Datasheet

Micrel, Inc. KS8999

January 2005

KS8999

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.

Figure 20. MLT3 Coding

MAC Frame

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

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

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 14. MAC Frame