D4 framing
- See SF.
Dual-attached concentrator. FDDI or CDDI concentrator capable of attaching to both rings of an FDDI or CDDI network. It can also be dual-homed from the master ports of other FDDI or CDDI concentrators.
- Defense Advanced Research
Projects Agency. U.S. government agency that funded
research for and experimentation with the Internet. Evolved from
ARPA, and then, in 1994, back to ARPA. See also ARPA.
DARPA Internet
- Obsolete term referring to the
Internet. See Internet.
- Dual attachment station.
Device attached to both the primary and the secondary FDDI rings.
Dual attachment provides redundancy for the FDDI ring: if the
primary ring fails, the station can wrap the primary ring to the
secondary ring, isolating the failure and retaining ring integrity.
Also known as a Class A station. Compare with SAS.
data
- Upper-layer protocol data.
database object
- 1. In general, a piece of information
that is stored in a database.
2. Chassis, card, or port defined in the configuration database of a LightStream 2020 ATM switch. Database objects have associated attributes that describe them.
data bus connector
- See DB
connector.
data channel
- See D
channel.
data circuit-terminating equipment
- See DCE.
data communications equipment
See DCE.
Data Country Code
- See DCC.
Data Encryption Standard
- See DES.
Data Exchange Interface
- See DXI.
- Layer 5 of the SNA
architectural model. This layer determines and manages interactions
between session partners, particularly data flow. Corresponds to the
session
layer of the OSI model. See also data
link control layer, path
control layer, physical
control layer, presentation
services layer, transaction
services layer, and transmission
control layer.
- Logical grouping of information sent
as a network layer unit over a transmission medium without prior
establishment of a virtual circuit. IP datagrams are the primary
information units in the Internet. The terms frame,
message,
packet,
and segment
are also used to describe logical information groupings at
various layers of the OSI reference model and in various technology
circles.
Datagram Delivery Protocol
- See DDP.
data-link connection identifier
- See DLCI.
- Layer 2 in the SNA
architectural model. Responsible for the transmission of data over a
particular physical link. Corresponds roughly to the data
link layer of the OSI model. See also data
flow control layer, path
control layer, physical
control layer, presentation
services layer, transaction
services layer, and transmission
control layer.
- Layer 2 of the OSI
reference model. This layer provides reliable transit of data
across a physical link. The data link layer is concerned with
physical addressing, network topology, line discipline, error
notification, ordered delivery of frames, and flow control. The IEEE
has divided this layer into two sublayers: the MAC sublayer and the
LLC sublayer. Sometimes simply called link layer. Roughly
corresponds 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.
data-link switching
- See DLSw.
Data Movement Processor
- See DMP.
Data Network Identification Code
- See DNIC.
data set ready
- See DSR.
data service unit
- See DSU.
data sink
- Network equipment that accepts data
transmissions.
data stream
- All data transmitted through a
communications line in a single read or write operation.
data terminal equipment
- See DTE.
data terminal ready
- See DTR.
dB
- decibels.
- Data bus connector.
Type of connector used to connect serial and parallel cables to a
data bus. DB connector names are of the format DB-x, where x
represents the number of (wires) within the connector. Each line is
connected to a pin on the connector, but in many cases, not all pins
are assigned a function. DB connectors are defined by various EIA/TIA
standards.
DC
- Direct current.
Electrical current that travels in only one direction. Direct
current is generally used in electronic circuits. See DC."
- Defense Communications Agency.
U.S. government organization responsible for DDN networks such as
MILNET. Now called DISA. See DISA.
- Data Country Code.
One of two ATM address formats developed by the ATM Forum for use by
private networks. Adapted from the subnetwork model of addressing in
which the ATM layer is responsible for mapping network layer
addresses to ATM addresses. See also ICD.
- Data communications equipment
(EIA expansion) or data circuit-terminating equipment (ITU-T
expansion). The devices and connections of a communications network
that comprise the network end of the user-to-network interface. The
DCE provides a physical connection to the network, forwards traffic,
and provides a clocking signal used to synchronize data transmission
between DCE and DTE devices. Modems and interface cards are examples
of DCE. Compare with DTE.
- 1. Data channel.
