1 - What is a PBX?
A PABX, usually called a PBX, is a Private Automatic Branch Exchange. A PBX is private because the enterprise owns it, not the telephone company.
The telephone company can still be a supplier or service provider. Originally,PBX equipment was analog, more recent PBX equipment is digital. A PBX is cost attractive because it is less expensive to use a
PBX than a separate phone line for every user in the enterprise and because it provides more services.
With a PBX, lines from the telephone company can be shared instead of having a separate line to the
telephone company for each user. APBX provides a place for trunk (multiple phone) lines to terminate
at the enterprise. APBX is a telephone system that services an enterprise by switching calls between
enterprise users on local lines and by sharing the external phone lines. ThePBX has the intelligence to
switch calls within the enterprise and outside the enterprise.
A PBX provides features and capabilities not available with direct connections to the Public Switched
Telephone Network ( PSTN.) A PBX moves telephone functions from the phone company to the
enterprise. APBX provides additional functions and features like interactive voice response, call waiting, conferencing or voice mail, paging, transferring calls, or three way calling that wouldn't be available with separate telephone lines. A PBX usually has a console for use by an operator.
Alternatives to a PBX include Centrex. Centrex provides a pool of lines from the central office to the
enterprise.Centrex can provide some of the same functions as a PBX, for example voice mail, call hold,
call waiting or call transfer.
Like the PSTN, legacy enterprise telephony (ET) systems are circuit switched. They both use a common infrastructure model. All the control protocols and features are combined into a single model. ET
systems usually cannot handle the same volume of traffic as PSTN switches. ET systems usually use
proprietary protocols where the PSTN relies on the standard SS7 protocol.
Larger PBX systems typically have more features and abilities than smaller PBX systems. This is the
way legacyPBX vendors market their systems. A feature you want may not be available on a PBX you
can afford. You can only get the features you need if you are willing to spend more money.
How Does VoIP Compare to a PBX?
ET systems, and , provide interoperability between a local system and the PSTN. Many features in a legacy PBX system are rarely used. Some features may have been developed for a single user
to make a single large sale. Because of this, does not yet have all the features of allPBX systems from all vendors. Because is an open platform features are easy to add and many new features are being added all the time. If does not yet have a feature you want it is either already
under development or easy to add. Any feature added to by any user will be available for yo
to use. This is because is an open source product distributed under a GPL license.
What is ?
New interfaces and technologies are easily added to . With you can take control of
your communications. Once a call is in your Linux sever with , anything can be done with it
gives you fine-grained control over every aspect of your communications
Scenario - A Home Office
Julie is an outside sales rep for a company in Chicago. She covers the Southwestern
region and lives in Phoenix. Julie has a DSL line coming in to her home
office. The head office has an server. The head office has a hig
speed Internet connection.
Julie has a telephone on her desk that connects to her DSL line. A caller contacts the
Chicago office by dialing the Chicago 800 toll free telephone number of th
head office. The caller listens to the directory of extensions for the sale
department. The directory gives choices for each of the regions. The calle
selects the Southwestern region. tells them the extension for Julie
announces her name, and then announces it will contact her.
The server in Chicago rings the telephone on Julie's desk. Since this call is
being made over the Internet over Julie's DSL line, there is no long distance
charge between Julie and the head office. If Julie doesn't answer within si
rings, the caller is given the choice of leaving a message or returning to th
Sales directory or talking with the operator.
An system is a fraction of the cost of legacy PBX systems. The additional hardware that turns
a small Linux server into a telephone system is inexpensive and readily available. Support is availabl
from different sources including Signate.
is incredibly efficient. A small PC will serve many telephone users. With you can
easily build a telephone system for the smallest or the largest enterprise, There are server
running thousands of phones right now. You can easily scale or combine systems to serve an
number of users in any number of locations.
