Re: [boost] problem with small width CRCs

From: Bert Klaps (bertk_at_[hidden])
Date: 2004-08-15 16:34:50

On Sat, Aug 14, 2004 at 10:44:48PM -0400, Daryle Walker wrote:
> On 8/10/04 11:32 AM, "Bert Klaps" <bertk_at_[hidden]> wrote:
> 
> > I ran into a problem using the CRC library for SDH LCAS control words
> > (3-bit CRC) and SDH Trail Trace Identifier messages (7-bit CRC).
> 
> Could you provide sample data and answer so a new test case can be added?

Sure, though in the end I picked random values for
polynomials etc and found that for CRC width or `Bits' < 8
and ReflectIn = false crc_basic and crc_optimal give
different results for those as well - crc_basic is correct
of course.

Example #1 is a SDH/SONET Low Order LCAS control word with CRC-3
taken from ITU-T G.707 (12/03) XIII.2:
That's a 29 bit word protected by a 3 bit CRC, together
32 bits or 4 bytes. The ITU-T text itself has 4 examples which
include a correct CRC-3. So the answer to these four sequences
should be zero.
a. 0x3a 0xc4 0x08 0x06, answer is 0
b. 0x42 0xc5 0x0a 0x41, answer is 0
c. 0x4a 0xc5 0x08 0x22, answer is 0
d. 0x52 0xc4 0x08 0x05, answer is 0
The LCAS CRC-3 parameters are:
- polynomial: 0x3 ( x^3 + x + 1 )
- initial remainder: 0
- final xor value: 0
- reflected input: false
- reflected remainder: false

Example #2 is a SDH/SONET J0/J1/J2/N1/N2/TR TTI (trace message)
with CRC-7, o.a. ITU-T G.707 Annex B, G.832 Annex A:
That's basically calculated over 16 bytes: the first
input byte is always 0x80, the following 15 bytes are
printable characters. Some examples:
a. "\x80""123456789ABCDEF" , answer is 0x62
b. "\x80""TTI UNAVAILABLE" , answer is 0x23
The TTI CRC-7 parameters are:
- polynomial: 0x9 ( x^7 + x^3 + 1 )
- initial remainder: 0
- final xor value: 0
- reflected input: false
- reflected remainder: false

> Does anyone know how to add patches to a file?  (It looked like it was in a
> "diff" format. 

Plain "diff -c"...

Regards
Bert Klaps