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GD82559ER — Networkin
g
Silicon
38
Datasheet
6.1.2.2 100BASE-TX Scrambler and MLT-3 Encoder
Data is scrambled in 100BASE-TX to reduce electromagnetic emissions during long transmissions
of high-frequency data codes. The scrambler logic accepts 5 bits from the 4B/5B encoder block and
presents the scrambled data to the MLT-3 encoder. The PHY unit implements the 11-bit stream
cipher scrambler as adopted by the ANSI XT3T9.5 committee for UTP operation. The cipher
equation used is:
X[n] = X[n-11] + X[n-9] (mod 2)
The encoder receives the scrambled Non-Return to Zero (NRZ) data stream from the Scrambler
and encodes the stream into MLT-3 for presentation to the driver. MLT-3 is similar to NRZI
coding, but three levels are output instead of two. There are three output levels: positive, negative
and zero. When an NRZ “0” arrives at the input of the encoder, the last output level is maintained
(either positive, negative or zero). When an NRZ “1” arrives at the input of the encoder, the output
steps to the next level. The order of steps is negative-zero-positive-zero which continues
periodically.
E 11100 1110
F 11101 1111
I 11111
Inter Packet Idle S
y
mbol
(
No 4B
)
J 11000
1st Start of Packet S
y
mbol
0101
K 10001
2nd Start of Packet S
y
mbol
0101
T 01101 1st End of Packet S
y
mbol
R 00111
2nd End of Packet S
y
mbol
and Flow Control
V 00000 INVALID
V 00001 INVALID
V 00010 INVALID
V 00011 INVALID
H 00100 INVALID
V 00101 INVALID
V 00110 INVALID
V 01000 INVALID
V 01100 INVALID
V 10000 PHY based Flow Control
V 11001 INVALID
Table 3. 4B/5B Encoder
Symbol 5B Symbol Code 4B Nibble Code