D


data direct VCC

A bidirectional point-to-point virtual control connection (VCC) set up between two LECs in ATM and one of three data connections defined by Phase 1 LAN Emulation. Because data direct VCCs do not guarantee QoS, they are generally reserved for UBR and ABR connections. Compare with: control distribute VCC and control direct VCC.

datagram

A logical collection of information transmitted as a Net- work layer unit over a medium without a previously established virtual circuit. IP datagrams have become the primary information unit of the Internet. At various layers of the OSI reference model, the terms cell, frame, message, packet, and segment also define these logical information groupings.

data link control layer

Layer 2 of the SNA architectural model, it is responsible for the transmission of data over a given physical link and compares somewhat to the Data Link layer of the OSI model.

Data Link layer

Layer 2 of the OSI reference model, it ensures the trustworthy transmission of data across a physical link and is primarily concerned with physical addressing, line discipline, network topology, error notification, ordered delivery of frames, and flow control. The IEEE has further 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 control layer of the SNA model. See also: Application layer, LLC, MAC, Network 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, 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 automatically initiate and end a circuit-switched session per the requirements of the sending station. By mimicking keep-alives, 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.

default route

The routing table entry used to direct frames whose next hop is not spelled out in the 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 customer premises equipment (CPE) and the telco’s carrier equipment.

demodulation

A series of steps that return a modulated 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: modulation.

demultiplexing

The process of converting a single multiplex signal, comprising more than one input stream, back into separate output streams. See also: multiplexing.

denial of service

(DoS) A type of network attack that can cause the receiving device to be non-responsive.

designated bridge

In the process of forwarding a frame from a segment to the route bridge, the bridge with the lowest path cost.

designated router

An OSPF router that creates LSAs for a multiaccess network and is required to perform other special tasks in OSPF operations. Multiaccess OSPF networks that maintain a minimum 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 necessary on a multiaccess network. This in turn reduces the quantity of routing protocol traffic and the physical size of the database.

destination address

The address for the network devices that will receive a packet.

DHCP

(Dynamic Host Configuration Protocol) A system based on network interface card addresses that is used to allocate IP addresses and other configuration information automatically for networked systems. DHCP is an update of the Bootstrap Protocol.

digital subscriber line (DSL)

DSL is a technology that allows ordinary PSTN copper to be used for high-speed data communications. In essence, it is a new way of using the existing infrastructure.

discontiguous addressing

Some routing protocols support the use of discontiguous addressing where different major network numbers are supported in a single network design.

discovery mode

Also known as dynamic configuration, this technique is used by an AppleTalk interface to gain information from a working node about an attached network. The information is subsequently used by the interface for self-configuration.

distance vector routing algorithm

In order to find a shortest-path spanning tree, this group of routing algorithms repeats on the number of hops in a given route, requiring each router to send its complete routing table with each update, but only to its neighbors. Routing algorithms of this type tend to generate loops, but they are fundamentally simpler than their link-state counterparts. See also: link-state routing algorithm and SPF.

distributed denial of service

(DDoS) A type of network attack that can cause the receiving device, devices or network to be non-responsive.

Distribution layer

The layer at which routing redistribution, network policies, and firewalls are placed and Internet access occurs in the Cisco three-layer hierarchical design model.

DLCI

Data-Link Connection Identifier: Used to identify virtual circuits in a Frame Relay network.

DMZ

(Demilitarized Zone) Used in many network designs and implementations, the DMZ usually includes publicly accessible servers.

DNS

Domain Name System: Used to resolve host names to IP addresses.

downtime

The amount of time during which a computer system is not available to users, because of a hardware or software failure.

DSAP

Destination Service Access Point: The service access point of a network node, specified in the destination field of a packet. See also: SSAP and SAP.

DSCP

Used by voice over IP QoS to define the Type of Service field of the IP packet as voice traffic.

DSR

Data Set Ready: When a DCE is powered up and ready to run, this EIA/TIA-232 interface circuit is also engaged.

DSU

Data Service Unit: This device is used to adapt the physical interface on a data terminal equipment (DTE) mechanism to a transmission facility such as T1 or E1 and is also responsible for signal timing. It is commonly grouped with the channel service unit and referred to as the CSU/DSU. See also: CSU.

DTE

Data Terminal Equipment: Any device located at the user end of a user-network interface serving as a destination, a source, or both. DTE includes devices such as multiplexers, protocol translators, and computers. The connection to a data network is made through data channel equipment (DCE) such as a modem, using the clocking signals generated by that device. See also: DCE.

DTR

Data Terminal Ready: An activated EIA/TIA-232 circuit communicating to the DCE the state of preparedness of the DTE to transmit or receive data.

DUAL

Diffusing Update Algorithm: Used in Enhanced IGRP, this convergence algorithm provides loop-free operation throughout an entire route’s computation. DUAL grants routers involved in a topology revision the ability to synchronize simultaneously, while routers unaffected by this change are not involved. See also: Enhanced IGRP.

DVMRP

Distance Vector Multicast Routing Protocol: Based primarily on the Routing Information Protocol (RIP), this Internet gateway protocol implements a common, condensed-mode IP multicast scheme, using IGMP to transfer routing datagrams between its neighbors. See also: IGMP.

DXI

Data Exchange Interface: Described in RFC 1482, DXI defines the effectiveness of a network device such as a router, bridge, or hub to act as an FEP to an ATM network by using a special DSU that accomplishes packet encapsulation.

dynamic addressing

A network interface device that receives its IP addresses and other configuration information automatically for networked systems.

dynamic routing

Also known as adaptive routing, this technique automatically adapts to traffic or physical network revisions.




CCDA. Cisco Certified Design Associate Study Guide
CCDA: Cisco Certified Design Associate Study Guide, 2nd Edition (640-861)
ISBN: 0782142001
EAN: 2147483647
Year: 2002
Pages: 201

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