XRT7250 datasheet(289/463 Pages) EXAR

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XRT7250

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DS3/E3 FRAMER IC

REV. 1.1.1

270

5.2.4

The Transmit E3 Framer Block

5.2.4.1

Brief Description of the Transmit E3

Framer

The Transmit E3 Framer block accepts data from any

of the following four sources, and uses it to form the

E3 data stream.

• The Transmit Payload Data Input block

• The Transmit Overhead Data Input block

• The Transmit HDLC Controller block

• The Internal Overhead Data Generator

The manner in how the Transmit E3 Framer block

handles data from each of these sources is described

below.

Handling of data from the Transmit Payload Data

Input Interface

For E3 applications, all data that is input to the Trans-

mit Payload Data Input Interface will be inserted into

the payload bit positions within the outbound E3

frames.

Handling of data from the Internal Overhead Bit

Generator

By default, the Transmit E3 Framer block will internal-

ly generate the overhead bytes. However, if the Ter-

minal Equipment inserts its own values for the over-

head bits or bytes (via the Transmit Overhead Data

Input Interface) or if the user enables and employs

the Transmit E3 HDLC Controller block, then these in-

ternally generated overhead bytes will be overwritten.

Handling of data from the Transmit Overhead Da-

ta Input Interface

For E3 applications, the Transmit E3 Framer block au-

tomatically generates and inserts the framing align-

ment bytes (e.g., the 10 bit FAS framing alignment

signal) into the outbound E3 frames. Hence, the

Transmit E3 Framer block will not accept data from

the Transmit OH Data Input Interface block for the

FAS signal.

However, the Transmit E3 Framer block will accept

(and insert) data from the Transmit Overhead Data In-

put Interface for both the “A” and “N” bit-fields.

If the user's local Data Link Equipment activates the

Transmit Overhead Data Input Interface block and

writes data into this interface for these bits or bytes,

then the Transmit E3 Framer block will insert this data

into the appropriate overhead bit/byte-fields, within

the outbound E3 frames.

Handling of data from the Transmit HDLC Control-

ler Block

The exact manner in how the Transmit E3 Framer

handles data from the Transmit HDLC Controller

block depends upon whether the Transmit HDLC

Controller is activated or not. If the Transmit DS3

HDLC Controller block is not activated, then the

Transmit E3 Framer block will insert a “1” into each

“N” bit-field, within each outbound E3 frame.

If the Transmit E3 HDLC Controller block is activated,

then data will be inserted into the “N” bit-fields as de-

scribed in Section 4.2.3.

5.2.4.2

Detailed Functional Description of the

Transmit E3 Framer Block

The Transmit E3 Framer receives data from the fol-

lowing three sources and combines them together to

form the E3 data stream.

• The Transmit Payload Data Input Interface block.

• The Transmit Overhead Data Input Interface block

• The Transmit HDLC Controller block.

• The Internal Overhead Data Generator.

Afterwards, this E3 data stream will be routed to the

Transmit E3 LIU Interface block, for further process-

ing.

Figure 119 presents a simple illustration of the Trans-

mit E3 Framer block, along with the associated paths

to the other functional blocks within the chip.

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