What is PABX?
A term used synonymously with Electronic Private Automated Branch Exchange (EPABX)
and Private Automated Branch Exchange (PABX). Similar to a central office exchange
but smaller. A central off exchange can accommodate 10,000 subscribers. PBX systems
are typically designed to accommodate from 20 to 10,000 subscribers or station
Private Branch Exchange. A small, privately-owned version of the phone company's
larger central switching office.
Piece of telecommunications equipment installed in a company's office, which makes
it possible to connect many internal telephone lines to city telephone lines
A Private Branch eXchange (also called PBX, Private Business eXchange or PABX for
Private Automatic Branch eXchange) is a telephone exchange that serves a particular
business or office, as opposed to one a common carrier or telephone company operates
for many businesses or for the general public.
PBX Functions ?
Functionally, the PBX performs three main duties:
Establishing connections (circuits) between the telephone sets of two users. (e.g.
mapping a dialed number to a physical phone, ensuring the phone isn't already busy)
Maintaining such connections as long as the users require them. (I.e. channeling
voice signals between the users)
Providing information for accounting purposes (e.g. metering calls)
In addition to these basic functions, PBXs offer many other capabilities, with different
manufacturers providing different features in an effort to differentiate their products.
Common capabilities include (manufacturers may have a different name for each capability):
- Auto Attendant
- Automatic call distributor
- Automated directory services (where callers can
be routed to a given employee by keying or speaking the letters of the employee's
- Automatic ring back
- Call accounting
- Call forwarding on absence
- Call forwarding on busy
- Call park
- Call pick-up
- Call transfer
- Call waiting
- Conference call
- Custom greetings
- Customised Abbreviated dialing (Speed Dialing)
- Direct Inward Dialing
- Direct Inward System Access (DISA) (the ability
to access internal features from an outside telephone line)
- Do Not Disturb (DND)
- Music on hold
- Night service
- Shared message boxes (where a department can
have a shared voicemail box)
- Voice mail
- Voice paging (PA system)
Alexander Graham Bell (1847-1922) is most famous for his invention of the telephone.
As a teenager of 18, Bell had been experimenting with the idea of transmitting speech.
In 1874, while working on a multiple telegraph he developed the basic ideas for
He and his assistant Thomas Watson finally proved successful on March 10, 1876,
when the first telephone message was transmitted: "Watson, come here; I want you.".
This led eventually to the establishment of the Bell Telephone Company, still in
existence today, which introduced the telephone to the world. story of our ongoing
quest to communicate better, from the first simple messages to today's sophisticated
Operator mannequins, Telecommunications Museum, Madrid, Spain Historically a PBX
was a company's manual switchboard. These were gradually replaced by automated electromechanical
and then electronic switching systems, called PABXs (Private Automatic Branch eXchange).
The old kind were then called PMBX (Private Manual Branch eXchange) As PMBXs are
almost unheard of these days, the terms PABX and PBX have become synonymous. When
PABXs were built using solid state and digital components the term EPABX came into
use, but PBX is still more widely recognized.
The PBX term was dominant for so long, that it is now being applied liberally to
systems providing complex services regarding telephony, even if they are not Private,
Branches or eXchanging anything.
PBXs were distinguished from smaller "key systems" by the fact that external lines
are not normally indicated or selectable at an individual extension. From a user's
point of view, calls on a key system are made by selecting a specific outgoing line
and dialing the external number; calls on a PBX are made by dialing the escape code
(usually 9, 0 in some systems) followed by the external number.
This automatically selects an outgoing trunk line, also called "outside line" or
in Bell System jargon DDCO (Direct Dial Central Office) upon which to complete the
call. This practice is becoming obsolete as number analysis systems change with
modern technology - one can use internal numbers or external numbers without escape
The first consumer PBX systems were for the analog telephone systems, typically
supporting four private analog and one public analog line. They have the size of
a small cigar box or smaller and are inexpensive.
Particularly in Europe these systems for analog phones were followed by consumer-grade
PBXs for ISDN. Using small PBXs for ISDN is a logical step, since already the Basic
Rate Interface of ISDN (which is the phone interface individuals and small businesses
typically get) provides two logical phone lines (two B channels) which can be used
in parallel. Small, entry-level systems are also extremely cheap .
With the pickup of VoIP by consumers, of course consumer VoIP PBXs have seen the
light, and PBX functions have become simple additional features of consumer-grade
routers and switches.
Proponents of recent open source projects (in particular, Asterisk and SIPfoundry)
claim that their initiatives have finally brought PBXs within the reach of individuals
and small businesses. However, some affordable off-the-shelf solutions have been
available since the beginning of the 90s.
The mentioned open source projects provide more flexibility and more features (often
not needed or understood by average users) on standard hardware platforms, plus
means to actually inspect and change the inner working of a PBX. They have also
opened business opportunities for newcomers to the market of mid-size PBX, since
they have lowered the entry barrier for new manufacturers .