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<td bgcolor="#336666"><font face="Arial" size="2" color="#FFFFFF"><b>4.1</b></font></td>
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<td bgcolor="#336666" width="100%"><strong>
<font face="Arial" size="2" color="#FFFFFF">ISDN Concepts</font></strong></td>
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<td bgcolor="#669999" height="25"><b>
<font face="Arial" size="2" color="#FFFFFF">4.1.1</font></b></td>
<td bgcolor="#669999"><img border="0" src="../../images/transdot.gif" width="10" height="1"></td>
<td bgcolor="#669999" height="25" width="100%"><strong>
<font face="Arial" size="2" color="#FFFFFF">Introducing ISDN</font></strong></td>
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<font face="Arial" size="2" COLOR="#000000">
There are several WAN technologies used to provide network access from
remote locations. One of these technologies is ISDN. </font>
<font face="Arial" size="2">ISDN can be used as a solution to the low
bandwidth problems that small offices or dial-in users have with
traditional telephone dial-in services.</font><p>
<font FACE="Arial" SIZE="2" COLOR="#000000">
<font face="Arial" size="2">
The traditional PSTN was based on an analog connection between the
customer premises and the local
exchange, also called the local loop. </font>
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"><font face="Arial" size="2">
The analog circuits introduce limitations on the bandwidth that can be
obtained on the local loop. Circuit restrictions do not permit analog
bandwidths greater than approximately 3000 Hz. ISDN technology
permits the use of digital data on the local loop, providing better
access speeds for the remote users.
<img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12"></font></font></p>
<p>
<font FACE="Arial" SIZE="2" COLOR="#000000">
<font face="Arial" size="2">
Telephone companies developed ISDN with the intention of creating a
totally digital network. ISDN allows digital signals to be transmitted
over existing telephone wiring. This became possible when the
telephone company switches were upgraded to handle digital signals.
ISDN is generally used for telecommuting and networking small and
remote offices into the corporate LAN.</font></font></p>
<p>
<font face="Arial" size="2" COLOR="#000000">
Telephone companies developed ISDN as part of an effort to standardize
subscriber services. This included the User-Network Interface (UNI),
better known as the local loop.
<font face="Arial" size="2">
The ISDN standards define the hardware and call setup schemes for
end-to-end digital connectivity. These standards help achieve the goal
of worldwide connectivity by ensuring that ISDN networks easily
communicate with one another. In an ISDN network, the digitizing
function is done at the user site rather than the telephone company.</font></font></p>
<p>
<font face="Arial" size="2" COLOR="#000000">
ISDN brings digital connectivity to local sites. The following list
provides some of the benefits of ISDN:
<img border="0" src="../../images/3.gif" align="absmiddle" width="12" height="12"></font></p>
<ul>
<li>
<font face="Arial" size="2" COLOR="#000000">
Carries a variety of user traffic signals, including data, voice,
and video </font></li>
<li>
<font face="Arial" size="2" COLOR="#000000">
Offers much faster call setup than modem connections </font></li>
<li>
<font face="Arial" size="2" COLOR="#000000">
B channels provide a faster data transfer rate
than modems </font></li>
<li>
<font face="Arial" size="2" COLOR="#000000">
B channels are suitable for negotiated Point-to-Point Protocol (PPP)
links </font></li>
</ul>
<p>
<font FACE="Arial" SIZE="2" COLOR="#000000">
<font face="Arial" size="2">
ISDN is a versatile service able to carry voice, video, and data
traffic. It is possible to use multiple channels to carry different
types of traffic over a single connection.</font></font></p>
<p>
<font face="Arial" size="2" COLOR="#000000">
ISDN uses out-of-band signaling, the delta (D channel), for call setup
and signaling. To make a normal telephone call, the user dials the
number one digit at a time. Once all the numbers are received, the
call can be placed to the remote user. ISDN delivers the numbers to
the switch at D-channel rates, thus reducing the time it takes to set
up the call.</font></p>
<p>
<font face="Arial" size="2" COLOR="#000000">
ISDN also provides more bandwidth than a traditional 56 kbps dialup
connection. ISDN uses bearer channels, also called B channels, as
clear data paths. Each B channel provides 64 kbps of bandwidth. With
multiple B channels, ISDN offers more bandwidth for WAN connections
than some leased services. An ISDN connection with two B channels
would provide a total usable bandwidth of 128 kbps.</font></p>
<p>
<font face="Arial" size="2" COLOR="#000000">
Each ISDN B channel can make a separate serial connection to any other
site in the ISDN network. Since PPP operates over both synchronous and
asynchronous serial links, ISDN lines can be used in conjunction with
PPP encapsulation.</font></p>
<p>
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<p><font color="#808080">
<img border="0" src="../../images/links_icon.gif" width="25" height="25">
<b>Web Links</b></font></p>
</font><font FACE="Arial" SIZE="2" COLOR="#808080">
<p>Integrated Services Digital Network (ISDN)<p>
<a target="_blank" href="http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/isdn.htm">
http://www.cisco.com/univercd/cc/
td/doc/cisintwk/ ito_doc/ isdn.htm</a></font></p>
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