Goodbye and Good Riddance To ISDN

I used to teach data communications, and one of the most boring lectures I had to give was related to ISDN. And so am I rather pleased that ISDN is coming to an end (with BT ending sales in 2020). I even wrote chapters in books on ISDN:

For those who have only grown up on Ethernet, here is ISDN with its two base 64kbps data channels (B) and a 16kbps (D) signalling channel:

Yes. I did say that … 128 kbps.

Not, the superfast 100Mbps or 1Gbps Ethernet, but a fraction of that. But why 64kbps? Well, that is the base rate of digital speech. Basically, Nyquist said that you must sample at twice the highest frequency of the signal in order to fully recover the signal. And so if we assume that speech has a maximum frequency of around 4kHz, so the sample rate is 8,000 times per second (8kHz). If we take an 8-bit sample of the speech every 125 microSeconds (8,000 samples per second), we get a base rate of 64kbps. This then fitted into TDM (Time Division Multiplexing) systems, and where we bundled up many data…