4B3T, which stands for 4 (four)
binary 3 (three)
ternary, is a
line encoding
In telecommunication, a line code is a pattern of voltage, current, or photons used to represent digital data transmitted down a communication channel or written to a storage medium. This repertoire of signals is usually called a constrained co ...
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. Work ...
PRI interface. 4B3T represents four binary bits using three pulses.
Description
It uses three states:
* + (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 In telecommunication, a paired disparity code is a line code in which at least one of the data characters is represented by two codewords of opposite disparity that are used in sequence so as to minimize the total disparity of a longer sequence o ...
to achieve an overall zero
DC bias: 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 voice-grade telephone servi ...
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 modulat ...
.
The sync sequence used is the 11-symbol
Barker code, +++−−−+−−+− 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 telecommunication, 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.
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 designed ...
*
MLT-3 encoding
*
B3ZS
References
{{Bit-encoding
Line codes
Integrated Services Digital Network