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ISDN Or Modem ? A Beginner's Precis
(by Julian H. Stacey )

Experts should instead go to Dan Kegel's ISDN Page


Periodically I get asked about benefits of ISDN as against staying analogue, so here's a few quick notes.


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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