Physical Medium Attachment (Pma); Scrambling - Samsung KS8910 User Manual

100BASE-TX DIGITAL BLOCKS

PHYSICAL MEDIUM ATTACHMENT (PMA)

SCRAMBLING

The Scrambler and Descrambler can be bypassed. This is controllable through a register bit.
Scrambler
The scrambler shall encode a plain-text bit stream derived from the TX_code_bit. The bit stream shall be encoded
by addition (modulo 2) of a key stream to produce a cipher-text bit stream. The key stream shall be the periodic
sequence of 2047 bits generated by the recursive linear function X[n] = X[n-11] + X[n-9] (modulo 2). The scrambler
shall generate the specified non-zero key stream whenever the Active Output Interface is required to transmit a
scrambled data stream.
The key steam sequence can be generated by an 11-bit Linear Feedback Shift Register(LFSR) whose input bit is
the exclusive-OR of its 11th and 9th previous bits, and which contains at least one non-zero bit. The functional
diagram for the LFSR is given Figure 4-5 whereas a functional block diagram of the scrambler s in Figure 4-6.
Q11
D10
Tx_clk
Tx_en
Tx_clk
Tx_en
TX_bit
4-10
Q10 Q9 Q8 Q7
D9 D8 D7
Figure 4-5. Linear Feedback Shift Register (LFSR)
LFSR
Figure 4-6. Scrambler Function of KS8910
KS8910 100/10 Mbps ETHERNET TRANSCEIVER
Q6 Q5 Q4
D6 D5 D4 D3
Rst
lfsr_out
Q3 Q2 Q1 Q0
D2 D1 D0
cl/st
cipher-text_out
Preliminary Spec. ver
1.4
lfsr_out