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July 3, 2001

IDT77V107

Functional Description

Functional Description

Functional Description

Functional Description

Transmission convergence (TC) sub layer

Transmission convergence (TC) sub layer

Transmission convergence (TC) sub layer

Transmission convergence (TC) sub layer

Introduction

Introduction

Introduction

Introduction

The TC sub layer defines the line coding, scrambling, data framing

and synchronization. Under control of a switch interface or Segmenta-

tion and Reassembly (SAR) unit, the 25.6Mbps ATM PHY accepts a 53-

byte ATM cell, scrambles the data, appends a command byte to the

beginning of the cell, and encodes the entire 53 bytes before transmis-

sion. These data transformations ensure that the signal is evenly distrib-

uted across the frequency spectrum. In addition, the serialized bit

stream is NRZI coded. An 8kHz timing sync pulse may be used for

isochronous communications.

Data Structure and Framing

Data Structure and Framing

Data Structure and Framing

Data Structure and Framing

Each 53-byte ATM cell is preceded with a command byte. This byte

is distinguished by an escape symbol followed by one of 17 encoded

symbols. Together, this byte forms one of seventeen possible command

bytes. Three command bytes are defined:

1. X_X (read: 'escape' symbol followed by another 'escape'): Start-

of-cell with scrambler/descrambler reset.

2.

X_4 ('escape' followed by '4'): Start-of-cell without scrambler/

descrambler reset.

3.

X_8 ('escape' followed by '8'): 8kHz timing marker. This

command byte is generated when the 8kHz sync pulse is

detected, and has priority over all line activity (data or command

bytes). It is transmitted immediately when the sync pulse is

detected. When this occurs during a cell transmission, the data

transfer is temporarily interrupted on an octet boundary, and the

X_8 command byte is inserted. This condition is the only allowed

interrupt in an otherwise contiguous transfer.

Below is an illustration of the cell structure and command byte usage:

{X_X} {53-byte ATM cell} {X_4} {53-byte ATM {X_8} cell} ...

In the above example, the first ATM cell is preceded by the X_X start-

of-cell command byte which resets both the transmitter-scrambler and

receiver-descrambler pseudo-random nibble generators (PRNG) to their

initial states. The following cell illustrates the insertion of a start-of-cell

command without scrambler/descrambler reset. During this cell's trans-

mission, an 8kHz timing sync pulse triggers insertion of the X_8 8kHz

timing marker command byte.

Figure 2

TC Transmit Block Diagram

PHY-ATM

Interface

Control,

HEC Gen. &

Insertion

Scrambler

4

Command

Byte

Insertion

4

PRNG

4

Scramble

Nibble

Next

Reset

4b/5b

Encoding

4

NRZI

Encoding

1

Tx +

Tx -

Line Rate

Clock

TxRef (8kHz)

5362 drw 05

3 Cells

Start of Cell

UTOPIA

Interface

.