XRT7250 datasheet(253/463 Pages) EXAR

253 / 463 page

XRT7250

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DS3/E3 FRAMER IC

REV. 1.1.1

234

5.0

E3/ITU-T G.751 OPERATION OF THE

XRT7250

Configuring the XRT7250 to Operate in the E3,

ITU-T G.751 Mode

The XRT7250 can be configured to operate in the E3/

ITU-T G.751 Mode by writing a “0” into bit-field 6 and

a “0” into bit-field 2, within the Framer Operating

Mode register, as illustrated below.

Prior to describing the functional blocks within the

Transmit and Receive Sections of the XRT7250, it is

important to describe the E3, ITU-T G.751 framing

format.

5.1

DESCRIPTION OF THE E3, ITU-T G.751 FRAMES

AND

ASSOCIATED OVERHEAD BITS

The role of the various overhead bytes are best de-

scribed by discussing the E3, ITU-T G.751 Frame

Format as a whole. The E3, ITU-T G.751 Frame con-

tains 1536 bits, of which 12 bits are overhead and the

remaining 1524 bits are payload bits.

Each E3, ITU-T G.751 Frame consists of the following

12 overhead bits.

• A 10 bit FAS (Framing Alignment Signal) pattern.

This pattern is assigned the constant pattern of

“1111010000”, and is used by the Receive E3

Framer block to acquire and maintain Frame Syn-

chronization with the incoming E3 frames.

• The “A” (or Alarm) Bit.

• The “N” (or National) Bit.

• The BIP-4 Bits (if configured).

The frame repetition rate for this type of E3 frame is

22375 times per second, thereby resulting in the

standard E3 bit rate of 34.368 Mbps. Figure 97 pre-

sents an illustration of the E3, ITU-T G.751 Frame

Format.

5.1.1

Definition of the Overhead Bits

Each of these Overhead Bits are further defined be-

low.Frame Alignment Signaling (FAS) Pattern Bits

The first 10 bits, within each E3, ITU-T G.751 frame

are known as the FAS (or Framing Alignment Signal-

ing) bits. The Receive E3 Framer block, while trying

to acquire or maintain framing synchronization with its

incoming E3 frames, will attempt to locate the FAS

bits. The FAS pattern is assigned the value

“1111010000”.

5.1.1.1

The “A” (Alarm) Bit

The “A” bit typically functions as a FERF (Far-End

Receive Failure) indicator bit. However, if the user

configures the XRT7250 Framer IC to transmit and

receive E3 frames which are carrying the BIP-4 value

(located at the end of a given E3 frame), then this bit

will also function as the FEBE indicator bit. A detailed

discussion on the practical use of the “A” is present-

ed in Section 4.2.2. Each of these roles of the “A” bit

are briefly discussed below.

The “A” Bit Functioning as the FERF bit-field

If the Receive E3 Framer block (at a Local Terminal)

is experiencing problems receiving E3 frame data

from a Remote Terminal (e.g., an LOS, OOF or AIS

FRAMER OPERATING MODE REGISTER (ADDRESS = 0X00)

BIT 7

BIT 6BIT 5BIT 4BIT 3

BIT2BIT 1BIT 0

Local

Loopback

DS3/E3*

Internal

LOS

Enable

RESET

Interrupt

Enable

Reset

Frame

Format

TimRefSel[1:0]

R/W

FIGURE 97. ILLUSTRATION OF THE E3, ITU-T G.751 FRAMING FORMAT.

Frame

Alignment

Signal

Data

BIP-4

if Selected

384 385

768

769

1152 1153

1532

1536

Framing Alignment Signal Pattern = 1111010000

XRT7250 Datasheet(PDF) 253 Page - Exar Corporation

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