13
ICS1890
Figure 2
0.1
1
10
100
0
2
4
Resistance (Ohm/m) v. Freq. (MHz)
100Base-T Twisted Pair Physical Media
Dependent [TP-PMD]
Stream Cipher Scrambler/Descrambler
When the
ICS1890
is operating in the 100Base-TX mode, a
stream cipher scrambler/descrambler that conforms to the ANSI
Standard X3T9.5 FDDI TP-PMD is employed. The purpose of
the stream cipher scrambler is to randomize the 100 Mbps data
on transmission resulting in a reduction of the peak amplitudes
in the frequency spectrum. The stream cipher descrambler
restores the received 5-bit code groups to their unscrambled
values. The stream cipher scrambler/descrambler is bypassed
in the 100M stream interface mode.
MLT-3 Encoder/Decoder
When the
ICS1890
is operating in the 100Base-TX mode, an
MLT-3 encoder and decoder is employed. The encoder converts
the NRZI transmitted bit stream to a three-level code resulting
in a reduction in the energy over the critical frequency range
of 20MHz to 100MHz.The MLT-3 decoder converts the received
three-level code back to an NRZI bit stream.
DC Restoration
The 100Base-TX specification 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 run lengths of zeroes and ones can cause the
production of a DC component. This DC component cannot
be transmitted through the isolation transformers and results
in baseline wander. Baseline wander decreases noise immunity
since the base-line moves closer to either the positive or
negative signal comparaters. Figure 1 is an exaggerated
simulation of the effect of baseline wander (the time period
would normally be much longer).
The
ICS1890
uses DC restoration to restore the lost DC
component of the recovered digital data thus correcting for
baseline wander.
Adaptive Equalizer
The
ICS1890
includes an adaptive equalizer to compensate
for signal amplitude and phase distortion incurred from the
transmission media. Signal equalization will actively occur for
twisted pair cable lengths of up to 105 meters.
At a data rate of 100 Mbps, the cable introduces significant
signal distortion due to high frequency roll off and phase
shift. The high frequency loss is mainly due to skin-effect
which causes the conductor resistance to rise as the square of
the frequency (see Figure 2).
Figure 1
Summary of Contents for PHYceiver ICS1890
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