Draw block diagram of functional architecture of ISDN and ISDN is a network concept providing a integration of data, voice and video. Its based on 64Kbps digital Communication channel. ISDN is a generic term for any network which connects homes and business together with a service companies such as bank, air-lines, stock market etc using a digital network. Integrated Services Digital Network - ISDN Definition and This diagram shows the different interfaces that are available in the integrated services digital network (ISDN). The two interfaces shown are BRI and PRI. These are all digital interfaces from the PSTN to the end customer's network termination. Network termination 1 (NT1) equipment devices can directly connect to the NT1 connection.