Darpa (Defense Advanced Research Projects
Agency) Formerly called Arpa, this US
government agency that funded research and experimentation with
the Arpanet and later, the connected Internet- The group within
Darpa responsible for the Arpanet is ISTO (information Systems
Techniques Office), formerly IPTO (Information Processing Techniques
Office).
DassiII A message
based signaling system following the ISO based model developed
by BT to provide multi-line IDA interconnection to the BT network.
Data compression A
way of reducing the amount of data to be transmitted by applying
one of severs techniques that reduce the number of bits needed
to represent the information. When the data is received It is
decompressed into its original form.
Database server A
database installed as a back-end or server component of a client-server
system, which can be accessed over a LAN by one or more client,
or front-end applications through the use of query language, typically
SQL. The server part of the program is responsible for updating
the records, ensuring that multiple access is available to authorized
users, protecting the data and communicating with other servers
holding relevant data. The client end of the program requests
records and then modifies them, while the server tracks records
down for the client and adds new ones.
Datagram A method
of sending data in which parts of the message are sent in random
order. The recipient machine has the task of reassembling the
parts in the correct sequence. The datagram is a connectionless,
single packet message or item of data that can traverse a network
at OS I Level Three, the Network Layer. It typically does not
involve end-to-end session establishment or delivery-confirmation
acknowledgment. As well as the information within the datagram,
there is a destination network address and usually a source network
address.
Data link A direct
serial data communications path between two devices without intermediate
switching nodes.
Data Link Layer Layer
Two of the ISO OSI model is responsible for the transmission of
information over a physical medium. After establishing the link
it ensures the error-free delivery of the information through
the use of error detection, error recovery and flow control. The
contention access methods such as CSMA/CD and Token passing are
Layer Two activities.
Data PBX A switching
system for data traffic that allows terminals and workstations
connected by individual cables to the Data PBX selectively to
link to one or more host computers over asynchronous circuits
through the use of contention.
DCA (Defense Communication Agency) The
US government agency responsible for the installation of Defense
Data Networks, like Arpanet and Milnet, and PSNs. The DCA writes
contracts for operation of the DDN and pays for network services.
DCA (Document Content Architecture) The
IBM approach to storing documents as two types of document group:
draft documents and final form documents. For presentation, the
draft document is transformed into a final document through an
office system.
DCE (Data Circuit Terminating Equipment) Communications
equipment installed in a user's premises responsible for establishing,
maintaining and terminating a connection. A modem is an example.
DCE (Distributed Computing Environment) A
suite of software utilities and operating system extensions that
will, in theory, create applications on networks of heterogeneous
hardware - PCs, Unix workstations, minicomputers and mainframes.
The DCE is the product of the OSF. The DCE is designed to simplify
the building of heterogeneous client/server applications and provides
seven general services: Remote Procedure Call, Security, Naming
(directory), Distributed File System, Threads, Time and PC Integration.
DDE (Dynamic Data Exchange) - A Microsoft messaging specification.
When DDE-compliant applications are combined, dynamic documents
can be created which update each other as data changes.
DDM (Distributed Data Management Architecture)
An IBM SNA LU 6.2 transaction providing
users with facilities to locate and access data in the network.
It involves two structures: DDM Source, and DDM Target. The Source
works with a transaction application to retrieve distributed data
and transmits commands to the Target program on another system
where the data that has been requested is stored. The Target interprets
the DDM commands, retrieves the data and sends it back to the
Source that originated the request.
DDCMP (Digital Data Communication Message
Protocol) The DecNet- specific Link Level
protocol that operates at Layer Two of the Digital Network Architecture.
DDN (Defense Data Network) Used
generally to refer to Milnet, Arpanet and the TCP/IP protocols
those networks use. More specifically refers to Milnet and associated
parts of the connected Internet that connect military installations.
DecNet Proprietary
peer-to-peer network technology originally developed for use in
wide area networking by the Digital Equipment Corporation (Dec)
and evolved to include significant Ethernet-based LAN capabilities.
It is the implementation of the Digital Network Architecture (DNA).
Dect (Digital European Cordless Telecommunications)
A standard governing pan-European digital
mobile telephony. Based on advanced TDMA technology, Dect covers
cordless PBXs, telepoint and residential cordless telephony.
Demand Priority Access
method providing support for time-sensitive applications such
as video and multimedia as part of the proposed 100BaseVG standard
offering l00Mbit/s over voice-grade UTP cable. By managing and
allocating access to the network centrally, at a hub rather than
from individual workstations, sufficient bandwidth for the particular
application is guaranteed on demand. Users, say its proponents,
can be assured of reliable, continuous transmission of information.
Demodulation Technique
for retrieving information from a modulated signal. Demonstrated
by the eponymous modem (modulator/demodulator).
Des (Data Encryption Standard) An
algorithm designed by the US National Bureau of Standards for
the encryption and de-encryption of data using a 64-bit key.
Device driver In the
context of computer networking a device driver is a software module
forming part of a computer operating system, or software that
interacts with the operating system. It aims to control communications
equipment, such as a LAN network adapter card and facilitate the
transfer of information to and from the network. Other examples
of device driver programs include software to support the activities
of printers, disks and mice.
DIA (Document Interchange Architecture) An
IBM term defining the sets of functions needed for document handling
in an IBM environment, including storage and distribution.
Digital signal A signal
with only two values, normally 0 and 1, during transmission, unlike
an analog signal whose values constantly vary.
Direct attachment The
IBM term for linking a device or LAN directly to a host computer
through an appropriate Control Unit, like a cluster controller.
