Connecting Your PBX to the Internet
by Brian McConnell
Imagine being able to call your Tokyo office for the cost of a local
phone call. Imagine being able to connect all of your office phone systems
worldwide into one large supersystem. Imagine having your customers call
you toll free from anywhere in the world using the internet.
All of these applications are now possible thanks to advances in internet
telephony technology. The key to successful use of internet telephony revolves
around making internet telephony invisible; in other words, just like an
ordinary phone call.
Most businesses already own a private branch exchange (PBX). A PBX is
to a business telephone system what an Ethernet hub is to a LAN. With a
PBX, a large number of users share a small number of limited resources
(outside phone lines, voice mail system, etc.).
Several companies have produced software and hardware products which
essentially fool a PBX into seeing the internet as a bank of ordinary phone
lines through which it can place and receive phone calls. By placing these
devices in front of a PBX, you can place and receive internet calls the
same way you make ordinary phone calls, using the phone which is sitting
right on your desk.
Let’s take a look at a few examples:
All of these applications are feasible today using a combination of internet
telephony software and low-cost voice cards designed with PBX integration
A customer is browsing your web site, and sees a button labeled "Call Customer
Service." He or she clicks on this URL, and a few seconds later is talking
to a customer service representative using Microsoft NetMeeting (Microsoft's
internet telephony applet).
You need to call an associate at your Hong Kong office; you pick up the
phone on your desk and dial extension 401. Moments later, you are talking
via the internet to the Hong Kong office.
You need to call a customer in Hong Kong; you dial 81 to get an outside
line from your Hong Kong office, and then dial the customer's local number.
You talk for several hours for the cost of a local phone call.
Internet telephony is not all that different from ordinary telephony. When
you place a call over the telephone network, your voice is digitized into
a stream of 8-bit numbers (about 8000 samples per second, for a overall
bit rate of 64 Kbps). These numbers are passed from node to node through
the telephone network until they reach their destination, where they are
translated back into an analog signal which is played out through the called
When you place a call over the internet, your voice is digitized, broken
into packets, and then transmitted using the TCP/IP protocol. It's a simple
process. What makes internet telephony difficult from a technical point
of view is working with limitations which are inherent in the internet
The internet is a packet switched network, and offers no performance
guarantees (i.e., once your packet is transmitted, it might arrive at its
destination in 10 ms, or in 1000 ms). This variability in delivery times
makes real-time audio delivery tricky. You can buffer the transmitted data
somewhat, but if you buffer it too much, you wind up with an annoying delay.
If you don't buffer enough, you wind up with dropouts (like talking to
someone on a cell phone while driving through a tunnel). The public telephone
network, on the other hand, transmits data in serial fashion, and provides
a guaranteed data rate of 64 Kbps.
Internet telephony software attacks this problem in several ways…
Buffering - on unreliable connections, applications may buffer data for
a few hundred milliseconds to allow for late packets. This technique works
effectively, but also introduces a delay of ½ to 1 second to a call.
Annoying, but tolerable unless the delay is really noticeable.
Compression - audio is compressed from the standard telephone rate of 64 Kbps
to as little as 5 Kbps with minimal loss of audio quality. By reducing the
amount of data that needs to be transmitted through the network, applications
perform more reliably over slow internet connections.
Forward packet loss compensation - some applications encode redundant data
as packets are transmitted. This enables the application on the receiving
end to reconstruct lost or delayed packets without waiting for their retransmission.
This makes applications perform better on a saturated network (where many
packets arrive late, or are discarded en route).
The good news is that advances in modem technology, and in the internet
itself, have made internet telephony practical on well-managed networks.
However, quality of service is still highly variable, and so it is important
to wisely choose your internet service provider. Better still, new internet
protocols such as RSVP provide a mechanism for providing guaranteed levels
of service (i.e., opening a socket which delivers a guaranteed data rate
of, for example, 9.6 Kbps each way). Some internet servics providers (ISPs), such as BBN, are already
implementing RSVP. It will make real-time applications such as telephony
not only practical, but better in some situations than using the public
The most important internet standard governing telephony is H.323. This
standard defines a method for setting up calls over the internet, and defines
a standard set of codecs (compression/decompression algorithms) which can
be used for internet phone calls. H.323 is already supported by industry
leading vendors such as Microsoft, Intel, and many others. With H.323,
anybody's internet phone will talk to anyone else's, an important and
necessary development for the industry.
Also important to internet telephony and videoconferencing is the RSVP
protocol. This protocol defines a mechanism for reserving a certain amount
of bandwidth to/from a server. The ability to reserve a dedicated piece
of bandwidth from the network will enable internet telephony applications
to perform as reliably as conventional telephone calls. As mentioned earlier,
BBN is one of the early leaders in adding RSVP to their network.
