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Re: [Discuss-gnuradio] How to receive existing AP's signal using gr-ieee

From: Ke Xu
Subject: Re: [Discuss-gnuradio] How to receive existing AP's signal using gr-ieee802-11?
Date: Sun, 13 Dec 2015 17:36:36 -0700 (MST)

Bastian Bloessl-2 wrote
> Hi,
> I only have some tips that you might want to try.
> - set the lo_offset to 11M
> - use the LMS equaliser (the other one is total non-sense :-)
> - change the gain
> - use the 5GHz band or assert that it’s really purge-G (non-compatible
> mode)
> The last one is important. In the 5GHz-band you usually have 11a, which is
> perfectly fine.
> In the 2.4GHz band 11g is often in a mode that is backwards compatible
> with 11b. This mode uses a different preamble that is currently not
> supported.
> Some APs (or a MacBook) can be configured to use 11g-only mode. These
> frames should work.
> Best,
> Bastian
>> On 09 Dec 2015, at 17:50, Ke Xu <

> kexukezhao@

> > wrote:
>> Hi everyone!
>>      I am a beginner of GNU Radio and i'm studying 802.11 recently.I
>> found
>> a very nice work in https://github.com/bastibl/gr-ieee802-11/tree/master
>> and
>> it does me a great favor. I connected two USRP B210,one run wifi_tx.grc,
>> the
>> other run wifi_rx.grc, it worked and i can see the decoded "hello world"
>> in
>> my terminal.But when i use the wifi_rx.grc to receive existing WIFI
>> signal(such as wifi in home) to get the AP' SSID,it failed,i got nothing
>> but
>> disorder code(i enabled the debug in OFDM Decode MAC block).It seems that
>> the flow breaks in the checking checksum step because the checksum is not
>> equal to 558161692,the OFDM Parse MAC block has never been excuted.
>>      What shall i do? Chould the wifi_rx.grc receive existing WIFI signal
>> correctly? I notice that the receiver is limited to BPSK and QOSK, does
>> this
>> caused the problem? I'm not familiar with CRC, can anyone explain why the
>> checksum is a constant?
>>     Thanks in advance! Any advice will be welcome! 
> _______________________________________________
> Discuss-gnuradio mailing list

> Discuss-gnuradio@

> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio

Thanks for your advice and time. 

I have tried what you have said. I use a wireless router as the AP, as it
does not support 11a, I cannot use the 5G band, but it support '11g-only',
so i set it to this mode. And in my testbed, there are many other APs, i
have no idea which mode they operate on. I also annotate the checksum
operation in 'OFDM Ddcode MAC' block because if don't, I will get nothing
output in my terminal. I set the sample rate of my USRP N210 to 20M. 

Sometimes I can get some meaningful string output but it is not sent by my
AP. I also get some beacon frames rarely, however its decoded SSID is messy
code. What confused me is that I can see a lot of decoded mac address which
belong to other APs in my testbed and none for my own AP.

Best regards,
Ke Xu

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