XRT7250 datasheet(428/463 Pages) EXAR

428 / 463 page

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XRT7250

DS3/E3 FRAMER IC

REV. 1.1.1

409

After receiving the LAPD Message frame, the LAPD

Receiver will check the state of the “C/R” bit-field,

within octet # 2 of the LAPD Message frame header

and will reflect this value in Bit 3 (Rx CR Type) within

the Rx E3 LAPD Status Register, as depicted below.

When this bit-field is “0”, it means that this LAPD

Message frame is originating from a customer instal-

lation. When this bit-field is “1”, it means that this

LAPD Message frame is originating from a network

terminal.

4. Identify the Type of LAPD Message Frame/PMDL

Message

Next, the LAPD Receiver will check the value of the

first octet within the PMDL Message field, of the

LAPD Message frame. Recall that from Section _,

that when operating the LAPD Transmitter, the user is

required to write in a byte of a specific value into the

first octet position within the Transmit LAPD Message

buffer. The value of this byte corresponds to the type

of LAPD Message frame/PMDL Message that is to be

transmitted to the Remote LAPD Receiver. This Mes-

sage-Type Identification octet is transported to the

Remote LAPD Receiver, along with the rest of the

LAPD frame. From this Message Type Identification

octet, the LAPD Receiver will know the type of size of

the newly received PMDL Message. The LAPD Re-

ceiver will then reflect this information in Bits 4 and 5

(RxLAPDType[1:0]) within the Rx E3 LAPD Status

Table 86 presents the relationship between the con-

tents of RxLAPDType[1:0] and the type of message

received by the LAPD Receiver.

NOTE: Prior to reading in the PMDL Message from the

Receive LAPD Message buffer, the user is urged to read

the state of the RxLAPDType[1:0] bit-fields in order to deter-

mine the size of this message.

5. Inform the Local Microprocessor/External Cir-

cuitry of the receipt of the new LAPD Message

frame.

RXE3 LAPD STATUS REGISTER (ADDRESS = 0X19)

BIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0

Not Used

Rx ABORT

RxLAPDType[1:0]

RxCR

Type

RxFCS

Error

End of

Message

Flag

Present

0001

000

RXE3 LAPD STATUS REGISTER (ADDRESS = 0X19)

BIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0

Not Used

Rx ABORT

RxLAPDType[1:0]

RxCR

Type

RxFCS

Error

End of

Message

Flag

Present

0000

000

TABLE 86: THE RELATIONSHIP BETWEEN THE CONTENTS OF RXLAPDTYPE[1:0] BIT-FIELDS AND THE PMDL

MESSAGE TYPE/SIZE

RXLAPDTYPE[1:0]

PMDL MESSAGE TYPE

PMDL MESSAGE SIZE

Test Signal Identification

76 Bytes

Idle Signal Identification

76 Bytes

CL Path Identification

76 Bytes

ITU-T Path Identification

82 Bytes

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