D4 Digital Channel Bank Family: Dataport - Digital Access Through D4

Today the Bell System T-carrier network has digital transmission facilities that serve most major cities in the United States. Since its introduction in 1962 this network has grown to include more than 400,000 channel banks and 80,000,000 channel miles of T-carrier repeatered lines. Digital facilities have enabled each new equipment generation to offer smaller size, less power dissipation, and more features. * Service mark of AT&T. 2703 To date these digital facilities have been used almost exclusively for voice-frequency (VF) channels. In general, data communication over these facilities is achieved with voiceband data modems that employ digital-to-analog (D/A) conversions to utilize the VF channels. Such analog techniques allow data rates of up to 9.6 kb/s over private line connections, but do not allow the full potential of the 64-kb/s T-carrier channel to be utilized. Moreover, the voiceband data terminals employ relatively complex modulation and detection schemes and essentially duplicate the analog-to-digital (A/D) processes performed by the Dchannel banks. There is a rapidly growing demand in the telecommunications industry to support high-speed digital data services. The Bell System, recognizing the inherent efficiencies in providing digital connectivity to serve the data communications market, began deploying the Digital Data System in 1974. By incorporating a family of dedicated data multiplexers (e.g., T1DM, T1WB4/5, SRDM) interconnected by digital facilities, the DDS eliminated the analog interface and the costly A/D hardware from data connections.