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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>The Digital Circuit-Switched Telephone Network</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-7.html" title="Chapter 7. Understanding Telephony" /><link rel="prev" href="asterisk-CHP-7-SECT-2.html" title="Digital Telephony" /><link rel="next" href="asterisk-CHP-7-SECT-4.html" title="Packet-Switched Networks" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">The Digital Circuit-Switched Telephone Network</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="asterisk-CHP-7-SECT-2.html">Prev</a> </td><th width="60%" align="center">Chapter 7. Understanding Telephony</th><td width="20%" align="right"> <a accesskey="n" href="asterisk-CHP-7-SECT-4.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-7-SECT-3"></a>The Digital Circuit-Switched Telephone Network</h2></div></div></div><p>For over a <a id="ch07_dcs" class="indexterm"></a>hundred<a id="I_indexterm7_tt1061" class="indexterm"></a> years, telephone networks were exclusively
circuit-switched. What this meant was that for every telephone call made,
a dedicated connection was established between the two endpoints, with a
fixed amount of bandwidth allocated to that circuit. Creating such a
network was costly, and where distance was concerned, using that network
was costly as well. Although we are all predicting the end of the
circuit-switched network, many people still use it every day, and it
really does work rather well.</p><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="asterisk-CHP-7-SECT-3.1"></a>Circuit Types</h3></div></div></div><p>In the PSTN, there <a id="I_indexterm7_tt1062" class="indexterm"></a>are many different sizes of circuits serving the various
needs of the network. Between the central office and a subscriber, one
or more analog circuits, or a few dozen channels delivered over a
digital circuit, generally suffice. Between PSTN offices (and with
larger customers), fiber-optic circuits are generally used.</p><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="asterisk-CHP-7-SECT-3.1.1"></a>The humble DS-0―the foundation of it all</h4></div></div></div><p>Since the standard<a id="I_indexterm7_tt1063" class="indexterm"></a> method of digitizing a telephone call is to record an
8-bit sample 8,000 times per second, we can see that a PCM-encoded
telephone circuit will need a bandwidth of eight times 8,000 bits per
second, or 64,000 bps. This 64 Kbps channel is referred to as a DS-0
(that’s “Dee-Ess-Zero”). The DS-0 is the fundamental building block of
all digital telecommunications circuits.</p><p>Even the ubiquitous analog circuit is sampled into a DS-0 as
soon as possible. Sometimes this happens where your circuit terminates
at the central office, and sometimes well before.<sup>[<a id="id4137669" href="#ftn.id4137669">97</a>]</sup></p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="asterisk-CHP-7-SECT-3.1.2"></a>T-carrier circuits</h4></div></div></div><p>The venerable T1 is<a id="I_indexterm7_tt1064" class="indexterm"></a> one of the more recognized digital telephony terms.
A<a id="I_indexterm7_tt1065" class="indexterm"></a> T1 is a digital circuit consisting of 24 DS-0s
multiplexed together into a 1.544 Mbps bitstream.<sup>[<a id="asterisk-CHP-7-FN-10" href="#ftn.asterisk-CHP-7-FN-10">98</a>]</sup> This bit stream is properly defined as a
<span class="emphasis"><em>DS-1</em></span>. Voice is encoded on a T1 using the μlaw
companding algorithm.</p><div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title"><a id="asterisk-CHP-7-NOTE-41"></a>Tip</h3><p>The European version of the T1 was developed by the<a id="I_indexterm7_tt1066" class="indexterm"></a><a id="I_indexterm7_tt1067" class="indexterm"></a> European Conference of Postal and Telecommunications
Administrations<sup>[<a id="asterisk-CHP-7-FN-11" href="#ftn.asterisk-CHP-7-FN-11">99</a>]</sup> (CEPT), and was first referred to as a
<span class="emphasis"><em>CEPT-1</em></span>. It is now called<a id="I_indexterm7_tt1068" class="indexterm"></a> an <span class="emphasis"><em>E1</em></span>.</p><p>The E1 is comprised of 32 DS-0s, but the method of PCM
encoding is different: E1s use alaw companding. This means that
connecting between an E1-based network and a T1-based network will
always require a transcoding step. Note that an E1, although it has
32 channels, is also considered a <span class="emphasis"><em>DS-1</em></span>. It is
likely that E1 is far more widely deployed, as it is used everywhere
in the world except North American and Japan.</p></div><p>The various other T-carriers (T2, T3, and T4) are multiples of
the T1, each based on the humble DS-0. <a href="asterisk-CHP-7-SECT-3.html#asterisk-CHP-7-TABLE-2" title="Table 7.2. T-carrier circuits">Table 7.2, “T-carrier circuits”</a> illustrates the relationships
between the different T-carrier circuits.</p><div class="table"><a id="asterisk-CHP-7-TABLE-2"></a><p class="title"><b>Table 7.2. T-carrier circuits</b></p><table summary="T-carrier circuits" border="1"><colgroup><col /><col /><col /><col /></colgroup><thead><tr><th align="left">
