A Made IT project http://www.made-it.com info@made-it.com

G.703 is a standard which originally described voice over digital networks. It's a CCITT recommendation which is associated with the PCM standard. Voice to digital conversion according to PCM requires a bandwidth of 64 kbps (+/- 100 ppm), resulting in the basic unit for G.703. By multiplication this results in e.g. T1 (1544 kbps) and E1 (2048 kbps).
Modern networks are working with voice and data and so is G.703.
G.703 is the electrical and functional description. Other characteristics are described in other G-standards.

G.703 can be transported over balanced (120 ohm TP) and unbalanced (dual 75 ohm coax) wires. The balanced version with a speed of 64kbps, is split in three different ways of transmission: co-directional (4-wire), central-directional (6/8 wire) and contra-directional (8-wire).

Co-Directional
This is a 4-wire version. Each direction (transmit, receive) consists of 2 wires twisted together, providing a balanced signal.

The data and timing are send in the same direction over the same wires. This makes the co-directional.

The bit coding is done in three steps.
Step1: A binary 1 is replaced by 1100 and a binary 0 by 1010.
Step2: Conversion into a three-level signal (AMI) by alternating the polarity of consecutive blocks.
Step3: Conversion to Violated AMI. Every 8th block of the polarity is alternated. The violated block marks the last bit in an octet.

Central-Directional
This is a rarely used version. The clock signals are supplied on different wires from a centralized clock. The centralized could be an atomic-clock. The reason for 8 or 6 wire version is the possibility to send a clock signal balanced in both directions at the same time, or in each direction separate. The first has 6-wires (2 clock, 4 data), the second is 8-wire (4 clock, 4 data).

The modulation technique used is AMI.

Contra-Directional
This is always an 8-wire version. There is ofcourse the transmit and receive pair and two pairs for the clock signals. All clock signals are send to the DTE. This means they are all originated by the DCE.

The modulation technique used is AMI.

Speeds higher than 64kbps
All other speeds use a different coding scheme and different pulse width, also the mark and space voltages may differ.

A quick overview for the most common used:

For a more detailed description see the corresponding documents about the E-series and T-series.

Pinning Specifications