Questions

GNU Radio
1

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 ?

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 ?!

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 ?!

Thanks in-advance
Amr ,

2

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