D4 Digital Channel Bank Family: Dataport - Channel Units for Digital Data System Subrates

The D4 channel bank can be equipped with over forty different channel units, each one designed as a customized transmission and signaling interface between the T-carrier transmission facility and the central office switch or local wire loop. These channel units are used for diverse applications, such as regular telephone grade voice band and radio program wide band, dial-pulse signaling and nonsignaling transmission only, switched service and private line, and analog and 2721 digital loop service. Dataports are the series of D4 channel units that allow a direct digital interface into the T-facility from the customer loop, bypassing the regular analog-to-digital conversion of voice channel units. As Ref. 1 describes, some dataports connect to the customer's location, while others link inside a telephone office. For each of the above two applications, two kinds of dataports exist, those that serve a customer using one T-facility time slot and those that use two time slots. This article describes the hardware implementation of the class of dataports that serves a customer at a data rate of 2.4, 4.8, or 9.6 kb/s, the so-called "subrate" dataports. A companion article details the designs for the class of dataports that serves a customer at 56 kb/s.2 The basic D4 bank functions are described in Ref. 3, while the system issues of dataport are fully treated in Ref. 1. This article briefly reviews the various applications and system issues of dataports. The basic hub office unit, the digital signal zero (DSO) dataport, is then described, as well as the basic end office dataport, known as the Office Channel Unit (ocu) dataport.