View source for MLT-3 encoding

{{Short description|MLT-3 Encoding way & links to Patent}}
[[Image:MLT3encoding.svg|thumb|right|360px|Example of MLT-3 Encoding. Light-colored lines indicate two previous states, where : {{math|1=''[[Leet|1337]]''<sub>10</sub> = ''10100111001''<sub>2</sub>}}]]
'''MLT-3 encoding''' (Multi-Level Transmit) is a [[line code]] (a signaling method used in a [[telecommunication]] system for transmission purposes) that uses three [[voltage]] levels. An MLT-3 interface emits less [[electromagnetic interference]] and requires less [[Bandwidth (signal processing)|bandwidth]] than most other [[Boolean logic|binary]] or [[Ternary logic|ternary]] interfaces that operate at the same [[bit rate]] (see [[Pulse-code modulation|PCM]] for discussion on bandwidth / [[Quantization (signal processing)|quantization]] tradeoffs), such as [[Manchester code]] or [[Bipolar encoding|Alternate Mark Inversion]].

[[Image:Eye pattern MLT3.png|thumb|right|360px|[[Eye pattern]] of a MLT-3 signal]]

MLT-3 cycles sequentially through the voltage levels −1, 0, +1, 0. It moves to the next state to transmit a 1 bit, and stays in the same state to transmit a 0 bit. Similar to simple [[Non-return-to-zero|NRZ]] encoding, MLT-3 has a coding efficiency of 1 bit/baud, however it requires four transitions ([[baud]]) to complete a full cycle (from low-to-middle, middle-to-high, high-to-middle, middle-to-low). Thus, the maximum fundamental frequency is reduced to one fourth of the baud rate. This makes signal transmission more amenable to copper wires.

The lack of transition on a 0 bit means that for practical use, the number of consecutive 0 bits in the transmitted data must be bounded; i.e. it must be pre-coded using a [[run-length limited]] code.  This results in an effective bitrate slightly lower than one bit per baud or four bits per cycle.

MLT-3 was first introduced by Crescendo Communications<ref>[https://books.google.com/books?id=gRAEAAAAMBAJ&dq=crescendo+proposal+copper+FDDI&pg=PA19], Network World, June 29, 1992, page 19</ref> as a coding scheme for [[FDDI]] copper interconnect (TP-PMD,<ref name="x3t12">
{{cite journal
| author = American National Standards Institute
| year = 1994
| title = FDDI twisted pair physical layer medium dependent (TP-PMD)
| journal =  American National Standard X3T12 (Incorporates X3.263)
}} - initial implementation; also see patent
</ref> aka [[CDDI]]). Later, the same technology was used in the [[Fast Ethernet#100BASE-TX|100BASE-TX]] physical medium dependent sublayer, given the considerable similarities between [[FDDI]] and [[Fast Ethernet|100BASE-[TF]X]] physical media attachment layer (section 25.3 of IEEE802.3-2002 specifies that ANSI X3.263:1995 TP-PMD should be consulted, with minor exceptions).

Signaling specified by [[Fast Ethernet#100BASE-T4|100BASE-T4]] Ethernet, while it has three levels, is not compatible with MLT-3. It uses selective [[Binary numeral system|base-2]] to [[Ternary numeral system|base-3]] conversion with direct mapping of base-3 digits to line levels ([[8B6T]] code).

== See also ==
* [[4B5B]]

== External links ==
* {{US patent|5280500}}

== References ==
<references/>
[[Category:Line codes]]

{{Bit-encoding}}

{{telecomm-stub}}