A feature that allows inward directed calls to a PBX to reach a specific PBX extension, this process is automated. Dialing within the network

Digital

Digital communication allows higher speeds of transmission of data with a lower rate of error than is possible with analog technology.  A digital signal is detected and regenerated and amplified, any "line" noise is eliminated by digital regeneration.

DOD

Direct Outward Dialing
Automated PBX feature that provides  outgoing calls to be dialed directly from the end user terminal.  Dialing out the network

DS

Digital Signal

DS1

Digital Signal 1
A digital signaling rate of 1.544 Mb/s

DSL

Digital Subscriber Line.  A high speed broadband end to end connection.  Runs on existing infrastructure and lines, and  is a means to integrate voice and data communications.

Facilities-Based Carrier - FBC

Carriers that use their own facilities to provide services.

FCC - Federal Communications Commission

Government Agency that regulates interstate communications: licenses, rates, tariffs, standards, limitations, etc.

Fiber Optic Cable

Cable in which one or more optical fibers (a filament of transparent dielectric material, usually glass or plastic) are used as the propagation medium

Frame Relay