Line code

An example of coding a binary signal using rectangularpulse-amplitude modulationwith polarnon-return-to-zerocode

An example ofbipolar encoding,or AMI.

Encoding of 11011000100 inManchester encoding

An example ofdifferential Manchester encoding

An example ofbiphase mark code

An example ofMLT-3 encoding

Intelecommunications,aline codeis a pattern of voltage, current, or photons used to represent digital datatransmitteddown acommunication channelor written to astorage medium.This repertoire of signals is usually called aconstrained codein data storage systems. Some signals are more prone to error than others as the physics of the communication channel or storage medium constrains the repertoire of signals that can be used reliably.

Common line encodings areunipolar,polar,bipolar,andManchester code.

Transmission and storage

After line coding, the signal is put through a physical communication channel, either atransmission mediumordata storage medium.The most common physical channels are:

• the line-coded signal can directly be put on atransmission line,in the form of variations of the voltage or current (often usingdifferential signaling).
• the line-coded signal (thebasebandsignal) undergoes furtherpulse shaping(to reduce its frequency bandwidth) and then ismodulated(to shift its frequency) to create anRF signalthat can be sent through free space.
• the line-coded signal can be used to turn on and off a light source infree-space optical communication,most commonly used in an infraredremote control.
• the line-coded signal can be printed on paper to create abar code.
• the line-coded signal can be converted to magnetized spots on ahard driveortape drive.
• the line-coded signal can be converted to pits on anoptical disc.

Some of the more common binary line codes include:

Signal	Comments	1 state	0 state
NRZ–L	Non-return-to-zerolevel. This is the standard positive logic signal format used in digital circuits.	forces a high level	forces a low level
NRZ–M	Non-return-to-zero mark	forces a transition	does nothing (keeps sending the previous level)
NRZ–S	Non-return-to-zero space	does nothing (keeps sending the previous level)	forces a transition
RZ	Return to zero	goes high for half the bit period and returns to low	stays low for the entire period
Biphase–L	Manchester. Two consecutive bits of the same type force a transition at the beginning of a bit period.	forces a negative transition in the middle of the bit	forces a positive transition in the middle of the bit
Biphase–M	Variant of Differential Manchester. There is always a transition halfway between the conditioned transitions.	forces a transition	keeps level constant
Biphase–S	Differential Manchester used in Token Ring. There is always a transition halfway between the conditioned transitions.	keeps level constant	forces a transition
Differential Manchester (Alternative)	Need a Clock, always a transition in the middle of the clock period	is represented by no transition.	is represented by a transition at the beginning of the clock period.
Bipolar	The positive and negative pulses alternate.	forces a positive or negative pulse for half the bit period	keeps a zero level during bit period

An arbitrary bit pattern in various binary line code formats

Each line code has advantages and disadvantages. Line codes are chosen to meet one or more of the following criteria:

• Minimize transmission hardware
• Facilitate synchronization
• Ease error detection and correction
• Achieve a targetspectral density
• Eliminate aDC component

Disparity

Most long-distance communication channels cannot reliably transport aDC component.The DC component is also called thedisparity,thebias,or theDC coefficient.The disparity of a bit pattern is the difference in the number of one bits vs the number of zero bits. Therunning disparityis therunning totalof the disparity of all previously transmitted bits.The simplest possible line code,unipolar,gives too many errors on such systems, because it has an unbounded DC component.

Most line codes eliminate the DC component – such codes are calledDC-balanced,zero-DC, or DC-free. There are three ways of eliminating the DC component:

• Use aconstant-weight code.Each transmittedcode wordin a constant-weight code is designed such that every code word that contains some positive or negative levels also contains enough of the opposite levels, such that the average level over each code word is zero. Examples of constant-weight codes includeManchester codeandInterleaved 2 of 5.
• Use apaired disparity code.Each code word in a paired disparity code that averages to a negative level is paired with another code word that averages to a positive level. The transmitter keeps track of the running DC buildup, and picks the code word that pushes the DC level back towards zero. The receiver is designed so that either code word of the pair decodes to the same data bits. Examples of paired disparity codes includealternate mark inversion,8b/10band4B3T.
• Use ascrambler.For example, the scrambler specified inRFC2615for64b/66b encoding.