<p>Carrier</p>
</th><th align="left">
<p>Equivalent data
bitrate</p>
</th><th align="left">
<p>Number of DS-0s</p>
</th><th align="left">
<p>Data bitrate</p>
</th></tr></thead><tbody><tr><td align="left">
<p>
<span class="strong"><strong>T1</strong></span>
</p>
</td><td align="left">
<p>24 DS-0s</p>
</td><td align="left">
<p>24</p>
</td><td align="left">
<p>1.544 Mbps</p>
</td></tr><tr><td align="left">
<p>
<span class="strong"><strong>T2</strong></span>
</p>
</td><td align="left">
<p>4 T1s</p>
</td><td align="left">
<p>96</p>
</td><td align="left">
<p>6.312 Mbps</p>
</td></tr><tr><td align="left">
<p>
<span class="strong"><strong>T3</strong></span>
</p>
</td><td align="left">
<p>7 T2s</p>
</td><td align="left">
<p>672</p>
</td><td align="left">
<p>44.736 Mbps</p>
</td></tr><tr><td align="left">
<p>
<span class="strong"><strong>T4</strong></span>
</p>
</td><td align="left">
<p>6 T3s</p>
</td><td align="left">
<p>4,032</p>
</td><td align="left">
<p>274.176 Mbps</p>
</td></tr></tbody></table></div><p>At densities above T3, it is very uncommon to see a T-carrier
circuit. For these speeds, optical carrier (OC) circuits may be
used.</p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="asterisk-CHP-7-SECT-3.1.3"></a>SONET and OC circuits</h4></div></div></div><p>The Synchronous Optical Network (SONET) <a id="I_indexterm7_tt1069" class="indexterm"></a><a id="I_indexterm7_tt1070" class="indexterm"></a>was developed out of a desire to take the T-carrier
system to the next technological level: fiber optics. SONET is based
on the bandwidth of a T3 (44.736 Mbps), with a slight overhead making
it 51.84 Mbps. This is referred to as an <span class="emphasis"><em>OC-1</em></span> or
<span class="emphasis"><em>STS-1</em></span>. As <a href="asterisk-CHP-7-SECT-3.html#asterisk-CHP-7-TABLE-3" title="Table 7.3. OC circuits">Table 7.3, “OC circuits”</a> shows, all higher-speed OC
circuits are multiples of this base rate.</p><div class="table"><a id="asterisk-CHP-7-TABLE-3"></a><p class="title"><b>Table 7.3. OC circuits</b></p><table summary="OC circuits" border="1"><colgroup><col /><col /><col /><col /></colgroup><thead><tr><th align="left">
<p>Carrier</p>
</th><th align="left">
<p>Equivalent data
bitrate</p>
</th><th align="left">
<p>Number of DS-0s</p>
</th><th align="left">
<p>Data bitrate</p>
</th></tr></thead><tbody><tr><td align="left">
<p>OC-1</p>
</td><td align="left">
<p>1 DS-3 (plus
overhead)</p>
</td><td align="left">
<p>672</p>
</td><td align="left">
<p>51.840 Mbps</p>
</td></tr><tr><td align="left">
<p>OC-3</p>
</td><td align="left">
<p>3 DS-3s</p>
</td><td align="left">
<p>2,016</p>
</td><td align="left">
<p>155.520 Mbps</p>
</td></tr><tr><td align="left">
<p>OC-12</p>
</td><td align="left">
<p>12 DS-3s</p>
</td><td align="left">
<p>8,064</p>
</td><td align="left">
<p>622.080 Mbps</p>
</td></tr><tr><td align="left">
<p>OC-48</p>
</td><td align="left">
<p>48 DS-3s</p>
</td><td align="left">
<p>32,256</p>
</td><td align="left">
<p>2488.320 Mbps</p>
</td></tr><tr><td align="left">
<p>OC-192</p>
</td><td align="left">
<p>192 DS-3s</p>
</td><td align="left">
<p>129,024</p>
</td><td align="left">
<p>9953.280 Mbps</p>
</td></tr></tbody></table></div><p>SONET was created in an effort to standardize optical circuits,
but due to its high cost, coupled with the value offered by many newer
schemes, such as Dense Wave Division Multiplexing (DWDM), there is
some controversy surrounding its future.</p></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="asterisk-CHP-7-SECT-3.2"></a>Digital Signaling Protocols</h3></div></div></div><p>As with any circuit, it<a id="ch07_digitalsignal" class="indexterm"></a> is not enough for the circuits used in the PSTN to just
carry (voice) data between endpoints. Mechanisms must also be provided
to pass information about the state of the channel between each
endpoint. (Disconnect and answer supervision are two examples of basic
signaling that might need to take place; Caller ID is an example of a
more complex form of signaling.)</p><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="asterisk-CHP-7-SECT-3.2.1"></a>Channel Associated Signaling (CAS)</h4></div></div></div><p>Also known as robbed-bit signaling, CAS<a id="I_indexterm7_tt1071" class="indexterm"></a> is what you will use to transmit voice on a T1 when
ISDN is not available. Rather than taking advantage of the power of
the digital circuit, CAS simulates analog channels. CAS works by
stealing bits from the audio stream for signaling purposes. Although
the effect on audio quality is not really noticeable, the lack of a
powerful signaling channel limits your flexibility.</p><p>When configuring a CAS T1, the signaling options at each end
must match.<a id="I_indexterm7_tt1072" class="indexterm"></a><a id="I_indexterm7_tt1073" class="indexterm"></a><a id="I_indexterm7_tt1074" class="indexterm"></a> E&M (Ear & Mouth or recEive & transMit)
signaling is generally preferred, as it offers the best supervision.
