电信专业英英词典D-6（在线收听）

　　Direct Inward Dialing (DID)

　　DID is PBX-to-central office trunks that allow incoming calls to a PBX to ring specific stations without attendant assistance. DID greatly reduces the number of required console attendants, compared with systems in which all calls must be extended by console attendants.

　　Direct Outward Dialing (DOD)

　　DOD is PBX-to-central office trunks that allow outgoing calls to be placed directly by PBX stations.

　　Dispersion

　　In dispersive media, dispersion are complex signals are distorted because the various frequency components which make up the signal have different propagation characteristics and paths. Due to the finite conductivity of copper, wire or cable media for guided wave transmission is fundamentally dispersive. Dispersion limits the upper bit rate that a medium can support by distorting the signal waveforms to the extent that transitions from one information state to another cannot be reliably detected by receiving equipment, (e.g., logical l logical 0 value changes).

　　DLCI

　　data link connection identifier

　　DOD

　　direct outward dialing

　　DQDS

　　distributed queue dual bus

　　DS"N" Digital Signal Hierarchy

　　A time division multiplexed hierarchy of standard digital signals used in telecommunications systems. DS1 level in the hierarchy corresponds to a 1.544 Mbps TDM signal which comprises 24 DS0 signals. DS0 refers to individual digital signals at channel rates of 64 kbps. Four DS1 signals digitally multiplexed produce a DS2 level signal, containing 96 DS0 channels, and requires a transmission medium that supports 6.312 Mbps. A DS3 level signal results from the digital multiplexing of 7 DS2 signals, supports 672 DS0 signals and requires a 44.736 Mbps transmission medium. Finally a DS4 level signal supports 6 DS3 level signals, 4032 DS0 signals and requires a 274.176 Mbps transmission medium. The DS hierarchy accounts for non synchronism in the multiplexing plan, hence the term "asynchronous digital hierarchy" and the use of overhead bits. Note that bit rates at higher levels are not integer multiples of 64 kbps.

　　DSP

　　digital signal processor

　　DSS/BLF

　　direct station selection/busy lamp field

　　DSS

　　digital subscriber service