Re: [Discuss-gnuradio] Questions

[Josh Blum]

On 06/17/2012 08:25 AM, Amr Youssef wrote:
> Hi all , i have some questions :
> 
>  In  " ./gr-digital/python/ofdm_packet_utils.py " file  :
> 
>  1-  in pkt_dt = ' ' . join ((payload_with_crc ,'\x55'))
> What is  '\x55'   representing to , i found it "U" in ASCII  .  Why U
>  specifically ?  and why we concatenate it with  payload_with_crc  ?
> 

Well 0x55 is 0b1010101. So perhaps 0x55 is a sentinel to identify the
end of payload. Also since the bits keep changing that probably has some
desirable property... "take everything with a grain of salt".

>  2- How  "random_mask_tuple"  is generated through the 15-bit Linear
> Feedback Shift Register (LFSR) .
> Are they random ? if yes , why are they  of length 8 bits (maximum number
>  255 ) instead of 15 bits ?!
> 

They are psudo-random. Everything is in bytes and all of these examples
operate on byte streams. And well, a 15 element lfsr can generate random
8 bit data just fine.

> 3-    Pad for USRP  , It is for USRP to pad zeros to reach a length of  512
> Bytes (or its multiples)  to send across the USB ! . Now , what about USRP
> 2 ?! and the process across the Ethernet , in other wards , am i forced to
> make    " Pad for USRP  =True "  while using USRP2  ?!
> 

For USRP1, the host holds onto byte remainder and only flushes out
multiple of 512 bytes. This is not necessary for newer USRP, or, if you
specify the EOB tag, even the USRP1 will flush its output.

-josh