On Thu, 12 Mar 2009 05:02:28 +0000 (UTC), David Lesher wrote,
Well, since you asked politely... ;-)
To answer Lisa's question, ISDN is basically The Telephone Network (PSTN), with the subscriber connections digital rather than analog. The guts of the network are 64000 bps digital, so why use analog at the edges? And if it's digital, why use ancient signaling techniques (tones) which were designed to run over analog circuits? It's computers at both ends, so it makes more sense to use computer-type signaling protocols. That's ISDN in a nutshell, 64000 bps circuit switched telephony. You can use it for voice or data calls. In Europe, ISDN became very common for ordinary phones. And the T1/E1 speed ISDN PRI is common for PBXs, again more in Europe than the US but pretty common here.
DSL is not the PSTN at all. It is simply using the old copper wire to carry faster data. ADSL can run "atop" an analog line (high frequency), and in Europe, it runs atop ISDN. (The splitters in Europe have a higher split frequency.) For good measure, there's a rare flavor of DSL (MVL) that runs *in* the ISDN frequency range (20-160 kHz) *and* can run atop analog phones.
ISDN was conceived in the early 1980s as a way to complete the digitization of the PSTN, a logical evolution. And 64000 bps (both ways) beats the crap out of modems. (The retail BRI has two B channels so you can combine them to get 128,000 bps.)
But then we got the RBOCs. They were born in 1984, just after a failed and long-forgotten boomlet in "integrated voice-data terminals", which basically were dumb timesharing terminals with handsets. Bellcore and some RBOCs got the idea that ISDN was built for that, so the two B channels had to be one voice and one data channel. Of course it took years for ISDN to be available, by which time IVDTs were no more common than Hupmobiles, but instead of positioning it as a modem substitute, they positioned it as a LAN substitute (yeah, right, dial-up 64000 instead of Ethernet). BOCs rhymes with rocks, as in "dumb as"...
So while ISDN trials began in 1987 or so and it was commercially ready for volume deployment in the US by 1990 or so, the RBOCs screwed up. Bell Atlantic in particular was fanatical about Centrex, and ISDN provided them with a standardized Centrex featurephone, so ISDN was made available for Centrex, period.
A couple of years later, Internet went commercial, and ISDN was way better than the 28 kbps modems of the day, so it became the "killer app". BUT the Bells *hated* Internet with a purple passion (and still do). So while demand for ISDN soared, for Internet use, they associated ISDN demand with Internet demand and treated it as an evil to be stomped out. One way was to tariff it as measured-service only, or, in BA's case, sometimes flat rate for $240/month. In NYNEXland, it was hard to order, and "loop qualified" only about half the time. At least BA would clean up the loop if they decided to take the order; NYNEX used lousy plant as a way to avoid it.
Eventually modems got faster and the Internet caught on even bigger. Modems actually cost the Bells more to support than ISDN, but since POTS lines were also used for voice, they accepted them. And once DSL caught on, ISDN declined. I switched from ISDN to a cable modem about a decade ago, after putting in cable telephone service, so I've been Bell-free since then.
In most places ISDN BRI has been de-tariffed (grandfathered) so you can't even order it. Modern US-market switches don't even support it; only the ILECs have it in their "legacy" (antique) 5ESS and DMS-100 switches. PRI is however very common, especially from CLECs; many VoIP networks use CLEC PRI to connect their gateways to the PSTN. That has recycled many of the PRIs formerly used for modems.
-- Fred Goldstein k1io fgoldstein "at" ionary.com ionary Consulting