Wireless Standards · Topic 1 of 3
802.11 a, b, g and n Standards
Compare the 802.11 a, b, g and n standards, their bands, modulation and maximum rates, and how mixed networks slow down when old clients join the cell.
- 5 min
- 802.11b uses 2.4 GHz and reaches 11 Mbps.
- 802.11a uses 5 GHz and reaches 54 Mbps.
- 802.11g uses 2.4 GHz and reaches 54 Mbps, and it works with 802.11b.
What it is
The 802.11 standards define how Wi-Fi devices talk, and each letter adds a band or a speed.
A device can only use the rates its standard allows. When two devices differ, they fall back to the best standard that both understand.
For the MTCWE you need to know the four main letters, which are a, b, g and n. They differ in band, modulation and speed.
How it works
Each standard uses a band and a modulation, and 802.11n adds several parallel streams.
The four standards
| Standard | Band | Modulation | Top rate per link |
|---|---|---|---|
| 802.11b | 2.4 GHz | DSSS and CCK | 11 Mbps |
| 802.11a | 5 GHz | OFDM | 54 Mbps |
| 802.11g | 2.4 GHz | OFDM, with b compatibility | 54 Mbps |
| 802.11n | 2.4 GHz and 5 GHz | OFDM with MIMO | Several hundred Mbps |
802.11b is the oldest of the four. It uses a simple modulation, so its rates are low, and it works in the crowded 2.4 GHz band.
802.11a moved to 5 GHz and used OFDM. It reached 54 Mbps, and it has more channels and less interference than 2.4 GHz. Its shorter wavelength loses more signal over the same distance.
802.11g brought the 802.11a speeds to 2.4 GHz. It was designed to work together with 802.11b, so one network could serve both kinds of device.
802.11n works in both bands. It uses MIMO (multiple input, multiple output), which means several transmit and receive chains work in parallel. The 802.11n chapter covers this in detail.
Data rates
Each standard has a fixed list of rates. A device picks a rate depending on signal quality. Better signal allows higher rates, and weaker signal drops it to lower, more robust ones.
For legacy standards the rate list looks like this:
- 802.11b has 1, 2, 5.5 and 11 Mbps.
- 802.11a and 802.11g have 6, 9, 12, 18, 24, 36, 48 and 54 Mbps.
The rate is the speed on air, not the throughput that a user sees. Real throughput is lower because of headers, waiting times and acknowledgements.
Mixed networks
When 802.11b and 802.11g clients share one cell, the access point must protect the slower devices. Frames take longer on air, so everyone's throughput falls. One old 802.11b client can slow the whole cell.
The cure is to stop the old rates from being used. In RouterOS the band setting decides which standards are allowed on the interface:
/interface wireless set wlan1 band=2ghz-onlyg
/interface wireless set wlan1 band=2ghz-b/g/nThe first command allows only g. The second allows b, g and n together. Allowing fewer standards gives better speed, but older clients can no longer connect.
Seeing the standard in RouterOS
The registration table shows the rate of each connected client:
/interface wireless registration-table printThe tx-rate and rx-rate values show the current rate in each direction. A client stuck at a low rate may be far away, or the signal may be poor.
When a 2.4 GHz network is slow for everyone, check whether an old 802.11b client is connected and whether the band setting still allows 802.11b rates.
Compatibility
A newer standard in the same band is usually compatible with an older one. An 802.11g device can talk to an 802.11b device. An 802.11a device cannot talk to an 802.11b device, because they use different bands.
An 802.11n device can speak to older devices in its band, but it then uses their rates for that client.
What to remember for the exam
Seven facts summarise the standards.
- 802.11b uses 2.4 GHz and reaches 11 Mbps.
- 802.11a uses 5 GHz and reaches 54 Mbps.
- 802.11g uses 2.4 GHz and reaches 54 Mbps, and it works with 802.11b.
- 802.11n works in 2.4 GHz and 5 GHz and uses MIMO.
- 802.11a and 802.11b cannot talk to each other, because the bands differ.
- A slow legacy client in a mixed cell reduces the speed for others.
- The
bandsetting selects which standards the interface accepts.
Turning on 802.11b support only to help one old client slows every other client in the cell.
Common mistakes
Four errors appear again and again.
Confusing rate with throughput. A 54 Mbps rate does not give 54 Mbps of user data.
Assuming a and b are compatible. They use different bands, so they never meet.
Leaving old rates enabled. A cell that allows 802.11b rates wastes airtime on slow frames.
Forgetting that n clients fall back. An 802.11n device talking to an 802.11g access point uses the 802.11g rates.
Check yourself
Work through these three questions.
Which standards use the 5 GHz band?
802.11a uses 5 GHz only, and 802.11n works in both 2.4 GHz and 5 GHz. 802.11b and 802.11g use 2.4 GHz.
Why can one old 802.11b client slow down a whole cell?
Its frames take longer on air, and the access point must protect it. Everyone's throughput falls as a result.
Which setting limits an interface to 802.11g only on 2.4 GHz?
band=2ghz-onlygallows only 802.11g. Using2ghz-b/g/nallows b, g and n together.
Topic 1 of 3 in Wireless Standards
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