Technical data

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G.703
The ITU standard G.703, describes the electrical and physical properties and a number
of transfer rates.
There are three basic physical types of the interface, codirectional, contradirectional
and centralised interfaces.
The standard specifies speeds from 64 kbit/s to 155 520 kbit/s. The standard was origi-
nally created to carry speech over a PCM-link.
The transmission medium can either be a 120 ohm balanced pair or an unbalanced
75 ohm coaxial cable.
Codirectional interface
Transmission takes place over a wire pair in each
direction. Data and timing information are super-
imposed. Data and timing information run in the
same direction and it falls upon the receiver to syn-
chronize the data and timing information.
Contradirectional interface
This type of transfer usesa4wirepair,timing infor-
mation is provided by the governing device.
Centralised clock interface
This variant of the interface uses 3 or 4 wiring pairs,
timing information is provided by the central unit. In
the 3-pair instance, timing is provided
common to both transmitting and receiving.
In the 4-pair instance individual timing is used
for the transmission and for the reception.
Information/Timing
Timing equipment
Controlling
equipment
Subordinate
equipment
Information
Timing
Information
Timing
Information/Timing
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