Line code
Encyclopedia
In telecommunication
, a line code (also called digital baseband modulation, also called digital baseband transmission method) is a code
chosen for use within a communications system
for baseband
transmission
purposes. Line coding is often used for digital data transport.
to be transported by an amplitude- and time-discrete signal that is optimally tuned for the specific properties of the physical channel (and of the receiving equipment). The waveform
pattern of voltage or current used to represent the 1s and 0s of a digital data on a transmission link is called line encoding. The common types of line encoding are unipolar
, polar, bipolar
, and Manchester encoding.
For reliable clock recovery
at the receiver, one usually imposes a maximum run length constraint
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 such clock recovery, while sequences without such a constraint could seriously hamper the detection quality.
After line coding, the signal is put through a "physical channel
", either a "transmission medium
" or "data storage medium". Sometimes the characteristics of two very different-seeming channels are similar enough that the same line code is used for them. The most common physical channels are:
Unfortunately, most long-distance communication channels cannot transport a DC component.
The DC component is also called the disparity, the bias, or the DC coefficient
.
The simplest possible line code, called unipolar
because it has an unbounded DC component, gives too many errors on such systems.
Most line codes eliminate the DC component — such codes are called DC balanced, zero-DC, zero-bias or DC equalized etc.
There are two ways of eliminating the DC component:
Line coding should make it possible for the receiver to synchronize itself to the phase
of the received signal. If the synchronization is not ideal, then the signal to be decoded will not have optimal differences (in amplitude) between the various digits or symbols used in the line code. This will increase the error probability in the received data.
It is also preferred for the line code to have a structure that will enable error detection.
Note that the line-coded signal and a signal produced at a terminal
may differ, thus requiring translation.
A line code will typically reflect technical requirements of the transmission medium
, such as optical fiber
or shielded twisted pair. These requirements are unique for each medium, because each one has different behavior related to interference, distortion, capacitance and loss of amplitude.
Optical line codes:
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
, a line code (also called digital baseband modulation, also called digital baseband transmission method) is a code
Code
A code is a rule for converting a piece of information into another form or representation , not necessarily of the same type....
chosen for use within a communications system
Communications system
In telecommunication, a communications system is a collection of individual communications networks, transmission systems, relay stations, tributary stations, and data terminal equipment usually capable of interconnection and interoperation to form an integrated whole...
for baseband
Baseband
In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...
transmission
Transmission (telecommunications)
Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
purposes. Line coding is often used for digital data transport.
Line coding
Line coding consists of representing the digital signalDigital signal
A digital signal is a physical signal that is a representation of a sequence of discrete values , for example of an arbitrary bit stream, or of a digitized analog signal...
to be transported by an amplitude- and time-discrete signal that is optimally tuned for the specific properties of the physical channel (and of the receiving equipment). The waveform
Waveform
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...
pattern of voltage or current used to represent the 1s and 0s of a digital data on a transmission link is called line encoding. The common types of line encoding are unipolar
Unipolar encoding
Unipolar encoding is a line code. A positive voltage represents a binary 1, and zero volts indicates a binary 0. It is the simplest line code, directly encoding the bitstream, and is analogous to on-off keying in modulation....
, polar, bipolar
Bipolar encoding
In telecommunication, bipolar encoding is a type of line code . A duobinary signal is such an encoding.- Advantages :...
, and Manchester encoding.
For reliable clock recovery
Clock recovery
Some digital data streams, especially high-speed serial data streams are sent without an accompanying clock signal. The receiver generates a clock from an approximate frequency reference, and then phase-aligns to the transitions in the data stream with a phase-locked loop...
at the receiver, one usually imposes a maximum run length constraint
Run Length Limited
Run length limited or RLL coding is a line coding technique that is used to send arbitrary data over a communications channel with bandwidth limits. This is used in both telecommunication and storage systems which move a medium past a fixed head. Specifically, RLL bounds the length of stretches ...
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 such clock recovery, while sequences without such a constraint could seriously hamper the detection quality.
After line coding, the signal is put through a "physical channel
Channel (communications)
In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...
", either a "transmission medium
Transmission medium
A transmission medium is a material substance that can propagate energy waves...
" or "data storage medium". Sometimes the characteristics of two very different-seeming channels are similar enough that the same line code is used for them. The most common physical channels are:
- the line-coded signal can directly be put on a transmission lineTransmission lineIn communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
, in the form of variations of the voltage or current (often using differential signalingDifferential signalingDifferential signaling is a method of transmitting information electrically by means of two complementary signals sent on two separate wires. The technique can be used for both analog signaling, as in some audio systems, and digital signaling, as in RS-422, RS-485, Ethernet , PCI Express and USB...
). - the line-coded signal (the "basebandBasebandIn telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...
signal") undergoes further pulse shapingPulse shapingIn digital telecommunication, pulse shaping is the process of changing the waveform of transmitted pulses. Its purpose is to make the transmitted signal better suited to the communication channel by limiting the effective bandwidth of the transmission. By filtering the transmitted pulses this way,...
