From very early in the development of telephony equipment
there have been users who have wanted or required their own
private systems. These were called Private Branch Exchanges
(PBXs) and Private Automatic Branch Exchanges (PABXs) depending
upon weither they were manual or automatic. Since all are
now automatic PABX is the correct term. However, today, PBX
and PABX may be used interchangeably with little or no confusion.
Modern systems are also called Call Routing Apparatus (CRAs)
and it is common for smaller systems to be referred to as
CRAs and larger installations as PBXs. This is a peception
issue by the general public and does not denote any particular
functionality. Indeed the smaller systems are often more feature
rich than the larger ones.
Originally PBXs would be the same equipment used by the network
provider as the exchange equipment, but located with and under
the control of the subscriber. With deregulation and the growth
in demand there are now a wide range of systems of all sizes
from many manufacturers which are marketed for this purpose.
From the demand for private exchanges there arose the need
for closed links to other offices and large customers with
high point to point traffic began leasing private circuits
to link their systems into private networks.
Private links are rented from the network operator who installs
and maintains them. Originally based on analogue technology
and signalling systems they were only economic for large users.
Digital technology allowed data and speech to be transmitted
across these links via multiplexer equipment together with
a reduction in costs which allowed smaller customers access
to their benefits. For the lower volume users or less busy
networks ISDN can be used to provide a dial up option which
connects via the public ISDN system when required and is idle
when not needed. Calls will be charged as normal, but this
can still be cheaper than leasing a line and not maximising
In recent times these private networks have been changing
from fixed point to point to switched systems still point
to point but not requiring an single route with connections
made as required. These dynamically routing links are called
Virtual Private Networks (VPNs) since they appear to the user
to be a static link but are in reality continually changing
the route used to connect the ends.
The UK network was created under the control of the General
Post Office. This was later split from the post office and
British Telecommunications PLC was formed. BT had a monopoly
over the provision of services and equipment until the industry
was deregulated in 1984.
Deregulation allowed manufacturers and service providers
access to the market with a range of goods and services and
customers a greater choice of equipment and features.
The result was a massive increase in equipment available
and the development of alternative networks whose operators
could become licensed as network providers. Although these
are separate networks they are all interlinked so that call
traffic can be passed between them.