G.703 is a CCITT standard for transmitting voice over digital carriers such as T1 and E1. G.703 provides the specifications for pulse code modulation (PCM) at data rates from 64 Kbps to 2.048 Mbps. G.703 service is typically used for interconnecting data communications equipment such as bridges, routers, and multiplexers. G.703 is transported over balanced (120 ohm twisted pair) or unbalanced (dual 75 ohm coax) cable. Whether the G.703 is balanced or unbalanced depends on your geographic location and the carrier that supplies the service. Balanced service is the most common around the world with the exception of the U.K. and the Netherlands. However, the new Open Network Provision standard of the EEC requires that balanced service be available throughout all of Europe.
At data rates of 64 kbps over balanced wire, there are three ways of transmission: co-directional, central directional, and contra-directional. Co-directional uses four twisted wires, two to transmit and two to receive. The data and timing are sent in the same direction over the same wires. Central directional is rarely used. Here the clock signals are supplied on different wires from a centralized clock such as an atomic clock. Central directional can use six or eight wires to send a clock signal in both directions or in separate directions. The six-wire version uses two for the clock signals and four for the data signals, and the eight-wire version uses four for clock signals and the other four for data. The contra-directional is an eight-wire version that uses two wires each for transmitting and receiving and two pairs for the clock signals. (Clock signals originate at the Data Communications Equipment (DCE) and are sent to the Data Terminal Equipment (DTE).)