Telecom Dictionary

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D type Channel Bank
A D type channel bank is channel termination equipment used for combining (multiplexing) 
individual analog channel signals on a time division basis. D type channel banks provide 
interfaces for "n" analog signal inputs. Each analog input signal is directed to a codec for en 
coding to PCM samples. A part of a Tl carrier system.

DACS 
digital access & cross-connect system

DARPA 
Defense Advanced Research Projects Agency

Data Compression
Data compression is a technique that removes redundancy in transmitted bit patterns to reduce 
transmission rates by 20% to 200%. For example, a modem designed to send and receive data 
at 1200 bps without data compression may be capable of supporting 2400 bps with data 
compression, using the same network analog voice-grade channel.

Data Service Units (DSU)
DSUs are channel service units (CSI's) and data service units (DSUs) are required to connect 
digital customer premises equipment (CPE ) to carrier networks. A hardware device providing an 
interface between a digital line and a unit of data terminal equipment. DSUs provide transmit and 
receive control logic, synchronization and timing recovery across data circuits. DSUs may also 
convert ordinary binary signals generated by CPE to special bipolar signals. Bipolar signals are 
designed specifically to facilitate transmission at up to 1.544 Mbps rates over UTP cable, a media 
originally intended for 3 kHz, voice bandwidth signals. See Channel Service Units (CSU).

Data Terminal Equipment (DTE)
DTE is any device that can send data, receive data or perform both functions. (Note: sometimes 
DTE implies digital terminal equipment, a type of CPE used with digital Service--see CSU and 
DSU.)

DB Loss
Decibel (dB) is a unit of measure of signal strength, usually the relationship between a 
transmitted signal and a standard signal source. Loss is measured when the signal received 
across a communication circuit is less than what was transmitted by the signal generator.

DCE 
data circuit terminating equipment

DCP 
digital communications protocol

DCS 
digital cross-connect system

DDD 
direct distance dialing

DDN 
Defense Data Network

Digit L
Digital L is another amplitude condition that represents a binary digit 0. See binary digit or bit.

Digital Carrier Systems
Digital Carrier Systems are carrier systems for digital signals that uses regenerative versus linear 
repeaters and time division multiplexing.

Digital Cross Connect System (DCS)
DCS is a new generation of switching/multiplex equipment that permits per-channel DS0 (64 
kbps) electronic cross-connection from one T1 transmission facility to another, directly from the 
constituent DSl signals. Commonly referred to as "DACS" (digital access and cross connect 
system), although this is a trademark of AT&T.

Digital Signal
A digital signal is a signal (electrical or otherwise) in which information is carried in a limited 
number of different (two or more) discrete states. The most fundamental and widely used form 
of the digital signal is binary, in which one amplitude condition represents a binary.

Digital Termination Service (DTS)
DTS is a service provided by some carriers permitting opera tors of private networks to use 
digital microwave equipment to gain access to carrier net works. The FCC has allocated a special 
microwave band for DTS.

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

DSU 
data service unit

DTMF 
dual tone multiple frequency

DTS 
digital termination service

Dual-Tone Multiple Frequency (DTMF)
the generic name for the tone signaling scheme used to signal from telephones to switching 
equipment, in which 10 decimal digits and two auxiliary characters are represented by selecting 
two frequencies of the following group: 697, 770, 852, 941,1209,1336,1447 Hz.