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Glossary

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Term Main definition
DB-25
Standard 25-pin connector used with serial and parallel ports.
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DB-9
Standard 9-pin connector used with Token Ring and serial connections.
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DC loop resistance
The total resistance of a conductor from the near end to the far end and back. For a single conductor, it is just the one-way measurement doubled. For a pair of conductors, it is the resistance from the near end to the far end on one conductor and from the far end to the near end on the other.
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Decibel (dB)
A standard unit used to express a relative measurement of signal strength or to express gain or loss in optical or electrical power. A unit of measure of signal strength, usually the relation between a received signal and a standard signal source. The decibel scale is a logarithmic scale, expressed as the logarithmic ratio of the strength of a received signal to the strength of the originally transmitted signal. For example, every 3dB equals 50 percent of signal strength, so therefore a 6dB loss equals a loss of 75 percent of total signal strength. See Chapter 1 for more information.
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Degenerate waveguides
A set of waveguides having the same propagation constant for all specified frequencies.
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Delay skew
The difference in propagation delay between the fastest and slowest pair in a cable or cabling system. See Chapter 1 for more information on delay skew.
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Delta
In fiber optics, equal to the difference between the indexes of refraction of the core and the cladding divided by the index of the core.
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Demand priority
A network access method used by Hewlett-Packard
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Demarcation point
A point where the operational control or ownership changes, such as the point of interconnection between telephone company facilities and a user
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Demodulate
To retrieve a signal from a carrier and convert it into a usable form.
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Demultiplex
The process of separating channels that were previously joined using a multiplexer.
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Demultiplexer
A device that separates signals of different wavelengths for distribution to their proper receivers.
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Dense wavelength division multiplexing (DWDM)
A form of multiplexing that separates channels by transmitting them on different wavelengths at intervals of about 0.8nm (100GHz).
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Detector
(1) A device that detects the presence or absence of an optical signal and produces a coordinating electrical response signal. (2) An optoelectric transducer used in fiber optics to convert optical power to electrical current. In fiber optics, the detector is usually a photodiode.
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Detector noise limited operations
Occur when the detector is unable to make an intelligent decision on the presence or absence of a pulse due to the losses that render the amplitude of the pulse too small to be detected or due to excessive noise caused by the detector itself.
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