ISDN, which stands for Integrated Services Digital Network, is a particular kind of digital phone and internet service that was the first technology to be introduced prior to ADSL or asynchronous digital subscriber line. The introduction of ADSL has largely made ISDN obsolete.
In a normal telephone line, analog signals are transmitted but the telephone company amplifies this signal and converts it into a digial signal. This is a process that actually results in a bit of lag and distortion to the signal. Examples of equipment that use analog signals include dial up modems and telephones. ISDN, on the other hand, uses digital signals that run through the existing copper lines that result in the significant increase in the throughput of data. Other benefits include the reduction of noise (or interference) and also the enhancement in the quality of the signal.
ISDN was a technology that was widely used in the mid 1990s because ADSL was still expensive and not widely available at that time. ISDN offered a faster method of accessing the internet, especially when compared to the then standard dial up connection. ISDN offered speeds of up to 128 kilobits per second. Compare this to the standard speed of about 30 to 53 kbps that is offered by a dial up modem.
The most prevalent type of ISDN service for the purpose of connecting to the internet is the Basic Rate Interface. This technology, which is also known as ISDN BRI, makes tow B-channels on the copper lines, with each channel capable of speed of 64 kbps. There is also a separate D-Channel for the phone line that carries 16 kbps. The separate channels mean that telephone and fax services can be used while still maintaining the connection online.