MLT-3 encoding (Multi-Level Transmit) is a

line code In telecommunications, a line code is a pattern of voltage, current, or photons used to represent digital data transmission (telecommunications), transmitted down a communication channel or written to a storage medium. This repertoire of signal ...

(a signaling method used in a

telecommunication Telecommunication, often used in its plural form or abbreviated as telecom, is the transmission of information over a distance using electronic means, typically through cables, radio waves, or other communication technologies. These means of ...

system for transmission purposes) that uses three

voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...

levels. An MLT-3 interface emits less electromagnetic interference and requires less

bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...

than most other

binary Binary may refer to: Science and technology Mathematics * Binary number, a representation of numbers using only two values (0 and 1) for each digit * Binary function, a function that takes two arguments * Binary operation, a mathematical op ...

or ternary interfaces that operate at the same

bit rate In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction ...

(see

PCM Pulse-code modulation (PCM) is a method used to Digital signal (signal processing), digitally represent analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio application ...

for discussion on bandwidth / quantization tradeoffs), such as

Manchester code In telecommunications and data storage, Manchester code (also known as phase encoding, or PE) is a line code in which the encoding of each data bit is either low then high, or high then low, for equal time. It is a self-clocking signal with no ...

Alternate Mark Inversion In telecommunication, bipolar encoding is a type of return-to-zero (RZ) line code, where two nonzero values are used, so that the three values are +, −, and zero. Such a signal is called a duobinary signal. Standard bipolar encodings are designed ...

. 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 NRZ encoding, MLT-3 has a coding efficiency of 1 bit/baud, however it requires four transitions (

baud In telecommunications and electronics, baud (; symbol: Bd) is a common unit of measurement of symbol rate, which is one of the components that determine the speed of communication over a data channel. It is the unit for symbol rate or modulat ...

) 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 Run-length limited (RLL) is a line coding technique that is used to send arbitrary data over a communications channel with bandwidth limits. RLL codes are defined by four main parameters: ''m'', ''n'', ''d'', ''k''. The first two, ''m''/''n'', ...

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
Network World, June 29, 1992, page 19 as a coding scheme for

FDDI Fiber Distributed Data Interface (FDDI) is a standard for data transmission in a local area network. It uses optical fiber as its standard underlying physical medium. It was also later specified to use copper cable, in which case it may be c ...

copper interconnect (TP-PMD, - initial implementation; also see patent aka CDDI). Later, the same technology was used in the

100BASE-TX In computer networking, Fast Ethernet physical layers carry traffic at the nominal rate of . The prior Ethernet speed was . Of the Fast Ethernet physical layers, 100BASE-TX is by far the most common. Fast Ethernet was introduced in 1995 as t ...

physical medium dependent sublayer, given the considerable similarities between

and 100BASE- F 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 100BASE-T4 Ethernet, while it has three levels, is not compatible with MLT-3. It uses selective base-2 to base-3 conversion with direct mapping of base-3 digits to line levels ( 8B6T code).

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References

Line codes {{telecomm-stub

See also

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References