Nortel Meridian 1 Succession 1000M Installation And Configuration Manual page 560

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CRC protocol specifications
CRC Calculation
553-3011-210 Standard 1.00
Appendix A: Communications protocol specifications
The following definitions apply to XModem protocol:
<soh>01H
<eot>04H
<ack>06H
<nak>15H
<C>43H
This protocol does not restrict the contents of data sent. Control characters are
not processed in the 128-byte data messages (they are handled the same as
other characters).
The Cyclic Redundancy Check (CRC) protocol is a form of block check
which provides more robust error detection than the original checksum. The
CRC-CCITT used by the modem protocol detects all single- and double-bit
errors, all errors with an odd number of bits, all burst errors of length 16 or
less, 99.97% of 17-bit error bursts, and 99.98% of 18-bit and longer bursts.
Each block of the transfer in CRC mode looks like:
<soh><blk #><255-blk #><--128 data bytes--><CRC hi><CRC lo>
in which:
<soh>= 01 hex
<blk #>= binary number, starts at 01 increments by 1, and wraps OFFH
to 00H (not 01)
<255-blk #>= ones complement of blk #
<CRC hi>= byte containing the 8 hi order coefficients of the CRC
<CRC lo>= byte containing the 8 lo order coefficients of the CRC
To calculate the 16-bit CRC, the message bits are considered to be the
coefficients of a polynomial. This message polynomial is first multiplied by
16
X and then divided by the generator polynomial (X
modulo 2 arithmetic. The remainder left after the division is the desired CRC.
Since a message block in the Modem Protocol is 128 bytes or 1024 bits, the
October 2003
16 12 5
+ X + X + 1) using