MLT-3 encoding

MLT -3 ( Multi- Level Transmission Encoding - 3 levels ) is a term used in telecommunications line code with three voltage levels which with the symbols ( , 0, - ), respectively. Through its three stages include the waveform formed on the ternary signals.

General

MLT -3 changes when a logic one in the data stream according to the signal level of the fixed sequence [ 0, , 0, -]. During the transmission of a logic zero, the instantaneous state of the line does not change. MLT -3 thus has as Non Return to Zero (NRZ ), an information content of one bit per symbol on, but the bandwidth of 1/ 4 is reduced with respect to the binary NRZ transmission. The MLT -3 coding line of the DC signal component, and the bandwidth is decreased.

The bandwidth reduction is due to the fixed sequence [ 0, , 0, -] when running through in the transfers only a logical one. Are transmitted only logical zeros does not change on the line, so these states do not contribute to the bandwidth. Thus, a timing recovery with the MLT - 3 is not possible, and require additional measures, such as a Precodierung the 4B5B code or statistical methods such as the " scrambling " of the user data.

Application to Fast Ethernet

The Ethernet standard 100 - Base-TX (100 Mbit / s on twisted pair cables) to the transmitted user data encoded with first - 4B5B code and then with MLT- third By the 4B5B encoding, the transmitted bit rate is reduced by 1/ 5, for a clock recovery is possible, and in addition are provided by the 4B5B code unique " special symbols " for the synchronization or transmission of signaling states are available:

Example Ethernet 100 - Base-TX line coding:

Data Stream: | 0 1 1 1 | 0 1 0 0 | 0 0 1 0 | 0 0 0 0 | 1 0 1 1 | 4- blocks: | 0111 | 0100 | 0010 | 0000 | 1011 | 4B5B code: | 01111 | 01010 | 10100 | 11110 | 10111 | MLT -3 level: | 0 0-0 | 0 00 | - 000 | 0-00 | 0-0 | Note: The order of transmission in 100BASE -TX is not easy. Lt.. IEEE 802.3 Section 3.3 is the least significant bit transmitted first in each case, except for the checksum (see IEEE 802.3, section 3.3: "Each octet of the MAC frame, with the exception of the FCS, is trans- mitted least significant bit first. "). However, according to IEEE 802.3 section 24.2.2.4 transmitted the most significant bit of a 5- bit code word ( 4B5B code) first. In the above example, the byte sequence is thus x47 x02 x? B ( hexadecimal representation ) transfer, if it is not the checksum. The line " data stream ", however, is not strictly correct only for the transmission of the checksum.

Even a null sequence generated by the 4B5B code conditionally level change:

Data Stream: | 0 0 0 0 | 0 0 0 0 | 0 0 0 0 | 0 0 0 0 | 0 0 0 0 | 4- blocks: | 0000 | 0000 | 0000 | 0000 | 0000 | 4B5B code: | 11110 | 11110 | 11110 | 11110 | 11110 | MLT -3 level: | 0-00 | 0-00 | 0-00 | 0-00 | 0-00 | In the MLT -3 level sequence can lead to non- DC- free transmission patterns in combination with code - 4B5B at Fast Ethernet certain data packets. By the non- DC voltage compatible with Ethernet transmission channel via pulse transformers is used for these data packets, which are also known as "killer packets ', shifts the dc level (English baseline wander ). This can at the receiver lead to a significant increase in bit error rate and a loss of connection. By determining the interfering DC component at the transmitter can be compensated for in the MLT -3 encoding the baseline wander.