User guide

98 RealCT Direct API Developer Guide
Chapter 4: E1 Networking
CRC
The CRC process treats the bits from one sub-multiframe as a
single binary number. It performs a calculation and transmits
the remainder in the first bit of each odd frame in the following
subframe. The receiving end performs the same calculation and
compares its results to the transmitted CRC from the remote
end. If the two ends calculate the same value, the lines are
communicating properly. If the receiving end calculates a
different value, the two ends are not communicating properly.
The first bit of the first six even frames contains a fixed
alignment signal. The first bit of the last two even frames, called
Si1 and Si2, are reserved to transmit the result of the CRC
comparison to the remote end. Table 18 shows the meanings for
Si1 and Si2.
Table 18. Si1 and Si2 Bits in a CEPT Multiframe
Si1 Si2 Meaning
1 1 CRC results are error free.
1 0 CRC results for SMFII contains errors.
CRC results for SMFI are error free.
0 1 CRC results for SMFI contains errors.
CRC results for SMFII are error free.
0 0 CRC results for both SMFI and SMFII contain
errors.