4B3T, which stands for 4 (four)
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 ...
3 (three)
ternary, is a
line encoding scheme used for
ISDN
Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. ...
PRI interface. 4B3T represents four binary bits using three pulses.
Description
It uses three output levels:
* + (positive pulse),
* 0 (no pulse), and
* − (negative pulse).
This means we have 2
4 = 16 input combinations to represent, using 3
3 = 27 output combinations. 000 is not used to avoid long periods without a transition. 4B3T uses a
paired disparity code to achieve an overall zero
DC bias
In signal processing, when describing a periodic function in the time domain, the DC bias, DC component, DC offset, or DC coefficient is the mean value of the waveform. A waveform with zero mean or no DC bias is known as a ''DC balanced'' or ''DC ...
: six triplets are used which have no DC component (0+−, 0−+, +0−, −0+, +−0, −+0), and the remaining 20 are grouped into 10 pairs with differing disparity (e.g. ++− and −−+). When transmitting, the DC bias is tracked and a combination chosen that has a DC component of the opposite sign to the running total.
This mapping from 4 bits to three ternary states is given in a table known as Modified Monitoring State 43 (MMS43).
A competing encoding technique, used for the ISDN
basic rate interface
Basic Rate Interface (BRI, 2B+D, 2B1D) or Basic Rate Access is an Integrated Services Digital Network (ISDN) configuration intended primarily for use in subscriber lines similar to those that have long been used for Plain old telephone service, ...
where 4B3T is not used, is
2B1Q
Two-binary, one-quaternary (2B1Q) is a line code used in the U interface of the Integrated Services Digital Network (ISDN) Basic Rate Interface (BRI) and the high-bit-rate digital subscriber line (HDSL). 2B1Q is a four-level pulse-amplitude modul ...
.
The sync sequence used is the 11-symbol
Barker code
In telecommunication technology, a Barker code or Barker sequence is a finite sequence of digital values with the ideal autocorrelation property. It is used as a synchronising pattern between the sender and receiver of a stream of bits.
Explanat ...
, +++−−−+−−+− or its reverse, −+−−+−−−+++.
Encoding table
Each 4-bit input group is encoded as a 3-symbol group (transmitted left to right) from the following table.
Encoding requires keeping track of the accumulated DC offset, the number of + pulses minus the number of − pulses in all preceding groups. The starting value is arbitrary; here we use the values 1 through 4, although −1.5, −0.5, +0.5 and +1.5 is another possibility.
This code forces a transition after at most five consecutive identical non-zero symbols, or four consecutive zero symbols.
Decoding table
Decoding is simpler, as the decoder does not need to keep track of the encoder state, although doing so allows greater
error detection
In information theory and coding theory with applications in computer science and telecommunications, error detection and correction (EDAC) or error control are techniques that enable reliable delivery of digital data over unreliable communi ...
. The 000 triplet is not a legal encoded sequence, but is typically decoded as binary 0000.
8B/6T
A similar paired-disparity code was used in the short-lived Fast Ethernet standard. This achieved the same ratio of 4/3 bits per
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 ...
while needing to keep track of only two levels of accumulated DC offset by using a larger 256-entry coding table. 6 ternary symbols allow 140 balanced codes (30 permutations of +0000−, 90 permutations of ++00−−, and 20 permutations of +++−−−), and 126 codes with disparity +1 (6 permutations of +00000, 60 permutations of ++000−, and 60 permutations of +++0−−). The codes with disparity +1 are inverted whenever the running disparity is high. From the total of 266 code words, 256 are used as follows:
See also
Other line codes that have 3 states:
*
hybrid ternary code
In telecommunications, the hybrid (H-) ternary line code is a line code that operates on a hybrid principle combining the binary non-return-to-zero-level (NRZL) and the polar return-to-zero (RZ) codes.
The H-ternary code has three levels for sig ...
*
bipolar encoding
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 designe ...
*
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 ba ...
*
B3ZS
References
{{Bit-encoding
Line codes
Integrated Services Digital Network