Re: [Discuss-gnuradio] why can't use iwconfig when I run the gr-ieee802-11

[Bastian Bloessl]

Hi,

On 06/07/2017 02:32 PM, zhan siyu wrote:
> Thanks for your reply. Let me explain what I 'm doing.
>
> I have two B210s connected with two computers. I want to measure the 
> throughput between the two computers over the usrp connection over 
> gr-ieee802-11. But no matter how hard I try, like tuning the parameters 
> and turning off my own wifi card and AP, I can only get 150K B/s, which 
> should be around 300K B/s if my theoretical calculation is right.  There 
> are no underrun or overrun errors. The throughput measurement tool I'm 
> using is iperf, which is an application to measure the end to end 
> throughput.
>
> Could you give me some hints?

I'm afraid I can't help a lot, since I have no idea what iperf is doing. 
I guess it floods the network stack with UDP packets. The GNU Radio 
transceiver, however, has no back pressure mechanism, so it will just 
drop the frames when they are injected into the GNU Radio flow graph.

Also, I don't understand what exact analytical number you are trying to 
reproduce here. In fact, I have no idea what this experiment is supposed 
to show/prove or what part of the transceiver you are trying to profile. 
If, at all, this experiment will tell you how fast the TX-side can 
produce frames.

This was investigated in

Gonzalo Arcos, Rodrigo Ferreri, Matías Richart, Pablo Ezzatti and 
Eduardo Grampín, "Accelerating an IEEE 802.11 a/g/p Transceiver in GNU 
Radio," Proceedings of 9th Latin America Networking Conference 
(LANC'16), Valparaíso, Chile, October 2016, pp. 13-19.

Turns out, you can improve the TX-side by improving the OFDM Carrier 
Allocator.

Best,
Bastian


> Best regards.
>
> Siyu
>
> 2017-06-07 14:02 GMT+08:00 Bastian Bloessl <address@hidden 
> <mailto:address@hidden>>:
>
>     Hi,
>
>     On 06/07/2017 03:04 AM, zhan siyu wrote:
>
>         Thanks. I just wonder why. Because I meet some performance
>         problem. I thought it maybe caused by my misconfiguration of the
>         gr-ieee802-11 code. Now, it seems not.
>
>
>     I'm a bit confused why the fact that the transceiver is not
>     configured through iwconfig ruled out any configuration issues, but
>     great that all seems to be set up now.
>
>
>         However, theoretically,  as my current sample rate is 10M and
>         BPSK. So the coding rate should be 10M/2 = 5M b/s. The
>         throughtput should be around 5M/8 = 625K B/s. Assuming the 12%
>         head cost, so the data throughput should be 625 * 88 % = 550K
>         B/s.  But as my experiment shows, the throughput is only 150K B/s.
>
>         I'm new to the communication. Is my calculation right ?
>
>
>     BPSK 1/3 is 3Mbit/s gross at 10MHz. The overhead per packet has to
>     be subtracted, i.e. the actual maximum rate depends on the frame size.
>
>
>         If it were right, then what might cause the gap?
>
>
>     Since you don't explain what you are doing, this is very hard to
>     tell. You would reach this theoretical throughput only if you send
>     frames back-to-back (which probably only works if you pregenerate
>     the sample stream). But also a WiFi card will insert inter-frame
>     space, so that the actual throughput will not match the theoretical
>     maximum physical layer throughput.
>
>     Best,
>     Bastian
>
>
>
>         One more question, I didn't run the volk_profile. Does it matter?
>
>         Best regards.
>
>         Siyu
>
>
>         2017-06-07 4:23 GMT+08:00 Bastian Bloessl <address@hidden
>         <mailto:address@hidden> <mailto:address@hidden
>         <mailto:address@hidden>>>:
>
>              Hi,
>
>              On 06/06/2017 03:55 PM, zhan siyu wrote:
>
>                  Hi all,
>
>                  I just found I can't use the iwconfig tap0 rate 20M to
>         setup the
>                  bandwidth of the tap0. The error message is :
>
>                  Error for wireless request "Set Bit Rate" (8B20) :
>                             SET failed on device tap0 ; Operation not
>         supported.
>
>                  But in their video , it can be set in this way. May I
>         know how
>                  to solve it ?
>
>
>              The WiFi transceiver is attached to the tun/tap interface,
>         which is
>              a virtual Ethernet device. This device doesn't support
>         WiFi-specific
>              configuration through iwconfig.
>
>              If you wanted this level of integration, you would have to
>         write a
>              kernel module that attaches the transceiver to a virtual
>         WiFi card.
>
>              Some group already did that, but they didn't release the
>         source code.
>
>              Best,
>              Bastian