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Internal
and External ISDN Adapters
There
are two types of ISDN hardware adapters: internal and external.
Internal ISDN adapters are cards that you put inside your
PC. External adapters connect though a port on the back of
your PC.
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Internal
ISDN adapters can take the fullest advantage of your ISDN
line. These are cards that go inside your PC. Internal
adapters require you to open your PC to install the card.
In order to install an internal adapter, you need a slot
free in your PC that supports the same type of bus (ISA,
EISA, PCI, for example) as the card you want to install.
Look for ISDN adapters with the Windows logo, which support
Plug and Play, so Windows 95 and later versions can automatically
detect and configure the adapter for you. |
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External
ISDN adapters are easy to install and do not require any
special software, but they do not provide the same level
of performance that internal adapters do. External ISDN
adapters look just like a modem to the PC (some manufacturers
even call them "ISDN modems"). Communication
programs control the external ISDN modem just like they
control an analog modem (typically with AT commands).
An external ISDN adapter plugs into a PC's serial or parallel
port. |
There
are certain limitations imposed by these ports. Most PC serial
ports will not transmit information faster than 115 kilobits/second,
which is less than ISDN's maximum data speed of 128 kilobits/second.
These serial ports impose overhead on the transfer of information
between the PC and the external adapter, further slowing data
speeds. An external ISDN modem can impact performance of your
system, because an external adapter places heavy requirements
on the CPU. To use an external ISDN adapter, it is recommended
you have a Pentium or faster PC.
There
are also potential interoperability issues with external adapters
because higher level protocols like PPP or authentication
are implemented in the modem itself. These protocols are evolving
quickly and can be difficult to update in the modem for new
protocols or to fix bugs.
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