When combined with low-cost Linux telephony hardware, creates a PBX at a fraction of the
price of traditionalPBX systens. While an system is a fraction of the cost of legacy systems, it
provides better functionality than the most expensive proprietary systems. includes feature
such as voicemail, interactive voice response IVR,) and conferencing which are very expensive in proprietary systems
Scenario - A Large Business
can benefit a large business with offices in several locations. In this scenario,
there are fifteen hundred employees. The main office is in New York. Distric
offices are in Chicago and Los Angeles. Support is done at the Denver office.
servers are in separate hosted facilities in New York and Chicago. The servers communicate with each other over a high-speed Internet connection. Various servers are needed to support this many users. The
servers communicate witheach other and each of the branch office
over a high-speed internet connection. The hosted facilities are hardened an
geographically separate from each other and the company offices.
With shared servers, if one fails the another takes over. This is much safer for
the company as there is no single point of failure. Even in the event of an outage at one of the main offices, telephone communications won't be disrupted.
If there is a problem in the office, employees can take their phones off their desk and
move them to their home or another office. If there is a problem at the Chicago office, key employees can relocate to the New York office. They can tak
their desk phones with them, or use phones already at the New York office
Business goes on.
Users seeking support can call local numbers in any of the regions. These calls are
routed to the support center in Denver. The calls are sent over the Internet so
there is no long distance charge to the company. The user has called a loca
number and has no long distance charge. This is called"toll bypass."
You can make calls through the telephone company, or make calls over the Internet.
With the appropriate hardware, supports telephony over the PSTN without any Internet connection. It is much cheaper to send telephone calls over the Internet than through the telephone companies. can pay for itself with the money you save on your phone bill.
With PBX's and Interactive Voice Response ( IVR) applications are rapidly created and
deployed. The powerful command line interface and feature rich text configuration files support rapi
configuration and real-time diagnostics
Web servers provide easy deployment of dynamic content, for example movie listings or weather
reports. can deploy dynamic content over the telephone, with the same ease. For example
can display contact or meeting information on the LCD panel of an IP telephone.
Supports a wide range of protocols for handling and transmitting voice over traditional telephony interfaces. supports US and European standard signalling types used in standard business phone systems. This allows to bridge between next generation voice-data integrated
networks and existing network infrastructure. not only supports traditional phone equipment
it provides this equipment with additional capabilities
Scenario - A Busy User
can benefit a busy user who travels frequently. A caller contacts the user's
system. prompts the caller for their name. The caller say
their name. then plays a message asking them to wait for a momen
while the called party is located.
The server rings the office telephone at the headquarters and at the branch
office, the home telephone and the cell phone of the user, all at the same time
If any of the phones are busy, the caller is directed to voicemail. If the use
doesn't answer any of the phones after six rings, the caller is prompted to leav
a voicemail message.
If the user answers any of the phones, the server announces the telephone
number of the calling party, if caller ID is available. Then the serve
plays back the name the called party recorded. The user presses one on th
keypad of their phone to accept the call, or three to refuse the call. If the use
refuses the call, the caller is directed to voicemail. The server sends
text message to the user's cell phone indicating there is new voicemail.
Inter- Exchange (IAX) is a Voice over IP protocol specific to . IAX allows to
merge voice and data traffic seamlessly across disparate networks. When using Packet Voice, data like URL information and images can be sent in-line with voice traffic. This supports advanced integratio
of voice and data that is not available in legacy systems
With four APIs for modular loading of telephony applications, hardware interfaces, file format handling, and codecs1. provides transparent switching
between all supported interfaces. This is how ties together diverse telephony systems into
single switching network
Scenario - An International Business
An electronics manufacturer has main offices in San Jose, California with international offices in London, Tokyo, Hong Kong and Munich. servers
are in hosted facilities in San Jose, and Tokyo. servers are in th
Hong Kong, Munich and London offices.
All the servers have high speed connections to the Internet. All the servers
have connections to local public telephone systems.
