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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Types of Phones</title><link rel="stylesheet" href="styles.css" type="text/css" /><meta name="generator" content="DocBook XSL Stylesheets V1.69.1" /><link rel="start" href="index.html" title="Asterisk™: The Future of Telephony" /><link rel="up" href="asterisk-CHP-2.html" title="Chapter 2. Preparing a System for Asterisk" /><link rel="prev" href="asterisk-CHP-2-SECT-3.html" title="Telephony Hardware" /><link rel="next" href="asterisk-CHP-2-SECT-5.html" title="Linux Considerations" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Types of Phones</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="asterisk-CHP-2-SECT-3.html">Prev</a> </td><th width="60%" align="center">Chapter 2. Preparing a System for Asterisk</th><td width="20%" align="right"> <a accesskey="n" href="asterisk-CHP-2-SECT-5.html">Next</a></td></tr></table><hr /></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="asterisk-CHP-2-SECT-4"></a>Types of Phones</h2></div></div></div><p>Since the title of this book is<a id="ch02_phonestypes" class="indexterm"></a><a id="ch02_telephonetype" class="indexterm"></a> <span class="emphasis"><em>Asterisk: The Future of Telephony</em></span>, we
would be remiss if we didn’t discuss the devices that all of this
technology ultimately has to interconnect: telephones!</p><p>We all know what a telephone is—but 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.</p><p>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).</p><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="asterisk-CHP-2-SECT-4.1"></a>Physical Telephones</h3></div></div></div><p>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.</p><p>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 <a href="asterisk-CHP-7.html" title="Chapter 7. Understanding Telephony">Chapter 7, <i>Understanding Telephony</i></a>.</p><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="asterisk-CHP-2-SECT-4.1.1"></a>Analog telephones</h4></div></div></div><p>Analog phones have been around <a id="I_indexterm2_tt165" class="indexterm"></a>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.</p><div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title"><a id="asterisk-CHP-2-NOTE-8"></a>Tip</h3><p>This contiguous connection <a id="I_indexterm2_tt166" class="indexterm"></a><a id="I_indexterm2_tt167" class="indexterm"></a>is referred to as a <span class="emphasis"><em>circuit</em></span>,
which the <span class="keep-together">telephone</span> network
used to use electromechanical switches to create—hence the
term<a id="I_indexterm2_tt168" class="indexterm"></a> <span class="emphasis"><em>circuit-switched network</em></span>.</p></div><p>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 <span class="emphasis"><em>analogous</em></span> 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.</p><p>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.</p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="asterisk-CHP-2-SECT-4.1.2"></a>Proprietary digital telephones</h4></div></div></div><p>As digital<a id="I_indexterm2_tt169" class="indexterm"></a><a id="I_indexterm2_tt170" class="indexterm"></a> switching systems developed in the 1980s and 1990s,
telecommunications companies developed digital <a id="I_indexterm2_tt171" class="indexterm"></a>Private Branch eXchanges (PBXes) and Key Telephone
Systems (KTSes). <a id="I_indexterm2_tt172" class="indexterm"></a><a id="I_indexterm2_tt173" class="indexterm"></a>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.</p><p>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 signal—that is, a numerical representation of the
analog waveform. We’ll leave a detailed discussion of digital signals
until <a href="asterisk-CHP-7.html" title="Chapter 7. Understanding Telephony">Chapter 7, <i>Understanding Telephony</i></a>; 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.</p><p>The chances of anyone ever making a proprietary digital phone
directly compatible with Asterisk are slim, but companies such
as<a id="I_indexterm2_tt174" class="indexterm"></a> Citel (<a href="http://www.citel.com" target="_top">http://www.citel.com</a>)<sup>[<a id="id4107084" href="#ftn.id4107084">35</a>]</sup> have created gateways that convert the proprietary