(to reduce its frequency bandwidth) and then modulatedModulationIn electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
(to shift its frequency bandwidth) to create the "RF signal" that can be sent through free space. - the line-coded signal can be used to turn on and off a light source in Free Space Optics, most commonly infrared remote controlRemote controlA remote control is a component of an electronics device, most commonly a television set, used for operating the television device wirelessly from a short line-of-sight distance.The remote control is usually contracted to remote...
. - the line-coded signal can be printed on paper to create a bar code.
- the line-coded signal can be converted to magnetized spots on a hard drive or tape driveTape driveA tape drive is a data storage device that reads and performs digital recording, writes data on a magnetic tape. Magnetic tape data storage is typically used for offline, archival data storage. Tape media generally has a favorable unit cost and long archival stability.A tape drive provides...
. - the line-coded signal can be converted to pits on an optical discOptical discIn computing and optical disc recording technologies, an optical disc is a flat, usually circular disc which encodes binary data in the form of pits and lands on a special material on one of its flat surfaces...
.
Unfortunately, most long-distance communication channels cannot transport a DC component.
The DC component is also called the disparity, the bias, or the DC coefficient
DC coefficient
When describing a periodic function in the frequency domain, the DC bias, DC component, DC offset, or DC coefficient is the mean value of the waveform...
.
The simplest possible line code, called unipolar
Unipolar encoding
Unipolar encoding is a line code. A positive voltage represents a binary 1, and zero volts indicates a binary 0. It is the simplest line code, directly encoding the bitstream, and is analogous to on-off keying in modulation....
because it has an unbounded DC component, gives too many errors on such systems.
Most line codes eliminate the DC component — such codes are called DC balanced, zero-DC, zero-bias or DC equalized etc.
There are two ways of eliminating the DC component:
- Use a constant-weight codeConstant-weight codeIn coding theory, a constant-weight code, also called an m of n code, is an error detection and correction code where all codewords share the same Hamming weight. The theory is closely connected to that of designs...
. In other words, each transmitted code wordCode wordIn communication, a code word is an element of a standardized code or protocol. Each code word is assembled in accordance with the specific rules of the code and assigned a unique meaning...
is corrected 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. For example, Manchester codeManchester codeIn telecommunication and data storage, Manchester code is a line code in which the encoding of each data bit has at least one transition and occupies the same time...
and Interleaved 2 of 5Interleaved 2 of 5Interleaved 2 of 5 is a continuous two-width barcode symbology encoding digits. It is used commercially on 135 film and on cartons of some products, while the products inside are labeled with UPC or EAN....
. - Use a paired disparity codePaired disparity codeIn telecommunication, a paired disparity code is a line code in which some or all of the characters are represented by two sets of digits of opposite disparity that are used in sequence so as to minimize the total disparity of a longer sequence of digits....
. In other words, the transmitter has to make sure that every code word that averages to a negative level is paired with another code word that averages to a positive level. Therefore it must keep track of the running DC buildup, and always pick 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. For example, AMI, 8B10B, 4B3T4B3T4B3T stands for 4 Binary 3 Ternary, a line encoding scheme used for ISDN BRI interface. 4B3T represents four binary bits using three pulses.It uses three states:* + ,* 0 , and* − ....
, etc.
Line coding should make it possible for the receiver to synchronize itself to the phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
of the received signal. If the synchronization is not ideal, then the signal to be decoded will not have optimal differences (in amplitude) between the various digits or symbols used in the line code. This will increase the error probability in the received data.
It is also preferred for the line code to have a structure that will enable error detection.
Note that the line-coded signal and a signal produced at a terminal
Terminal (telecommunication)
In the context of telecommunications, a terminal is a device which is capable of communicating over a line. Examples of terminals are telephones, fax machines, and network devices - printers and workstations....
may differ, thus requiring translation.
A line code will typically reflect technical requirements of the transmission medium
Transmission medium
A transmission medium is a material substance that can propagate energy waves...
, such as optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
or shielded twisted pair. These requirements are unique for each medium, because each one has different behavior related to interference, distortion, capacitance and loss of amplitude.
Common line codes
- AMI
- Modified AMI codeModified AMI codeModified AMI codes are Alternate Mark Inversion line codes in which bipolar violations may be deliberately inserted to maintain system synchronization. There are several types of modified AMI codes, used in various T-carrier and E-carrier systems....
s: B8ZS, B6ZS, B3ZS, HDB3 - 2B1Q2B1QTwo-binary, one-quaternary is a physical layer encoding used for Integrated Services Digital Network Basic Rate Interface implementations...
- 4B5B4B5BIn telecommunication, 4B5B is a form of data communications Block Coding. 4B5B maps groups of four bits onto groups of 5 bits, with a minimum density of 1 bits in the output. When NRZI-encoded, the 1 bits provide necessary clock transitions for the receiver. For example, a run of 4 bits such as...
