MT9079AC datasheet(7/54 Pages) MITEL

7 / 54 page

MT9079

4-243

ST-BUS bit 7 is synonymous with PCM 30 bit 1; bit 6

with bit 2: and so on. See Figure 33.

PCM 30 time slot zero is reserved for basic frame

alignment, CRC-4 multiframe alignment and the

communication of maintenance information. In most

configurations time slot 16 is reserved for either

channel associated signalling (CAS or ABCD bit

signalling) or common channel signalling (CCS). The

remaining 30 time slots are called channels and

carry either PCM encoded voice frequency signals or

digital data signals. 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 1.

Basic Frame Alignment

Time slot zero 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 can be see in

Table 4. Bit two is used to distinguish between a FAS

(bit two = 0) and a 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 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

national bits, which telephone authorities used to

communicate

maintenance,

control

and

status

information. A single national bit can also be used as

a 4 KHz maintenance channel or data link. Bit three,

the ALM bit, is used to indicate the near end basic

frame synchronization status to the far end of a link.

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 be

used

for

other

PCM 30

Timeslots

1 2 3 ....15

17 18 19 ....31

Voice/Data

Channels

1 2 3 ....15

16 17 18 ....30

Table 1 - Time slot to Channel Relationship

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 4.

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 near end by the E-bits.

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

a submultiframe one received at the far end; and if

E2 = 0, a CRC-4 error was discovered in a

submultiframe two received at the far end. No

submultiframe sequence numbers or re-transmission

capabilities are supported with layer 1 PCM 30

protocol. See CCITT G.704 and G.706 for more

details on the operation of CRC-4 and E-bits.

CAS Signalling Multiframing

The purpose of the signalling multiframing algorithm

is to provide a scheme that will allow the association

of a specific ABCD signalling nibble with the

appropriate PCM 30 channel. Time slot 16 is

reserved

for

the

communication

Channel

Associated Signalling (CAS) information (i.e., ABCD

signalling bits for up to 30 channels). Refer to CCITT

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