XRT7250 datasheet(169/463 Pages) EXAR

169 / 463 page

XRT7250

DS3/E3 FRAMER IC

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REV. 1.1.1

150

error-checking/detection of a DS3 data stream, as it

is transmitted from one Terminal Equipment to adja-

cent Terminal Equipment (e.g., point-to-point check-

ing). Hence, the user cannot insert his/her value for

the P-bits into the outbound DS3 data stream, via the

Transmit Overhead Data Input Interface.

In contrast to “P” bits, “CP” bits are used perform er-

ror-checking/detection of a DS3 data stream from the

Source Terminal Equipment to the Sink Terminal

Equipment. In applications where a given DS3 data

stream is received via one port, and is output via an-

other port, it is necessary that the “CP” bit-values re-

main constant. The only way to insure this to (1) ex-

tract out the “CP” bit values, via the Receiving Line

Card and (2) insert these CP-bit values into the out-

bound DS3 data stream, via the Transmit Overhead

Data Input Interface block. Hence, the Transmit Over-

head Data Input Interface block will permit the user to

externally insert the “CP” bits into the outbound DS3

data stream.

The Alarm and signaling related Overhead bits

Bits that are used to transport the alarm conditions

can be either internally generated by the Transmit

Section within the XRT7250, or can be externally

generated and inserted into the outbound DS3 data

stream, via the Transmit Overhead Data Input Inter-

face. The DS3 frame overhead bits that fall into this

category are:

• The X bits

• The FEAC bits

• The FEBE bits.

The Data Link Related Overhead Bits

The DS3 frame structure also contains bits which can

be used to transport User Data Link information and

Path Maintenance Data Link information. The UDL

(User Data Link) bits are only accessible via the

Transmit Overhead Data Input Interface. The Path

Maintenance Data Link (PMDL) bits can either be

sourced from the Transmit LAPD Controller/Buffer or

via the Transmit Overhead Data Input Interface.

Table 19 lists the Overhead Bits within the DS3 frame.

Additionally, this table also indicates whether or not

these overhead bits can be sourced by the Transmit

Overhead Data Input Interface or not.

NOTES:

* The XRT7250 contains mask register bits that per-

mit the user to alter the state of the internally generat-

ed value for these bits.

+ The Transmit LAPD Controller/Buffer can be config-

ured to be the source of the DL bits, within the out-

bound DS3 data stream.

In all, the Transmit Overhead Data Input Interface

permits the user to insert overhead data into the out-

bound DS3 frames via the following two different

methods.

• Method 1 - Using the TxOHClk clock signal

• Method 2 - Using the TxInClk and the TxOHEnable

signals.

Each of these methods are described below.

4.2.2.1

4.2.2.1 Method 1 - Using the TxOHClk

Clock Signal

The Transmit Overhead Data Input Interface consists

of the five signals. Of these five (5) signals, the fol-

lowing four (4) signals are to be used when imple-

menting Method 1.

• TxOH

• TxOHClk

• TxOHFrame

TABLE 19: A LISTING OF THE OVERHEAD BITS WITHIN THE DS3 FRAME, AND THEIR POTENTIAL SOURCES, WITHIN THE

XRT7250 IC

OVERHEAD BIT

INTERNALLY GENERATED

ACCESSIBLE VIA THE TRANSMIT OVERHEAD

DATA INPUT INTERFACE

BUFFER/REGISTER

ACCESSIBLE

PYes

Yes*

Yes

FYes

Yes*

MYes

Yes*

FEAC

Yes

FEBE

Yes

Yes+

UDL

Yes

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