Increasing throughput on 3-radio system?

Increasing throughput on 3-radio system?

[Ben Greear]

I've put 3 ath9k AR9380 NICs into a single system (core-i7).  Each is
configured for an AP on a separate 5Ghz channel (40Mhz wide).

Individually, I can get about 300Mbps transmit towards the AP on any
particular radio, but when running all together, total throughput is
only about 475Mbps.

I assume there is some cross-channel bleeding or similar.

The APs are close by, so I was wondering if perhaps there was a way
to configure the radios to be less sensitive and thus pay less
attention to channel cross-talk?

Any suggestions for things to try are appreciated!

Thanks,
Ben

-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com

Re: Increasing throughput on 3-radio system?

[Ben Greear]

On 01/25/2013 10:23 AM, Sam Leffler wrote:
> On Fri, Jan 25, 2013 at 10:11 AM, Ben Greear <greearb@candelatech.com <mailto:greearb@candelatech.com>> wrote:
>
>     I've put 3 ath9k AR9380 NICs into a single system (core-i7).  Each is
>     configured for an AP on a separate 5Ghz channel (40Mhz wide).
>
>     Individually, I can get about 300Mbps transmit towards the AP on any
>     particular radio, but when running all together, total throughput is
>     only about 475Mbps.
>
>     I assume there is some cross-channel bleeding or similar.
>
>
> Yes.
>
>
>     The APs are close by, so I was wondering if perhaps there was a way
>     to configure the radios to be less sensitive and thus pay less
>     attention to channel cross-talk?
>
>     Any suggestions for things to try are appreciated!
>
>
> If you cannot fully isolate the antenna the usual approach is to schedule radio usage so tx/rx is done on all radios at the same time. Ubiquity and Mikrotik do
> this and there have been many research papers that discuss this.

It seems basically impossible to fully isolate the NICs in a normal
system.  The u.fl/IPEX pigtails bleed for sure.  The NICs probably bleed
just as bad or worse, and I can't think of a good way to increase isolation
without somehow encasing each NIC in a small RF-proof box (and somehow keeping the
RF from running down the pci-e ribon cables, etc).

Playing tricks with scheduling seems pretty nifty, but probably not realistic
for my particular use case (emulating lots and lots of stations that should run
against whatever AP(s) the customers may be using).

If there were a way to decrease rx-sensitivity, wouldn't that be pretty
similar to having all NICs transmit at once (in other words, the NICs
could better ignore the fainter cross-talk signals and focus on the
high-powered signals on their particular channel)?

Thanks,
Ben

-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com

Re: Increasing throughput on 3-radio system?

[Adrian Chadd]

Hi,

Strong enough out of band RF is still going to bleed over and it may
saturate the receive amplifiers before it hits the chip and the
configurable analog band-pass filter.

Hence the suggestion to always TX at the same time. That way you don't
have the problem of TX from one radio being strong enough to
saturation. You can tune the PHY (via magic handwavy parameters that
strangely enough are partially described in the ANI code :-) to reduce
its sensitivity to blocker signals, but it doesn't help you if the
signal is close enough by to trigger the wideband / out of band
interference check. If you try and receive during that you're going to
have very poor behaviour.

Thanks,



Adrian

Re: Increasing throughput on 3-radio system?

[Ben Greear]

On 01/25/2013 01:55 PM, Adrian Chadd wrote:
> Hi,
>
> Strong enough out of band RF is still going to bleed over and it may
> saturate the receive amplifiers before it hits the chip and the
> configurable analog band-pass filter.
>
> Hence the suggestion to always TX at the same time. That way you don't
> have the problem of TX from one radio being strong enough to
> saturation. You can tune the PHY (via magic handwavy parameters that
> strangely enough are partially described in the ANI code :-) to reduce
> its sensitivity to blocker signals, but it doesn't help you if the
> signal is close enough by to trigger the wideband / out of band
> interference check. If you try and receive during that you're going to
> have very poor behaviour.

Could you point me to which registers/vars/keywords in (ath9k) ani
do this?  I'd like to play around with them a bit.

As for transmitting at the same time, is that even possible
using multiple ath9k NICs in a PC?  It would seem to me that you'd
need some kind of low-level communication between the hardware/firmware
on the NICs themselves to accomplish that?

Thanks,
Ben

-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com

Re: Increasing throughput on 3-radio system?

[Adrian Chadd]

On 25 January 2013 14:08, Ben Greear <greearb@canelatech.com> wrote:

> Could you point me to which registers/vars/keywords in (ath9k) ani
> do this?  I'd like to play around with them a bit.

Just look at the ANI code and the ANI patent. It covers what the
registers do. There are obviously more PHY register nowdays, but those
are a good starting point.

> As for transmitting at the same time, is that even possible
> using multiple ath9k NICs in a PC?  It would seem to me that you'd
> need some kind of low-level communication between the hardware/firmware
> on the NICs themselves to accomplish that?

No, you just need a cute hack to un-gate the TX queues at the same
time. You'd also probably have to disable CCA (hah!) so they all
transmit at the same time, and configure the TXQ backoff to be linear
exponential, rather than random exponential backoff.


Adrian