The scrambler is required to control the radiated emissions at the media connector and on the twisted-pair cable. By scrambling the data, the total energy launched onto the cable is randomly distributed over a wide frequency range. Without the scrambler, energy levels at the MDI and on the cable could peak beyond FCC limitations at frequencies related to repeating 5B sequences (that is, continuous transmission of IDLEs).

The scrambler is configured as a closed loop linear feedback shift register (LFSR) with an 11-bit polynomial. The output of the closed loop LFSR is X-ORd with the serial NRZ data from the code-group encoder. The result is a scrambled data stream with sufficient randomization to decrease radiated emissions at certain frequencies by as much as 20 dB.

A serial descrambler is used to de-scramble the received MLT3 decoded data. The descrambler has to generate an identical data scrambling sequence (N) in order to recover the original unscrambled data (UD) from the scrambled data (SD) as represented in the equations:

SD = (UD ⊕ N) (1)

UD = (SD ⊕ N) (2)

Synchronization of the descrambler to the original scrambling sequence (N) is achieved based on the knowledge that the incoming scrambled data stream consists of scrambled IDLE data (which is continuous stream of 1’s). After the descrambler has recognized 88 consecutive IDLE symbols (or ~18 5-bit IDLE codes) where an IDLE symbol is bit decoded as ‘b1 after de-scrambling, it will synchronize to the receive data stream and generate unscrambled data in the form of unaligned 5B code-groups. In order to maintain synchronization, the descrambler must continuously monitor the validity of the unscrambled data that it generates. To ensure this the received symbols are continuously monitored to check for any non-IDLE symbols received before start of packet i.e. /J/K/. If 3 consecutive errors occur i.e. descrambled bits are received as ‘b0 but don’t represent /J/ symbol the scrambler is forced to unlock state and reset to reacquire a synchronization. Similarly, if the symbol following /J/ doesn’t match /K/ (i.e. the 2nd SSD symbol) the scrambler is forced to unlock state and reset to reacquire a synchronization.