T-1 and ISDN are service lines, which support telecommunication equipments and phone systems that, provide voice and data services (Shepler, 2003). T-1 differs greatly from ISDN, in terms of circuit configuration. ISDN (Integrated Services Digital Network) is divided into two levels, in which, voice and data can be sent and received in a single fiber-optic cable. These levels are BRI and PRI (Shepler, 2003). BRI (Basic Rate Interface) is the most common level, whose data or voice transmission speeds are low. It therefore supports residential and small businesses data and voice needs. On the other hand, PRI (Primary Rate Interface) transmits data and voice at very high speeds. It is usually carried in T-1 trunk lines whereby, it divides T-1 digital signal into 24 different channels. PRI is suitable for large businesses and large institutions, which have large data and voice requirements (Shepler, 2003).
T-1 service, which is actually more of hardware than a service, is used to support PRI service in ISDN (Shepler, 2003). This support involves helping PRI to deliver data and voice services to different locations. A single T-1 line consists of 24 channels. The channels can be used separately: each with its own purpose, or in combination. For instance, all the 24 channels can be used to deliver individual telephone threads, bandwidth for data networks, or internet connection to a given location. Alternatively, the 24 channels may be split into two groups, either to deliver data or voice, in any combination. When used to deliver bandwidth data networks, each T-1 thread is capable of transmitting data at a speed of 64 Kilobytes per second. This means that, a T-1 line, which is fully dedicated to data delivery, is capable of transmitting data at a speed of 1.54 Megabytes per second (Shepler, 2003).