XRT7250 datasheet(410/463 Pages) EXAR

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XRT7250

DS3/E3 FRAMER IC

REV. 1.1.1

391

Once the HDB3 (or AMI) encoded data has been de-

coded into a binary data-stream, the Receive E3

Framer block will use portions of this data-stream in

order to synchronize itself to the remote terminal

equipment. At any given time, the Receive E3 Fram-

er block will be operating in one of two modes.

• The Frame Acquisition Mode: In this mode, the

Receive E3 Framer block is trying to acquire syn-

chronization with the incoming E3 frame, or

• The Frame Maintenance Mode: In this mode, the

Receive E3 Framer block is trying to maintain frame

synchronization with the incoming E3 Frames.

Figure 194 presents a State Machine diagram that

depicts the Receive E3 Framer block's E3/ITU-T

G.832 Frame Acquisition/Maintenance Algorithm.

6.3.2.1

The Framing Acquisition Mode

The Receive E3 Framer block is considered to be op-

erating in the Frame Acquisition Mode, if it is operat-

ing in any one of the following states within the E3

Frame Acquisition/Maintenance Algorithm per

Figure 194.

• FA1, FA2 Octet Search State

• FA1, FA2 Octet Verification State

• OOF Condition State

• LOF Condition State

Each of these Framing Acquisition states, within the

Receive E3 Framer Framing Acquisition/Maintenance

State Machine are discussed below.

The FA1, FA2 Octet Search State

When the Receive E3 Framer block is first powered

up, it will be operating in the FA1, FA2 Octet Search

state. While the Receive E3 Framer is operating in

this state, it will be performing a bit-by-bit search for

the FA1 and FA2 Framing Alignment octets. FA1 is

assigned the value “0xF6”, and FA2 is assigned the

value of “0x28”. Figure 195, which presents an illus-

tration of the E3, ITU-T G.832 Framing Format, indi-

cates that these two octets will occur at the beginning

of each E3 frame, and that the FA2 octet will appear

immediately after the FA1 octet.

FIGURE 193. A SIMPLE ILLUSTRATION OF THE RECEIVE E3 FRAMER BLOCK AND THE ASSOCIATED PATHS TO THE

OTHER FUNCTIONAL BLOCKS

Receive E3 Framer

Block

Receive E3 Framer

Block

To Receive E3 HDLC

Buffer

Receive Overhead Data

Output Interface

Receive Payload Data

Output Interface

From Receive E3

LIU Interface Block

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