I had been doing a similar experiment with 802.11b/g. With CWMIN=CWMAX=AIFS=0, and the backoff disable flag used, I was surprised to get only a 11microsecond interpacket gap at the 1Mbps/11Mbps rates and about 17microsecond interpacket gap.

In effect, I am seeing no DIFS interval between packets. Since you said you have set DIFS=SIFS, I would like to know how you did it and I want to check if I have accidently done so myself.

The rx_tasklet function is a tasklet which is scheduled from the ath_intr interrupt and hence may not be called fast enough for your custom response packet. Moreover, if you are creating a packet after you receive the required data, we have observed a delay greater than 75microsecond between "wanting" to create the response and the actual sending it out on hardware.

Please let me know if I am missing anything.

-Ashutosh Dhekne

On Sun, Feb 8, 2009 at 10:13 PM, Bo Han <hanbo00@gmail.com> wrote:
Hi all,

For my current project, I need to send out response frames to a given type of data frames in the MadWifi driver as quick as possible.  I am doing it in the ath_rx_tasklet(...) function after knowing that I have received the wanted data frames and then sending out the customized response frames through a dedicated hardware queue.  However, I noticed that for IEEE 802.11a the inter frame space between the response and data frames is usually higher than 100 microseconds on my T43 laptop, but the gap between ACK and data frames is only around 12 microseconds. 

I know that ACK is generated in hardware to guarantee that the gap is SIFS and there may be some delay from the hardware to the driver for packet reception, depending on my system configuration.  Does anyone know how to reduce this delay?  I have disabled backoff and used DIFS = SIFS for the response frames.


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