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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 (1554 kbps) and E1 (2048 kbps) and multiple E1 with Cisco.

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.

Some definitions

G.704 Framing
G.706 CRC-4 procedure
G.732 Fault handling

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).


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.


Some Electrical Characteristics

Mark 1.0 Vdc
Space 0 Vdc +/- 0.10 Vdc
Pulse width 3.9 usec

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.




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).


Some Electrical Characteristics

Mark 1.0 Vdc
Space 0 Vdc +/- 0.10 Vdc
Pulse width 15.6 usec

The modulation technique used is AMI.


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.

Some Electrical Characteristics

Mark 1.0 Vdc
Space 0 Vdc +/- 0.10 Vdc
Pulse width 3.9 usec

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:

Some Electrical Characteristics for T1

Cabling co-directional
Mark 3.0 Vdc
Space 0 Vdc +/- 0.30 Vdc
Pulse width 647 nsec
Encoding AMI (bipolar) or B8ZS
Speed 1544 kbps +/- 50 ppm

Some Electrical Characteristics for E1

Cabling Coaxial or one symmetrical pair (4 wires) for each direction
Mark Balanced: 3.0 Vdc
Unbalanced: 2.37 Vdc
Space Balanced: 0 Vdc +/- 0.237 Vdc
Unbalanced: 0 Vdc +/- 0.3 Vdc
Pulse width 488 nsec
Encoding AMI or High Density Bipolar of order 3 (HDB3)
Speed 2048 kbps +/- 50 ppm

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

Pinning Specifications

Signal RJ45 Description DTE RJ45 BNC Description DTE BNC
RxA Receive Input Negative 1 Receive Input Tip
RxB Receive Input Positive 2 Receive Ground Ring
TxA Transmit Output Negative 4 Transmit Output Tip
TxB Transmit Output Positive 5 Transmit Ground Ring
S1 Transmit Ground 3    
S2 Receive Ground 6    
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