MLT-3 code
MLT-3 ( M ulti l evel T ransmission Encoding - 3 levels) is a line code used in communications technology with three voltage levels which are identified by the symbols (+, 0, -). Due to its three levels, the signal shape formed is one of the ternary signals .
General
MLT-3 changes the signal level according to the fixed sequence [at a logic one in the data stream 0, +, 0, −]. When a logical zero is transmitted, the current state of the line does not change. Thus has MLT-3 as Non Return to Zero (NRZ) an information content of one bit per symbol on, but that is bandwidth to 1 / 2 reduced compared to NRZ. The MLT-3 line coding in particular reduces the bandwidth compared to a more direct transmission of the line code.
The bandwidth reduction is due to the fixed sequence [ 0, +, 0, −] when passing through in the transfers only a logical one. If only logical zeros are transmitted, nothing changes on the line, which means that these states do not contribute to the bandwidth. This means that clock recovery is not possible with MLT-3 alone and requires additional measures such as precoding with the 4B5B code or statistical methods such as “ scrambling ” the user data.
Use with Fast Ethernet
The Ethernet standard 100BASE-TX (100 Mbit / s on twisted pair cables ) first encodes the user data to be transmitted with Code-4B5B and then with MLT-3. By inserting an additional bits, the coded bit rate is increased compared to the Nutzdatenbitrate to 1 / 4 to 125 Mbit / s, this clock recovery is possible, and in addition are by the 4B5B code unique "special symbols" for synchronization or signaling of transmission states available :
Example Ethernet 100BASE-TX line coding:
Datenstrom: | 0 1 1 1 | 0 1 0 0 | 0 0 1 0 | 0 0 0 0 | 1 0 1 1 | 4er-Blöcke: | 0111 | 0100 | 0010 | 0000 | 1011 | 4B5B-Code: | 01111 | 01010 | 10100 | 11110 | 10111 | MLT-3 Pegel: | 0+0−0 | 0++00 | −−000 | +0−00 | ++0−0 |
Note: The order of transmission in 100BASE-TX is not very straightforward. Lt. IEEE 802.3 section 3.3 the least significant bit is transmitted first, with the exception of the checksum (see IEEE 802.3 section 3.3: "Each octet of the MAC frame, with the exception of the FCS, is transmitted least significant bit first."). However, according to IEEE 802.3 Section 24.2.2.4, the most significant bit of a 5-bit code word (4B5B code) is transmitted first. In the above example, the byte sequence x47 x02 x? B (hexadecimal representation) is transmitted if it is not the checksum. Strictly speaking, however, the "data stream" line is only correct for the transmission of the checksum.
Also a zero sequence generated by the code 4B5B caused level change:
Datenstrom: | 0 0 0 0 | 0 0 0 0 | 0 0 0 0 | 0 0 0 0 | 0 0 0 0 | 4er-Blöcke: | 0000 | 0000 | 0000 | 0000 | 0000 | 4B5B-Code: | 11110 | 11110 | 11110 | 11110 | 11110 | MLT-3 Pegel: | +0−00 | +0−00 | +0−00 | +0−00 | +0−00 |
With the MLT-3 level sequence in combination with Code-4B5B with Fast Ethernet, certain data packets can lead to transmission patterns that are not free of DC voltage. Due to the transmission channel not suitable for direct voltage in Ethernet via pulse transformers , these data packets, which are also known as "killer packets", lead to shifts in the direct voltage level ( baseline wander ). This can lead to a significant increase in the bit error rate at the receiver and to an interruption of the connection. By determining the disruptive direct component at the transmitter, the baseline wander can be compensated for with the MLT-3 coding .
Individual evidence
- ↑ European Patent EP1195959 : Baseline wander correction for MLT3 signals , 2007
literature
- John G. Proakis, Masoud Salehi: Communication System Engineering . 2nd Edition. Prentice Hall, 2002, ISBN 0-13-095007-6 .