MT9075B datasheet(12/78 Pages) MITEL

12 / 78 page

MT9075B

Preliminary Information

data. Channel alignment and bit numbering is

consistent

with

time

slot

alignment

and

bit

numbering. However, channels are numbered 1 to 30

and relate to time slots as per Table 3.

Table 3 - Time Slot to Channel Relationship

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 7. 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

MT9075B 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.

CRC-4 Multiframing

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

position one of the four FASs of the following

submultiframe before it is transmitted (see Table 7).

The submultiframe is then transmitted and, at the far

end, the same process occurs. That is, a CRC-4

remainder

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.

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 E1 = 0, a CRC-4 error was discovered in

a submultiframe 1 received at the far end; and if E2 =

0, a CRC-4 error was discovered in a submultiframe

received

the

far

end.

submultiframe

sequence numbers or re-transmission capabilities

are supported with layer 1 PCM 30 protocol. See

ITU-T G.704 and G.706 for more details on the

operation of CRC-4 and E-bits.

PCM 30

Timeslot

1 2 3...15

17 18 19...31

Voice/Data

Channels

1 2 3...15

16 17 18...30

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