82555 — Networking Silicon
14
Datasheet
4.2.2
100BASE-TX Scrambler and MLT-3 Encoder
Data is scrambled in 100BASE-TX in order 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 82555 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 MLT-3 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
E
11100
1110
F
11101
1111
I
11111
Inter Packet Idle Symbol
(No 4B)
J
11000
1st Start of Packet Symbol
0101
K
10001
2nd Start of Packet Symbol
0101
T
01101
1st End of Packet Symbol
R
00111
2nd End of Packet Symbol
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
Flow Control S
V
11001
INVALID
Table 2. 4B/5B Encoder
Symbol
5B Symbol Code
4B Nibble Code
