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G.703 is an encoding standard developed by the International Telecommunication Union (ITU) that was widely used in Europe and not used at all in North America. See full details about this standard.

What is G.703?

G.703 is an encoding standard developed by the International Telecommunication Union (ITU) for interfacing data communications equipment (DCE) with digital high-speed synchronous communication services.

G.703 is not used in North America but was widely used in Europe, and it covers specifications for digital transmission from rates of 64 Kbps to 2.048 Mbps. Private Branch Exchange (PBX) systems often use 64-Kbps leased lines utilizing the G.703 standard, as do E-carrier services such as E1 communication links.

Some U.S. vendors sell converters for connecting synchronous V.35, RS-449, or X.21 interfaces to G.703 in order to sell their switching equipment in Europe.


G.703 has been updated to include support for U.S. standard T-carrier service speeds, such as T1 transmission at 1.544 Mbps.

G.703 at 2,048Kbit/s

The G.703 standard specifies which encoding is to be used for the transmission. The two key requirements for line encoding are to make sure that there are enough clock hits on the transmission paths. If e.g. 0 is presented as no voltage a long sequence of 0 s will prevent the transmission of any timing information. In addition, line encoding prevents any DC offset information on the line during code transmission. For example: If we encoded 1 as +voltage and 0 as no voltage a moving DC component could be included in each pattern of 0 s and 1 s. This process is called ground line movement and is unwanted in telecommunications systems. An encoding system known as HDB3 was chosen for the G.703 standard at 2,048Kbit/s. HDB3 stands for High Density Bipolar with a level of 3. A digital 0 is transmitted as 0V (no voltage). A digital 1 is transmitted as alternately a positive voltage (+V) and a negative voltage (-V). This kind of encoding is called AMI (Alternate Mark Inversion).

HDB3 is an AMI code variation. In AMI coding a long sequence of 0 s results in no transmission on the line and consequently there is no time information. In HDB3 coding, in each sequence consisting of more than three 0 s the fourth 0 is replaced by a 1 in forbidden direction (i.e. the same direction as the preceding 1).

This interface is either carried out on a balanced wire pair or on a coaxial pair.

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