Wireless Standards · Topic 2 of 5
Modulation and Data Rates
Learn how modulation, coding rate, channel width, streams and guard interval set the Wi-Fi data rate, and how MCS and rate settings work in RouterOS.
- 5 min
- A denser modulation carries more bits per symbol and needs a higher signal quality.
- Order of density: BPSK, QPSK, 16 QAM, 64 QAM, 256 QAM and 1024 QAM.
- The coding rate is the share of sent bits that carry real data.
What it is
Modulation is the method that puts data onto a radio wave, and the data rate is the speed that results.
A device does not use one fixed speed. It picks the fastest rate that the signal quality can support, and it moves up and down as conditions change.
The same idea explains why a client close to the access point is fast and a distant client is slow.
How it works
The rate is the product of how many bits each symbol carries and how many symbols you can send.
Modulation types
A denser modulation carries more bits in each symbol, but it needs a cleaner signal.
| Modulation | Bits per symbol | Needs |
|---|---|---|
| BPSK | 1 | The lowest signal quality |
| QPSK | 2 | Low |
| 16 QAM | 4 | Medium |
| 64 QAM | 6 | High |
| 256 QAM | 8 | Higher |
| 1024 QAM | 10 | The highest signal quality, used in 802.11ax |
Coding rate
Some of the bits that are sent are error correction bits. The coding rate is the share of the bits that carry real data. A rate of 3/4 means that three of every four bits are data.
A lower coding rate adds more protection and so gives a lower speed. A higher coding rate gives more speed but less protection.
What decides the rate
Five things together set the rate.
- the modulation
- the coding rate
- the channel width
- the number of spatial streams
- the guard interval
Doubling the channel width roughly doubles the rate. Doubling the number of streams also doubles it.
MCS index
The MCS index is a number that names a combination of modulation and coding rate. A device shows its rate with an MCS value, together with the width and the number of streams.
For 802.11n, MCS 0 to 7 apply to one stream. These values are for a 20 MHz channel and the normal guard interval.
| MCS | Modulation and coding | Rate in Mbps |
|---|---|---|
| 0 | BPSK 1/2 | 6.5 |
| 1 | QPSK 1/2 | 13 |
| 2 | QPSK 3/4 | 19.5 |
| 3 | 16 QAM 1/2 | 26 |
| 4 | 16 QAM 3/4 | 39 |
| 5 | 64 QAM 2/3 | 52 |
| 6 | 64 QAM 3/4 | 58.5 |
| 7 | 64 QAM 5/6 | 65 |
With a 40 MHz channel, MCS 7 gives 135 Mbps. With a short guard interval it gives 150 Mbps. With two streams the rate doubles again.
Guard interval
The guard interval is a short pause between symbols that prevents one symbol from interfering with the next. A shorter guard interval gives a higher rate, but it works only when the signal is clean.
Older standards
802.11b uses DSSS and has rates up to 11 Mbps. 802.11a and 802.11g use OFDM with rates from 6 to 54 Mbps.
Rate adaptation
The device measures the quality of the link and moves between rates. Retries and errors push the rate down, and clean frames push it up.
A client that sits on a weak link uses a low rate. A low rate takes more airtime for the same data, and that slows other clients too.
Throughput is always lower than the rate shown. Overhead and retries use part of the airtime.
Controlling rates in RouterOS
On the legacy wireless package you can limit which rates the radio offers.
/interface wireless set wlan1 supported-rates-a/g=12Mbps,18Mbps,24Mbps,36Mbps,48Mbps,54Mbps basic-rates-a/g=12Mbps
/interface wireless registration-table print statssupported-rates-a/glists the rates that the radio may use.basic-rates-a/glists the rates that every client must support.ht-supported-mcsandht-basic-mcsdo the same for the MCS values of 802.11n.- The registration table shows the rate that each client is using now.
Removing the lowest rates makes a weak client leave sooner, and it reduces the airtime that slow clients use. Take care, because a very high lowest rate shrinks the useful cell.
Do not set the basic rates too high. Clients at the edge of coverage may not be able to join at all.
What to remember for the exam
Seven facts cover this topic.
- A denser modulation carries more bits per symbol and needs a higher signal quality.
- Order of density: BPSK, QPSK, 16 QAM, 64 QAM, 256 QAM and 1024 QAM.
- The coding rate is the share of sent bits that carry real data.
- Channel width, streams, modulation, coding rate and guard interval together set the rate.
- The MCS index names a modulation and coding rate combination.
- Devices adapt the rate to the signal quality, and low rates use more airtime.
supported-ratesandbasic-ratescontrol the rates that the radio offers on the legacy package.
Common mistakes
These four errors are common.
Expecting high rates at long distance. A weak signal forces simple modulation.
Treating the rate as throughput. Real throughput is lower.
Setting basic rates too high. Clients at the edge cannot join.
Forgetting that slow clients cost airtime. A low rate uses more time for the same data.
Check yourself
Work through these three questions.
Why does a client far from the access point have a lower rate?
The signal is weaker, so only a simple modulation works. A simple modulation carries fewer bits per symbol.
What does the coding rate 3/4 mean?
Three of every four bits sent carry real data. The other bit is for error correction.
What do
basic-rates-a/gandsupported-rates-a/gdo?supported-rates-a/glists the rates the radio may use.basic-rates-a/glists the rates that every client must support.
Topic 2 of 5 in Wireless Standards
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