802.11n · Topic 1 of 3
802.11n Features and Data Rates
Learn the main 802.11n features, how MIMO spatial streams and MCS indexes set the data rate, and how to read and limit the n rates in RouterOS.
- 5 min
- 802.11n works in both 2.4 GHz and 5 GHz.
- It gets its speed from MIMO, channel bonding, frame aggregation and the short guard interval.
- MCS 0 to 7 are one stream, and MCS 8 to 15 are two streams.
What it is
802.11n is the Wi-Fi standard that added parallel radio streams, wider channels and aggregation to reach much higher speeds.
The older standards top out at 54 Mbps on air. 802.11n raises that by using several antennas at once, doubling the channel width and sending frames more efficiently.
It works in both the 2.4 GHz and the 5 GHz band, and it remains compatible with the older standards in the same band.
How it works
Four features raise the speed, and a table of MCS indexes describes the resulting rates.
The main features
| Feature | What it adds |
|---|---|
| MIMO | Several transmit and receive chains send separate streams at once |
| Channel bonding | Two 20 MHz channels join into one 40 MHz channel |
| Frame aggregation | Many frames are sent with less overhead |
| Short guard interval | A shorter gap between symbols gives a small speed gain |
MIMO (multiple input, multiple output) uses the fact that radio waves take several paths. Each stream travels on its own path, so two streams carry twice the data. The number of streams cannot exceed the number of chains at either end.
Spatial streams and MCS
Rates in 802.11n are described by an MCS (modulation and coding scheme) index. The index covers the modulation, the coding and the number of streams.
- MCS 0 to 7 use one stream.
- MCS 8 to 15 use two streams.
- MCS 16 to 23 use three streams.
- MCS 24 to 31 use four streams.
Within each group, a higher number means a faster modulation that needs a cleaner signal. MCS 0 is the slowest and the most robust. MCS 7 is the fastest single stream rate.
Example rates
| MCS | Streams | 20 MHz, long guard | 40 MHz, long guard | 40 MHz, short guard |
|---|---|---|---|---|
| 0 | 1 | 6.5 Mbps | 13.5 Mbps | 15 Mbps |
| 7 | 1 | 65 Mbps | 135 Mbps | 150 Mbps |
| 15 | 2 | 130 Mbps | 270 Mbps | 300 Mbps |
Doubling the streams doubles the rate. Going from 20 to 40 MHz a bit more than doubles it. The short guard interval adds roughly a tenth.
These are rates on air. User throughput is lower, because of overhead and waiting.
A device can only use as many streams as its chains allow, so a one chain client never reaches the two stream rates.
Reading n rates in RouterOS
The registration table shows the rate with its details:
/interface wireless registration-table print detailThe tx-rate value shows the speed, the channel width, the number of streams and whether the short guard interval is in use. A rate that shows two streams and 40 MHz is much faster than one that shows one stream and 20 MHz.
Limiting the n rates
The interface has settings that choose the MCS rates:
/interface wireless set wlan1 ht-supported-mcs=mcs-0,mcs-1,mcs-2,mcs-3,mcs-4,mcs-5,mcs-6,mcs-7
/interface wireless set wlan1 ht-guard-interval=longht-supported-mcslists the MCS rates the interface may use.ht-basic-mcslists the rates that every client must support.ht-guard-intervalchooses between allowing the short interval, withany, and using only the long one.
A list of mcs-0 to mcs-7 allows only one stream. Removing the very low rates is a way to push far clients away, in the same way as with the legacy rate lists.
Why the rate may be low
| Observation | Likely cause |
|---|---|
| One stream only | One chain active, or one chain weak or broken |
| 20 MHz when 40 MHz is expected | No bonding, or bonding was dropped because of interference |
| Low MCS in both directions | Weak signal or noise |
| Rate lower than the client supports | Chain, width or MCS settings limit it |
Compatibility with older clients
An 802.11n access point still serves 802.11a, b and g clients in its band. Those clients use the older rates. Their slow frames take more airtime, so a mixed cell is slower than a pure n cell.
To get full n speed, allow only n on the band, for example 2ghz-onlyn or 5ghz-onlyn.
In the 5 GHz band, 5ghz-onlyn removes the older rates. Use it only if all of your clients support n.
What to remember for the exam
Seven facts cover this topic.
- 802.11n works in both 2.4 GHz and 5 GHz.
- It gets its speed from MIMO, channel bonding, frame aggregation and the short guard interval.
- MCS 0 to 7 are one stream, and MCS 8 to 15 are two streams.
- The number of streams cannot exceed the number of chains.
- Doubling the streams doubles the rate.
ht-supported-mcsandht-basic-mcsset the n rates.- A mixed cell with old clients is slower than an n only cell.
Common mistakes
Four errors are typical.
Expecting full speed from one chain. A single chain device cannot use the two stream rates.
Confusing rate with throughput. The on air rate is higher than what the user gets.
Forgetting the old clients. Legacy frames waste airtime in a mixed cell.
Setting basic MCS too high. Clients with a weaker signal can no longer connect.
Check yourself
Work through these three questions.
Which MCS values use two streams?
MCS 8 to 15 use two streams. MCS 0 to 7 use one stream.
What limits the number of streams a link can use?
The number of chains at both ends, because a stream needs a chain at each side. A one chain device cannot use two stream rates.
Which setting lists the MCS rates that an interface may use?
ht-supported-mcslists them.ht-basic-mcslists the rates that every client must support.
Topic 1 of 3 in 802.11n
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