I've begun hacking on a lot of ISDN equipment that I've purchased (phones, terminal adapters, protocol analyzers, etc.) One thing that's struck me odd is that a lot of the equipment notes it compatible with specific PBX, or Switch models (i.e. ESS5, Nortel) as opposed to claiming compatibility with DSS1 or something. This strikes me as odd, because if ISDN is standardized from the physical layer up, one would expect the switches to be mostly running on these standards and terminal devices to have at least a baseline of compatibility with any standard switch.

I only have one possible hunch. I recall a story from someone who had decided to build their own professional TCP/IP stack. They discovered later on that the modern internet did not quite fit into the RFCs but also relied on a ton of unofficial extensions. Perhaps there was a similar sort of thing going on with ISDN networks?

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    Mind to add what country and/or networks that question is about/related to? Over all the years I used (and developed) ISDN hardware (in Europe/Germany) I never seen any of this.
    – Raffzahn
    Feb 27, 2021 at 22:31
  • Sorry, this is likely to be North American equipment. The one European device I have does note EuroDSS1 compatibility.
    – an earwig
    Feb 27, 2021 at 22:54
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    Would you mind adding that to the question? Beside being always a god idea, it might be especially helpful here - RC.Se is a worldwide success, isn't it?
    – Raffzahn
    Feb 27, 2021 at 23:09
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    DSS1 (and later the pan-European EDSS1) were never formally "international standards" and the USA never recognized them. Hence ISDN never became as widespread in the USA as in Europe and non-European countries which adopted DSS1 as a standard.
    – alephzero
    Feb 27, 2021 at 23:13
  • @alephzero, that sounds like an answer, rather than a comment to improve the question. Even if that's all you write, it's still worth copying into the Answer box! Feb 28, 2021 at 17:25

1 Answer 1


From memory (it was a few decades ago...) the ISDN situation in North America was not particularly uniform. Some of the big switch vendors (like AT&T) had flavors of ISDN that were kind of mostly compatible with the standards but had differences that the devices needed to specifically know about (I don't know what the differences actually were). And since end users had little or no say in what kind of switch they were connected to, the device vendors had no choice but to accommodate the various switch incompatibilities.

As mentioned in the comments, the situation in Europe was much more uniform and ISDN was widely used there, at least compared to North America. Then ADSL and cable came along and eliminated any further need for anybody to think much about ISDN.

  • Well, DSL in general--ADSL for consumers, SDSL for a fair number of companies running servers and such. Mar 1, 2021 at 5:56

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