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TUCoPS :: Phreaking Technical System Info :: intrpbx.txt

Introduction to PBXes





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[Introduction to PBXs]
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A telephone exchange serving an individual organization and having connections
to a public telephone exchange is called a Private Branch Exchange (PBX).  The
PBX performs a switching function by connecting any extension in the private
organization to an outside line.  A PBX is actually a private switch that
connects a group of telephones within an individual organization.  Calls placed
outside this individual group are connected to a telephone company's central
office switch through trunks.  A PBX may be operated by an attendant from the
private organization or the switching system may be done automatically.  Other
terms that are commonly used interchangeably with PBX are: Private Automatic
Branch Exchange (PABX), Private Automatic Exchange (PAX), and Computerized
Branch Exchange (CBX).  Although these terms were originally used to identify
specific switch structures, today they are often used as synonyms.

PBXs can use any of three basic switching methods: step-by-step (SxS),
Cross-bar (X-bar), and computer controlled, to perform the basic function of
switching.  However, in addition to detecting calls and establishing a
transmission link between two telephones, PBXs can do much more.

The common control, often called a central processing unit (CPU), controls the
switching matrix that connects the stat ons and trunks.  The switching matrix
of a PBX performs the same job as does an operator at a manual switchboard or a
common control central office switch.  The CPU, however, gets its instructions
from the "stored program", which contains directions for activities, such as
detecting calls, sending them over the best available route, and recording
billing information.  These computerized electronic switches are used to
perform routine, as well as unique, functions that simply weren't practical or
even possible with electromechanical switches.

Just as in the public switched network, PBX switches make connection between
instruments, or "key telephone sets".  We're all familiar with key telephone
sets, whether we know them by name or not.  They're the business telephones
that have six push-button keys lined up below the dial--a red button marked
"hold" and five buttons or lines with flashing lights.

Systems with PBXs and key sets have a great deal of flexability in planning for
their needs because they can set up their codes to accomplish the functions
needed in their particular situations.  In fact, the PBX can be programmed so
that each individual extension within a system can take advantage of features
applicable to its own business needs.

Some of the features that are availiable with PBXs and key systems are: call
transfer, which allows internal or external calls to be transferred from one
telephone to any other phone in the system; automatic push-button signaling,
which indicates the status of all phones in the system with display lights and
buttons; one-way voice paging, which can be answered by dialing the operator
from the nearest telephone in the system; camp-on, in which a call made to a
busy phone automatically waits until the line is idle; and internal and
external conference capabilities, which enables outside callers to conference
with several inside users.

Some features automatically handle incoming telephone calls.  Automatic call
waiting not only holds calls made to a busy extension until the extension is
free, but also signals the person being called that a call is waiting and
informs the caller that he is on hold.  Automatic call forwarding will send
calls to employees who are temporarily in locations other than their offices,
provided they "inform" the PBX where they can be found.  Automatic call
distribution automatically send an incoming call to the first extension that's
not busy--a useful feature for situations in which any one of a group of
persons in the organization can adequately respond to incoming calls.  Another
example is automatic call back, which allows a caller who reaches a busy line
to ask the PBX to return his or her call when the line is free.

Still other features provide services such as night telelphone answering,
telephone traffic monitoring, and network or hot-line connection.  These
examples are but a sample from the features possible with computerized PBXs.

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This is a very brief description of how to use and what to expect on a PBX.
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Basically, you call the PBX and you will have to enter a code that can be
anywhere from 4 to 6 digits (Note: some PBXs do not require codes).  Then you
will hear a dial tone.  From here you would under normal circumstances dial:
9 + 1 (or 0) + NPA-PRE-SUFF, for long distance dialing or dial 8 for local
dialing.

The most common use of the PBX is to call Alliance Teleconferencing,
a teleconference service offered by AT&T.  To do this dial:
0700-456-1000,1002,1003,2000,2001,2002.

Note: PBX codes are usually very simple and usually 4 digits.
EX: 0000, 1111, 1234, etc

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