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• PABX – Private Automatic Branch Exchange
• EPABX – Electronic Private Automatic Branch Exchange
PBXs make connections among the internal telephones of a private organization— usually
a business—and also connect them to the public switched telephone network (PSTN)
via trunk lines. Because they incorporate telephones, fax machines, modems, and
more, the general term "extension" is used to refer to any end point on the branch.
PBXs are differentiated from "key systems" in that users of key systems manually
select their own outgoing lines, while PBXs select the outgoing line automatically.
Hybrid systems combine features of both.
Initially, the primary advantage of PBXs was cost savings on internal phone calls:
handling the circuit switching locally reduced charges for local phone service.
As PBXs gained popularity, they started offering services that were not available
in the operator network, such as hunt groups, call forwarding, and extension dialing.
In the 1960s a simulated PBX known asCentrex provided similar features from the
central telephone exchange.
Two significant developments during the 1990s led to new types of PBX systems. One
was the massive growth of data networks and increased public understanding of packet
switching. Companies needed packet switched networks for data, so using them for
telephone calls was tempting, and the availability of the Internet as a global delivery
system made packet switched communications even more attractive. These factors led
to the development of the VoIP PBX. (Technically, nothing was being "exchanged"
any more, but the abbreviation PBX was so widely understood that it remained in
use.)
The other trend was the idea of focusing on core competence. PBX services had always
been hard to arrange for smaller companies, and many companies realized that handling
their own telephony was not their core competence. These considerations gave rise
to the concept of hosted PBX. In a hosted setup, the PBX is located at and managed
by the telephone service provider, and features and calls are delivered via the
Internet. The customer just signs up for a service, rather than buying and maintaining
expensive hardware. This essentially removes the branch from the private premises,
moving it to a central location.
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