PBX4Linux-3.0.pdf

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PBX
4
Linux
Software based ISDN Private Branch Exchange for Linux
By Andreas Eversberg (Jolly)
(http://isdn.jolly.de)
Documentation for Version 3.0
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Index
1.
2.
3.
4.
5.
6.
7
8
9
Introduction
Hardware Requirements
Download & Installation
Configuration
Using PBX4Linux
Tuning
Debugging
Architecture
Support
10 References & Relates Projects
Appendix
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1
Introduction
1.1 What is PBX4Linux?
PBX4Linux is pure software based ISDN PBX, that connects external lines, internal
telephones, and optionally voice over IP. It is designed to run with Linux only because it uses
the kernel’s mISDN passive driver by Karsten Keil. It can work together with OpenH323, that
is a voice over IP implementation complied to the H.323 ITU-T standard. The primary goal is
a working ISDN PBX that connects BRI and PRI lines using call routing. Basic applications,
like voice box and conferences are implemented.
1.2 Philosophy behind
At the end of 2001, I found out, that my ISDN card has a chipset capable of connecting
telephones to it. This is called the NT-Mode. So my card can be used to transfer information
between telephones and my Linux box. Normally ISDN cards transfer information between a
computer and the public telephone system. But at this time there was no protocol for Linux
kernel that could talk to ISDN telephones.
My first idea was to use a telephone to make voice over IP calls. Instead of messing with
headsets, I wanted to just pick up the phone, get a dial tone, and dial the IP number. On the
other side, I wanted to have another phone, that rings and shows me the IP number of the
caller. I know, that there are IP phone already but they are more expensive than ISDN phones.
So I started to expand the Linux kernel to be able to handle telephones connected to my ISDN
card. The patch was written for the old ‘HiSax’ driver. I hated that patch because it was
another dirty addition. I was glad that Karsten Keil wrote the new modular ISDN driver that
gave me a well defined API and real multipoint NT-mode. I added some features to the new
driver, so real time cross connections, conferences, DTMF decoding and tone generation is
possible.
While writing the expansion of the kernel driver, I designed the PBX4Linux, that is now also
capable of connecting calls between connected telephones and external ISDN lines connected
to the public telephone system, as well as voice over IP using OpenH323 library.
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I wanted to have a PBX, that provides features I am missing in standard products, like letting
a call ring on my telephone at home and at the same time on my mobile via external call. The
called phone, that picks up first, gets connected, the other gets released. Another idea was
callback, which helps me to reduce the costs of mobile calls.
Many features followed: Deletion of digits while dialing after pick-up, an answering machine
that records during the announcement to trick the caller, and conferences with no member
limit but the resources of available channels.
During this book, you will learn about the features it has, how it works and, how to set up
your own software based ISDN PBX.
1.3 Who wrote it?
My name is Andreas Eversberg and I live in the northern part of Germany, called ‘Schleswig-
Holstein’. I currently work for a telephone company, called Versatel, which is a full service
provider for telephone lines, internet and leased lines in Schleswig-Holstein and other parts of
Germany. I do administration of the network elements like subscriber line cards and ADSL
routers. I started writing the PBX and the kernel driver, six months before I got the job at
Versatel. I see this job as a start of managing life and getting used to “work” and of course to
learn and earn money.
My email address is ‘jolly@jolly.de’. My homepage is ‘http://www.jolly.de’. The PBX4Linux
page is ‘http://isdn.jolly.de’.
1.4 How much does it cost?
It costs nothing at all. It is
GPL!
As long as you use it for non commercial purpose, I will not
charge you. Using this PBX in your company to handle the daily business is not a commercial
purpose in this context. See the appendix for copyright information.
1.5 Some definitions of terms in this document
PBX
Private Branch Exchange (PBX), also called PABX, is a small private telephone system. It
features internal calls without getting changed, because calls are routed internally. It has
connectivity to the telephone network in most cases. It provides more features than the
telephone network provides.
H.323
H.323 is a standard for making calls via packet switched IP networks. It specifies, how voice
(and call control) is transferred over IP. H.323 is defined by the International
Telecommunication Union (ITU). To be able to interconnect two persons using H.323, both
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