
Preliminary Information
MT9071
45
The CRC-4 frame alignment verification functions as follows. Initially, the CRC-4 operation must be activated
and CRC-4 multiframe alignment must be achieved at both ends of the link. At the local end of a link, all the bits
of every transmit submultiframe are passed through a CRC-4 polynomial (multiplied by X
4
then divided by X
4
+
X + 1), which generates a four bit remainder. This remainder is inserted in bit position one of the four FASs of
the following submultiframe before it is transmitted (see Table 8 - E1 CRC-4 FAS and NFAS Structure).
The submultiframe is then transmitted and, at the far end, the same process occurs. That is, a CRC-4
remainder is generated for each received submultiframe. These bits are compared with the bits received in
position one of the four FASs of the next received submultiframe. This process takes place in both directions of
transmission.
When more than 914 CRC-4 errors (out of a possible 1000) are counted in a one second interval, the framing
algorithm will force a search for a new basic frame alignment (see Section 5.5 E1 Framing Algorithms).
The result of the comparison of the received CRC-4 remainder with the locally generated remainder will be
transported to the far end by the E-bits. Therefore, if E
1
= 0, a CRC-4 error was discovered in a submultiframe
1 received at the far end; and if E
2
= 0, a CRC-4 error was discovered in a submultiframe 2 received at the far
end. No submultiframe sequence numbers or re-transmission capabilities are supported with layer 1 PCM30
protocol (see ITU-T G.704 and G.706 for more details on the operation of CRC-4 and E-bits).
5.4.3
E1 Automatic CRC-4 Interworking
The MT9071 framing algorithm supports automatic interworking of interfaces with and without CRC-4
processing capabilities when control register bit AUTC (see Table 79 - E1 Alarms and Framing Control - R/W
Address Y00) is set to zero. That is, if an interface with CRC-4 capability, achieves valid basic frame alignment,
but does not achieve CRC-4 multiframe alignment by the end of a predefined period, the distant end is
CRC-4
CRC-4
Frame/Type
PCM30 Timelsot Zero
1
2
3
4
5
6
7
8
S
0/FAS
1/NFAS
2/FAS
3/NFAS
4/FAS
5/NFAS
6/FAS
7/NFAS
8/FAS
9/NFAS
10/FAS
11/NFAS
12/FAS
13/NFAS
14/FAS
15/NFAS
C
1
0
C
2
0
C
3
1
C
4
0
C
1
1
C
2
1
C
3
E
1
C
4
E
2
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
A
0
A
0
A
0
A
0
A
0
A
0
A
0
A
1
1
0
1
1
S
a4
1
S
a4
1
S
a4
1
S
a4
1
S
a4
1
S
a4
1
S
a4
1
S
a4
S
a5
1
S
a5
1
S
a5
1
S
a5
1
S
a5
1
S
a5
1
S
a5
1
S
a5
S
a6
0
S
a6
0
S
a6
0
S
a6
0
S
a6
0
S
a6
0
S
a6
0
S
a6
S
a7
1
S
a7
1
S
a7
1
S
a7
1
S
a7
1
S
a7
1
S
a7
1
S
a7
S
a8
1
S
a8
1
S
a8
1
S
a8
1
S
a8
1
S
a8
1
S
a8
1
S
a8
Table 8 - E1 CRC-4 FAS and NFAS Structure
indicates position of CRC-4
multiframe alignment signal.