D

DARPA

Defense Advanced Research Projects Agency, a funding agency for the computer networking experiments performed over the ARPAnet.

database

A collection of data, generally on the same subject, stored in computer-readable form, and usually indexed or arranged in some other logical order. Computer or network users can use the index or logical arrangement to find a particular item of data that they desire.

data link level

The second level of the ISO layered model of communications protocols. The first level (physical level) performs the service of delivering data to this level, and this level performs the service of collecting the data into frames, for the benefit of higher levels.

data link relay

See bridge .

datagram

A method of transmitting messages in which sections of the message are allowed to be transmitted through the transmission system in scattered order and the correct order is reestablished by the receiving station. Contrast with virtual circuit .

DC voltages

Electrical signals sent at a constant level that is always positive (or always negative) relative to earth potential, in contrast to AC voltages, which continuously vary in equal amounts on either side of earth potential.

DDCMP

Digital Data Communications Message Protocol. In this data link layer protocol, message headers contain a character count indicating the size of the message. This permits the transmission of arbitrary character sequences in the message without concern that some may be interpreted as control sequences. The protocol may be used in serial or parallel, asynchronous, or synchronous transmission.

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