2.4. Types of
Phone
Since the title of this book is Asterisk: The Future of Telephony, we would be
remiss if we didn't discuss the devices that all of this technology
ultimately has to interconnect: telephones !
We all know what a telephone isbut will it be
the same five years from now? Part of the revolution that Asterisk
is contributing to is the evolution of the telephone, from a simple
audio communications device into a multimedia communications
terminal providing all kinds of yet-to-be-imagined functions.
As an introduction to this exciting concept, we
will briefly discuss the various kinds of devices we currently call
"telephones" (any of which can easily be integrated with Asterisk).
We will also discuss some ideas about what these devices may evolve
into in the future (devices that will also easily integrate with
Asterisk).
2.4.1. Physical Telephones
Any physical device whose primary purpose is
terminating an on-demand audio communications 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.
2.4.1.1. 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.
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.
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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.
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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.
2.4.1.2. Proprietary digital
telephones
As digital switching systems developed in the
1980s and 1990s, telecommunications companies developed digital
Private Branch eXchanges (PBXs) and Key Telephone Systems
(KTSs) . The proprietary telephones
developed for these systems were completely 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 phone
is different is that inside the telephone, the analog signal is
sampled and converted into a digital signalthat is, a numerical
representation of the analog waveform. We'll leave a detailed
discussion of digital signals until Chapter 7; for now,
suffice it to say that the primary advantage of a digital signal is
that it can be transmitted over limitless distances with no loss of
signal quality.
The chances of anyone ever making a proprietary
digital phone directly compatible with Asterisk are fairly small,
but companies such as Citel (http://www.citel.com) have created
gateways that convert the proprietary signals to SIP.
2.4.1.3. 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.
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There are two types of ISDN: Primary Rate Interface (PRI) and Basic Rate Interface (BRI). PRI is commonly
used to provide trunking facilities between PBXs and the PSTN, and
is widely deployed. BRI is not at all common in North America, but
has enjoyed some success in Europe.
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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 amongst
the major 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). While a
wealth of BRI devices have been developed, 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.
BRI is still sometimes used in place of analog
circuits to provide trunking. Whether or not this is a good idea
depends mostly on how your local phone company prices the service,
and what features it is willing to provide.
2.4.1.4. IP telephones
IP telephones are heralds of the most exciting
change in the telecommunications industry. In the very near future,
standards-based IP telephones will be 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
mobility 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 communicate the way
you want.
The early-model IP phones that have been
available for several years now do not represent 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.
2.4.2. Soft Phones
A soft phone is
a software program that provides telephone functionality on a
non-telephone device, such as a PC or PDA. So how do we recognize
such a beast? What might at first glance seem a simple question
actually raises many. A soft phone should probably have some sort
of dial pad, and it should provide an interface that reminds users
of a telephone. But will this always be the case?
The term "soft phone" can be expected to evolve
rapidly, as our concept of what exactly a telephone is undergoes a
revolutionary metamorphosis. As an example of this evolution,
consider the following: would we correctly define popular
communication programs such as Instant Messenger as soft
phones ?
IM provides the ability to initiate and receive standards-based
VoIP connections. Does this not qualify it as a soft phone?
Answering that question requires knowledge of the future that we do
not yet possess. Suffice it to say that while at this point in
time, soft phones are expected to look and sound like traditional
phones, that conception is likely to change in the very near
future.
As standards evolve and we move away from the
traditional telephone and toward a multimedia communications
culture, the line between soft phones and physical telephones will
become blurred indeed. For example, we might purchase a
communications terminal to serve as a telephone, and install a soft
phone program onto it to provide the functions we desire.
Having thus muddied the waters, the best we can
do at this point is to define what the term "soft phone" will refer
to in relation to this book, with the understanding that the
meaning of the term can be expected to undergo a massive change
over the next few years. For our purposes, we will define a soft
phone: any device that runs on a personal computer, presents the
look and feel of a telephone, and provides as its primary function
the ability to make and receive full-duplex audio
communications (formerly known as
"phone calls") through E.164
addressing.
2.4.3. Telephony Adaptors
A telephony
adaptor (usually referred to as an ATA, or Analog Terminal
Adaptor) can loosely be described as an end-user device that
converts communications circuits from one protocol to another. Most
commonly, these devices are used to convert from some digital (IP
or proprietary) signal to an analog connection that you can plug a
standard telephone or fax machine into.
These adaptors could be described as gateways,
for that is their function. However, popular usage of the term
telephony gateway would probably
best describe a multi-port telephony adaptor, generally with more
complicated routing functions.
Telephony adaptors will be with us for as long
as there is a need to connect incompatible standards and old
devices to new networks. Eventually, our reliance on these devices
will disappear, as did our reliance on the modemobsolescence
through irrelevance.
2.4.4. Communications Terminals
Communications
terminal is an old term that disappeared for a decade or two
and is being reintroduced here, very possibly for no other reason
than that it needs to be discussed so that it can eventually
disappear againonce it becomes ubiquitous.
First, a little history. When digital PBX
systems were first released, manufacturers of these machines
realized that they could not refer to their endpoints as
telephonestheir proprietary nature prevented them from connecting
to the PSTN. They were therefore called terminals
, or stations . Users, of course, weren't having any of it.
It looked like a telephone and acted like a telephone, and
therefore it was a telephone. You
will still occasionally find PBX sets referred to as terminals, but
for the most part they are called telephones.
The renewed relevance of the term
"communications terminal" has nothing to do with anything
proprietaryrather, it's the opposite. As we develop more creative
ways of communicating with each other, we gain access to many
different devices that will allow us to connect. Consider the
following scenarios:
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If I use my PDA to connect to my voicemail and
retrieve my voice messages (converted to text), does my PDA become
a phone?
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If I attach a video camera to my PC, connect to
a company's web site, and request a live chat with a customer
service rep, is my PC now a telephone?
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If I use the IP phone in my kitchen to surf for
recipes, is that a phone call?
The point is simply this: we'll probably always
be "phoning" each other, but will we always be using "telephones"
to do so? |
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