ISDN stands for Integrated Services Digital Network. ISDN is a method of sending voice and data information on a digital phone line. Two 64Kbps B-channels with one 16Kbps D channel is provided with basic ISDN service. The D channel is normally used for control signals but can be used to carry data. The B channels carry data.
Basic ISDN service is called basic rate access (BRA) and primary ISDN service is called primary rate access (PRA). Primary ISDN service has 2 D channels and a number of B channels that vary from country to country.