Technical data

(5) Scrambling/Descrambling

Scrambling the data before transmission helps to eliminate large narrow-band signal power peaks for

repeated data patterns, and spreads the signal power more uniformly over the entire channel band-

width.

The scrambler encodes a plaintext NRZ bit stream by addition (modulo 2) of 2047 bits generated by

the recursive linear function X[n]= X[n-11] + X[n-9] (modulo 2). The scrambler generates the specified

non-zero key stream whenever the active output interface is required to transmit a scrambled data

stream. The seed for the scrambler is generated from the PHY address, ensuring that in multiple-PHY

applications each PHY will have its individual scrambler sequence.

The descrambler descrambles the NRZ ciphertext bit stream coming from the MLT-3 decoder by addi-

tion (modulo 2) of a key stream to re-produce a plaintext bit stream. During the reception of IDLE sym-

bols, the descrambler synchronizes its descrambler key to the incoming stream. Once synchronization

is achieved, the descrambler locks on this key and is able to descramble incoming data.

Special logic in the descrambler ensures synchronization with the remote PHY by searching for IDLE

symbols within a window of 4000 bytes (40us). This window ensures that a maximum packet size of

1514 bytes, allowed by the IEEE 802.3 standard, can be received with no interference.

This core has a scrambler and descrambler bypass mode for testing purposes.

(6) MLT3 Encoding/Decoding

In the transmit direction, the serial 125 MHz NRZI data stream is encoded to MLT-3. MLT-3 is a trilevel

code where a change in the logic level represents a code bit “1” and the logic output remaining at the

same level represents a code bit “0”.

In the receive direction, the MLT-3 code is converted to an NRZI data stream. The NRZI to MLT-3 con-

version is illustrated in

Figure 2.

Figure 2: MLT-3 Encoding Example

Technical data subject to change Version 1.0.0