How PBX systems work
Business telephone systems very often take use of technologies
like PABX, multiline telephones and digital telephones.
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General information on business phone
systems
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PBX systems
The terms "PBX" and "Key" both refer to hardware that enables
several telephones to be connected to a smaller number of telephone
lines. The term "Key" is was originally used to describe
the manual keys or push-buttons on systems like the 1A series
Key telephone. The key telephone system is a direct evolution
from having multiple phoneson the desk. The earliest system
had a series of "keys" (switches) mounted ina box,
which allowed one to choose which of several lines was connected
tothe phone.
One position of the key "hung up" the line while
another connectedit to the phone. Usually an intermediate position
of the switch allowed one toplace a call on "hold".
In early "mechanical" key system such as the 1A1 and
1A2 the user?s phone is actually connected to the line through
a "hard" connection (switch contacts) when a line
is in use. In older key systems such as the 1A1 and 1A2 six
conductors for each line go to each phone on which it appears!Modern
key telephone systems have a Key Switching Unit that all lines
areconnected to. Standard loop start lines are normally used.
All phones in thesystem connect to this KSU. Each phone in a
Key system typically has accessto several lines, through the
KSU. The phones and KSU work together asfollows.
Each phone has an indicator for each line to which it has access.These
indicators allow the user to see that state of each line to
which they have access. Modern key systems may only require
2 or 4 conductors to each phone.Today?s "Key" system
is more like a small PBX with programmable features such as
distinctive ringing, hunt groups, and automatic line selection.
Key systems allow multiple phones to efficiently share phonecompany
lines. Each line has an identity (the phone number) but thetelephones
do not. Private Branch Exchange (PBX) is a private telephone
network used within a company. Users of such a network share
a certain number of external lines for making outside calls.
This is typically less expensive than connecting an external
telephone line with every telephone in an office.
A PBX consists of a switch box and punch block located where
the telephone lines come into the building. Usually this PBX
system is a device tucked away in an obscure closet together
with rest of telephone infrastructure. PBX telephones have an
identity of their own. They areextensions with a unique extension
number. Each has access to the PBX. ThePBX has trunks to the
phone company. The PBX actually switches callsthrough itself
based on the users demands, not based on a pre-configuredwiring
plan. Station-to-station calls as well as station-to-trunk calls
arepossible. Modern business telephone systems can still generally
be categorized as Keysystems or PBXs. Note that the advanced
features available on many modernKey systems can blur the difference
unless one looks carefully.
Electronic PBX wiring from PBX to pohones is typically 4 to
8 wires using RJ-11 or RJ-45 modular telephone jacks. These
are not usually standard telephone wires (can be similar to
normal lines in some systems). Even a line from a simple analog
PBX line does not usually look like a standard phone line. The
voice path on an analog PBX is typically referred to as a dry
pair. Dry refers to the lack of DC current or ring voltage found
on regular phone lines.In addition to this there can be another
pair that carries the control information and power to telephone.
On an electronic PBX telephone, two wires are often used as
control lines, which send keypress data to the PBX, and ringer
and LED data back to the phone.
This control information is required to set up or answer a
call. On a digital PBX, your voice is converted to data right
in the base of the phone. This kind of specialized digital handsets
can run hundreds of dollars, compared to $20 for a standard
analog handset.In either case is not possible to use normal
POTS telephone or normal telephone recording accessories with
PBX lines. You need special phones supplies by PBX manufacturers.
If you want to record the calls, you need to do it through the
handset cord of your telephone. There are also some PBX systems
which provide normal analogue line connection to the lines from
the PBX. Network PBXs now generally support standard analog
telephones, rather than requiring specialized digital units.
Whenever working with PBX system lines, make sure you know what
kind of phones are used in this particular system. There are
different kind of PBX systems in use. The most wellknown acronyms
you might encounter are:
- PABX (Private Automatic Branch eXchange): is a telephone exchange operated within an organisation, used for switching calls between internal lines and between internal and PSTN lines. A PABX can route calls without manual intervention, based entirely on the number dialed.
- EPABX (Electronics Private Automatic Branch eXchange): EPABX is a PABX system which is built using electronic controlling and signal switching (to differentiate them from some old relay based designs)
- PNX (Packet Network Exchange): A communication switching platform that combines PBX and VoIP functionalitues.
- PMBX (Private Manual Branch eXchange): In some old companies the might still be PMBX, which involves company employed operators manually switching each call using a manual switchboard.
- KTS (Key Telephone System): KTSs are generally smaller versions of a PBX that provides direct access to CO telephone lines.
System vendors and users report, however, that the reliability of newer systems
is excellent, often reaching the "five nines" level
of 99.999 percent uptime. Many vendors recommend running their
PBX systems on separate servers, rather than sharing a server
with other networked applications. This practice dramatically
decreases the potential clash of data and communication systems.
Vendors also often provide recommendations for hardware based
on their compatibility and reliability testing. Most traditional
PBXs were built on proprietary technology, but network PBX solutions
take an open-systems approach. At one time, some network systems
required specialized cards or handsets, but now network PBX
vendors that have fully adopted the IP protocol are actively
supporting other industrywide standards.