Having said that, in an Asterisk environment the most likely reason
for you to use CAS would be for a channel bank, which means you are
most likely going to have to use FXS signaling.</p><p>CAS is very rarely used on PSTN circuits anymore, due to the
superiority of ISDN-PRI. One of the limitations of CAS is that it does
not allow the dynamic assignment of channels to different functions.
Also, Caller ID information (which may not even be supported) has to
be sent as part of the audio stream. CAS is commonly used on the T1
link in channel banks.</p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="asterisk-CHP-7-SECT-3.2.2"></a>ISDN</h4></div></div></div><p>The Integrated Services Digital Network (ISDN) <a id="I_indexterm7_tt1075" class="indexterm"></a><a id="I_indexterm7_tt1076" class="indexterm"></a>has been around for more than 20 years. Because it
separates the channels that carry the traffic (the bearer channels, or
B-channels) from the channel that carries the signaling information
(the D-channel), ISDN allows for the delivery of a much richer set of
features than CAS. In the beginning, ISDN promised to deliver much the
same sort of functionality that the Internet has given us, including
advanced capabilities for voice, video, and data transfer.</p><p>Unfortunately, rather than ratifying a standard and sticking to
it, the respective telecommunications manufacturers all decided to add
their own tweaks to the protocol, in the belief that their versions
were superior and would eventually dominate the market. As a result,
getting two ISDN-compliant systems to connect to each other was often
a painful and expensive task. The carriers who had to implement and
support this expensive technology, in turn, priced it so that it was
not rapidly adopted. Currently, ISDN is rarely used for much more than
basic trunking—in fact, the acronym ISDN has become a joke in the
industry: “It Still Does Nothing.”</p><p>Having said that, ISDN has become quite popular for trunking,
and it is now (mostly) standards-compliant. If you have a PBX with
more than a dozen lines connected to the PSTN, there’s a very good
chance that you’ll be running an ISDN-<span class="emphasis"><em>PRI</em></span>
(Primary Rate Interface) circuit. Also, in places where DSL and cable
access to the Internet are not available (or are too expensive), an
ISDN-<span class="emphasis"><em>BRI</em></span> (Basic Rate Interface) circuit might
provide you with an affordable 128 Kbps connection. In much of North
America, the use of BRI for Internet connectivity has been deprecated
in favor of DSL and cable modems (and it is never used for voice), but
in many European countries it has almost totally replaced analog
circuits.</p><div class="sect4" lang="en" xml:lang="en"><div class="titlepage"><div><div><h5 class="title"><a id="asterisk-CHP-7-SECT-3.2.2.1"></a>ISDN-BRI/BRA</h5></div></div></div><p>Basic Rate Interface (or Basic Rate Access) <a id="I_indexterm7_tt1077" class="indexterm"></a><a id="I_indexterm7_tt1078" class="indexterm"></a>is the flavor of ISDN, and is designed to service
small endpoints such as workstations.</p><p>The BRI flavor of the ISDN specification is often referred to
simply as “ISDN,” but this can be a source of confusion, as ISDN is
a protocol, not a type of circuit (not to mention that PRI circuits
are also correctly referred to as ISDN!).</p><p>A Basic Rate ISDN circuit consists of two 64 Kbps B-channels
controlled by a 16-Kbps D-channel, for a total of 144 Kbps.</p><p>Basic Rate ISDN has been a source of much confusion during its
life, due to problems with standards compliance, technical
complexity, and poor documentation. Still, many European telecos
have widely implemented ISDN-BRI, and thus it is more popular in
Europe than in North America.</p></div><div class="sect4" lang="en" xml:lang="en"><div class="titlepage"><div><div><h5 class="title"><a id="asterisk-CHP-7-SECT-3.2.2.2"></a>ISDN-PRI/PRA</h5></div></div></div><p>The Primary Rate Interface <a id="I_indexterm7_tt1079" class="indexterm"></a><a id="I_indexterm7_tt1080" class="indexterm"></a>(or Primary Rate Access) flavor of ISDN is used to
provide ISDN service over larger network connections. A Primary Rate
ISDN circuit uses a single DS-0 channel as a signaling link (the
D-channel); the remaining channels serve as B-channels.</p><p>In North America, Primary Rate ISDN is commonly carried on one
or more T1 circuits. Since a T1 has 24 channels, a North American
