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Glossary

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and flow control. The IEEE has further segmented
this layer into the MAC sublayer and the LLC sub-
layer. Also known as the Link layer. Can be compared
somewhat to the data link control 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 circuit-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, any-
thing that provides a connection between the proces-
sor and main storage with any peripherals.

DDP

Datagram Delivery Protocol: Used in the

AppleTalk suite of protocols as a connectionless pro-
tocol that is responsible for sending datagrams
through an internetwork.

DDR

dial-on-demand routing: A technique that

allows a router to automatically initiate and end a cir-
cuit-switched session per the requirements of the send-
ing station. By mimicking keepalives, the router fools
the end station into treating the session as active. DDR
permits routing over ISDN or telephone lines via a
modem or external ISDN terminal adapter.

Discard Eligibility: Used in Frame Relay net-

works 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 Com-
mitted Information Rate (CIR) is oversubscribed or set
to 0.

debug

The Cisco IOS command that provides the

administrator with low-level, detailed information
about processes that run on the router.

default route

The static routing table entry used to

direct frames whose next hop is not spelled out in the
dynamic routing table.

delay

The time elapsed between a sender's initiation

of a transaction and the first response they receive.
Also, the time needed to move a packet from its source
to its destination over a path. See also: latency.

demarc

The demarcation point between the cus-

tomer premises equipment (CPE) and the telco's car-
rier equipment.

demodulation

A series of steps that return a modu-

lated signal to its original form. When receiving, a
modem demodulates an analog signal to its original
digital form (and, conversely, modulates the digital
data it sends into an analog signal). See also: modula-
tion.

demultiplexing

The process of converting a single

multiplex signal, comprising more than one input
stream, back into separate output streams. See also:
multiplexing.

designated bridge

In the process of forwarding a

frame from a segment to the route bridge, the bridge
with the lowest path cost.

designated port

Used with the Spanning-Tree Pro-

tocol (STP) to designate forwarding ports. If there are
multiple links to the same network, STP will shut a
port down to stop network loops.

designated router

An OSPF router that creates

LSAs for a multi-access network and is required to
perform other special tasks in OSPF operations.
Multi-access OSPF networks that maintain a mini-
mum of two attached routers identify one router that
is chosen by the OSPF Hello protocol, which makes
possible a decrease in the number of adjacencies nec-
essary on a multi-access network. This in turn reduces
the quantity of routing protocol traffic and the physi-
cal size of the database.

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