data flow control layer. Layer 5 of the SNA architectural model. This layer determines and manages interactions between session partners, particularly data flow. Corresponds to the session layer of the OSI model. See also data link control layer, path control layer, physical control layer, presentation services layer, transaction services layer, and transmission control layer.
datagram. A logical grouping of information sent as a network layer unit over a transmission medium without prior establishment of a virtual circuit. IP datagrams are the primary information units in the Internet. The terms frame, message, packet, and segment are also used to describe logical information groupings at various layers of the OSI reference model and in various technology circles.
Datagram Delivery Protocol. See DDP.
data-link connection identifier. See DLCI.
data link control layer. Layer 2 in the SNA architectural model. This layer is responsible for the transmission of data over a particular physical link. Corresponds roughly to the data link layer of the OSI model. See also data flow control layer, path control layer, physical control layer, presentation services layer, transaction services layer, and transmission control layer.
data link layer. Layer 2 of the OSI reference model. This layer provides reliable transit of data across a physical link. The data link layer is concerned with physical addressing, network topology, line discipline, error notification, ordered delivery of frames, and flow control. The IEEE has divided this layer into two sublayers: the MAC sublayer and the LLC sublayer. Sometimes simply called link layer. Roughly corresponds to the data link control layer of the SNA model. See also application layer, LLC, MAC, network layer, physical layer, presentation layer, session layer, and transport layer.
data-link switching. See DLSw. Data Network Identification Code. See DNIC. data set ready. See DSR. data service unit. See DSU.
data sink. Network equipment that accepts data transmissions.
data stream. All data transmitted through a communications line in a single read or write operation.
data terminal equipment. See DTE. data terminal ready. See DTR. dB. decibels.
DB connector. data bus connector. A type of connector used to connect serial and parallel cables to a data bus. DB connector names are of the format DB-x, where x represents the number of wires within the connector. Each line is connected to a pin on the connector, but in many cases, not all pins are assigned a function. DB connectors are defined by various EIA/TIA standards.
DCA. Defense Communications Agency. A U.S. government organization responsible for DDN networks such as MILNET. Now called DISA. See DISA.
DCC. Data Country Code. One of two ATM address formats developed by the ATM Forum for use by private networks. Adapted from the subnetwork model of addressing, in which the ATM layer is responsible for mapping network layer addresses to ATM addresses. See also ICD.
DCE. data communications equipment (EIA expansion) or data circuit-terminating equipment (ITU-T expansion). The devices and connections of a communications network that comprise the network end of the user-to-network interface. The DCE provides a physical connection to the network, forwards traffic, and provides a clocking signal used to synchronize data transmission between DCE and DTE devices. Modems and interface cards are examples of DCE. Compare with DTE.
D channel. 1. data channel. Full-duplex, 16-kbps (BRI) or 64-kbps (PRI) ISDN channel. Compare with B channel, E channel, and H channel. 2. In SNA, a device that connects a processor and main storage with peripherals.
DDM. Distributed Data Management. Software in an IBM SNA environment that provides peer-to-peer communication and file sharing. One of three SNA transaction services. See also DIA and SNADS.
DDN. Defense Data Network. A U.S. military network composed of an unclassified network (MILNET) and various secret and top-secret networks. DDN is operated and maintained by DISA. See also DISA and MILNET.
DDP. Datagram Delivery Protocol. An Apple Computer network layer protocol that is responsible for the socket-to-socket delivery of datagrams over an AppleTalk internetwork.
DDR. dial-on-demand routing. A technique whereby a Cisco router can automatically initiate and close a circuit-switched session as transmitting stations demand. The router spoofs keepalives so that end stations treat the session as active. DDR permits routing over ISDN or telephone lines using an external ISDN terminal adapter or modem.
deadlock. 1. Unresolved contention for the use of a resource. 2. In APPN, when two elements of a process each wait for action by or a response from the other before they resume the process.
decibels. Abbreviated dB.
DECnet. Group of communications products (including a protocol suite) developed and supported by Digital Equipment Corporation. DECnet/OSI (also called DECnet Phase V) is the most recent iteration and supports both OSI protocols and proprietary Digital protocols. Phase IV Prime supports inherent MAC addresses that allow DECnet nodes to coexist with systems running other protocols that have MAC address restrictions. See also DNA.
DECnet routing. A proprietary routing scheme introduced by Digital Equipment Corporation in DECnet Phase III. In DECnet Phase V, DECnet completed its transition to OSI routing protocols (ES-IS and IS-IS).
decryption. The reverse application of an encryption algorithm to encrypted data, thereby restoring that data to its original, unencrypted state. See also encryption.
dedicated LAN. A network segment allocated to a single device. Used in LAN switched network topologies.
dedicated line. A communications line that is indefinitely reserved for transmissions, rather than switched as transmission is required. See also leased line.
de facto standard. A standard that exists by nature of its widespread use. Compare with de jure standard. See also standard.
default route. A routing table entry that is used to direct frames for which a next hop is not explicitly listed in the routing table.
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