ICS1892 datasheet(46/148 Pages) ICST

46 / 148 page

ICS1892, Rev. D, 2/26/01

February 26, 2001

Chapter 7

Functional Blocks

ICS1892 Data Sheet

7.4

Functional Block: 100Base-TX TP-PMD Operations

The ICS1892 supports both 10Base-T and 100Base-TX operations. The ICS1892 TP-PMD modules

perform 100Base-TX Twisted-Pair Physical Media Dependent (TP-PMD) stream cipher

scrambling/descrambling and MLT-3 encoding/decoding (3-level, multi-level transition) in compliance with

the ANSI Standard X3.263: 199X FDDI TP-PMD as called out in the ISO/IEC 8802-3 standard. The

ICS1892 TP-PMD also performs DC restoration and adaptive equalization on the received signals.

Note:

For an overview of the 100Base-TX, see Section 5.5, “100Base-TX Operations”.

For more information on the Twisted-Pair Interface, see Section 6.6, “Twisted-Pair Interface”.

7.4.1

100Base-TX Operation: Stream Cipher Scrambler/Descrambler

When the ICS1892 is operating in 100Base-TX mode, it employs a stream cipher scrambler/descrambler

that complies with the ANSI Standard X3.263: 199X FDDI TP-PMD. The purpose of the stream cipher

scrambler is to spread the transmission spectrum to minimize electromagnetic compatibility problems. The

stream cipher descrambler restores the received serial bit stream to its unscrambled form.

The MAC/Repeater Interface bypasses the stream cipher scrambler/descrambler when in the 100M

Symbol Interface mode.

7.4.2

100Base-TX Operation: MLT-3 Encoder/Decoder

When operating in the 100Base-TX mode, the ICS1892 TP-PMD sublayer employs an MLT-3 encoder and

decoder. During data transmission, the TP-PMD encoder converts the NRZI bit stream received from the

PMA sublayer to a 3-level Multi-Level Transition code. The three levels are +1, 0, and -1. The results of

MLT-3 encoding provide a reduction in the transmitted energy over the critical frequency range from 20

MHz to 100 MHz. The TP-PMD MLT-3 decoder converts the received three-level signal back to an NRZI bit

stream.

7.4.3

100Base-TX Operation: DC Restoration

100Base-TX operations employ a stream cipher scrambler to minimize peak amplitudes in the frequency

spectrum. However, the nature of the stream cipher and MLT-3 encoding is such that long sequences of

consecutive zeros or ones can exist. These unbalanced data patterns produce an undesirable DC

component in the data stream.

Baseline wander adversely affects the noise immunity of the receiver, because the ‘baseline’ signal moves

or ‘wanders’ from its nominal DC value.

The ICS1892 employs a unique technique to restore the DC component ‘lost’ by the medium. As a result,

the design is very robust and immune to noise, independent of the data.

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