MT9074AP datasheet(17/122 Pages) MITEL

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Advance Information

MT9074

Basic Frame Alignment

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).

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.

Bits four to eight of the NFAS (i.e., Sa4 - Sa8) are

additional spare bits which may be used as follows:

•Sa4 to Sa8 may be used in speciﬁc point-to-point

applications

(e.g.

transcoder

equipments

conforming to G.761).

•Sa4 may be used as a message-based data link

for operations, maintenance and performance

monitoring.

•Sa5 to Sa8 are for national usage.

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.

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.

Bit position one of the NFAS can be either a CRC-4

multiframe

alignment

signal,

E-bit

international usage bit. Refer to an approvals

laboratory and national standards bodies for speciﬁc

requirements.

Table 11 - FAS and NFAS Structure

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

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

CRC

Frame/

Type

PCM 30 Channel Zero

45678

0/FAS

11011

1/NFAS

01A

Sa4 Sa5 Sa6 Sa7 Sa8

2/FAS

11011

3/NFAS

01A

Sa4 Sa5 Sa6 Sa7 Sa8

4/FAS

11011

5/NFAS

11A

Sa4 Sa5 Sa6 Sa7 Sa8

6/FAS

11011

7/NFAS

01A

Sa4 Sa5 Sa6 Sa7 Sa8

8/FAS

11011

9/NFAS

11A

Sa4 Sa5 Sa6 Sa7 Sa8

10/FAS

11011

11/NFAS

11A

Sa4 Sa5 Sa6 Sa7 Sa8

12/FAS

11011

13/NFAS

Sa4 Sa5 Sa6 Sa7 Sa8

14/FAS

11011

15/NFAS

Sa4 Sa5 Sa6 Sa7 Sa8

indicates position of CRC-4 multiframe alignment signa

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