Full-duplex, 16-kbps (BRI) or 64-kbps (PRI) ISDN channel. Compare to
B
channel, E
channel, and H
channel.
2. In SNA, a device that connects a processor and main storage with peripherals.
- Distributed Data Management.
Software in an IBM SNA environment that provides peer-to-peer
communication and file sharing. One of three SNA transaction
services. See also DIA
and SNADS.
- Defense Data Network.
U.S. military network composed of an unclassified network (MILNET)
and various secret and top-secret networks. DDN is operated and
maintained by DISA. See also DISA
and MILNET.
- Datagram Delivery Protocol.
Apple Computer network layer protocol that is responsible for the
socket-to-socket delivery of datagrams over an AppleTalk
internetwork.
- Dial-on-demand routing.
Technique whereby a Cisco router can automatically initiate and
close a circuit-switched session as transmitting stations demand.
The router spoofs keepalives so that end stations treat the session
as active. DDR permits routing over ISDN or telephone lines using an
external ISDN terminal adaptor or modem.
- Discard eligible. See
tagged
traffic.
deadlock
- 1. Unresolved contention for the use
of a resource.
2. In APPN, when two elements of a process each wait for action by or a response from the other before they resume the process.
- Command that displays RIP routing
updates as they are sent and received.
decibels
- Abbreviated dB.
- Group of communications products
(including a protocol suite) developed and supported by Digital
Equipment Corporation. DECnet/OSI (also called DECnet Phase V) is
the most recent iteration and supports both OSI protocols and
proprietary Digital protocols. Phase IV Prime supports inherent MAC
addresses that allow DECnet nodes to coexist with systems running
other protocols that have MAC address restrictions. See also DNA.
DECnet routing
- Proprietary routing scheme introduced
by Digital Equipment Corporation in DECnet Phase III. In DECnet
Phase V, DECnet completed its transition to OSI routing protocols
(ES-IS and IS-IS).
decorative raceway
- Type of wall-mounted channel with
removable cover used to support horizontal cabling. Decorative
raceway is big enough to hold two cables.
- The reverse application of an
encryption algorithm to encrypted data, thereby restoring that data
to its original, unencrypted state. See also encryption.
dedicated LAN
- Network segment allocated to a single
device. Used in LAN switched network topologies.
- Communications line that is
indefinitely reserved for transmissions, rather than switched as
transmission is required. See also leased
line.
- Standard that exists by nature of its
widespread use. Compare with de
jure standard. See also standard.
- Routing table entry that is used to
direct frames for which a next hop is not explicitly listed in the
routing table.
Defense Advanced Research Projects
Agency
- See DARPA.
Defense Communications Agency
- See DCA.
Defense Data Network
- See DDN.
Defense Information Systems Agency
See DISA.
Defense Intelligence Agency
- See DIA.
- Standard that exists because of its
approval by an official standards body. Compare with de
facto standard. See also standard.
delay
- The time between the initiation of a
transaction by a sender and the first response received by the
sender. Also, the time required to move a packet from source to
destination over a given path.
demand priority
- Media access method used in
100VG-AnyLAN that uses a hub that can handle multiple transmission
requests and can process traffic according to priority, making it
useful for servicing time-sensitive traffic such as multimedia and
video. Demand priority eliminates the overhead of packet collisions,
collision recovery, and broadcast traffic typical in Ethernet
networks. See also 100VG-AnyLAN.
demarc
- Demarcation point between carrier
equipment and CPE.
demodulation
- Process of returning a modulated
signal to its original form. Modems perform demodulation by taking
an analog signal and returning it to its original (digital) form.
See also modulation.
- The separating of multiple input
streams that have been multiplexed into a common physical signal
back into multiple output streams. See also multiplexing.
dense mode PIM
- See PIM
dense mode.
Department of Defense
- See DoD.
Department of Defense Intelligence
Information System Network Security for Information Exchange
- See DNSIX.
Dependent LU
- See DLU.
Dependent LU Requester
- See DLUR.
Dependent LU Server
- See DLUS.
