XRT7250 datasheet(352/463 Pages) EXAR

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XRT7250

DS3/E3 FRAMER IC

REV. 1.1.1

333

6.1.1.1

Frame Alignment (FA1 and FA2) Bytes

FA1 and FA2 are known as the frame alignment

bytes. The Receive E3 Framer, while trying to ac-

quire or maintain framing synchronization with its in-

coming E3 frames, will attempt to locate these two

bytes. FA1 is assigned the value “0xF6” and FA2 is

assigned the value “0x28”.

6.1.1.2

Error Monitor (EM) Byte

The EM byte contains the results of BIP-8 (Bit-Inter-

leaved Parity) calculations over an entire E3 frame.

The Bit Interleaved Parity (BIP-8) byte field supports

error detection, during the transmission of E3 frames,

between the Local Terminal Equipment and the Re-

mote Terminal Equipment.

The Transmit E3 Framer will compute the BIP-8 value

over the 537 octet structure, within each E3 frame.

The resulting BIP-8 value is then inserted into the EM

byte-field within the very next E3 frame. BIP-8 is an

eight bit code in which the nth bit of the BIP-8 code

reflects the even-parity bit calculated with the nth bit

of each of the 537 octets within the E3 frame. Thus,

the BIP-8 value presents the results for 8 separate

even-bit parity calculations.

The Receive E3 Framer will compute its own version

of the EM bytes for each E3 frame that it receives. Af-

terwards, it will compare the value of its locally com-

puted EM byte with the EM byte that it receives in the

very next E3 frame. If the two EM byte values are

equal, then the Receive E3 Framer will conclude that

this E3 frame was received in an error-free manner.

Further, the Receive E3 Framer will block will inform

the Remote Terminal Equipment of this fact by having

the Local Terminal Equipment set the FEBE (Far-

End-Block Error) bit, within the MA Byte of an Out-

bound E3 frame (to the Remote Terminal Equipment)

to “0”. Please see Section 5.1.1 for a discussion of

the MA Byte.

However, if the Receive E3 Framer block detects an

error in the incoming EM byte, then it will conclude

that the corresponding E3 frame is errored. Further,

the Receive E3 Framer block will inform the Remote

Terminal (e.g., the source of this erred E3 frame) of

this fact by having the Local Terminal Equipment

(e.g., the Transmit E3 Framer block) set the FEBE bit,

within an Outbound E3 frame (destined to the Re-

mote Terminal) to “1”.

NOTE: A detailed discussion on the practical use of the EM

byte is presented in Section 5.2.2.

6.1.1.3

The Trail-Trace Buffer (TTB) Byte

This byte-field is used to repetitively transmit a Trail-

access point identifier so that a trail receiving terminal

can verify its continued connection to the intended

transmitter. The trail access point identifier uses the

16-byte numbering format as tabulated in Table 68.

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

TABLE 68: DEFINITION OF THE TRAIL TRACE BUFFER BYTES, WITHIN THE E3, ITU-T G.832 FRAMING FORMAT

TRAIL TRACE BITS

BYTE NUMBER

BIT 7BIT 6BIT 5BIT 4BIT 3BIT 2BIT 1BIT 0

1 (Frame Start

Marker)

C6C5C4C3C2C1C0

XXXXXXXX

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