XRT7250 datasheet(395/463 Pages) EXAR

395 / 463 page

XRT7250

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DS3/E3 FRAMER IC

REV. 1.1.1

376

NOTE: When this bit is set, it overrides all of the other bits in

this register.

6.2.4.2.1.3

TxMARx - Bit 0

This read/write bit-field permits the user to force the

XRT7250 Framer IC to transmit either a FERF (Far-

End Receive Failure) or a FEBE (Far-End Block Er-

ror) indication to the remote terminal equipment.

6.2.4.2.2

Configuring the Transmit Trail Trace

Buffer Message

The XRT7250 Framer IC contains 16 bytes worth of

Transmit Trail Trace Buffer registers and 16 bytes

worth of Receive Trail Trace Buffer registers. The role

of the Receive Trail Trace Buffer registers are de-

scribed in Section 5.3.7.

The XRT7250 Framer IC contains 16 Transmit Trail

Trace Buffer registers (e.g., Tx TTB-0 through TxTTB-

15). The purpose of these registers are to provide a

16-byte Trail Access Point Identifier to the Remote

Terminal Equipment. The Remote Terminal Equip-

ment will use this information in order to verify that it

is still receiving data from its intended transmitter.

The specific use of these registers follows.

For Trail Trace Buffer Message purposes, the Trans-

mit E3 Framer block will group 16 consecutive E3

frames, into a Trail Trace Buffer super-frame. When

the Transmit E3 Framer block is generating the first

E3 frame, within a Trail Trace Buffer super-frame, it

will read in the contents of the Tx TTB-0 Register (Ad-

dress = 0x38) and insert this value into the “TR” byte-

field of this very first Outbound E3 frame. When the

Transmit E3 Framer is generating the very next E3

frame (e.g., the second E3 frame, within the Trail

Trace Buffer super-frame), it will read in the contents

of the Tx TTB-1 register (Address = 0x39) and insert

this value into the TR byte-field of this Outbound E3

frame. As the Transmit E3 Framer block is creating

each subsequent E3 frame, within this Trail Trace

Buffer super frame, it will continue to increment to the

very next Transmit Trail Trace Buffer register. The

Transmit E3 Framer block will then read in the con-

tents of this particular Transmit Trail Trace Buffer reg-

ister (Tx TTB-n) and insert this value into the TR byte-

field of the very next Outbound E3 frame. After the

Transmit E3 Framer block has created the 16th E3

frame, within a given Trail Trace Buffer super-frame

(e.g., it has read in the contents of Tx TTB-15 register

and has inserted this value into the “TR” byte of the

16th E3 frame), it will begin to create a new Trail Trace

Buffer super-frame, by reading the contents of the Tx

TTB-0 register, and repeating the above-mentioned

procedure.

The contents of the Tx TTB-0 register will typically be

of the form [1, C6, C5, C4, C3, C2, C1, C0]. The “1”

in the MSB (Most Significant bit) position of this byte

is used to designate that this octet is the frame-start

marker (e.g., is the first of the 16 TR bytes, within a

Trail Trace Buffer super-frame). The remaining Trail

Trace Buffer registers (TxTTB-1 through TxTTB-15)

will typically contain a “0” in their MSB positions. The

remaining bits within the Tx TTB-0 register C6

through C0 are the CRC-7 bits calculated over the

contents of all 16 TR bytes, within the previous Trail

Trace Buffer super-frame. The contents of the re-

maining Trail Trace Buffer registers (e.g., Tx TTB-1

through Tx TTB-15) will typically contain the 15 ASCII

characters required for the E.164 numbering format.

NOTES:

1. The XRT7250 Framer IC will not compute the CRC-

7 value, to be written into the Tx TTB-0 register.

The user’s system must compute this value prior to

writing it into the Tx TTB-0 register.

2. The user, when writing data into the Tx TTB regis-

ters, must take care to insure that only the Tx TTB-

0 register contains an octet with a “1” in the MSB

(most significant bit) position. All remaining Tx TTB

registers (e.g., Tx TTB-1 through Tx TTB-15) must

contain octets with a “0” in the MSB position. The

reason for this cautionary note is presented in Sec-

tion 5.3.2.9.

6.2.5

The Transmit E3 Line Interface Block

The XRT7250 Framer IC is a digital device that takes

E3 payload and overhead bit information from some

terminal equipment, processes this data and ulti-

mately, multiplexes this information into a series of

Outbound E3 frames. However, the XRT7250 Framer

IC lacks the current drive capability to be able to di-

rectly transmit this E3 data stream through some

transformer-coupled coax cable with enough signal

strength for it to be received by the remote receiver.

Therefore, in order to get around this problem, the

Framer IC requires the use of an LIU (Line Interface

Unit) IC. An LIU is a device that has sufficient drive

capability, along with the necessary pulse-shaping

circuitry to be able to transmit a signal through the

transmission medium in a manner that it can be reli-

ably received by the far-end receiver. Figure 178 pre-

sents a circuit drawing depicting the Framer IC inter-

facing to an LIU (XRT7300 DS3/E3/STS-1 Transmit

LIU).

XRT7250 Datasheet(PDF) 395 Page - Exar Corporation

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