Metroliner telephone article 4136

You can't use the D channel as a dedicated circuit, but it can be used to carry X.25 packet data. A lot of ATM machines used to have that type of connectivity, but that's gone out of practice as dialup or DSL are much more cost-effective.

An ISDN BRI typically costs a lot more than two POTS lines, though most of the extra services people order on POTS lines are included in that price. And then there's the significant difference in CPE prices, the more complicated wiring for phone usage, etc.

A large part of the pricing problem is that ISDN was considered a "business" service by most-all of the Baby Bells, and business pricing is way out of the league of the average residential consumer. It was (and still is) also a nightmare to order, and you need a pretty savvy customer to get it working even in the absolute best case. I've installed hundreds of the BRIs all over the US, and even as a telecom engineer with years of practice I still rarely got one in five working within a week of install. Compare to DSL, which any idiot can install in minutes by themselves at a 90%+ success rate.

Even worse, ISDN is like T1 in that the pair must be conditioned -- no taps or loads on the line. Very, very few POTS lines are capable of supporting ISDN without sending a guy in a truck up and down the run between the CO and the customer premises to remove all the goofy stuff that accumulated over the years of POTS service on the pair, buttuming they can even find it all. DSL, OTOH, works even over very dirty pairs with taps and loads all over the place.

And then you have the relative bandwidth of DSL and ISDN -- my VDSL2 line at home runs 25Mb-s -- over 200 times faster than a bonded BRI.

They already owned the market. Why bother?

There's only two flavors of ISDN, BRI at 2B+1D and PRI at 23B+1D (or 30B+1D overseas). A PRI is a channelized T1-E1, except the last channel is used for signalling. Unfortunately, a PRI is often significantly more expensive than a CT1 in the US, and many equipment vendors charge twice as much for PRI interfaces (or the software license) as for CT1, even though they're the same physically. PRIs are remarkably easy to get working, though, if you can afford them.

Add on top of that that many of the Baby Bells charged per minute for ISDN usage for calls that would have been free on a POTS line, and ISDN was stillborn. Nowadays, where most data traffic is packet switched, nailing up a switched circuit isn't even justified technically; DSL is much cleaner.

S

PS: What does any of this have to do with trains?

-- Stephen Sprunk "God does not play dice." --Albert Einstein CCIE #3723 "God is an inveterate gambler, and He throws the K5SSS dice at every possible opportunity." --Stephen Hawking

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