Re:Re: [Discuss-gnuradio] PACKET FORMAT "error"

[zzw.1012]

 Hi, Thomas
    thanks for your help ! you are right!
     I add printf code in the function of usrp_basic.cc like this:
int usrp_basic_tx::write(const void *buf, int len, bool *underrun)
{
 ...
 printf ("len = %d\n", len);
 if (len < 0 || (len % 512) != 0)
 {
  fprintf(stderr, "usrp_basic_tx::write: invalid length=  %d\n", len);
 } 
}

then I run the example of benchmark_tx.py :

./benchmark_tx.py -f 2500M -s 1500 -M 0.001

it really send 194560 bytes  = 190k for the default case using 4 samples per symbol and it send 380k with -M 0.002.

but , I still have some question. first , after making packet with 1520 bytes, then whether is it going on gmsk(default) modulation? I think the 1520-bytes packet changes to 190kbytes occur in the process of gmsk. Is it right? how to  calculate the number from 1520 to 194560?

second, in the process of sending packet, the printf information as follow:
(./benchmark_tx.py -f 2500M -s 1500 -M 0.001)
len = 16384
len = 15872
len = 16384
len = 16384
len = 16384
len = 15872
len = 16384
len = 512
len = 16384
len = 16384
len = 16384
len = 16384
len = 14848

Is there have some discipline in sending data? I find it seems unorderly.

 

thank you again

 

best wishes

 

 

> 
> On Sun, May 9, 2010 at 11:28 PM, zzw.1012 <address@hidden> wrote:
> > Hi,
> >     I'm studying benchmark_tx.py now. I find that the packet size is not
> > right (at least I'd like to think so) in the process of making packet, which
> > can be seen in pkt.py and packet_utils.py. In the packet consist of 2 bytes
> > packed_preamble, 8 bytes packed_access_code, 4 bytes header, 4 bytes
> > outband_crc with default 1500 bytes size, padding bytes and endbyte"\x55" .
> > I use default gmsk modulatiom. So, the packet have 2 + 8 + 4 + 1504 + 1 + 1
> > = 1520bytes.
> >     However, in the function of _npadding_bytes(pkt_bytes_len,
> > samples_per_symbol, bits_per_symbol) , there have such description:
> > "generate sufficient padding such that each packet ultimately ends up being
> > a multiple of 512 bytes when sent across the USB, we send 4-byte samples
> > across the USB (16-bit I and 16-bit Q), thus we want to pad so that after
> > modulation the resulting packet is a multiple of 128 samples". Also , in the
> > function "int usrp_basic_tx::write(const void *buf, int len, bool
> > *underrun)"in the usrp_basic.cc,  there have similar code like "if (len < 0
> > || (len % 512) != 0){fprintf(stderr, "usrp_basic_tx::write: invalid length
> > =  %d\n", len}".
> >     they both tell me that the data across the USB must be a multiple of 512
> > bytes .but in the example of benchmark_tx.py, the packet size is 1520
> > bytes.  what's wrong ?
> 
> 1520 bytes only refers to the packet size. The transmitted sample
> stream at the physical layer includes a number of other factors
> including conversion to bits, samples per symbol, and sample size. So
> for the default case, a 1520 byte packet using 4 samples per symbol
> yields 48640 samples or 194560 bytes, which is a multiple of 512.
> 
>   Thomas

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