- 4B3T4B3T4B3T stands for 4 Binary 3 Ternary, a line encoding scheme used for ISDN BRI interface. 4B3T represents four binary bits using three pulses.It uses three states:* + ,* 0 , and* − ....
- 6b/8b encoding
- Hamming CodeHamming codeIn telecommunication, Hamming codes are a family of linear error-correcting codes that generalize the Hamming-code invented by Richard Hamming in 1950. Hamming codes can detect up to two and correct up to one bit errors. By contrast, the simple parity code cannot correct errors, and can detect only...
- 8b/10b encoding8B/10B encodingIn telecommunications, 8b/10b is a line code that maps 8-bit symbols to 10-bit symbols to achieve DC-balance and bounded disparity, and yet provide enough state changes to allow reasonable clock recovery. This means that the difference between the count of 1s and 0s in a string of at least 20 bits...
- 64b/66b encoding64b/66b encodingIn data networking and transmission, 64b/66b is a line code that transforms 64-bit data to 66-bit line code to provide enough state changes to allow reasonable clock recovery and facilitate alignment of the data stream at the receiver....
- 128b/130b encoding
- Coded mark inversionCoded mark inversionIn telecommunication, coded mark inversion is a non-return-to-zero line code. It encodes zero bits as a half bit time of zero followed by a half bit time of one, and while one bits are encoded as a full bit time of a constant level...
(CMI) - Conditioned DiphaseConditioned DiphaseConditioned Diphase is a line code for digital baseband transmission. Conditioned diphase uses a line encoding technique that encodes the digital data to be transmitted with a clock signal...
- Eight-to-Fourteen ModulationEight-to-Fourteen ModulationEight-to-fourteen modulation is a data encoding technique – formally, a channel code – used by compact discs and pre-Hi-MD MiniDiscs. EFMPlus is a related code, used in DVDs and SACDs. EFM and EFMPlus were both invented by Kees A...
(EFM) used in Compact DiscCompact DiscThe Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,... - EFMPlus used in DVDDVDA DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....
- RZ — Return-to-zeroReturn-to-zeroFor the Delp/Goudreau band, see RTZReturn-to-zero describes a line code used in telecommunications signals in which the signal drops to zero between each pulse. This takes place even if a number of consecutive 0's or 1's occur in the signal. The signal is self-clocking...
- NRZ — Non-return-to-zeroNon-return-to-zeroIn telecommunication, a non-return-to-zero line code is a binary code in which 1's are represented by one significant condition and 0's are represented by some other significant condition , with no other neutral or rest condition. The pulses have more energy than a RZ code...
- NRZI — Non-return-to-zero, inverted
- Manchester codeManchester codeIn telecommunication and data storage, Manchester code is a line code in which the encoding of each data bit has at least one transition and occupies the same time...
(also variants Differential ManchesterDifferential Manchester encodingDifferential Manchester encoding, also called biphase mark code or FM1, is a line code in which data and clock signals are combined to form a single 2-level self-synchronizing data stream. It is a differential encoding, using the presence or absence of transitions to indicate logical value...
& Biphase mark code) - pulse-position modulationPulse-position modulationPulse-position modulation is a form of signal modulation in which M message bits are encoded by transmitting a single pulse in one of 2^M possible time-shifts. This is repeated every T seconds, such that the transmitted bit rate is M/T bits per second...
(a generalization of Manchester code) - Miller encoding (also known as Delay encoding or Modified Frequency ModulationModified Frequency ModulationModified Frequency Modulation, commonly MFM, is a line coding scheme used to encode the actual data-bits on most floppy disk formats, hardware examples include Amiga, most CP/M machines as well as IBM PC compatibles. Early hard disk drives also used this coding.MFM is a modification to the original...
, and has variant Modified Miller encoding) - MLT-3 EncodingMLT-3 encodingMLT-3 encoding is a line code that uses three voltage levels...
- Hybrid Ternary Codes
- Surround by complement (SBC)
- TC-PAMTC-PAMTC-PAM is an acronym for Trellis Coded Pulse Amplitude Modulation. It is the modulation format that is used in both HDSL2 and SHDSL, and provides vigorous presentation over an assortment of loop circumstances...
Optical line codes:
- Carrier-Suppressed Return-to-Zero
- Alternate-Phase Return-to-Zero
See also
- channel coding
- source codingSource codingIn information theory, Shannon's source coding theorem establishes the limits to possible data compression, and the operational meaning of the Shannon entropy....
- modulationModulationIn electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
- Physical layerPhysical layerThe physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....
- Self-synchronizing codeSelf-synchronizing codeIn telecommunications, a self-synchronizing code is a line code in which the symbol stream formed by a portion of one code word, or by the overlapped portion of any two adjacent code words, is not a valid code word...
and bit synchronization
External links
- Line Codes Analyzer: tool for graphical comparison of line codes