Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

Fundamentals of distributed network protocol 97
GENERATION
RECEIPT
x2 16
K
K
M
M
P
000..00
16
T
K
K
M
M
R
16
Invert
R
Q
16
R
16
Invert
(Discard)
Quotient
Remainder
P
QC
RC
(Discard)
Quotient
Remainder
16
R
If RC = 0 M is GOOD
If RC = 0 M is BAD
T
K
M
16
R
K
M
Transmit
Notes: P = 1 0011 1101 0110 0101 = 13D65 (hex)
Transmission Order of CRC
Data
(M)
LSB MSB
CRC
Figure 5.18
CRC code processing
Generation of message:
• Start with user data block M of k bits (k = 64 for header, 8–128 for blocks
in body)
• Multiply by 2 16 (ie append 16 zeros to end to form k + 16-bit number)
• Divide by generator polynomial P to obtain quotient Q and remainder R
modulo-2 division is used)
• Discard Q, keep R
• Invert R to obtain R′
• Append R′ to M forming message T′ to be transmitted
Where generator polynomial:
P = x 16 + x 13 + x 12 + x 10 + x 8 + x 5 + x 2 + 1
This forms a 17-bit number which is 13D65 (in hex)