signals to<a id="I_indexterm2_tt175" class="indexterm"></a> Session Initiation Protocol (SIP).<sup>[<a id="asterisk-CHP-2-FN-19" href="#ftn.asterisk-CHP-2-FN-19">36</a>]</sup></p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="asterisk-CHP-2-SECT-4.1.3"></a>ISDN telephones</h4></div></div></div><p>Prior to VoIP, the<a id="I_indexterm2_tt176" class="indexterm"></a> 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.</p><div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title"><a id="asterisk-CHP-2-NOTE-9"></a>Tip</h3><p>There are two types of ISDN: <span class="emphasis"><em><span class="emphasis"><em>Primary Rate
Interface</em></span></em></span> (PRI) and<a id="I_indexterm2_tt177" class="indexterm"></a><a id="I_indexterm2_tt178" class="indexterm"></a><a id="I_indexterm2_tt179" class="indexterm"></a><a id="I_indexterm2_tt180" class="indexterm"></a> <span class="emphasis"><em><span class="emphasis"><em>Basic Rate
Interface</em></span></em></span> (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.</p></div><p>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.</p><p>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.</p><p>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.</p><p>BRI is still sometimes used in place of analog circuits to
provide trunking to a PBX. 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.<sup>[<a id="id4107235" href="#ftn.id4107235">37</a>]</sup></p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="asterisk-CHP-2-SECT-4.1.4"></a>IP telephones</h4></div></div></div><p>IP telephones are heralds <a id="I_indexterm2_tt181" class="indexterm"></a>of the most exciting change in the telecommunications
industry. 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 mobility solutions,
to purpose-built sets for particular industries, to flexible
all-in-one multimedia systems.</p><p>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.</p><p>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.</p><p>The future is far more promising.</p></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="asterisk-CHP-2-SECT-4.2"></a>Softphones</h3></div></div></div><p>A <span class="emphasis"><em>softphone</em></span> is a software<a id="I_indexterm2_tt182" class="indexterm"></a> 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 softphone 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?</p><p>The term <span class="emphasis"><em>softphone</em></span> can be expected to evolve
rapidly, as our concept of what exactly a telephone is undergoes a
revolutionary metamorphosis.<sup>[<a id="id4107324" href="#ftn.id4107324">38</a>]</sup> As an example of this evolution, consider the following:
would we correctly define popular communication programs such as Instant
Messenger as softphones? IM provides the ability to initiate and receive
standards-based VoIP connections. Does this not qualify it as a
softphone? 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, softphones are expected to look and sound like traditional phones,
that conception is likely to change in the very near future.</p><p>As standards evolve and we move away from the traditional
telephone and toward a multimedia communications culture, the line
between softphones and physical telephones will become blurred indeed.
For example, we might purchase a communications terminal to serve as a
telephone and install a softphone program onto it to provide the
functions we desire.</p><p>Having thus muddied the waters, the best we can do at this point
is to define what the term <span class="emphasis"><em>softphone</em></span> 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 softphone as 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”)<sup>[<a id="asterisk-CHP-2-FN-21" href="#ftn.asterisk-CHP-2-FN-21">39</a>]</sup> through E.164 addressing.<sup>[<a id="asterisk-CHP-2-FN-22" href="#ftn.asterisk-CHP-2-FN-22">40</a>]</sup></p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="asterisk-CHP-2-SECT-4.3"></a>Telephony Adaptors</h3></div></div></div><p>A <span class="emphasis"><em>telephony adaptor</em></span> <a id="I_indexterm2_tt183" class="indexterm"></a><a id="I_indexterm2_tt184" class="indexterm"></a>(usually referred to as an <a id="I_indexterm2_tt185" class="indexterm"></a><a id="I_indexterm2_tt186" class="indexterm"></a>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.</p><p>These adaptors could be described as gateways, for that is their
function. However, popular usage of the term <span class="emphasis"><em>telephony
gateway</em></span> would<a id="I_indexterm2_tt187" class="indexterm"></a> probably best describe a multiport telephony adaptor,
