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ISDN Or Modem ? A Beginner's Precis
(by Julian H. Stacey )
Experts should instead go to Dan Kegel's ISDN
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Introduction
Periodically I get asked about benefits of ISDN as against
staying analogue, so here's a few quick notes.
Info.
ISDN in Germany needs an NTBA (guess: network terminator block
adaptor) box in the house, which needs mains power (or draws
power from the in house exchange, which is plugged into the
mains power supply). If power fails: no phones work. If mains
fails, traditional phones on analogue lines still work, ISDN
does not.
With your existing exchange, if mains fails, I believe you
can just unplug the exchange & plug phones direct into line
socket ?
ISDN phones cost a lot, but can go direct on a wire coming
out of the NTBA, whereas your old analogue phones are obsolete
with ISDN, unless you connect them via a PBX with analogue in
house lines.
PBXs with ISDN local in house internal routing (intercom
function) are twice the price of analogue PBXs.
I think here ISDN costs about 160% of single analogue line
charge, why ? well phone co. saves money on wires. But, if you
have a break in the copper line, well, one pair of copper wires
carries both channels, ... one break ... = loss of both lines.
(There are 2 channels each of 64 kilobit/sec, controlled by a
signalling channel of around 8 kbit/s)
With ISDN in Germany one can have about 10 or so incoming
numbers even though there's only a channel capacity of 2
simultaneous calls. This abundance of numbers is used by your
PBX to route calls direct to "modem in loft", fax in garage",
"lodger in annex" or wherever else the large house/small firm
wants to allocate calls to, without need of a telephonist. In
DE, we used to get 3 numbers basic, extras were DM5/month, but
now free.
A modem running on an analogue line supplied by an ISDN PBX
will run (at best) at same speed as before, or quite possibly
slower, depending on quality of PBX. Only exception could be if
you were running over 10 Km of copper in the outback of Oz,
(where ISDN just might achieve better throughput). So be
careful of that "faster" claim.
If you want "faster", then ISDN will deliver up to 64 Kbit/s
now, (though Compuserve ISDN in Munich only runs at 32K I
think) & ISDN will never deliver more at any time in the
future; (except by channel bundling (128K), when both channels
are used, when the phone company will charge you for 2
simultaneous calls, & you can no longer receive or make a
phone call/fax for the duration of the data call.
Modems currently run at maybe 30 to 50 Kbit/s, but could in
future go beyond 64 Kbit/s on local calls, (where copper
connected & not multiplex restricted by the phone company
in the public exchange)
Note 32Kbps by synchronous ISDN is better than 32Kbps by
analogue modem: For Analogue modem speeds, bear in mind, for
each 8 data bits, about 3 packaging bits are wasted as start
stop parity bits.
With ISDN (that uses HDLC) bits are also used for packaging
but less than analogue on average.
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