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Re: [Discuss-gnuradio] 802.11 Question

From: Steve Glass
Subject: Re: [Discuss-gnuradio] 802.11 Question
Date: Thu, 7 Jun 2007 14:19:41 +1000

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.


On 6/7/07, jafa < address@hidden> wrote:
Steve Glass wrote:
> 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?
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

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.


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