ISDN test equipment verifies the communication services provided by ISDN equipment. The Integrated services digital network (ISDN) standard provides a guide for international telecommunications which connect customer telephone services to a telecommunications network through a digital mechanism. Typically, the primary interface between analog and digital hardware to digital lines is an ISDN terminal adapter (i.e. modem), which is capable of routing data, voice, and video transmissions to a digital device across the same circuit, irrespective if the output device is digital or analog. Standards defining the requirements for protocols commonly analyzed by ISDN testing equipment include ISO Q.921 and Q.931. With ISDN test equipment, these protocols operate commonly at the physical, data link, and network layers of the Open Systems Interconnection (OSI) reference model.