In fact the connections between Central Offices are now all digital, allowing much more
efficient use of equipment and transmission paths. The insight behind ISDN was to
extend this all the way to the customer. As a result the same single pair of wires ena-
bled for BAI can now carry two phone numbers (on two "B" or "bearer" channels) in-
stead of one.
And one or both of these channels can be used for computer-to-computer communica-
tion (for example internet web browsing, corporate network communications, or video-
conferencing), at speeds up to 128 kbps.
This is nominally 2-1/2 times faster than the fastest analogue dialup speed but in reality
it is even faster because there are fewer errors on the now completely digital path. A
PRI installation enables 30 B channels, typically for business use.
Accomplishing this miracle of modern technology requires special equipment at the
customer’s premise, and understanding what is needed requires a picture.
The ISDN standard developed in 1984 defines 4 "interfaces" which appear physically as
jacks or "ports" on ISDN equipment:
This is the delivery-point of the phone company’s copper wire pair. In Europe it
is connected to a Network Termination 1 (NT-1), owned only by the phone
monopoly and rented to the customer, who never sees it. In Thailand both TOT and
TelecomAsia offer the U interface so the customer may rent the NT-1 (100 baht per
month), buy it (5,000 baht), or buy a Terminal Adapter (see below) which includes the
NT-1. (Many brands now do.)
The NT-1 presents two 8-pin RJ-45 phone jacks (physically identical to the con-
nectors and cabling used for 10BaseT Ethernet, larger than the 6-pin jack we
normally use for an analogue phone line), which are the S or T interface (technically dif-
ferent but in practice the same for us). Here one may plug in a special ISDN feature
phone offering numerous capabilities not readily available in standard analogue tele-
phone instruments, such as call forwarding, call barring by number, call conferencing,
multiple phone number identities, and multiple user profiles. These phone sets are
rather costly (5,000 to 10,000 baht) but for business or professional use they pay their
way. Up to eight devices may be connected to the S/T bus simultaneously using T
jacks. The NT-1 arbitrates among them allowing only two devices to access the two B
channels simultaneously. Devices connected directly to the S/T bus are called TE1.
Two telsets can be plugged in, and in the event of a power failure, one set will still
work, as the European standard which Thailand adopted requires emergency power to
be supplied to one telset from the Central Office. The NT-1 itself requires local com-
mercial power and so its other functions will not be available. Pins 1-2 and 7-8 of the
RJ-45 connector may be wired to provide emergency power.
If one wanted only phone service one could stop here. Two telsets can be plugged in
and both can be used simultaneously, appearing as either the same or different direc-
tory numbers to a called party depending upon programming.
However the fun just starts now. One can also buy a Terminal Adapter, a modem-like
device connecting a computer’s serial or USB port to the digital phone line. To the
computer it looks like a modem, and it allows high-speed digital access to the internet
or (by dialing another ISDN number) to another computer, for example for high-speed
file transfer or videoconferencing between company branches.
3
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