time taken for set frequency ioctl calls

Jouni Malinen jkmaline at cc.hut.fi
Sun Jun 8 21:34:43 EDT 2003

On Thu, Jun 05, 2003 at 12:41:34PM -0700, Ishwar Ramani wrote:

> I was experimenting with the ioctl for changing the frequency of the card
> in the hostap mode. The time from the ioctl call being sent to the first
> packet being received is 23 ms.

First, I would like to know why you are trying to measure this..
It is not exactly a normal operation for an AP to change its operating
channel frequently. This would end up killing all station connections.

> I solved the user level delays by setting the process
> priority to the highest. It takes around 12.5 msec for the function call
> to complete. I also verified this by using timestamps in the driver. The
> biggest contributor to this delay is reset_port. But the surprising part
> is that it takes another 10 ms before receiving the first packet in the
> new new channel. can somebody throw some light on this?

That operation is not designed to be quick since it does not need to
be.. Setting RID, disabling port0, and enabling port0 are all going to
sleep while waiting the hardware to complete the command. At least part
of the extra 10 ms is probably caused by time needed for the hardware to
calibrate itself on new frequency.

If you want to change channels more than once a second or so, you should
probably use a different method for this. Depending on your goels, this
might be low-level test command to change the channel or selecting a
different hardware implementation ;-).

Jouni Malinen                                            PGP id EFC895FA

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