MLT-3 encoding
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 than most other binary or ternary interfaces that operate at the same bit rate (see PCM for discussion on bandwidth / quantization tradeoffs), such as Manchester code or Alternate Mark Inversion.
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) 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[1] as a coding scheme for FDDI copper interconnect (TP-PMD,[2] aka CDDI). Later, the same technology was used in the 100BASE-TX physical medium dependent sublayer, given the considerable similarities between FDDI and 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 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).
See also
- 4B5B
External links
- U.S. patent 5,280,500
References
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- Unipolar encoding
- Bipolar encoding
- On-off keying
- Mark and space
- Return to zero (RZ)
- Non-return-to-zero, level (NRZ/NRZ-L)
- Non-return-to-zero, inverted (NRZ-I)
- Non-return-to-zero, space (NRZ-S)
- Manchester
- Differential Manchester/biphase (Bi-φ)
- Conditioned diphase
- 4B3T
- 4B5B
- 2B1Q
- Alternate mark inversion
- Modified AMI code
- Coded mark inversion
- MLT-3 encoding
- Hybrid ternary code
- 6b/8b encoding
- 8b/10b encoding
- 64b/66b encoding
- Eight-to-fourteen modulation
- Delay/Miller encoding
- TC-PAM
- See also: Baseband
- Baud
- Bit rate
- Digital signal
- Digital transmission
- Ethernet physical layer
- Pulse modulation methods
- Pulse-amplitude modulation (PAM)
- Pulse-code modulation (PCM)
- Serial communication
- Category:Line codes
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