Data Encryption Standard. Standard cryptographic algorithm developed by the U.S. NBS.
designated bridge
- The bridge that incurs the lowest path
cost when forwarding a frame from a segment to the route bridge.
designated router
- OSPF router that generates LSAs for a
multiaccess network and has other special responsibilities in
running OSPF. Each multiaccess OSPF network that has at least two
attached routers has a designated router that is elected by the OSPF
Hello protocol. The designated router enables a reduction in the
number of adjacencies required on a multiaccess network, which in
turn reduces the amount of routing protocol traffic and the size of
the topological database.
- Address of a network device that is
receiving data. See also source
address.
destination MAC
- See DMAC.
destination port
- Number of the called port.
destination service access point
- See DSAP.
deterministic load distribution
- Technique for distributing traffic
between two bridges across a circuit group. Guarantees packet
ordering between source-destination pairs and always forwards
traffic for a source-destination pair on the same segment in a
circuit group for a given circuit-group configuration.
Deutsche Industrie Norm
- See DIN.
Deutsche Industrie Norm connector
- See DIN
connector.
device
- See node.
- Document Interchange
Architecture. Defines the protocols and data formats needed
for the transparent interchange of documents in an SNA network. One
of three SNA transaction services. See also DDM
and SNADS.
dial backup
- Feature supported by Cisco routers
that provides protection against WAN downtime by allowing the
network administrator to configure a backup serial line through a
circuit-switched connection.
dial-on-demand routing
- See DDR.
dial-up line
- Communications circuit that is
established by a switched-circuit connection using the telephone
company network.
differential encoding
- Digital encoding technique whereby a
binary value is denoted by a signal change rather than a particular
signal level.
differential Manchester encoding
- Digital coding scheme where a
mid-bit-time transition is used for clocking, and a transition at
the beginning of each bit time denotes a zero. The coding scheme
used by IEEE 802.5 and Token Ring networks.
Diffusing Update Algorithm
- See DUAL.
Digital Network Architecture
- See DNA.
digital signal
- Language of computers comprising only
two states, on and off which are indicated by a series of voltage
pulses.
digital signal level 0
- See DS-0.
digital signal level 1
- See DS-1.
digital signal level 3
- See DS-3.
- See SPF.
- Deutsche Industrie Norm.
German national standards organization.
- Deutsche Industrie Norm
connector. Multipin connector used in some Macintosh and
IBM PC-compatible computers, and on some network processor panels.
- Search request sent to a specific node
known to contain a resource. A directed search is used to determine
the continued existence of the resource and to obtain routing
information specific to the node. See also broadcast
search.
direct memory access
- See DMA.
directory services
- Services that help network devices
locate service providers.
- Defense Information Systems
Agency. U.S. military organization responsible for
implementing and operating military information systems, including
the DDN. See also DDN.
discard eligible
- See DE.
discovery architecture
- APPN software that enables a machine
configured as an APPN EN to automatically find primary and backup
NNs when the machine is brought onto an APPN network.
- Method by which an AppleTalk interface
acquires information about an attached network from an operational
node and then uses this information to configure itself. Also called
dynamic configuration.
- The combination of a hard disk drive,
a floppy disk drive, and a disk power supply on a LightStream 2020
ATM switch. Each NP card in a LightStream 2020 chassis has its own
disk assembly.
Distance Vector Multicast Routing
Protocol
- See DVMRP.
distance vector
routing algorithm
- Class of routing algorithms that
iterate on the number of hops in a route to find a shortest-path
spanning tree. Distance vector routing algorithms call for each
router to send its entire routing table in each update, but only to
its neighbors. Distance vector routing algorithms can be prone to
routing loops, but are computationally simpler than link state
routing algorithms. Also called Bellman-Ford routing algorithm.
See also link
state routing algorithm and SPF.
- Problem with a communication signal
resulting from nonuniform transmission speeds of the components of a
signal through a transmission medium. Also called group delay.
distributed computing (processing)
Distributed Data Management
- See DDM.
Distributed Queue Dual Bus
- See DQDB.
- Data-link connection
identifier. Value that specifies a PVC or SVC in a Frame
Relay network. In the basic Frame Relay specification, DLCIs are
locally significant (connected devices might use different values to
specify the same connection). In the LMI extended specification,
DLCIs are globally significant (DLCIs specify individual end
devices). See also LMI.
- Data-link switching.
