ICS1893AF Data Sheet - Release
Chapter 7 Functional Blocks
7.4 Functional Block: 100Base-TX TP-PMD Operations
The ICS1893AF supports both 10Base-T and 100Base-TX operations. For 100Base-TX operations, the
TP-PMD module performs 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 defined in the
specification for 100Base-TX Twisted-Pair Physical Media Dependent (TP-PMD) Sublayer. The
ICS1893AF’s TP-PMD also performs DC restoration (that is, baseline wander correction) and adaptive
equalization on the received signals.
Note:
1. For an overview of 100Base-TX operations, see Section 5.5, “100Base-TX Operations”.
2. For more information on the Twisted-Pair Interface, see Section 6.3, “Twisted-Pair Interface”.
7.4.1 100Base-TX Operation: Stream Cipher Scrambler/Descrambler
When the ICS1893AF 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 ICS1893AF “seeds” (that is, initializes) the Transmit Stream Cipher Shift register by using the
ICS1893AF PHY address from Table 8-16, which minimizes crosstalk and noise in repeater applications.
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 ICS1893AF 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
The ICS1893AF’s 100Base-TX operations uses 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 known as ‘baseline wander’.
Baseline wander adversely affects the noise immunity of the receiver, because the ‘baseline’ signal moves
or ‘wanders’ from its nominal DC value. The ICS1893AF uses a unique technique to restore the DC
component ‘lost’ by the medium. As a result, the design is very robust, immune to noise and independent
of the data stream.
ICS1893AF, Rev D 10/26/04
October, 2004
Copyright © 2004, Integrated Circuit Systems, Inc.
All rights reserved.
44
IDT [ INTEGRATED DEVICE TECHNOLOGY ]
