Wifi throughput test issue (FreeBSD 8.2)

[Viet-Ha Pham]

Hi Bernhard,

Thanks so much for your quick reply.

I did not test this scenario with 9.0 or later so I don't have any idea if 
it's working ok or not. I'll do it next.

For the debug options, I'll can do it right away and get back with more 
infor.


HAPV



-----Original Message----- 
From: Bernhard Schmidt
Sent: Wednesday, November 30, 2011 8:34 AM
To: Viet-Ha Pham
Cc: freebsd-net at freebsd.org ; Adrian Chadd
Subject: Re: Wifi throughput test issue (FreeBSD 8.2)

On Wed, Nov 30, 2011 at 13:37, Viet-Ha Pham <hapvbk at yahoo.co.uk> wrote:
> Hello,
>
>
> I am doing some throughput measurement for Wifi network. I am using 
> jperf/iperf to generate traffic (at the transmitter) and measure the 
> throughput (at the receiver). The transmitter and receiver are running 
> FreeBSD 8.2, using CM9-GP as the Wifi NIC. We are testing AWGN channel. 
> So, the transmitter and receiver are connected by cables, txpower on both 
> sides is set lower than the max (ranging from 0 to 10), AWGN noise is 
> injected in the middle of the cable.
>
> We measure the maximum throughput at each transmission rate. Transmitter 
> is a STA, receiver is an AP. We fix the unicast rate at the transmitter to 
> 54 Mbps (using "ifconfig ... ucastrate" command). The packet size is set 
> to 400 Bytes, UDP protocol is used and the transmit bandwidth is set to 30 
> Mbps in order to flood the transmission channel (to push it to its limit). 
> At first, at the receiver (AP) the measured throughput is about 12 Mbps. 
> Then I fix the unicast rate at the receiver (using ifconfig again) to 54 
> Mbps. The measured throughput increases to 20 Mbps. I set the unicast rate 
> at the receiver to autoselect again, the throughput is still 20 Mbps.
>
> When I switch the rate at the receiver, there are multiple message 
> displayed on the screen of the transmitter: "wlan0: 
> ieee80211_new_state_locked: pending RUN -> AUTH transition lost". This 
> message is repeated for many times (at least 20).
>
> I repeated the test with all transmission rates, change the packet size 
> and still observe the same thing: a throughput improvement of about 30% is 
> observed when I switch the unicast rate at the receiver and the 
> improvement is still there when I switch the unicast rate back to 
> autoselect. The same thing is observed in modes a and g. The same thing is 
> observed if the unicast rate at the receiver is set different from the 
> unicast rate at the transmitter.
>
>
>
> I spent time to look at the kernel source codes but I did not find a solid 
> answer for this. Please help me to identify the problem and the answer.
>
>
> My question is:
>
> 1. Why do I have that throughput improvement when I switch the rate like 
> that?
> 2. Is it a bug in the driver?

I think so, the driver might ignore/do some fancy stuff with a fixed 
ucastrate.

@Adrian, do you know if ath_rate_sample has some debug options in that 
regard?

> 3. What does the error/infor message on the transmitter mean?

It might mean a lot of things, you should probably enable additional
debug options to get a clue about what's going on. Btw, do you also
see that behavior on HEAD (or at least the latest 9.x release/rc)?

-- 
Bernhard