The Metropolitan Digital Truck Plant

The metropolitan trunk plant consists of a combination of central office equipment and outside plant cables and electronics. These provide a collection of voice-frequency (VF) circuits to interconnect central offices. At first these circuits were provided solely by loaded VF wire pairs in underground cables located in ducts. As the network grew and traffic increased, ducts became congested and ways were developed to provide additional circuits on the same wire pairs. In 1962, T1 carrier was introduced to provide these circuits and now accounts for a large fraction of the metropolitan trunk plant capacity. In 1975, TIC, operating at twice the bit rate of Tl, was introduced and provides a growing fraction of the plant capacity. The T-carrier plant now contains approximately 160,000 T-carrier systems consisting of some 6,000 interoffice spans providing interoffice connections throughout metropolitan areas. Each T-carrier system provides 24 voice circuits by means of a digital channel bank at each end interconnected by a T-carrier span line (or several span lines in tandem traversing intermediate offices). A D-channel bank converts the 24-voice circuit inputs to a DSl-level, 1.544-Mb/s pulse stream, which is transmitted over the T-carrier span line and is reconverted to 24 voice circuits by the channel bank at the other end. TIC provides 48 channels at the DS1C level, 3.152 Mb/s. The recently introduced METROPOLITAN TRUNK PLANT 933