N-Devices (QRT5) perform better than AC-Devices (QRT5ac) in noisy environments

RouterOS Wireless Networking
1

Hi,

Recently I had a problem with a new 7km link to a tower using two QRT5ac. The overall perfomance was quite bad. After several days of tweaking (changing frequencies and parameters, I also tried to switch to N-only), I decided to mount a QRT5 (with N-only) at the tower - and all problems are gone. The link performance improved to 100MBit up/down (UDP-BW Test) at the same frequency and bandwidth (40MHz), before especially the uplink quality to the tower was quite low (at about 10MBit, very fluctuating). The 5GHz band at the tower is quite full (many other links mounted here). It seems that the AC-Devices are much more sensitive to disturbances from other 5GHz equipment than the N-Devices.
Another point: It seems that the links are also need some “training”. I also installed a QRT5 at the client side. At the day of installaion the rates are quite ok (CCR at 70% and about 70MBit/s UDP-BW test), one day later, I got CCR at 95% and 100MBit/s at the UDP-BW test, and now 5 days later it is still the same - so it seems there must be some link training in the N-Devices…
My settings are a little bit special: to have latencies with low jitter I always set nv2 to 1ms and I diabled all HT MCS above MCS5 (above MCS13) so that my max connection rate is 240MBit/s. I now get about 60MBit/s TCP speed (transfer rate of a browser in win 7) over this link.
My question: What is wrong with the AC chipsets? Does anyone here have similar experiences?

Leo.

2

The physic is the problem and there is no way around.
For higher modulation rates you must have higher SNR (more cleaner channel).
So in noisy environment it is actually better to use lower (manually set) rates/modulations
which are almost the same in both 802.11ac and 802.11n radios.
And because there is almost no manual control over 802.11ac rates and power settings in RouterOS
older manually fine-tuned 802.11n radios in most cases are working better in noisy environment.

Regards,
M.

3

a tumb rule in noisy enviroment. Lower output power and get more antenna gain.

4

That’s what we’d also saw. Links primeraly setup in ac but not happy with the results and just switch to n-only mode and more stable and better CCQ links… I hardly use ac anywhere since in most cases ‘n’ is good enouhg or works better…

5

As another poster said, Not being able to manually fine tune the modulation rates on 802.11ac is the problem.

We replaced several 5 to 7km RB800/802.11n links with Netmetal 5AC’s only to see significantly worse performance.

With 802.11n we used to test the link to work out what the maximum modulation rate was that we could run without packet loss and then lock it in so the radios would not try to use anything higher.