XRT7250 datasheet(412/463 Pages) EXAR

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XRT7250

DS3/E3 FRAMER IC

REV. 1.1.1

393

When the Receive E3 Framer block detects the “FA1”

octet, and determines that this octet is immediately

followed by the “FA2” octet, then it will transition to the

FA1, FA2 Octet Verification state, per Figure 195.

The FA1, FA2 Octet Verification State

Once the Receive E3 Framer block has detected an

“0xF628” pattern (e.g., the concatenation of the FA1

and FA2 octets), it must verify that this pattern is in-

deed the “FA1” and “FA2” octets and not some other

set of bytes, within the E3 frame, mimicking the

Frame Alignment bytes. Hence, the purpose of the

FA1, FA2 Octet Verification state.

When the Receive E3 Framer block enters this state,

it will then quit performing its bit-by-bit search for the

Frame Alignment bytes. Instead, the Receive E3

Framer block will read in the two octets that occur 537

bytes (e.g., one E3 frame period later) after the candi-

date Frame Alignment patterns were first detected. If

these two bytes match the assigned values for the

“FA1” and “FA2” octets, then the Receive E3 Framer

block will conclude that it has found the Frame Align-

ment bytes and will then transition to the In-Frame

state. However, if these two bytes do not match the

assigned values for the “FA1” and “FA2” octets then

the Receive E3 Framer block will concluded that it

has been fooled by data mimicking the Frame Align-

ment bytes, and will transition back to the FA1, FA2

Octet Search state.

In Frame State

Once the Receive E3 Framer block enters the In-

Frame state, then it will cease performing Frame Ac-

quisition functions, and will proceed to perform Fram-

ing Maintenance functions. Therefore, the operation

of the Receive E3 Framer block, while operating in

the In-Frame state, can be found in Section 5.3.2.2

(The Framing Maintenance Mode).

OOF (Out of Frame) Condition State

If the Receive E3 Framer while operating in the In-

Frame state detects four (4) consecutive frames,

which do not have the valid Frame Alignment (FA1

and FA2 octet) patterns, then it will transition into the

OOF Condition State. The Receive E3 Framer

block’s operation, while in the OOF condition state is

a unique mix of Framing Maintenance and Framing

Acquisition operation. The Receive E3 Framer block

will exhibit some Framing Acquisition characteristics

by attempting to locate (once again) the Frame Align-

ment octets. However, the Receive E3 Framer block

will also exhibit some Frame Maintenance behavior

by still using the most recent frame synchronization

for its overhead byte and payload byte processing.

The Receive E3 Framer block will inform the Micro-

processor/Microcontroller of its transition from the In-

Frame state to the OOF Condition state, by generat-

ing a Change in OOF Condition Interrupt. When this

occurs, Bit 3 (OOF Interrupt Status), within the Rx E3

FIGURE 195. ILLUSTRATION OF THE E3, ITU-T G.832 FRAMING FORMAT

FA1

FA2

1 Byte

59 Bytes

60 Columns

9 Rows

530 Octet Payload

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