XRT7250 datasheet(425/463 Pages) EXAR

425 / 463 page

XRT7250

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DS3/E3 FRAMER IC

REV. 1.1.1

406

the Inbound E3 frames. In this case, the Inbound

message bits will be carried by either the “GC” or the

“NR” byte-fields within each E3 Frame. The remote

LAPD Transmitter will transmit a LAPD Message to

the Near-End Receiver via either one of these bytes

within each E3 Frame. The LAPD Receiver will re-

ceive and store the information portion of the re-

ceived LAPD frame into the Receive LAPD Message

Buffer, which is located at addresses: 0xDE through

0x135 within the on-chip RAM. The LAPD Receiver

has the following responsibilities.

• Framing to the incoming LAPD Messages

• Filtering out stuffed "0’s" (within the information

payload)

• Storing the Frame Message into the Receive LAPD

Message Buffer

• Perform Frame Check Sequence (FCS) Verification

• Provide status indicators for

End of Message (EOM)

Flag Sequence Byte detected

Abort Sequence detected

Message Type

C/R Type

The occurrence of FCS Errors

The LAPD receiver's actions are facilitated via the fol-

lowing two registers.

• Rx E3 LAPD Control Register

• Rx E3 LAPD Status Register

Operation of the LAPD Receiver

The LAPD Receiver, once enabled, will begin search-

ing for the boundaries of the incoming LAPD mes-

sage. The LAPD Message Frame boundaries are de-

lineated via the Flag Sequence octets (0x7E), as de-

picted in Figure 200.

Where: Flag Sequence = 0x7E

SAPI + CR + EA = 0x3C or 0x3E

TEI + EA = 0x01

Control = 0x03

The 16 bit FCS is calculated using CRC-16, x16 +

x12 + x5 + 1

The local µP (at the remote terminal), while assem-

bling the LAPD Message frame, will insert an addi-

tional byte at the beginning of the information (pay-

load) field. This first byte of the information field indi-

cates the type and size of the message being trans-

ferred. The value of this information field and the

corresponding message type/size follow:

CL Path Identification

= 0x38 (76 bytes)

IDLE Signal Identification = 0x34 (76 bytes)

Test Signal Identification = 0x32 (76 bytes)

ITU-T Path Identification = 0x3F (82 bytes)

Enabling and Configuring the LAPD Receiver

Before the LAPD Receiver can begin to receive and

process incoming LAPD Message frames, the user

must do two things.

1. The byte-field within each E3 frame which will be

carrying the comprising octets of the LAPD Mes-

sage frame must be specified and

2. The LAPD Receiver must be enabled.

Each of these steps are discussed in detail below.

1. Specifying which byte-field, within each E3 frame,

will be carrying the LAPD Message frame.

The LAPD Receiver can receive the LAPD Message

frame octets via either the GC-byte-field or the NR-

byte-field, within each incoming E3 frame. The user

makes this selection by writing the appropriate bit to

FIGURE 200. LAPD MESSAGE FRAME FORMAT

Flag Sequence (8 bits)

SAPI (6-bits)

C/R

TEI (7 bits)

Control (8-bits)

76 or 82 Bytes of Information (Payload)

FCS - MSB

FCS - LSB

Flag Sequence (8-bits)

XRT7250 Datasheet(PDF) 425 Page - Exar Corporation

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