XRT7250 datasheet(272/463 Pages) EXAR

272 / 463 page

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XRT7250

DS3/E3 FRAMER IC

REV. 1.1.1

253

The E3, ITU-T G.751 Frame consists of 1536 bits. Of

these bits, 1524 are payload bits and the remaining

12 are overhead bits. The XRT7250 has been de-

signed to handle and process both the payload type

and overhead type bits for each E3 frame. Within the

Transmit Section within the XRT7250, the Transmit

Payload Data Input Interface has been designed to

handle the payload data. Likewise, the Transmit

Overhead Input Interface has been designed to han-

dle and process the overhead bits.

The Transmit Section of the XRT7250 generates or

processes the various overhead bits within the E3

frame, in the following manner.

The Frame Alignment Signaling (FAS) Overhead

Bits

The FAS (Framing Alignment Signaling) bits are al-

ways internally generated by the Transmit Section of

the XRT7250. Hence, the user cannot insert his/her

value for the FAS bits into the outbound E3 data

stream, via the Transmit Overhead Data Input Inter-

face.

The “A” (Alarm) Overhead bit

The “A” bit is used to transport the FERF (Far-End

Receive Failure) condition. This bit-field can be either

internally generated by the Transmit Section within

the XRT7250, or can be externally generated and in-

serted into the outbound E3 data stream, via the

Transmit Overhead Data Input Interface. The Data

Link Related Overhead Bits

The “N” (National) Overhead bit

The E3 frame structure also contains the N bit which

can be used to transport a proprietary User Data Link

information and or Path Maintenance Data Link infor-

mation. The UDL (User Data Link) bits are only ac-

cessible via the Transmit Overhead Data Input Inter-

face. The Path Maintenance Data Link (PMDL) bits

can either be sourced from the Transmit LAPD Con-

troller/Buffer or via the Transmit Overhead Data Input

Interface.

Table 47 lists the Overhead Bits within the E3 frame.

In addition, this table also indicates whether or not

these overhead bits can be sourced by the Transmit

Overhead Data Input Interface.

FIGURE 112. SIMPLE ILLUSTRATION OF THE TRANSMIT OVERHEAD DATA INPUT INTERFACE BLOCK

Transmit

Overhead

Data Input

Interface Block

Transmit

Overhead

Data Input

Interface Block

TxOHFrame

TxOHEnable

TxOH

TxOHClk

TxOHIns

To Transmit DS3 Framer Block

XRT7250 Datasheet(PDF) 272 Page - Exar Corporation

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