# Questions

Hi , i have some questions :

1- In the following .cc file : ./gr-digital/lib/digital_crc32.cc

Regarding CRC-32 , why the generated polynomial is of length 36 Bits
instead of 32 Bits (as expected) ! " it is written as :
// Automatically generated CRC function
// polynomial: 0x104C11DB7
the above two lines exist in the code . "

2- In the following .py file :
./gr-digital/python/ofdm_packet_utils.py

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

Amr,

On Sat, May 5, 2012 at 12:00 PM, Amr Youssef
[email protected] wrote:

Hi , i have some questions :

1- In the following .cc file :./gr-digital/lib/digital_crc32.cc

Regarding CRC-32 , why the generated polynomial is of length 36 Bits
instead of 32 Bits (as expected) ! " it is written as :
// Automatically generated CRC function
// polynomial: 0x104C11DB7
the above two lines exist in the code . "

Amr,
That code is old and I’m not sure where it’s from, but I’ll respond
like I know what I’m talking about, anyways (mostly from experience
with these kinds of registers).

That’s not 36 bits, actually, it’s 33 bits, and the 33rd bit is a 1.
It’s typical to have always have a 1 in the last place for shift
registers like this. The algorithm itself is likely created using a
32-bit value.

2- In the following .py file :./gr-digital/python/ofdm_packet_utils.py

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