line coding

Definition of line coding in The Network Encyclopedia.

What is Line Coding?

A method of placing digital signals on a wire. Line coding specifies the relationship between the binary information in a data bitstream and the square-wave voltage variations on the wire that represent this information electrically.

For example, Integrated Services Digital Network (ISDN) technologies use several different line coding schemes. The U interface, which is located at the ISDN line termination point at the customer premises where a two-wire metallic cable terminates with an RJ-11 jack, uses the 2 binary, 1 quaternary (2B1Q) line coding scheme for Basic Rate Interface ISDN (BRI-ISDN) and the Bipolar with 8 Zero Substitution (B8ZS) scheme for Primary Rate Interface ISDN (PRI-ISDN) in the United States. European ISDN uses 4 binary, 3 ternary (4B3T) for BRI-ISDN and High Density Bipolar 3 (HDB3) for PRI-ISDN.

In the 2B1Q line coding scheme, a block of two binary bits can represent four different values: 00, 01, 10, and 11. These four values are mapped to one quaternary value, which is encoded using four different voltages. The first bit represents a positive or negative voltage, and the second bit represents either 1-volt or 3-volt line potential. The following table shows the four possible combinations.

Binary Data and Corresponding Voltage Level for 2B1Q Line Coding

Binary Data Represented Voltage of Electrical Pulse

The result of using 2B1Q line coding for BRI-ISDN is that a single electrical pulse represents 2 binary bits instead of 1 binary bit. This effectively doubles the possible bandwidth of the communication channel, as shown in the illustration.


The term “line coding” sometimes refers to signal modulation technologies used in Digital Subscriber Line (DSL) technologies.