802.11n force 40mhz channel

RouterOS Wireless Networking
1

Hi All,

We’re using a pair of SXT-ac about 200m away from each other. We have a problem with wild variation of tx-rate/rx-rate (modulation). As a result, throughput drastically suffers.

Signal level is very good : CCQ both ways is around 90%. The channels are not noisy (85db SNR). I tried decreasing signal power, but it resulted only in lower performance.

I have managed to stop the modulation instability by switching into 11n mode and limiting ht-supported-mcs to mcs-0,mcs-15. This has stabilized throughput, but the SXT keeps selecting 20mhz channel. I need to force it to use 40Mhz channel to get anything more than 80-90 mbps.

So far it seems that there is no such parameter; if I enable 20/40mhz-Ce or 20/40mhz-eC , SXTs would occasionally switch to 40mhz, but still stay on 20mhz most of the time.

Is there some trick, or at least a planned feature, to force minimum 40Mhz channel width?

2

You can try creating Channels manually
Wireless > Channels

3

I’ve tried the following:

/interface wireless channels add band=5ghz-onlyn frequency=5540 list=custom name=custom_5540 width=40

I get “unsupported channel” when I configure it on the wlan interface.

Further investigation resulted in conclusion that for SXT PtP I get best performance when I use 802.11 (not nv2), and also remove everything apart from 54Mbps from a/g rates. This makes the SXT use only 130Mbps+ rates and results in very stable performance. I only wish that I could force it to always use 40Mhz channels, so that I get stable 270+ rate.

Overall the wireless config is as follows:

set [ find default-name=wlan1 ] ampdu-priorities=0,1,2,3,4,5,6,7 band=5ghz-onlyn basic-rates-a/g=54Mbps channel-width=20/40mhz-Ce frequency-mode=superchannel \
    ht-basic-mcs=mcs-0,mcs-15 ht-supported-mcs=mcs-0,mcs-15 hw-retries=15 l2mtu=1600 preamble-mode=short rate-set=configured \
    supported-rates-a/g=54Mbps wireless-protocol=802.11
4

Sorry mate, but in my opinion it’s impossible. MCS is not TX-rate. You can achive 30MHz channel with custom channels with some chipsets. MCS means that it can always lower from wider to narrower channel, but only in that way.

5

Actually there is a direct relationship between MCS and TX-rate. Modulation Coding Scheme combined with channel width gives you the TX-rate: https://en.wikipedia.org/wiki/IEEE_802.11n-2009

Hard-fixing the channel width to 40 Mhz is technically possible as long as the chip supports it, I hope this is a software limitation and not hardware. I find current automatic rate selection algorithm used by Mikrotik to be very unstable as I see very wild rate jumps with 70db SNR. It seems as if very few lost packets can cause the tx-rate/MCS to plummet.

6

I have just received a response from Mikrotik support on this matter - they will put this feature in their to-do list for wireless enhancements.

7

Five years later…
Is there any method to force 40MHz channel (5GHz-only-N mode, single chain) at MikroTik RouterOS 6.46.2 ?

8

You can force technology (by setting band= … available values on hAP ac2 are 5ghz-a 5ghz-a/n 5ghz-a/n/ac 5ghz-n/ac 5ghz-onlyac 5ghz-onlyn).
AFAIK generally you can’t force 40MHz channel because standard 802.11 channels are 20MHz and one configures how many adjacent channels are usable and their relative placement (by setting channel-width= while 40MHz possibilities are 20/40mhz-Ce 20/40mhz-XX 20/40mhz-eC 40mhz-turbo).

9

Sorry, but You haven’t understand me…
I wuld like to use ONLY 40Mhz-Ce or 40-MHzeC, but NOT 20/40MHz, because at noisy environment it very, very frequentry chanfing beteween 20/40MHz. But i would like to use only 40Mhz.

10

I understand you very well. But, as I wrote: 802.11 standard defines 20MHz channels and as long as you wish to remain standard, it will be 20/40MHz and AP has to allow devices to use only single channel.

