Three important ISDN supplementary services are technically possible here because the
equipment and software packages were purchased, but they are not presently enabled.
The first is Multiple Subscriber Numbering (MSN), an unusual and very useful capabili-
ty which allows mapping up to eight separate telephone numbers to an ISDN PRI (BAI)
installation. In this way eight different TE1 or TE2 devices can each have a unique
directory number, for example, plain or digital telephone instruments or DUN ports on
a networked computer server. Each device will answer only its own incoming call.
Only two devices can be online simultaneously (since there are only two B channels)
but a small office could have for example one fax machine, one DUN computer server
port, and six telephone sets serving six different persons, each with a unique telephone
number.
Unfortunately TOT’s technical and legal staffs have been quarreling for years about
how to price this service (at press time still without a date for resolution), so this ad-
vanced and very useful technical capability remains unavailable.
A second useful but unexploited ISDN capability is called AO/DI (for Always
On/Dynamic ISDN). Currently a customer (for example a company using ISDN for
communication between branches or with a third party) must call the desired ISDN
number and hold up the circuit as long as data might be transmitted even though the
bursty nature of data transmissions means the circuit is mostly idle. AO/DI allows the
so-called Data or "D" channel (16 kbps, always on for control and signalling purposes)
to wake up a B channel upon demand. TOT has not allowed customers to use this
capability.
Third, the D-channel can be used for low-speed signalling (e.g. e-mail or Point of Sale
cardreader query) via the X.25 protocol, and this is one of the most popular commercial
applications for ISDN abroad. Again, the Thai ISDN network is technically capable of
supporting this useful application but TOT neither offers it nor allows TelecomAsia to
do so.
In short, Thailand’s ISDN hardware is as technically advanced as any in the world but
legal and management obstacles as usual prevent the nation from reaching its potential.
HARDWARE
If you want to use ISDN for data transmission such as corporate branch data transfers
or internet access, you will need a Terminal Adapter to convert the ISDN synchronous
datastream to the asynchronous protocol desktop computers employ.
The accompanying table presents representative Terminal Adapters now available (not
including routers and bridges, which are used principally in corporate networks) in
Bangkok. Many more devices are available, and new ones are coming all the time, so
check around for current offerings and prices. The TOT Marketing Department
provides a lengthy list of hardware vendors.
The point of the hardware table is to illustrate the important choices the buyer faces in
fulfilling his technical requirement and to give a ballpark idea of pricing by feature set.
The main choices to consider follow.
NT-1 or not You may buy or rent an NT-1 and connect a digital telephone directly to it.
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