MHz.
/interface wireless set wlan1 channel-width=20mhz
/interface wireless set wlan1 channel-width=10mhz
/interface wireless set wlan1 channel-width=20/40mhz-CeThe 802.11n chapter explains the Ce, eC and XX forms. They tell RouterOS where the second 20 MHz channel sits.
Half and quarter channels
A 10 MHz channel is half the width of a normal one. A 5 MHz channel is a quarter. These widths lower the data rates by the same factor, so a 54 Mbps rate becomes 27 Mbps at 10 MHz and 13.5 Mbps at 5 MHz.
In return, narrow channels give more usable channels in a small slice of spectrum. They can also improve range and sensitivity, which helps on long links.
Both ends must use the same width, and ordinary clients such as phones usually cannot join a 5 or 10 MHz network.
Trade offs
| Width | Speed | Channels available | Noise picked up | Typical use |
|---|---|---|---|---|
| 5 MHz | Lowest | Most | Least | Long, robust links |
| 10 MHz | Low | Many | Less | Long links, crowded bands |
| 20 MHz | Normal | Normal | Normal | General access |
| 40 MHz | Highest | Fewest | Most | Short, clean 802.11n links |
Width is a decision between capacity and reliability. Do not use 40 MHz in the 2.4 GHz band when neighbours are present, because only a few channels exist there and a wide channel overlaps with many others.
Checking what the interface uses
/interface wireless print detail where name=wlan1
/interface wireless monitor wlan1The first command shows the configured band and width. The second shows the live state, including the frequency in use.
Stations and the access point
A station follows the access point for most settings. It still needs a band that includes the access point's standards. If the width does not match on a link between two RouterOS devices, the link may not form or may run at poor rates, so set both ends the same.
What to remember for the exam
Six facts cover this topic.
- The band setting chooses the frequency range and the standards.
- The width setting is
channel-width, and the older standards use 20, 10 and 5 MHz. - A 10 MHz channel halves the rates, and a 5 MHz channel quarters them.
- Narrow channels give more channels in the spectrum and better sensitivity.
- Wide channels give more speed but pick up more noise and leave fewer channels.
- Both ends of a link must use matching settings.
In the crowded 2.4 GHz band, stay with 20 MHz. The 40 MHz option is for 5 GHz or very quiet places.
Common mistakes
Four mistakes are common.
Choosing 40 MHz in 2.4 GHz. Only a few channels fit, so a wide channel collides with neighbours.
Mixing widths at both ends. A link between two RouterOS devices with different widths can fail to connect or run poorly.
Expecting normal clients on narrow channels. A phone or laptop usually cannot join a 5 MHz or 10 MHz network.
Forgetting the band. A station set to a band that excludes the access point's standard will never see it.
Check yourself
Work through these three questions.
What happens to the data rates when the channel width is 10 MHz?
The rates are halved. A 54 Mbps rate becomes 27 Mbps.
Why would you use a narrow channel on a long link?
Narrow channels pick up less noise and improve sensitivity. They also leave more channels available in a small slice of spectrum.
Which setting chooses whether an interface uses only 802.11n on 5 GHz?
band=5ghz-onlynselects only 802.11n on 5 GHz. The width is a separatechannel-widthsetting.
Topic 2 of 3 in Wireless Standards
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