Remembering ISDN | Hackaday

We are definitely spoiled these days in terms of Internet access. In much of the world gigabit speeds are common and even cheap plans are likely to be measured in 100s of megabits. But there was a time not long ago when a fast modem received at 56 kilobits per second. If you couldn’t justify a dedicated T1 line and you had a lot of money, you might have thought about ISDN – the Integrated Services Digital Network. [Tedium] has a great retrospective now that the UK has decided to sunset ISDN in 2025. ISDN started in the UK in the mid-1980s.

ISDN offered two 64-kilobit channels that could be bonded to reach 128 kilobits. There was also a slower third channel for commands and signaling (although you could use it for data, too, using an X.25-like protocol). If you wanted phone service, your voice was on one 64K channel and the data on the other. No need to tie up your phone just to get online. Voice was digitized at 8 kHz with 8 bits of G.711 encoding.


One problem with ISDN was that it took a while to appear in the market, while conventional modems kept getting faster and faster. Despite having a theoretical maximum of 64k per channel, in reality, it sometimes topped out at 56K, just like an analog modem because of limits in the T1 backbone in the central office. In addition, cross-talk from the send and receive made long runs difficult. Someone eventually realized that it was because of the similar frequencies in use and that most people didn’t need the same upload speed as download. That was the birth of ADSL, but that’s another story.

For whatever reason, ISDN never really caught on. A common industry joke was that it stood for Innovations Subscribers Don’t Need. The truth is, the tech did offer more reliable connections and better voice quality. But in the end, it just didn’t pan out. It did spawn ADSL, but in the end, it was relegated to niche markets. Video conferencing and broadcasters used the technology and, of course, the phone company made use of it in a different way to manage the backend. You can see an AT&T video about ISDN below.

It is hard to remember when getting on the Internet was a technical project. Don’t even get us started on T1 which still topped out at around 1.5 megabits per second.

By