To understand how ISDN is able to provide 128Kb speeds, you must first understand the difference between ISDN and the more common analogue phone service. 

ISDN is a DIGITAL service.

Regular modems convert digital data into audible tones that are then sent down the line. At the other end, another modem converts the audible tones back into digital data.

To transmit data, however, it is far more efficient, and simpler to keep it in its original digital state. An ISDN line is a digital line - your computer connects to the ISDN line (via a terminal adaptor) without having to convert the data into sound first.

ISDN has two 64Kb 'channels' that may be used in various ways. The diagram above shows the home highway service. The 'Terminal adaptor' allows you to connect two standard telephones, each with their own number.

The analogue voices from the telephones are converted into digital data before being sent down the BT line.

In addition there are two digital ports. Each port can be connected to an ISDN card within the PC. The digital data is sent to and from the PCs without any conversion into audible tones.

Ways of using the two ISDN channels.

 You can use one channel to talk while you’re using the second channel to browse the Internet or send data files.  OR you can aggregate both lines into a single high-speed 128kbps link.

Connecting to the internet with ISDN is much quicker compared to a conventional modem. Typically it takes a second or two compared to 30 seconds with a regular modem.