ISDN layers ISDN layers refer to the standards established to guide the manufacturers of ISDN equipment. The layers include both physical connections, such as wiring, and logical connections, which are programmed in computer software. When equipment is designed to the ISDN standard for one of the layers, it works with equipment for the layers above and below it. There are three layers at work in ISDN for Norstar. To support ISDN service, all three layers must be working properly.