Polarity

Bipolar line codes have two polarities, are generally implemented as RZ, and have a radix of three since there are three distinct output levels (negative, positive and zero). One of the principle advantages of this type of code is that it can eliminate any DC component. This is important if the signal must pass through a transformer or a long transmission line.

Unfortunately, several long-distance communication channels have polarity ambiguity. Polarity-insensitive line codes compensate in these channels. There are three ways of providing unambiguous reception of 0 and 1 bits over such channels:

• Pair each code word with the polarity-inverse of that code word. The receiver is designed so that either code word of the pair decodes to the same data bits. Examples includealternate mark inversion,Differential Manchester encoding,coded mark inversionandMiller encoding.
• differential codingeach symbol relative to the previous symbol. Examples includeMLT-3 encodingandNRZI.
• Invert the whole stream when invertedsyncwordsare detected, perhaps usingpolarity switching

Run-length limited codes

For reliableclock recoveryat the receiver, arun-length limitationmay be imposed on the generated channel sequence, i.e., the maximum number of consecutive ones or zeros is bounded to a reasonable number. A clock period is recovered by observing transitions in the received sequence, so that a maximum run length guarantees sufficient transitions to assure clock recovery quality.

RLL codes are defined by four main parameters:m,n,d,k.The first two,m/n,refer to the rate of the code, while the remaining two specify the minimaldand maximalknumber of zeroes between consecutive ones. This is used in bothtelecommunicationsand storage systems that move a medium past a fixedrecording head.

Specifically, RLL bounds the length of stretches (runs) of repeated bits during which the signal does not change. If the runs are too long, clock recovery is difficult; if they are too short, the high frequencies might be attenuated by the communications channel. Bymodulatingthedata,RLL reduces the timing uncertainty in decoding the stored data, which would lead to the possible erroneous insertion or removal of bits when reading the data back. This mechanism ensures that the boundaries between bits can always be accurately found (preventingbit slip), while efficiently using the media to reliably store the maximal amount of data in a given space.

Early disk drives used very simple encoding schemes, such as RLL (0,1) FM code, followed by RLL (1,3) MFM code which were widely used inhard disk drivesuntil the mid-1980s and are still used in digital optical discs such asCD,DVD,MD,Hi-MDandBlu-rayusingEFMandEFMPLuscodes.Higher density RLL (2,7) and RLL (1,7) codes became thede facto standardsfor hard disks by the early 1990s.

Synchronization

Main article:Clock recovery

Line coding should make it possible for the receiver to synchronize itself to thephaseof the received signal. If the clock recovery is not ideal, then the signal to be decoded will not be sampled at the optimal times. This will increase the probability of error in the received data.

Biphase line codes require at least one transition per bit time. This makes it easier to synchronize the transceivers and detect errors, however, the baud rate is greater than that of NRZ codes.

Other considerations

A line code will typically reflect technical requirements of the transmission medium, such asoptical fiberorshielded twisted pair.These requirements are unique for each medium, because each one has different behavior related to interference, distortion, capacitance and attenuation.

Common line codes

• 2B1Q
• 4B3T
• 4B5B
• 6b/8b encoding
• 8b/10b encoding
• 64b/66b encoding
• 128b/130b encoding
• Alternate mark inversion(AMI)
• Coded mark inversion(CMI)
• EFMPlus,used inDVDs
• Eight-to-fourteen modulation(EFM), used incompact discs
• Hamming code
• Hybrid ternary code
• Manchester codeanddifferential Manchester
• Mark and space
• MLT-3 encoding
• Modified AMI codes:B8ZS, B6ZS, B3ZS, HDB3
• Modified frequency modulation,Miller encoding and delay encoding
• Non-return-to-zero(NRZ)
• Non-return-to-zero, inverted(NRZI)
• Pulse-position modulation(PPM)
• Return-to-zero(RZ)
• TC-PAM

Optical line codes

• Alternate-Phase Return-to-Zero(APRZ)
• Carrier-Suppressed Return-to-Zero(CSRZ)
• Three of Six, Fiber Optical(TS-FO)

See also

• Physical layer
• Self-synchronizing codeand bit synchronization

References

• This article incorporatespublic domain materialfromFederal Standard 1037C.General Services Administration.Archived fromthe originalon 2022-01-22.(in support ofMIL-STD-188).

External links

• Line Coding Lecture No. 9
• Line Coding in Digital Communication
• CodSim 2.0: Open source simulator for Digital Data Communications Model at the University of Malaga written in HTML