In telecommunication, a line code is a code chosen for use within a
communications system for transmitting a digital signal down a line. Line coding
is often used for digital data transport. Some line codes are digital
baseband modulation or digital baseband transmission methods, and these are
baseband line codes that are used when the line can carry DC components.
Line coding Line coding consists of representing the
digital signal to be transported, by a waveform that is optimally tuned for the
specific properties of the physical channel. The pattern of voltage, current
or photons used to represent the 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: the line-coded signal can directly be
put on a transmission line, in the form of variations of the voltage or current.
the line-coded signal undergoes further pulse shaping and then modulated to
create an "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 optical communication, most commonly used in an infrared remote
control. 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 drive.
the line-coded signal can be converted to pits on an optical disc.
Disparity The disparity of a bit pattern is the
difference in the number of one bits vs the number of zero bits. The running
disparity is the running total of the disparity of all previously transmitted
words. 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, 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 called DC-balanced, zero-DC, DC-free,
zero-bias, DC equalized, etc. There are three ways of eliminating the DC
component: Use a constant-weight code. In other
words, each transmitted code word 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 code and Interleaved 2 of 5.
Use a paired disparity code. 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, 4B3T, etc.
Use a scrambler. For example, the scrambler specified in RFC 2615 for
64b/66b encoding. Polarity
Unfortunately, several long-distance communication channels have polarity
ambiguity. To compensate, several people have designed polarity-insensitive
transmission systems. There are three ways of providing unambiguous reception
of "0" bits or "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, such as alternate mark inversion, Differential Manchester
encoding, coded mark inversion, Miller encoding, etc.
differential coding each symbol relative to the previous symbol, such as MLT-3
encoding, NRZI, etc. invert the whole stream when inverted
syncwords are detected Synchronization
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 between the various
digits or symbols used in the line code. This will increase the error probability
in the received data. line coding
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.
Common line codes Optical line codes:
Carrier-Suppressed Return-to-Zero Alternate-Phase Return-to-Zero
Three of Six, Fiber Optical See also
Channel coding Source coding
Modulation Physical layer
Self-synchronizing code and bit synchronization
References This article incorporates public domain
material from the General Services Administration document "Federal
Standard 1037C". External links
Line Coding Lecture No. 9 Line Coding in Digital Communication