802.11 Question

Hi,

I’m considering using the USRP together with the 802.11 stack to
prototype
some 802.11 MAC layer changes. I’m sending from an 11b card at the base
rate
to my USRP/RFX2400.

One shortcoming at present is that the code doesn’t generate ACKs for
received frames. There are quite strict timing requirements for ACK
generation: for 11b at the base rate the ACK should be sent within a
period
measured in microseconds. I can increase the ACKTimeout but apparently
there’s a hard limit in my 802.11 equipment at 746us beyond which I will
incur a rentransmission.

I’m prepared to wade into the code but its a fool’s errand if the
latencies
will be too high to ACK within a few hundred microseconds. I’m worried
that
the block latencies and USB round-trip delay would seem to make this
impossible. If anyone here can give me an informed opinion then I’ll be
really happy. So, does anyone know if it is it possible to generate an
ACK
within 700us or so of receiving a data frame?

Kind regards

Steve

Steve G. wrote:

I’m prepared to wade into the code but its a fool’s errand if the
latencies will be too high to ACK within a few hundred microseconds.
I’m worried that the block latencies and USB round-trip delay would
seem to make this impossible. If anyone here can give me an informed
opinion then I’ll be really happy. So, does anyone know if it is it
possible to generate an ACK within 700us or so of receiving a data frame?
Hi Steve,

One trick for testing if you can’t get the latency down - use a second
802.11b radio card and use the same MAC address in your gnuradio
implementation.

The 802.11b radio will automatically ACK every packet for its MAC
address.

If the gnuradio based MAC has the same MAC address then you can then use
the gnuradio for transmission and reception of real data without needing
to transmit ACK packets.

The catch - if the gnuradio misses a packet you don’t have the option of
not acking so it is sent again.

Nick

Thanks Nick,

I actually want to generate ACKs at the USRP and then only for some
frames.
Using a pair of real 802.11 cards I can increase the ACKTimeout and
generate
the ACKs in software at the receiving WNIC but it’s a little yukky.

If the USRP will work then its my preferred option.

Steve

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