Interoperability standard, described in RFC 1434, that provides a
method for forwarding SNA and NetBIOS traffic over TCP/IP networks
using data link layer switching and encapsulation. DLSw uses SSP
(Switch-to-Switch Protocol) instead of SRB, eliminating the major
limitations of SRB, including hop-count limits, broadcast and
unnecessary traffic, timeouts, lack of flow control, and lack of
prioritization schemes. See also DLSw+,
SRB,
and SSP
(Switch-to-Switch Protocol).
- Data Link Switching Plus.
Cisco implementation of the DLSw standard for SNA and NetBIOS
traffic forwarding. DLSw+ goes beyond the standard to include the
advanced features of the current Cisco RSRB implementation, and
provides additional functionality to increase the overall
scalability of data-link switching. See also DLSw.
- Dependent LU. An LU
that depends on the SSCP to provide services for establishing
sessions with other LUs. See also LU
and SSCP.
- Dependent LU Requester.
The client half of the Dependent LU Requestor/Server enhancement to
APPN. The DLUR component resides in APPN ENs and NNs that support
adjacent DLUs by securing services from the DLUS. See also APPN,
DLU,
and DLUS.
DLUR node
- In APPN networks, an EN or NN that
implements the DLUR component. See also DLUR.
- Dependent LU Server.
The server half of the Dependent LU Requestor/Server enhancement to
APPN. The DLUS component provides SSCP services to DLUR nodes over
an APPN network. See also APPN,
DLU,
and DLUR.
DLUS node
- In APPN networks, a NN that implements
the DLUS component. See also DLUS.
- Direct memory access.
The transfer of data from a peripheral device, such as a hard disk
drive, into memory without that data passing through the
microprocessor. DMA transfers data into memory at high speeds with
no processor overhead.
- Destination MAC. The
MAC address specified in the Destination Address field of a packet.
Compare with SMAC.
See also MAC
address.
- Data Movement Processor.
Processor on the Catalyst 5000 that, along with the multiport packet
buffer memory interface, performs the frame-switching function for
the switch. The DMP also handles translational bridging between the
Ethernet and FDDI interfaces, IP segmentation, and intelligent
bridging with protocol-based filtering. See also Catalyst
5000.
- Digital Network Architecture.
Network architecture developed by Digital Equipment Corporation. The
products that embody DNA (including communications protocols) are
collectively referred to as DECnet. See also DECnet.
- Data Network Identification
Code. Part of an X.121 address. DNICs are divided into two
parts: the first specifying the country in which the addressed PSN
is located and the second specifying the PSN itself. See also X.121.
- Domain Naming System.
System used in the Internet for translating names of network nodes
into addresses. See also authority
zone.
- Department of Defense
Intelligence Information System Network Security for Information
Exchange. Collection of security requirements for
networking defined by the U.S. Defense Intelligence Agency.
Document Interchange Architecture
- See DIA.
- Department of Defense. U.S. government
organization that is responsible for national defense. The DoD has
frequently funded communication protocol development.
domain
- 1. In the Internet, a portion of the
naming hierarchy tree that refers to general groupings of networks
based on organization-type or geography.
2. In SNA, an SSCP and the resources it controls.
3. In IS-IS, a logical set of networks.
Domain
- Networking system developed by Apollo
Computer (now part of Hewlett-Packard) for use in its engineering
workstations.
Domain Naming System
- See DNS.
domain specific part
- See DSP.
- Refers to the common notation for IP
addresses in the form <a.b.c.d> where each number a
represents, in decimal, 1 byte of the 4-byte IP address. Also called
dotted notation or four-part dotted notation.
dotted notation
- See dot
address.
downlink station
- See ground
station.
downstream physical unit
- See DSPU.
- Distributed Queue Dual Bus.
Data link layer communication protocol, specified in the IEEE 802.6
standard, designed for use in MANs. DQDB, which permits multiple
systems to interconnect using two unidirectional logical buses, is
an open standard that is designed for compatibility with carrier
transmission standards, and is aligned with emerging standards for
BISDN. SIP (SMDS Interface Protocol) is based on DQDB. See also MAN.
- Dynamic random-access memory.
