However it you want to transmit data, you may buy a Terminal Adapter which in-
cludes the NT-1, or buy one without and rent/buy the NT-1 from your ISDN provider.
Computer interface The big choices here are internal PCI card (least expensive, high
speed but limited capability, no R port), external USB (small, inexpensive, needs no
commercial power), external serial (consider whether your software platform is sup-
ported) or Ethernet (fast, more expensive, available in ISDN routers and bridges, cur-
rently Bht 15,000 and up and not shown in the hardware table).
If speed is important to you, devote special consideration to the interface issue. Internal
cards access system memory directly over the bus and so can operate at the full 128
kbps, as can USB devices. External Terminal Adapters operating over a serial port are
limited by the port’s speed of 115.2 kbps.
However the external adapter must convert data from synchronous to asynchronous
mode to communicate with the serial port, forcing it to insert start and stop bits and
dropping the top rate down to 94 kbps. This will not matter unless you bond two
channels and want to link at 128 kbps.
One workaround is a special coprocessed serial port (about US$125.00); another is an
ISDN router which provides access to the full ISDN bandwidth over the computer’s
Ethernet interface.
Ease of installation Most of the hardware comes with idiot-proof install routines but
since they are making better idiots every day, check carefully whether your hardware
and software are supported and even get an assurance from your hardware vendor to
install for you. For Windows most installs proceed uneventfully using PnP (Plug and
Pray).
Software-supported features ISDN includes many capabilities; ensure those you need
are supported, especially (if you need it) BOD/BACP which allows you to seize both B
channels to achieve up to a 128 kbps transfer rate, for example if you expect to make
lengthy file transfers. (One would select the Multilink PPP connection instead of the
PPP connection in Windows.)
Bundled software Some hardware comes with useful utilities to support telephone, fax,
answering machine, videophone and other tasks.
However this software is generally crippled to work only with the bundled hardware; it
must be upgraded at a fee to operate with other vendors’ hardware or to achieve the
most extensive feature set. (For example RVS-COM must be upgraded to the Standard
version to support videophone via USB.)
Don’t expect a lot from bundled voice or video software, which is available only for
Windows. When using internal speaker and microphone at any reasonable volume
level, calls suffer intolerable echo or oscillations. This is not the fault of ISDN but re-
sults from audible feedback at the audio-equipped computer, which is not adequately
handled (or handled at all) by local DSP echo cancellation. Using headphones or a
handset solves this problem, which then is replaced by the next: excessive line delay.
(Remember when you listened to Mission Control talking to the astronauts on the
moon?)
7
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