For example, the H.323 standard.Many products also support
Primary Rate ISDN (PRI) and standard computer-based telephony
interfaces (TAPI and TSAPI). Computer-based PBX systems also
tend to support a variety of third-party applications that integrate
with the system. Network PBX systems, which are most often sold
and installed by systems integrators, consist of a server connected
to telephone handsets and incoming telephone trunk lines through
a specialized interface device. Calls are managed at the desktop
level, typically using a Windows application.
These systems, which cost approximately $30,000 to $60,000,
are intended for medium and large business applications with
hundreds or thousands of users. PC PBX systems are less expensive
systems for small-office environments. These systems generally
cost a few thousand dollars and add cards and software to an
existing server. They typically lack scalability and are best
for single-site installations such as branch offices with a
couple of dozen to a hundred users.In the PBX environment a
line from the CO is called a trunk and a phone is called a line,
extension, or station. Many modern electronic PBX systems support
Primary Rate ISDN (PRI), which is priced more attractively than
other ISDN services. Many EPABX systems have some form of serial
interface to communicate with the device itself. In simplest
form this can be just an interface to a simple printer that
prints out the made calls. In more complicated systems this
can allow controlling of the device in more advanced ways.
The serial interface used is most often RS232. Usually there
is a proprietary protocol (specific to EPABX system) to communicate
with EPABX system. So when you interface to EPABX system you
need to have software which supports this EPABX systems or you
need to develop your custom application for this (to make this
you need some specifications of the protocol your EPABX uses).
Usually the communication with EPABX is used for extracting
Calling Line Identification (CLI) information, Dial Number Information
(DNI) and controlling voice speech paths in the EPABX system.There
has been a fundamental change in the way an enterprise views
aPBX today. Ditto for the position that a PBX commands within
an enterprise. Few years ago a PBX was just a voice switch;
but today, it can be a switch that can also facilitate data
and video communication. However, not all PBXs are like that.
With hundreds of choices available in the PBX market, choosing
the one that would not onlymeet your current business requirements
but also take care of the future needs, is not going to bean
easy job. Buy a proven to work platform instead of just a set
of boxes. Bet on non-proprietary open standards because they
give you more possibilities for future upgrades. Buy a solution
that would be inter-operable with applications from a diverse
set of vendors and solution providers.Avoid proprietary and
closed-architecture based solutions, because additions and changes
can be eitherimpossible or very difficult on proprietary and
closed standards.
At one time, choosing a proprietary standalone PBX phone system
was like Russian roulette--make a poor choice, and you could
be locked into escalating technical support costs, poor migration
paths, and complex system management. Service-level agreements
with vendors are also important.PBX field is changing quicly.
Many networking people nowadays say that the future of enterprise
telephony is clearer than ever: Treaditional circuit-switched
PBXs are nearing obsolescence. The makers of IP based telephony
equipment say thasto go out and buy a new circuit-switched PBX
now is investing in a dead-end technology. IP technology is
increasingly becoming the standard for corporate voice communications.
But how fast is the question. Should you believe the vendors
and embrace it as the newest way for ISPs to increase revenues
and for businesses to cut costs, or should you listen to the
analysts who say it's still too soon to take the plunge?
The answer probably has more to do with your situation than
with the technology. These new systems use standard Ethernet
connections and deliver more functionality, such as universal
messaging. And while many network-based PBX systems may cost
nowadays about the same as traditional PBXs on a per-seat basis,
prices are plummeting quicly. Network PBX systems may even be
more affordable when you also consider the potential system
management cost savings, easy integration of remote offices,
and reduced long distance charges. With this new technology
definately some savings are possible, but the true cost advantages
can be difficult to assess.
Useful Links
- Dialing for fewer dollars - You keep your company running by keeping tabs on a finely tuned array of high-tech equipment, but tucked away in an obscure closet is the one piece you may depend on most and probably know the least about: your phone system.
- EE476 Final Project AT90LS8515 PBX (Private Branch Exchange) - A project documentation that describes construction of a 4 line telephone systems with full signaling and switching functions similar to those of the central office systems.
- How IP PBX works
- Is Your PBX Ready for the Junk Heap? - So, what's the deal with VoIP? Should you believe the vendors and embrace it as the newest way for ISPs toincrease revenues and for businesses to cut costs, or should you listen to the analysts who say it's still too soon to take the plunge? The answer probably has more to do with your situation than with the technology.
- Network Prep and QoS Assurance - Yes, you should look at your network before installing an IP PBX. But the news is probably good. If not, here's a make-ready recipe, and some products to help.
- On speaking terms: a network-based PBX lexicon
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- Pack up your PBX--VoIP is here - IP technology is increasingly becoming the standard for corporate voice communications.
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- PbxTech - Free Technical Forums and News for PBX Administrators
- Voice&Data GoldBook - IP Is Future but Circuit Switch Stays
- VoIP--do it right - VoIP can help your company save on telephone costs, leverage its existing network infrastructure, and add communications features that enhance productivity--assuming, of course, that it's done right. If you're planning to take the plunge and swap out your old PBX for a VoIP system, you need to keep your eye on what's critical--and know the pitfalls to avoid.
- Take the Low-Cost Route to Building Your Own PBX
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