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Glossary

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Data Link layer

Layer 2 of the OSI reference model, it ensures the trust-

worthy transmission of data across a physical link and is primarily con-
cerned with physical addressing, line discipline, network topology, error
notification, ordered delivery of frames, and flow control. The IEEE has fur-
ther segmented this layer into the MAC sublayer and the LLC sublayer. Also
known as the Link layer. Can be compared somewhat to the data link con-
trol layer of the SNA model. See also: Application layer, LLC, MAC, Net-
work layer, Physical layer, Presentation layer, Session layer, and Transport
layer.

DCC

Data Country Code: Developed by the ATM Forum, one of two ATM

address formats designed for use by private networks. Compare with: ICD.

DCE

data communications equipment (as defined by the EIA) or data cir-

cuit-terminating equipment (as defined by the ITU-T): The mechanisms and
links of a communications network that make up the network portion of the
user-to-network interface, such as modems. The DCE supplies the physical
connection to the network, forwards traffic, and provides a clocking signal
to synchronize data transmission between DTE and DCE devices. Compare
with: DTE.

D channel

1) Data channel: A full-duplex, 16Kbps (BRI) or 64Kbps (PRI)

ISDN channel. Compare with: B channel, E channel, and H channel. 2) In
SNA, anything that provides a connection between the processor and main
storage with any peripherals.

DDP

Datagram Delivery Protocol: Used in the AppleTalk suite of protocols

as a connectionless protocol that is responsible for sending datagrams
through an internetwork.

DDR

dial-on-demand routing: A technique that allows a router to auto-

matically initiate and end a circuit-switched session per the requirements of
the sending station. By mimicking keepalives, the router fools the end station
into treating the session as active. DDR permits routing over ISDN or tele-
phone lines via a modem or external ISDN terminal adapter.

Discard Eligibility: Used in Frame Relay networks to tell a switch that

a frame can be discarded if the switch is too busy. The DE is a field in the
frame that is turned on by transmitting routers if the Committed Informa-
tion Rate (CIR) is oversubscribed or set to 0.

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