A private circuit is a link between two locations joining
two or more pieces of customer equipment. The need for these
type of links arise from the communications requirements of
the network customers. The physical link is provided by the
network provider and is exclusively for the use of the customer
leasing it. This exclusivity means that there is a premium
placed upon these types of circuit because they must be maintained
by the network provider for a single customer. This maintenance
commitment is costly. The benefit is derived from high usage
of the link as the calls are free once the rental has been
paid. It is therefore only a practical proposition between
sites where the call traffic is heavy and normal network
charges for the calls would exceed the cost of circuit rental.
With modern digital services and routing equipment a dial
on demand service can be used for lower traffic users, where
each call is charged at the usual rates. This provides similar
functionality to the permanent connection at a lower cost.
A private wire or access line is the simplest form of private
circuit. It is a straight wire pair from point A to point
B. The wires will be routed along the runs used by the network
carrier for other telephone lines in the area.
The circuit is a plain pair of wires with signalling being
supplied from the equipment located at its ends. Although
it shares the ducts of the network provider and is maintained
by the same there is no physical connection between the two.
The length of a private wire is limited by the ability of
the connected equipment to drive power down the line. This
gives a practical length of upto 2 kilometres. The line will
have a typical loop resistance of between 500 ohms and 1600
ohms. Equipment which cannot operate with these levels of
resistance cannot be used with private wires.