KS8993

Micrel

KS8993

32

April 2009

MLT3 Coding

For 100BaseTX operation the NRZI (Non-Return to Zero Invert on ones) signal is line coded as MLT3. The net result of using

MLT3 is to reduce the EMI (Electro Magnetic Interference) of the signal over twisted pair media. In NRZI coding, the level

changes from high to low or low to high for every “1” bit. For a “0” bit there is no transition. MLT3 line coding transitions through

three distinct levels. For every transition of the NRZI signal the MLT3 signal either increments or decrements depending on

the current state of the signal. For instance if the MLT3 level is at its lowest point the next two NRZI transitions will change the

MLT3 signal initially to the middle level followed by the highest level (second NRZI transition). On the next NRZI change, the

MLT3 level will decrease to the middle level. On the following transition of the NRZI signal the MLT3 level will move to the lowest

level where the cycle repeats. The diagram below describes the level changes. Note that in the actual 100BaseTX circuit there

is a scrambling circuit and that scrambling is not shown in this diagram.

A

3

8

E

9

4

T3

R3

I1

I1

1010 0011 1000 1110 1001 0100 UUUU UUUU UUUU UUUU

10110101011001011100100110101001101001111111111111

Hex Value

Binary 4B

Binary 5B

NRZ

NRZI

MLT3

Figure 11. MLT3 coding

Field

Octect Length

Description

Preamble/SFD

8

Preamble and Start of Frame Delimiter

DA

6

48-bit Destination MAC Address

SA

6

48-bit Source MAC Address

802.1p tag

4

VLAN and priority tag (optional)

Length

2

Frame Length

Protocol/Data

46 to 1500

Higher Layer Protocol and Frame Data

Frame CRC

4

32-bit Cyclical Redundancy Check

ESD

1

End of Stream Delimiter

Idle

Variable

Inter Frame Idles

Table 11. MAC Frame for 802.3

The MAC (Media Access Control) fields are described in the table below.