ISDN testers verify 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 testers 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. ISDN testers such as ISDN analyzers can perform tests at several different levels. At the physical level, ISDN testers may be used by telecommunications installers to test wire pairs for impedance, continuity, and electrical loading. Error-free seconds and bit error rates are often tested in the data link layer to qualify local loops. Within the network layers, ISDN testers can usually be configured to test circuits for conformance, performance, and interoperability. Most network test equipment, including ISDN test equipment, provides for conformance testing to insure an ISDN device operates within standards and guidelines. Performance testing is a common feature of a PLC emulator, and is typically used to evaluate the effectiveness of a specified number of frames, packets, and messages between units over a given time benchmark. PLC emulators test the performance specifically on bearer (B) channels, delta/demand (D) channels, and high-frequency (H) channels as defined within the standards of ISDN services. Most ISDN testers are also designed to facilitate ease of interoperability testing by ensuring that ISDN devices can communicate with each according to specifications. Some suppliers of ISDN testers commonly refer to their equipment as ISDN emulators, which do not require an actual ISDN to be established. The cost of such ISDN test equipment is less prohibitive than full-fledged ISDN testers. Other systems for testing ISDN use fully-qualified test equipment designed to troubleshoot or evaluate real-time ISDN services. Specialized ISDN testers are also available.