Because the servers are connected over the Internet, there are no long distance charges for calls between the offices. Any user in any office can call any
user in any other office. These calls are routed over the Internet, that is they
are toll bypass calls
The support staff for this company is all at the San Jose headquarters. Instead of having support staff in the London office, management decides to perform all
English language support from San Jose. Users in London can call the London telephone number for the company. If they wish to contact support, thei
call i routed to the San Jose office over the company's VPN. This is a toll
1. A CODEC is a compressor-decompressor. A CODEC is used to digitize voice into data or convert digitized voice back to
an analog signal.
VoIP (Voice Over IP) systems like can use a computer to send and receive telephone calls over
a data network.Telephone calls are sent over the network as data using IP, the Internet Protocol. Telephone calls are sent from one IP phone to another IP phone as data.
An system often services many IP telephones, as many as a thousand or more. Standard analog telephones or other devices like fax machines can be connected with an inexpensive adaptor. With
such a system, anyone in the office can call anyone else in the office. Calling outside the office, fo
example anyone with a regular telephone, is described below
IP phones are not connected to wires you rent from the phone company, to the telephone company
itself, or to telephone wires you have in your office. They are connected to your data network.
You can call from a VoIP phone on your network to any other phone connected to your VoIP system.
VoIP calls go over your local data network, not the PSTN (Public Switched Telephone Network,) and
not your local telephone wires.
You don't need a connection to the PSTN to make calls to other phones connected your local VoIP
system. If you have two different office buildings, or offices on different floors, and they are connecte
to your local area network, you call phones, or fax machines, in the other area. Those calls still trave
over your data network.
|Figure: 01-1 IP Phones in the Office|
Connecting your Office Telephone System to the Internet
As shown in the illustration, your telephone system can easily be connected to the Internet.
Any telephone can be easily connected to the Internet. You can connect an IP phone directly to the
Internet. You can connect any standard analog phone or fax machine to the Internet with an inexpensive VoIP adaptor.
If your system is connected to the Internet, any VoIP enabled telephone that is connected to
the Internet can be allowed to connect to your system. You can easily call any other VoIP
phone serviced by your system, no matter where that phone is. You can easily assure that th
connections are secure and that unauthorized users are excluded. Any phone controlled by your system can call any other VoIP or analog phone controlled by your system.
It doesn't matter where a network connected phone is located. For example, you can have an
phone system in your office in New York and an office in Shanghai. Your system in Ne
York is connected to the Internet, and your Shanghai office is connected to the Internet. A phone in
Shanghai connects to your New York system over the Internet.The phone in your Shanghai
office now works exactly like any phone in your New York office. When you dial the number for
phone in the Shanghai office from your New York phone, the phone rings in Shanghai.
With a little bit of the right equipment you can install a phone at your home office and plug it into the
Internet. Your office phone, now at home, communicates with your office system over the
Internet. Now, using your phone at home is just like using your phone in your office. No one would be
able to tell where you are! You can take your phone on a trip and call from anywhere you have an Internet connection.
You can call anyone who uses a VoIP system, even if it isn't an system. Your system
has to have a connection to their VoIP system. This can be a local network connection, or both system
can be connected to the Internet. The call is sent over the data network or Internet, not the PSTN.
Both systems must have the correct permissions and configurations.
Because the VoIP telephone call is sent over your data network or the Internet, there is never a long
distance charge or a toll charge. The charge for the telephone call is included in the price you pay fo
your network or Internet connection. This is one place you save money, no more toll charges or long
Connecting Your System to the PSTN
As shown in the following illustration, users should be able to place calls to telephones connected to the PSTN. This requires a connection to the PSTN. Your system has to be connected to the PSTN. This is easy to do.
users need a telephone number if calls are to be accepted from the PSTN. You have to rent
telephone numbers from a telephone company. You can rent a connection to your telephone company
This connection is usually some wires they buried in the ground or wires they hung from poles.
Boards you add to the server running connect the server to the connection you rent from the
phone company. When someone dials your telephone number from the PSTN, your desk phone rings.
|Figure: 01-2 Connecting to the Public Telephone Network|