In noisy environments, where interference degrades transfer speeds, it is sometimes better to use narrower channels (i.e. 20MHz), because that means 3dB higher Tx power (keep in mind that total Tx power is then spread into half of bandwidth meaning twice as high power amplitude) which in turn means 3dB better SINR for receiver and that means better throughput. The end effect might be slightly lower peak throughput but higher (and more stable) average throughput with RTT with lower jitter.

11

I have never seen any latency issues with 802.11n in 20+40MHz mode. There simply should be no difference between radio receiving/sending 20MHz or 40MHz frames.
If you want low latency link, first of all NEVER use nv2 (TDMA it uses adds a lot of latency). For my p2p links in 802.11n I always use NSTREME because it can have low latency even during heavy link load and when not under load, minimum latency is comparable to plain 802.11. If you are getting latency spikes even in plain 802.11, you have interference problem and these are due to retransmissions. Use spectral scan with max hold to see what’s going on. Remember that not just wifi uses the ISM bands, so you have to look for things like radars that will not appear in scan list. Or using higher power or lower modulations may help or even fixed 20MHz channel if you prioritize latency over throughput.

12

could you explain this in more detail as I cannot find adequate documentation on this phenomena

13

I have just found a got explanation in IEEE 802.11n/ac Data Rates under Power Constraints:

When a station is already using the maximum allowed power, it can either use all this power over a 20 MHz channel, or spread it over the different 20 MHz sub-channels of the selected wide channel. For example, an HT station operating in the 2.4 GHz band, can either use 100 mW over a single 20 MHz channel, or divide this power over the two sub-channels belonging to a 40 MHz channel (each 20 MHz sub-channel is allowed to use 50 mW only). We note that this power is also shared between the different spatial streams. So if the station sends 4 Nss on a 40 MHz channel, the part of each stream per sub-channel is 12.5 mW only (i.e. 100/(24) = 12.5). The
same thing is true for 802.11ac stations which can either use all the 200 mW with a single stream over a 20 MHz channel, or spread the power over up to 8 streams and 8 sub-channels (i.e. 160 MHz channel). So each stream is allowed to use 3.125 mW only per sub-channel. This power spreading may limit the transmission range significantly.

and here in a nice table https://www.duckware.com/tech/wifi-in-the-us.html#channelwidthrange

14

Sorry, but You haven’t understand me…
I wuld like to use ONLY 40Mhz-Ce or 40-MHzeC, but NOT 20/40MHz, because at noisy environment it very, very frequentry chanfing beteween 20/40MHz. But i would like to use only 40Mhz.

.
40MHz-Ce or 40MHz-eC would be 80 MHz wide

20MHz-Ce, 20 MHz-eC, 20 MHz-xx is 40 MHz wide.

The “20/40MHz Ce” setting is confusing, should be “20MHz Ce/40MHz”

Why use single chain with the SXT-ac, it’s a dual chain device. 20MHz-Ce/2S (40MHz/2S) has double the speed of 20MHz-Ce/1S (40MHz/1S)
(See http://mcsindex.com/, for the 40MHz column single stream HT-MCS07 (150Mbps) versus 20 MHz dual stream HT-MCS15 (144Mbps))

Look at the 802.11ac standards to find out how a transmitter decides to use one 20MHz channel or two 20MHz channels. It’s different from 802.11n. For 802.11n both channels must be clear for transmission, in 802.11ac the clear channels will be used, they don’t have both to be free.

When transmissions fail, the transmitter will step down the number of streams, MCS rate, and channel width, … until it works, or it totally fails = disconnect.
The algorithm on what to stepped down first, is not documented. It’s different from older ROS versions <5.9. (MUM presentation somewhere: Wireless Tips and Tricks for RouterOS v6
MUM South Africa 2013 Johannesburg Uldis Cernevskis MikroTik)

If the CCQ is excellent, but still the interface rate is stepped down, then there is interference from time to time (other transmitters, even other than 802.11, like nv2 or some remote controls)
If the other is in continuous transmission, the CCQ would drop, because of the many failed transmissions. (CCQ for that connection in the registration table)

“802.11 will reduce speed until it works”. You can set the max speed, but must let 802.11 step down, or you will frequently lose connection.