Metroliner telephone article 4126

One of the issues that held back widespread use of ISDN was that typical digital trunks installed during the 80's were not 8 bit clean. That was mostly due to the use of Robbed Bit Signaling on E&M trunks rather than the relatively new Common Channel Signaling (No. 6, which has now been replaced with SS7). Hence ISDN required routing to use only whatever 8 bit clean trunks were actually available... if any. After about 1989 most newly installed trunk equipment was 8 bit clean, but of course it took a long time before old equipment was upgraded.

I'm not sure about the 4E switch, but the DMS-200 tandem switching systems also required a special ISDN Digital Trunk Controller to handle ISDN traffic, as the normal DTC could not. Hence inter exchange ISDN data calls were not always possible either, again until upgrades were installed.

Of course few if any upgrades were ever installed just to provide ISDN, because the telecom industry hadn't yet realized that the Internet existed, and nobody imagined that *every* residential customer would want a data connection.

Oddly, what most of that meant in practical terms was that any location enjoying modern digital trunking in the late 70's through the middle 80's, would not have been a good place for ISDN customers, while locations that came to digital late in the game would have been more likely to be able to support ISDN in the 90's.

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