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<h1 class="topictitle1">Integrated Services Digital Network</h1>
<div><p>Integrated Services Digital Network (ISDN) provides switched end-to-end
digital connectivity. Unlike other services, however, ISDN can carry both
voice and data over the same connection.</p>
<div class="section"><p>There are different types of ISDN services, with Basic Rate Interface
(BRI) being the most common. BRI consists of two 64 Kbps B channels to carry
customer data and a D channel to carry signaling data. The two B channels
can be linked together to give a combined rate of 128 Kbps. In some areas,
the telephone company may limit each B channel to either 56 Kbps or 112 Kbps
combined. There is also a physical constraint in that the customer location
must be within 5400 m (18 000 ft) of the central office switch. This distance
can be extended with repeaters. You can connect to ISDN with a device called
a terminal adapter. Most terminal adapters have an integrated network termination
unit (NT1) that allows direct connection into a telephone jack. Typically,
terminal adapters connect to your computer over an asynchronous RS-232 link
and use the AT command set for setup and control, much like conventional analog
modems. Each brand has its own AT command extension for setting
up parameters that are unique to ISDN. In the past, there were many interoperability
problems between different brands of ISDN terminal adapters. These problems
were due mostly to the variety of rate adaptation protocols that were in
V.110 and V.120 as well as bonding schemes for the two B channels.</p>
</div>
<div class="section"><p>The industry has now converged to synchronous PPP protocol with
PPP Multilink for linking two B channels. Some terminal adapter manufactures
integrate V.34 (analog modem) capability into their terminal adapters. This
enables customers with a single ISDN line to handle either ISDN or conventional
analog calls by taking advantage of the simultaneous voice/data capabilities
of ISDN services. New technology also enables a terminal adapter to operate
as the digital server side for 56K(X2/56Flex) clients.</p>
</div>
<div class="section"><p>Typically, you need to connect to an ISDN terminal adapter over
an RS-232 serial interface using asynchronous protocol at rates up to 230.4
Kbps. However, the maximum iSeries™ server baud rate for asynchronous over RS-232
is 115.2 Kbps. Unfortunately, this restricts the maximum byte transfer rate
to 11.5k bytes/sec, while the terminal adapter with multi-linking is capable
of 14/16k bytes uncompressed. Some terminal adapters support synchronous over
RS-232 at 128 Kbps, but iSeries server maximum baud rate for synchronous
over RS-232 is 64 Kbps. </p>
</div>
<div class="section"><p>The iSeries server
is capable of running asynchronous over V.35 at rates up to 230.4 Kbps, but
terminal adapter manufacturers generally do not offer such a configuration.
Interface converters that convert RS-232 to V.35 interface might be a reasonable
solution for the problem, but this approach has not been evaluated for the iSeries server.
Another possibility is to use terminal adapters with V.35 interface synchronous
protocol at rate of 128 Kbps. Although this class of terminal adapters exists,
it does not appear that many offer synchronous Multilink PPP.</p>
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<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="rzaiyconnalts.htm" title="PPP can transmit datagrams over serial point-to-point links.">Connection alternatives</a></div>
</div>
<div class="relref"><strong>Related reference</strong><br />
<div><a href="rzaiysw56.htm" title="When you do not need a full-time connection, you can save money by using switched digital service, which is generally called Switch-56 (SW56).">Switched-56</a></div>
<div><a href="rzaiyadapters.htm" title="ISDN provides you with a digital connection that allows you to communicate by using any combination of voice, data, and video, among other multimedia applications.">ISDN terminal adapters</a></div>
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