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Data Service Unit (DSU)
A component that formats and controls data for transmission over digital lines. It is used in
conjunction with a Channel Service Unit (CSU).
Data Terminal Equipment (DTE)
A device
at the user end of a user−network interface
that serves as a data source, a destination, or both. These devices include computers, protocol
translators, and multiplexers.
datagram
Information groupings that are transmitted as a unit at the Network layer.
DB−9
A connector that has nine pins and is used for a serial−port or parallel−port connection between PCs
and peripheral devices.
DB−25
A connector that has 25 pins and is used for a serial−port or parallel−port connection between PCs
and peripheral devices.
dedicated line
Generally used in WANs to provide a constant connection between two points.
default gateway
Normally a router or a multihomed computer to which packets are sent when they are destined for a
host that’s not on their segment of the network.
demand node
Any end user or interface that requests and accesses network resources such as servers or printers.
destination address
The network address to which the frame is being sent. In a packet, this address is encapsulated in a
field of the packet so all nodes know where the frame is being sent.
Destination Service Access Point (DSAP)
A one−byte field in the frame that combines with the service access point (SAP) to inform the
receiving host of the identity of the destination host.
dialed number identification service
The method for delivery of automatic number identification using out−of−band signaling.
dial−up networking
The connection of a remote node to a network using POTS or PSTN.
diameter
A unit of measurement between the root switch and child switches, calculated from the root bridge
with the root bridge counting as the first switch. Each subsequent child switch out from the root
bridge is added to come up with the diameter number.
Digital Subscriber Line (DSL)
A public network technology that delivers high bandwidth over conventional copper wiring at limited
distances.
distributed switching
An implementation in which switching decisions are made at the local port or the line module.
Distribution layer
Functions as the separation point between the Core and Access layers of the network OSI Reference
Model. The devices in the Distribution layer implement the policies that define how packets are to be
distributed to the groups within the network.
domain
A logical grouping of interfaces in a network or intranet to identify a controlled network of nodes that
are grouped as an administrative unit.
Dual Attached Stations (DAS)
A connection that allows a device to connect to both FDDI counter−rotating rings.
dual−attachment concentrator (DAC)
A device connected to the FDDI counter−rotating rings. The DAC serves as a hub to provide passive
connections to the rings for peripheral devices.
dual−homed
A FDDI end station attached to two DACs for redundancy.
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