There were some short lived quirky technologies at the end of the dial up era, such as using multiple 56k modems bonded into a single connection, the increase throughput. Since this required two telephone lines on both ends, in addition to four modems, only the most data hungry went to that degree of effort. Most simply got broadband when it was offered.
As someone that worked at a dial up ISP and had an ISDN connection at home, modem bonding was really fascinating. At the ISP side, these were all T1 lines, so every line was technically 64k capable. The analog 56k modems rarely made a true 56k connection. However, there were some customers that lived one telephone pole away from the CO, and we would see them connected at 57600. One close customer would show at some oddball speed like 58k or 59k (though we don't know what they actually got at their end).
But for ISDN, each channel was the full 64k and that would bond two channels to 128k. I had a cisco modem that would let me use either channel as an analog phone. So it would connect up both channels, and when a call came in, drop down to one 64k channel. It was incredibly reliable. Even when I got DSL, I kept the ISDN for quite a while because it had more consistent upload speed.
I was on ISDN for about a year, using a 3Com modem. Compared to dial-up, it was amazing as the connect time was only 3-4 seconds. No more listening to the buzzy connect sequence trying to guess what rate it would negotiate.
But something happened to the line and BellSouth could not get it working again. After a couple of rounds of "It passes our tests" and "It still doesn't work" I gave up and went to Time-Warner's RoadRunner cable internet.
My ISDN ISP was Uzi Nissan, of nissan-dot-com fame.
I worked on the drivers and firmware for a few ISDN cards back in 1997-1999. One of my performance optimizations for the BRI cards cut ping times from ~30ms to ~16ms. The Motorola MC68302's puny 68000 CPU took a long time to memcpy() packets from the buffer the HDLC controller would DMA them into back to the shared memory with the host. Eliminating that copy was more of an improvement than I expected.
The PRI (T1) cards using Intel i960 CPUs, which were quite nice embedded CPUs. The HDLC controller was capable of using individual 64k channels or bonding them together for fractional of full T1.
It was a bit late in the lifetime of ISDN, but I learned a heck of a lot about embedded systems from that job.
ISDN was the standard for "next generation" phone lines. It was never really meant for data. The design was to handle converted analog signals their new digital network. The demand for true broadband upended that plan, tough, and not it's just voice over much faster IP technologies.