PRI circuit typically consists of 23 B-channels and 1 D-channel. For
this reason, PRI is often referred to as 23B+D.<sup>[<a id="asterisk-CHP-7-FN-12" href="#ftn.asterisk-CHP-7-FN-12">100</a>]</sup></p><div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title"><a id="asterisk-CHP-7-NOTE-42"></a>Tip</h3><p>In Europe, a 32-channel E1 circuit is used, so a Primary
Rate ISDN circuit is referred to as 30B+D (the final channel is
used for <span class="keep-together">synchronization</span>).</p></div><p>Primary Rate ISDN is very popular, due to its technical
benefits and generally competitive pricing at higher densities. If
you believe you will require more than a dozen or so PSTN lines, you
should look into Primary Rate ISDN pricing.</p><p>From a technical perspective, ISDN-PRI is always preferable to
CAS.</p></div></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="asterisk-CHP-7-SECT-3.2.3"></a>Signaling System 7</h4></div></div></div><p>Signaling System 7 (SS7) is<a id="I_indexterm7_tt1081" class="indexterm"></a><a id="I_indexterm7_tt1082" class="indexterm"></a> the signaling system used by carriers. It is
conceptually similar to ISDN, and it is instrumental in providing a
mechanism for the carriers to transmit the additional information ISDN
endpoints typically need to pass. However, the technology of SS7 is
different from that of ISDN; one big difference is that SS7 runs on a
completely separate network from the actual trunks that carry the
calls.</p><p>SS7 support in Asterisk is on the horizon, as there is much
interest in making Asterisk compatible with the carrier networks. An
open source version of SS7 (<a href="http://www.openss7.org" target="_top">http://www.openss7.org</a>) exists,
but work is still needed for full SS7 compliance, and as of this
writing it is not known whether this version will be integrated with
Asterisk. Another promising source of SS7 support comes from Sangoma
Technologies, which offers SS7 functionality in many of its
products.</p><p>It should be noted that adding support for SS7 in Asterisk is
not going to be as simple as writing a proper driver. Connecting
equipment to an SS7 network will not be possible without that
equipment having passed extremely rigorous certification processes.
Even then, it seems doubtful that any traditional carrier is going to
be in a hurry to allow such a thing to happen, mostly for strategic
and political <a id="I_indexterm7_tt1083" class="indexterm"></a>reasons.<a id="I_indexterm7_tt1084" class="indexterm"></a></p></div></div><div class="footnotes"><br /><hr width="100" align="left" /><div class="footnote"><p><sup>[<a id="ftn.id4137669" href="#id4137669">97</a>] </sup>Digital telephone sets (including IP sets) do the
analog-to-digital conversion right at the point where the handset
plugs into the phone, so the DS-0 is created right at the phone
set.</p></div><div class="footnote"><p><sup>[<a id="ftn.asterisk-CHP-7-FN-10" href="#asterisk-CHP-7-FN-10">98</a>] </sup>The 24 DS-0s use 1.536 Mbps, and the remaining .008 Mbps is
used by framing bits.</p></div><div class="footnote"><p><sup>[<a id="ftn.asterisk-CHP-7-FN-11" href="#asterisk-CHP-7-FN-11">99</a>] </sup>Conférence Européenne des Administrations des Postes et
des Télécommunications.</p></div><div class="footnote"><p><sup>[<a id="ftn.asterisk-CHP-7-FN-12" href="#asterisk-CHP-7-FN-12">100</a>] </sup>PRI is actually quite a bit more flexible than that, as it
is possible to span a single PRI circuit across multiple T1
spans. This can give rise, for example, to a 47B+D circuit
(where a single D-channel serves two T1s) or a 46B+2D circuit
(where primary and backup D-channels serve a pair of T1s). You
will sometimes see PRI described as nB+nD, because the number of
B- and D-channels is, in fact, quite variable. For this reason,
you should never refer to a T1 carrying PRI as “a PRI.” For all
you know, the PRI circuit spans multiple T1s, as is common in
larger PBX deployments.</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-7-SECT-2.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="asterisk-CHP-7.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="asterisk-CHP-7-SECT-4.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Digital Telephony </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Packet-Switched Networks</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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