Advance Information
MT9074
Basic Frame Alignment
PCM 30 Channel Zero
CRC
Frame/
Type
CRC
Time slot 0 of every basic frame is reserved for basic
frame alignment and contains either a Frame
Alignment Signal (FAS) or a Non-Frame Alignment
Signal (NFAS). FAS and NFAS occur in time slot zero
of consecutive basic frames as shown in Table 12.
Bit two is used to distinguish between FAS (bit two =
0) and NFAS (bit two = 1).
1
C1
0
2
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
3
0
4
5
6
7
8
0/FAS
1
1
0
1
1
1/NFAS
2/FAS
A
0
Sa4 Sa5 Sa6 Sa7 Sa8
C2
0
1
1
0
1
1
3/NFAS
4/FAS
A
0
Sa4 Sa5 Sa6 Sa7 Sa8
C3
1
1
1
0
1
1
Basic frame alignment is initiated by a search for the
bit sequence 0011011 which appears in the last
seven bit positions of the FAS, see the Frame
Algorithm section. Bit position one of the FAS can be
either a CRC-4 remainder bit or an international
usage bit.
5/NFAS
6/FAS
A
0
Sa4 Sa5 Sa6 Sa7 Sa8
C4
0
1
1
0
1
1
7/NFAS
8/FAS
A
0
Sa4 Sa5 Sa6 Sa7 Sa8
C1
1
1
1
0
1
1
9/NFAS
10/FAS
11/NFAS
12/FAS
A
0
Sa4 Sa5 Sa6 Sa7 Sa8
Bits four to eight of the NFAS (i.e., Sa4 - Sa8) are
additional spare bits which may be used as follows:
C2
1
1
1
0
1
1
A
0
Sa4 Sa5 Sa6 Sa7 Sa8
•
•
•
S
to S may be used in specific point-to-point
a4 a8
C3
1
1
0
1
1
applications (e.g. transcoder equipments
conforming to G.761).
13/NFAS E1
14/FAS C4
15/NFAS E2
A
0
Sa4 Sa5 Sa6 Sa7 Sa8
1
1
0
1
1
S
may be used as a message-based data link
a4
for operations, maintenance and performance
monitoring.
A
Sa4 Sa5 Sa6 Sa7 Sa8
Table 11 - FAS and NFAS Structure
S
to S are for national usage.
a8
a5
indicates position of CRC-4 multiframe alignment signa
A maintenance channel or data link at 4,8,12,16,or
20 kHz for selected Sa bits is provided by the
MT9074 in E1 mode to implement these functions.
Note that for simplicity all Sa bits including Sa4 are
collectively called national bits throughout this
document.
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.
Bit three (designated as “A”), the Remote Alarm
Indication (RAI), is used to indicate the near end
basic frame synchronization status to the far end of a
link. Under normal operation, the A (RAI) bit should
be set to 0, while in alarm condition, it is set to 1.
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.
Bit position one of the NFAS can be either a CRC-4
multiframe alignment signal, an E-bit or an
international usage bit. Refer to an approvals
laboratory and national standards bodies for specific
requirements.
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
17
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