The ISDN Internet connection was quite remarkable, so it definitely deserves a separate article for explanation of all of its details
Development of ISDN
Generally a standard telephone connection is made up of many pieces working together. It goes something like, the phone line goes from your phone out to the network interface box, which was typically installed on the side or on the pole around your house and a central switch which belonged to the telephone company.
In a standard metropolitan areas, a large number of central offices exist with a central switch. All of these central switches connect with each other through a high-capacity trunk lines. There was a time when the entire phone line was analogue until 1970. Over time phone companies upgraded their trunk lines to digital systems. Today, the whole telephone system is digital.
Since the old telephone line could not produce more than 28. kbps, over time, customers began to demand higher throughput. All the phone companies were quite hyped up to come up with a new solution and address the demand. They quickly found out that the answer was there already. Which was to adapt the digital system.
They began to install new and special gears, which facilitated the demand up to 64 Kbps per line, which was over the same copper wires that was already in use by the telephone lines. So, all of these together, meaning, the process of sending telephone transmission across full-fledged digital lines which needs to be end-to-end is called the Integrated Services Digital Network (ISDN).
ISDN channels
Every ISDN service consists of two types of channels. Bearer (B) and Delta (D) channels. Channel B is used to carry data and voice information at 64 kbps. While channel D is responsible to carry the setup and information of configuration data at 16 Kbps. Either one or two of the B channels are allowed to choose by the ISDN providers. One of the most common kind of setup is two of the Bs and one D, which was called a basic rate interface (BRI). The idea with this was, this setup used only one physical line but the catch was, each of the B channel would send 64 kbps which to sum up, doubled the throughput, at 128 kbps.
ISDN capacities
ISDN connects much faster than modems and eliminates the long, boring dial-up with phone modems. Monthly cost is certainly expensive than a phone line and also the installation and configuration cost was quite high. The most common issue this technology faced was the limitation. Anyone wanted the connection needed to be at least, 18,000 feet within the proximity of the central office who provided the ISDN.
Physical connection for the ISDN is almost similar to one of those analogue modems. ISDN wall sockets looks something like of a standard RJ-45 jack. As for your computer, the most common interface is a device called terminal adapter (TA). TA looks almost like regular modems and like any of the modems, they come in external as well as internal variants. There are also TN which would directly connect to the LAN.
Upgrade to 128kbps was a huge deal in times like 1990s. Today, we have super fast connection with greater throughput with DSL and cables. ISDN is rare to none, is found out in the wild other than the place where they need heavy security like of the military.