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MTT-14B e_Manual D07-00-083P RevA00
CRC-4 Error Checking in a MultiFrame Format
M-
SM-
FRM
FRM FRM
Bit 1
Bit 2 Bit 3
0
c1
0
1
0
1
2
c2
0
3
0
1
1
4
c3
0
5
1
1
6
c4
0
0
7
1
8
c1
0
9
1
1
10
c2
0
11
1
1
2
12
c3
0
13
E
1
14
c4
0
15
E
1
Notes:
•
SMF-FRM+1: Sub-Multiframe #1.
•
Sa: Spare bit reserved for national use.
•
A: Remote Alarm (FAS: Remote Alarm Indication).
•
Frame Alignment Signal Pattern: 0011011
•
CRC-4 Frame Alignment Signal: 001011
•
CRC multiframe is not aligned with MFAS timeslot 16 multiframe.
•
SM-FRM 2: Sub-Multiframe 2
•
E: E-bit Errors.
•
c1, c2, c3, c4: CRC bits
Figure 63 CRC-4 Multiframe Format
A CRC-4 (Cyclic Redundancy Check-4) is often used in E1 trans-
mission to identify possible bit errors. CRC-4 allows the detection
of errors within the 2.048 Mbit/s signal while it is in service.
CRC-4 is based on a mathematical calculation performed on
each sub-multiframe of data. The equipment which originates
the E1 data calculates the CRC-4 bits for one sub-multiframe.
Next it inserts the CRC-4 bits in the CRC-4 positions in the next
sub-multiframe. Receiving equipment performs the reverse math-
ematical computation on the sub-multiframe. It examines the
CRC-4 bits which were transmitted in the next sub-multiframe.
Next it compares the transmitted CRC-4 bits to the calculated
value. Discrepancies in the two values indicate a CRC-4 error.
TIME SLOT 0
Bits
Bit 4
Bit 5 Bit 6
Bit 7
0
1
1
0
1
A
Sa4
Sa5
Sa6
Sa7
0
1
1
0
1
A
Sa4
Sa5
Sa6
Sa7
0
1
1
0
1
A
Sa4
Sa5
Sa6
Sa7
0
1
1
0
1
A
Sa4
Sa5
Sa6
Sa7
0
1
1
0
1
A
Sa4
Sa5
Sa6
Sa7
0
1
1
0
1
A
Sa4
Sa5
Sa6
Sa7
0
1
1
0
1
A
Sa4
Sa5
Sa6
Sa7
0
1
1
0
1
A
Sa4
Sa5
Sa6
Sa7
Bit 8
1
Sa8
1
Sa8
1
Sa8
1
Sa8
1
Sa8
1
Sa8
1
Sa8
1
Sa8
Page 88 of 101
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