Trouble shooting low rate MCS9 in 802.11ac

Vu Hai NGUYEN vh.nguyen at
Wed Aug 20 07:13:13 PDT 2014

>> I'd cleaned all rules of iptables and removed all modules of iptables from kernel by command modprobe -r
>> But there's still some process of netfilter process running. I disabled netfilter on bridge interface by writing 0 to these files:
>> /proc/sys/net/bridge/bridge-nf-call-arptables
>> /proc/sys/net/bridge/bridge-nf-call-iptables
>> /proc/sys/net/bridge/bridge-nf-call-ip6tables
>> and the performance increases from 500 to 550 Mbps (iperf in tcp reports between 530 ~ 570Mbps).
>> There is no more process like  "br_nf_pre_routing" and "br_nf_forward_ip".
>> Do you think I can do anything more to strip down netfilter?

>What do you get for UDP?  That seems like pretty good TCP throughput.

In VHT80 mode and rate forced to NSS3:MCS7, I can obtain 750 Mbps with less than 1% data lost (in both way, AP to STA and vice versa).
Normally I can obtain 2/3 theory rate in practical (for expl NSS3:MCS3 has 390 Mbps in theory, and if I force to that rate, my TCP throughput
is around 270 Mbps). So I should obtain 650 Mbps in practical for TCP with NSS3:MCS7 (975 Mbps in theory). 
570 Mbps is so far to 650 Mbps :(

>Also, do you get different results if you use Open v/s WPA2 encryption?

>A test I just ran showed decreased performance on one of my systems
>when I ran with open v/s wpa2 (400Mbps v/s 350Mbps, UDP goodput).
>This was using hardware-crypt, so I expected it to be as fast as Open.
>I'm running customized firmware and driver, so maybe that is cause of
>slowdown...curious what results others are seeing though!

In my case, I don't get much differences between Open vs WPA2 encryption, rate is almost the same (different around 10 ~ 20 Mbps and hard to see)

>> I installed tc from package iproute2 and did : tc qdisc but it doesn't improve the performance
>> One more question: Could the antennas create any effect on the MCS in use?
>> We are using this antenna from PCTEL:
>> And in the specification, I saw that it supports the standard 802.11n, so may be this is the reason
>> that the best rate for me is MCS7?

>Antenna and orientation matter, but the one you list advertises itself
>as dual-band, so it looks like it should work well in the 5Ghz band
>that AC uses.
>You might try changing orientation and/or trying some different antenna.

That antenna has 6 access so I connect my AP and STA to that antenna, it means
that they are very close to each other. I've tried to connect my STA with others 3 smalls 
antennas dual band (
and change direction but the rate I obtain is much lower, I can not reach high throughput as above.
Route de Mayres - B.P. 9

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