MT9074_05 Datasheet, PDF (30/151 Pages) Zarlink Semiconductor Inc – T1/E1/J1 Single Chip Transceiver

MT9074
Data Sheet
CRC
PCM30 Channel Zero
CRC Frame/
Type _{1 2 3 4 5 6 7 8}
0/FAS
1/NFAS
C₁ 0
01
2/FAS
3/NFAS
C₂ 0
01
4/FAS
5/NFAS
C₃ 0
11
6/FAS
7/NFAS
C₄ 0
01
8/FAS
9/NFAS
C₁ 0
11
10/FAS C₂ 0
11/NFAS 1 1
12/FAS C₃ 0
13/NFAS E₁ 1
14/FAS C₄ 0
15/NFAS E₂ 1
011011
A S_a4 S_a5 S_a6 S_a7 S_a8
011011
A S_a4 S_a5 S_a6 S_a7 S_a8
011011
A S_a4 S_a5 S_a6 S_a7 S_a8
011011
A S_a4 S_a5 S_a6 S_a7 S_a8
011011
A S_a4 S_a5 S_a6 S_a7 S_a8
011011
A S_a4 S_a5 S_a6 S_a7 S_a8
011011
A S_a4 S_a5 S_a6 S_a7 S_a8
011011
A S_a4 S_a5 S_a6 S_a7 S_a8
Table 11 - FAS and NFAS Structure
indicates position of CRC-4 multiframe alignment signa
CRC-4 Multiframing in E1 Mode
The primary purpose for CRC-4 multiframing is to provide a verification of the current basic frame alignment,
although it can also be used for other functions such as bit error rate estimation. The CRC-4 multiframe consists of
16 basic frames numbered 0 to 15, and has a repetition rate of 16 frames X 125 microseconds/frame = 2 msec.
CRC-4 multiframe alignment is based on the 001011 bit sequence, which appears in bit position one of the first six
NFASs of a CRC-4 multiframe.
The CRC-4 multiframe is divided into two submultiframes, numbered 1 and 2, which are each eight basic frames or
2048 bits in length.
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 X4 then divided by X4 + 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 12).
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 Frame Algorithm section for more details.
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