CRC-32 Calculation, Test Cases and HEC Tutorial

• HEC Tutorial
• Charles Michael Heard's 32 bit CRC Calculation Code in C
• CRC-32 Test Cases, Angie Tso

CRC-32 Code Notes

In < my00-220994104419@myuen.gte.com>  Michael Yuen wrote:

Michael> Hi All,  I want to implement functions to encode and
Michael> decode CRC for AAL5. Are there any written code available
Michael> from FTP sites?  I am aware of Vince.  Are there any
Michael> others?  Thanks.

In response Berry Kercheval stated:

Berry> The AAL5 CRC is the same as the Ethernet CRC.  You can find example
Berry> code in many PD or freeware packages such as XModem or Kermit;  or
Berry> by asking archie about CRC.  You should be aware that while these
Berry> examples all use the "look up 1 byte at a time" mode to speed things
Berry> up, the programs to build the lookup tables seem to feed the bits in
Berry> in the opposite order from that which the Ethernet and AAL5 CRC expects.

The accompanying code in crc32h.c takes care of the bit-ordering problem.  It
will definitely do the right thing if correctly used, but it is very easy to
get tripped unless you pay attention to the following things:

1.)  The intial value of crc_accum which you pass to update_crc is 0xFFFFFFFFL,
not zero. This is true whether the code is used for CRC generation or checking.

2.)  In order to generate the AAL5 CRC you must first run update_crc over 
the whole CPCS-PDU, including PADS, CPCS-UU, and all other trailer fields 
except the CRC field itself.  You must then append the ones complement 
of the output from update_crc as the CRC field of the CPCS-PDU.

3.)  In order to check the AAL5 CRC you must run update_crc over the whole
CPCS-PDU including the CRC field and check that the remainder 
is 0xC704DD7BL, as specified in recommendation I.363.

4.)  You must run gen_crc_table before using update_crc.

Note that update_crc has been designed to allow for incremental calculation
one cell at a time should you wish to do so.  All that you need to do is to
save the return value from one invocation of update_crc and pass it as the
crc_accum argument to the next invocation of update_crc.

-- C. M. Heard