generally with more complicated routing functions.</p><p>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 modem—obsolescence through irrelevance.</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="asterisk-CHP-2-SECT-4.4"></a>Communications Terminals</h3></div></div></div><p><span class="emphasis"><em>Communications terminal</em></span> is<a id="I_indexterm2_tt188" class="indexterm"></a> 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 again—once it
becomes ubiquitous.</p><p>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 telephones—their proprietary nature
prevented them from connecting to the PSTN. They were therefore
called<a id="I_indexterm2_tt189" class="indexterm"></a><a id="I_indexterm2_tt190" class="indexterm"></a> <span class="emphasis"><em>terminals</em></span>, or
<span class="emphasis"><em>stations</em></span>. Users, of course, weren’t having any of
it. It looked like a telephone and acted like a telephone, and therefore
it <span class="emphasis"><em>was</em></span> a telephone. You will still occasionally
find PBX sets referred to as terminals, but for the most part they are
called telephones.</p><p>The renewed relevance of the term <span class="emphasis"><em>communications
terminal</em></span> has nothing to do with anything proprietary—rather,
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:</p><div class="itemizedlist"><ul type="disc"><li><p>If I use my PDA to connect to my voicemail and retrieve my
voice messages (converted to text), does my PDA become a
phone?</p></li><li><p>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?</p></li><li><p>If I use the IP phone in my kitchen to surf for recipes, is
that a phone call?</p></li></ul></div><p>The point is simply this: we’ll probably always be “phoning” each
other, but will we always be <a id="I_indexterm2_tt191" class="indexterm"></a>using<a id="I_indexterm2_tt192" class="indexterm"></a> “telephones”
to do so?</p></div><div class="footnotes"><br /><hr width="100" align="left" /><div class="footnote"><p><sup>[<a id="ftn.id4107084" href="#id4107084">35</a>] </sup>Citel has produced a fantastic product that is limited by
the fact that it is too expensive. If you have old proprietary PBX
telephones, and you want to use them with your Asterisk system,
Citel’s technology can do the job, but make sure you understand
how the per-port cost of these units stacks up against replacing
the old sets with pure VoIP telephones.</p></div><div class="footnote"><p><sup>[<a id="ftn.asterisk-CHP-2-FN-19" href="#asterisk-CHP-2-FN-19">36</a>] </sup>The SIP is currently the most well-known and popular
protocol for VoIP. We will discuss it further in <a href="asterisk-CHP-8.html" title="Chapter 8. Protocols for VoIP">Chapter 8, <i>Protocols for VoIP</i></a>.</p></div><div class="footnote"><p><sup>[<a id="ftn.id4107235" href="#id4107235">37</a>] </sup>If you are in North America, give up on this idea, unless
you have a lot of patience and money, and are a bit of a
masochist.</p></div><div class="footnote"><p><sup>[<a id="ftn.id4107324" href="#id4107324">38</a>] </sup>Ever heard of Skype?</p></div><div class="footnote"><p><sup>[<a id="ftn.asterisk-CHP-2-FN-21" href="#asterisk-CHP-2-FN-21">39</a>] </sup>OK, so you think you know what a phone call is? So did we.
Let’s just wait a few years, shall we?</p></div><div class="footnote"><p><sup>[<a id="ftn.asterisk-CHP-2-FN-22" href="#asterisk-CHP-2-FN-22">40</a>] </sup>E.164 is the ITU standard that defines how phone numbers are
assigned. If you’ve used a telephone, you’ve used E.164
addressing.</p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="asterisk-CHP-2-SECT-3.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="asterisk-CHP-2.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="asterisk-CHP-2-SECT-5.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Telephony Hardware </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Linux Considerations</td></tr></table></div><div xmlns="" id="svn-footer"><hr /><p>You are reading <em>Asterisk: The Future of Telephony</em> (2nd Edition for Asterisk 1.4), by Jim van Meggelen, Jared Smith, and Leif Madsen.<br />
This work is licensed under the <a href="http://creativecommons.org/licenses/by-nc-nd/3.0/">Creative Commons Attribution-Noncommercial-No Derivative Works License v3.0</a>.<br />
To submit comments, corrections, or other contributions to the text, please visit <a href="http://oreilly.com/catalog/9780596510480/">http://www.oreilly.com/</a>.</p></div></body></html>
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