Glossary E

D

data compression

See: compression.

data direct VCC

A bidirectional point-to-point virtual channel 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. See also: control direct VCC and control distribute VCC.

data encapsulation

The process in which the information in a protocol is wrapped, or contained, in the data section of another protocol. In the OSI reference model, each layer encapsulates the layer immediately above it as the data flows down the protocol stack.

data frame

Protocol Data Unit encapsulation at the Data Link layer of the OSI reference model. Encapsulates packets from the Network layer and prepares the data for transmission on a network medium.

datagram

A logical collection of information transmitted as a Network 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 to: 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 to: DTE.

D channel

(1) data channel: A full-duplex, 16Kbps (BRI) or 64Kbps (PRI) ISDN channel. Compare to: 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 enables 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.

DE

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 Information Rate (CIR) is oversubscribed or set to 0.

DE bit

The DE bit marks a frame as discard eligible on a Frame Relay network. If a serial link is congested and the Frame Relay network has passed the Committed Information Rate (CIR), then the DE bit will always be on.

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 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. Contrast with: multiplexing.

denial-of-service attack

A denial-of-service attack, or DoS, blocks access to a network resource by saturating the device with attacking data. Typically, this is targeted against the link (particularly lower-bandwidth links) or the server. DDoS attacks, or distributed denial-of-service attacks, make use of multiple originating attacking resources to saturate a more capable resource.

designated bridge

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

designated ports

Used with the Spanning Tree Protocol (STP) to designate forwarding ports. If there are multiple links to the same network, STP will shut down a port 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 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 multi-access 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.

dial backup

Dial backup connections are typically used to provide redundancy to Frame Relay connections. The backup link is activated over an analog modem.

differentiated services

The name applied to the mechanism whereby routers in an internetwork forward data in queues configured to respond to specific bits-the Differentiated Services Code Points (DSCP)-in an IP header.

DiffServe

See: differentiated services.

digital

A digital waveform is one in which distinct ones and zeros provide the data representation. See also: analog.

directed broadcast

A data frame or packet that is transmitted to a specific group of nodes on a remote network segment. Directed broadcasts are known by their broadcast address, which is a destination subnet address with all the bits turned on. Compare to: broadcast.

discard eligible

In a Frame Relay network, frames which have the Discard Eligible flag set may be discarded at congested switches.

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 the shortest path, 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.

distribution layer

Middle layer of the Cisco three-layer hierarchical model, which helps you design, install, and maintain Cisco hierarchical networks. The distribution layer is the point where Access layer devices connect. Routing is performed at this layer.

distribution list

Access list used to filter incoming and outgoing route table entries on a router.

DLCI

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

DNS

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

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.

DSL

Digital Subscriber Line: DSL technologies are used to provide broadband services over a single copper pair, typically to residential customers. Most vendors are providing DSL services at up to 6Mbps downstream, but the technology can support 52Mbps service.

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. Compare to: 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 entries

Used in layer 2 and layer 3 devices to dynamically create a table of either hardware addresses or logical addresses.

dynamic routing

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

dynamic VLAN

An administrator will create an entry with a server, which will be configured with the hardware addresses of all devices on the internetwork. The server then assigns dynamically used VLANs.