To bridge your PBX or intercom system to the internet, you need a piece
of equipment which will fool the PBX into seeing the internet as an ordinary
phone line, or POTS (plain old telephone service) line. You simply connect
this device to one of the trunk (outside line) ports on your PBX, and start
placing phone calls.
The best PBX-internet gateway to come out yet is Quicknet Technologies
Internet PhoneJACK. This plug-and-play expansion card fully emulates
a normal phone line. When a call arrives, it applies ring voltage to the
line. When you take the line offhook, it provides dialtone, call progress
tones, everything a PBX (or any telephone device) expects to hear when
placing or receiving a call.
So, you simply put the Internet PhoneJACK in front of your PBX,
and then program your phone system so that if you dial 9 + phone number,
calls go over ordinary phone lines, and if you dial 8 + IP address, calls
go over the internet instead. The PBX doesn’t know or care that calls are
going over the internet; to it, the internet is simply a bank of phone lines.
Want to call the Hong Kong office? Just dial 8 followed by a speed-dial
code for that office. You hear ringing, and a few seconds later are talking
to someone at that office, just as if you had called them directly.
Several vendors have gotten into the internet telephony software market,
Microsoft and Intel among them. Microsoft’s
NetMeeting product is an excellent internet phone for point-to-point
applications. NetMeeting works with the Internet PhoneJACK, and is useful
for applications where you want to call from your desk directly to another
Read the "Official Microsoft NetMeeting 2.1 Book"
One of the neatest applications for these new products is the ability to
bridge many office phone systems into one large supersystem. Provided each
office has a wide area network (WAN) or internet connection (a de facto requirement
nowadays), placing a call from one office to another is as simple as picking
up the phone on your desk and dialing an extension number.
VoiceNet is uniquely suited to this task on account of its direct inward
dialing and direct outward dialing features. It enables you to set up speed-dial
codes which map directly to a user at a specific extension at the
For example, using the AltiGen Windows NT-based PBX, you could set up
a series of virtual extensions which are forwarded into the VoiceNet gateway….
X201: mapped to VoiceNet node athens.phonezone.com x101
X202: mapped to VoiceNet node tokyo.phonezone.com x105
Click here to view a screen shot of AltiGen AltiServ. Using AltiGen's configuration utilities, it is easy to map internal
extensions to outside offices via the internet.
Thus, dialing the Hong Kong office becomes as simple as dialing an internal
Global toll-free calling
Another great application for internet telephony gateways is the creation
of global toll-free "numbers" which customers anywhere in the world can
call using NetMeeting or other internet telephony applications. This is
somewhat simpler to implement using Microsoft NetMeeting and the Internet
You simply connect the Internet PhoneJACK to one of the outside line
ports on your phone system, and publish the address of the machine in which
the card is installed. Incoming calls are routed directly to your receptionist
or to an automated attendant.
This is a great example of how you can use the worldwide web to boost sales
and enhance customer service. By adding a "Call Us" button to a web page,
you can make it incredibly easy for customers with internet telephony software
on their PCs to call you. Placing a phone call becomes as simple as clicking
on a URL.
Tips for improving sound quality
One of the key factors which will determine the reliability of your PBX-internet
link is the quality of your internet service provider. Unfortunately, ISPs can be
inconsistent in their level of service. Some
provide excellent service, however many provide marginal service at best. Be sure to
shop around to find the right ISP for you.
For inter-office bridging, you'll see the best performance if you connect
all of your offices to the same ISP. By sending data
along the same ISP's network backbone, you'll minimize packet delivery times,
and generally see better performance than you will if you use different
ISPs at each office. The reason behind this is simple. If you share the
same ISP, data takes a less circuitous route from point A to point B, and
this minimizes delivery delays.
It is also important to provision an adequate amount of bandwidth for
your office. If you have 10 people sharing a 28.8 K dial-up link, you'll
see slowdowns and dropouts when other users download large files while
you are talking. General rule of thumb: if you have more than a couple
people in your office, get Frame Relay or some other high-speed
service. Look at it this way, the money you save on long-distance tolls
(particularly international tolls) will more than make up for the added
expense and equipment.
Internet telephony technology has come a long way since its relatively
recent introduction as an entertainment application. Technologies for integrating
internet telephony into traditional PBX systems makes it practical for
businesses to use the internet for commercial applications.
Bottom line: for $200, you can get an Internet PhoneJACK, download
Microsoft NetMeeting, and start experimenting with turning the internet
into an extension of your existing phone system.