RAM that stores information in capacitors that must be periodically
refreshed. Delays can occur because DRAMs are inaccessible to the
processor when refreshing their contents. However, DRAMs are less
complex and have greater capacity than SRAMs. See also SRAM.
drop
Point on a multipoint channel where a connection to a networked device is made.
drop cable
- Generally, a cable that connects a
network device (such as a computer) to a physical medium. A type of
AUI. See also AUI.
- Digital signal level 0.
Framing specification used in transmitting digital signals over a
single channel at 64-kbps on a T1 facility. Compare with DS-1
and DS-3.
- Digital signal level 1.
Framing specification used in transmitting digital signals at
1.544-Mbps on a T1 facility (in the United States) or at 2.108-Mbps
on an E1 facility (in Europe). Compare with DS-0
and DS-3.
See also E1
and T1.
DS-1 domestic trunk interface
- See DS-1/DTI.
- DS-1 domestic trunk interface.
Interface circuit used for DS-1 applications with 24 trunks.
- Digital signal level 3.
Framing specification used for transmitting digital signals at
44.736-Mbps on a T3 facility. Compare with DS-0
and DS-1.
See also E3
and T3.
- Destination service access
point. The SAP of the network node designated in the
Destination field of a packet. Compare to SSAP.
See also SAP
(service access point).
- Domain specific part.
The part of a CLNS address that contains an area identifier, a
station identifier, and a selector byte.
- 1. Downstream physical unit.
In SNA, a PU that is located downstream from the host.
2. Cisco IOS software feature that enables a router to function as a PU concentrator for SNA PU 2 nodes. PU concentration at the router simplifies the task of PU definition at the upstream host while providing additional flexibility and mobility for downstream PU devices. This feature is sometimes referred to as DSPU concentration. See also PU and SNA.
DSPU concentration
- Data set ready. EIA/TIA-232
interface circuit that is activated when DCE is powered up and ready
for use.
- Data service unit.
Device used in digital transmission that adapts the physical
interface on a DTE device to a transmission facility such as T1 or
E1. The DSU is also responsible for such functions as signal timing.
Often referred to together with CSU, as CSU/DSU. See also CSU.
DSX-1
- Cross-connection point for DS-1
signals.
Data terminal equipment. Device at the user end of a user-network interface that serves as a data source, destination, or both. DTE connects to a data network through a DCE device (for example, a modem) and typically uses clocking signals generated by the DCE. DTE includes such devices as computers, protocol translators, and multiplexers. Compare with DCE.
Dual tone multifrequency. Use of two simultaneous voice-band tones for dialing (such as touch tone).
- Data terminal ready.
EIA/TIA-232 circuit that is activated to let the DCE know when the
DTE is ready to send and receive data.
- Diffusing Update Algorithm.
Convergence algorithm used in Enhanced IGRP that provides loop-free
operation at every instant throughout a route computation. Allows
routers involved in a topology change to synchronize at the same
time, while not involving routers that are unaffected by the change.
See also Enhanced
IGRP.
dual-attached concentrator
- See DAC.
dual attachment station
- See DAS.
dual counter-rotating rings
- Network topology in which two signal
paths, whose directions are opposite one another, exist in a
token-passing network. FDDI and CDDI are based on this concept.
dual-homed station
- Device attached to multiple FDDI rings
to provide redundancy.
dual homing
Network topology in which a device is connected to the network by way of two independent access points (points of attachment). One access point is the primary connection, and the other is a standby connection that is activated in the event of a failure of the primary connection.
Dual IS-IS
- See Integrated
IS-IS.
dual tone multifrequency
- See DTMF.
- Distance Vector Multicast
Routing Protocol. Internetwork gateway protocol, largely
based on RIP, that implements a typical dense mode IP multicast
scheme. DVMRP uses IGMP to exchange routing datagrams with its
neighbors. See also IGMP.
- Data Exchange Interface.
ATM Forum specification, described in RFC 1483, that defines how a
network device such as a bridge, router, or hub can effectively act
as an FEP to an ATM network by interfacing with a special DSU that
performs packet segmentation and reassembly.
dynamic address resolution
- Use of an address resolution protocol
to determine and store address information on demand.
dynamic configuration
- See discovery
mode.
dynamic random-access memory
- See DRAM.
- Routing that adjusts automatically to network topology or traffic changes. Also called adaptive routing.