Any physical device whose primary purpose is terminating an on-demand audio com-
munications circuit between two points can be classified as a physical telephone. At a
minimum, such a device has a handset and a dial pad; it may also have feature keys, a
display screen, and various audio interfaces.
This section takes a brief look at the various user (or endpoint) devices you might want
to connect to your Asterisk system. We’ll delve more deeply into the mechanics of
analog and digital telephony in Chapter 7.
Analog telephones
Analog phones have been around since the invention of the telephone. Up until about
20 years ago, all telephones were analog. Although analog phones have some technical
differences in different countries, they all operate on similar principles.
This contiguous connection is referred to as a circuit, which the
telephone network used to use electromechanical switches to create—
hence the term circuit-switched network.
When a human being speaks, the vocal cords, tongue, teeth, and lips create a complex
variety of sounds. The purpose of the telephone is to capture these sounds and convert
them into a format suitable for transmission over wires. In an analog telephone, the
transmitted signal is analogous to the sound waves produced by the person speaking.
If you could see the sound waves passing from the mouth to the microphone, they
would be proportional to the electrical signal you could measure on the wire.
Analog telephones are the only kind of phone that are commonly available in any retail
electronics store. In the next few years, that can be expected to change dramatically.
Proprietary digital telephones
As digital switching systems developed in the 1980s and 1990s, telecommunications
companies developed digital Private Branch eXchanges (PBXes) and Key Telephone
Systems (KTSes). The proprietary telephones developed for these systems were com-
pletely dependent on the systems to which they were connected and could not be used
on any other systems. Even phones produced by the same manufacturer were not cross-
compatible (for example, a Nortel Norstar set will not work on a Nortel Meridian 1
PBX). The proprietary nature of digital telephones limits their future. In this emerging
era of standards-based communications, they will quickly be relegated to the dustbin
of history.
The handset in a digital telephone is generally identical in function to the handset in
an analog telephone, and they are often compatible with each other. Where the digital
ISDN telephones
Prior to VoIP, the closest thing to a standards-based digital telephone was an ISDN-
BRI terminal. Developed in the early 1980s, ISDN was expected to revolutionize the
telecommunications industry in exactly the same way that VoIP promises to finally
achieve today.
There are two types of ISDN: Primary Rate Interface (PRI) and
Basic Rate Interface (BRI). PRI is commonly used to provide trunking
facilities between PBXes and the PSTN, and is widely deployed all over
the world. BRI is not at all popular in North America, but is common
in Europe.
While ISDN was widely deployed by the telephone companies, many consider the
standard to have been a flop, as it generally failed to live up to its promises. The high
costs of implementation, recurring charges, and lack of cooperation among the major
industry players contributed to an environment that caused more problems than it
solved.
BRI was intended to service terminal devices and smaller sites (a BRI loop provides two
digital circuits). A wealth of BRI devices have been developed, but BRI has largely been
deprecated in favor of faster, less expensive technologies such as ADSL, cable modems,
and VoIP.
BRI is still very popular for use in video-conferencing equipment, as it provides a fixed
bandwidth link. Also, BRI does not have the type of quality of service issues a VoIP
connection might, as it is circuit-switched.
IP telephones
IP telephones are heralds of the most exciting change in the telecommunications in-
dustry. Already now, standards-based IP telephones are available in retail stores. The
wealth of possibilities inherent in these devices will cause an explosion of interesting
applications, from video phones to high-fidelity broadcasting devices, to wireless mo-
bility solutions, to purpose-built sets for particular industries, to flexible all-in-one
multimedia systems.
The revolution that IP telephones will spawn has nothing to do with a new type of wire
to connect your phone to, and everything to do with giving you the power to commu-
nicate the way you want.
The early-model IP phones that have been available for several years now do not rep-
resent the future of these exciting appliances. They are merely a stepping-stone, a
familiar package in which to wrap a fantastic new way of thinking.
The future is far more promising.