XRT74L74 Datasheet, PDF(192/498 Page) Exar Corporation – 4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

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XRT74L74 Datasheet, PDF (192/498 Pages) Exar Corporation – 4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

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XRT74L74

REV. P1.1.1

4 CHANNEL, ATM UNI/PPP DS3/E3 FRAMING CONTROLLER

ADVANCED CONFIDENTIAL

XRT74L74 CONFIGURATION

The XRT74L74 DS3/E3 Framer IC can be configured

to support any of the following four framing formats.

â¢ DS3/C-Bit Parity

â¢ DS3/M13

â¢ E3/ITU-T G.832

â¢ E3/ITU-T G.751

As a consequence, the discussion of the XRT74L74

Framer IC will be organized as follows:

â¢ Section 4.0 - DS3 Mode Operation of the

XRT74L74

â¢ Section 5.0 - E3, ITU-T G.751 Operation of the

XRT74L74

â¢ Section 6.0 - E3, ITU-T G.832 Operation of the

XRT74L74

â¢ Section 7.0 - Framer Local Loop-back Mode

Operation

â¢ Section 8.0 - High Speed HDLC Controller Mode

of Operation

5.0 DS3 OPERATION OF THE XRT74L74

This section will discuss in detail, the operation of the

XRT74L74 Framer IC, when it has been configured to

operate in the DS3 Mode.

Configuring the XRT74L74 to Operate in the DS3

Mode

The XRT74L74 can be configured to operate in the

DS3 Mode by writing a "1" into bit-field 6 within the

Framer Operating Mode register, as illustrated below.

FRAMER OPERATING MODE REGISTER (ADDRESS = 0X00)

BIT 7

BIT 6

BIT 5

BIT 4

BIT 3

BIT2

BIT 1

BIT 0

Local Loopback

R/W

x

DS3/E3*

R/W

1

Internal LOS

Enable

R/W

x

RESET

R/W

0

Interrupt

Enable Reset

R/W

x

Frame Format

R/W

x

TimRefSel[1:0]

R/W

R/W

x

x

Prior to describing the functional blocks within the

Transmit and Receive Sections of the XRT74L74, it is

important to describe the following two framing for-

mats.

â¢ M13

â¢ C-Bit Parity

5.1 DESCRIPTION OF THE DS3 FRAMES AND ASSOCI-

ATED OVERHEAD BITS

The role of the various overhead bits are best de-

scribed by discussing the DS3 Frame Format as a

whole. The DS3 Frame contains 4760 bits, of which

56 bits are overhead and the remaining 4704 bits are

payload bits. The payload data is formatted into

packets of 84 bits and the overhead (OH) bits are in-

serted between these payload packets. The

XRT74L74 Framer supports the following two DS3

framing formats:

â¢ C-bit Parity

â¢ M13

Figures 47 and 48 present the DS3 Frame Format for

C-bit Parity and M13, respectively.

FIGURE 47. DS3 FRAME FORMAT FOR C-BIT PARITY

X

I

F1

I AIC I

F0

I

NA

I

F0

I FEAC I

F1

I

I

X

I

F1

I UDL I

F0

I

NA

I

F0

I UDL I

F1

I

190

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