Disk server A device
equipped with disks and a program permitting users to create and
store files on those disks. Each user has access to their own
section of disk on the disk server. The aim is to give users access
to disk space that they would not normally have on their PC. The
disk server is linked to the PCs via a LAN. The next level of
sophistication would be a file server.
Diskless workstation A
PC or workstation attached to a LAN that has neither floppy nor
hard disks, but relies on disk storage provided by a file server
attached to the same LAN. When the diskless workstation is first
initialized it uses a remote boot program stored in a remote boot
prom/eprom on its network adapter card to initialize a session
with the file server. The workstation then loads its operating
system, such as MS-Dos, from the server and executes the normal
server login procedure.
Distributed database A
database stored on more than one networked computer. The database
is split up across these machines, and not replicated.
Distributed name service A
technique for storing network node names so that the information
is stored throughout the network, and can be requested from, and
supplied by, any node.
Disoss (Distributed Office Support Systems)
IBM software typically forming part of
an IBM Office System Node.
Distributed computing The
trend away from having big, centralized computers such as mini-computers
and mainframes to bring processing power to the desk top. Often
confused with distributed processing.
Distributed processing An
approach that allows one application program to execute on multiple
computers linked together by a network. The networked computers
share the work between them.
DLS (Data Link Switching) An
enhancement to source routing which transports source route packets
over a resilient IP/OSPF network and provides local termination
of LLC2 sessions to avoid LLC timeouts in large or busy networks.
It is the ideal mechanism for mixed LAN-to-LAN and interactive
SNA traffic since it can recover from network problems quickly
using OSPF. It is rapidly becoming accepted as a major standard.
DMA (Direct Memory Access) A
technique for high-speed data transfer between a device such as
LAN network adapter card and the computer memory. DMA bypasses
the Central Processing Unit of the computer, PC or workstation,
allowing the device to transfer a block of information directly
across the bus into system memory.
DMI (Desktop Management Interface) A
set of APIs outlined by the DMTF, comprising three components:
service layer, component interface and management interface.
DNA (Digital Network Architecture) The
network architecture of Digital Equipment Corporation with eight
layers. The DNA is similar in structure to OSI at lower levels,
except that the top three layers of the DNA correspond to the
top two layers in the OSI model.
DNS (Domain Name System) The
online distributed database system used by Internet to map names
into IP addresses. DNS servers throughout the connected Internet
implement a hierarchical namespace that allows sites freedom in
assigning machine names and addresses. DNA also supports separate
mappings between mail destinations and IP addresses.
Domain A group of
nodes on a network that form an administrative entity. It could
also be a number of servers grouped together and named to simplify
network administration and security. Every computer on the LAN
belongs to at least one domain. Being logged in on one domain,
however, does not limit resources in other domains to which the
user has access permissions.
Dos - Disk Operating System comprising
one or a suite of programs managing a disk-based computer system.
Dos schedules and supervises work, allocating computer resources
and the operation of peripherals. Versions of Dos from different
vendors exist: Microsoft's MS-Dos is the most common. Dos 3.1
was the first version of MS and PC Dos able to support LAN functions
separate, of course, from the network's own operating system -
notably including record and file locking which is now standard
on multi-user systems.
Dos LAN Manager A
Dos version of Microsoft's network operating system LAN Manager.
It gives Named Pipes (an applications interface) support to Dos
machines, enabling them to use the client/server environment.
Downlink Transmission
from a satellite to an Earth Station.
DPA (Demand Protocol Architecture) A
technique for loading protocol stacks dynamically as they are
required. It is associated with adapter cards in workstations
and servers. Only the protocol stacks that are needed for a particular
communications sessions are loaded. Examples of stacks that could
be loaded include TCP/IP, XNS, SPX/IPX and NetBios.
DPNSS (Digital Private Networks Signaling
System) Signaling standard for digital
private networks within the UK formulated jointly by BT and PABX
manufacturers.
DQDB (Distributed Queue Dual Bus) The
standard for future Mans which operates as a dual bus, each carrying
data in both directions. A queuing system maintains transmission
order. Some similarity with ATM encourages evolution between the
technologies.
Drop cable A cable
that links a network adapter to an external transceiver attached
to a co-axial LAN such as Ethernet. Also called an Attachment
Unit Interface cable or transceiver cable.
DS1 (Digital Signal 1) Transmission
standard at T1 speeds, or 1.544Mbit/s
DS3 (Digital Signal 3) Transmission
standard at T3 speeds, or 44.736Mbit/s. DS3 allows the combination
of 28 DSls or a single DS3 facility - also known as a T3 circuit.
DSE (Digital Switching Exchange) A
node in a telecommunications network.
DSU (Data Service Unit) Data
transmission equipment used to interface to a digital circuit
at customer site. It converts the customer's datastream, such
as X.21 to E1 or T1 for transmission through the CSU, which is
often contained, functionally within the DSU device. DSUs can
convert data to or from a native port on a router to an E1, E2
or E3 leased line, primary rate ISDN or SMDS, DSU functionality
can be built into devices such as some routers or multiplexers.
In Europe a DSU can convert El bandwidth into RS.449, X.21, V.35
or other serial interface via a router. A DSU with an HSSI interface
will deliver E2 or E3 bandwidth from the WAN to an HSSI router
on a LAN.
DTE (Data Terminal Equipment) A
piece of equipment where a communications path ends. The user's
equipment is collectively termed DTE and can include PCs and display
terminals.
DTMF (Dual Tone Multi-Frequency) A
term for push button or Touchtone (an AT&T trademark) dialing.
The pushed button makes a tone, actually the combination of two
tones - of high and low frequency. They are necessary to access
advanced network features such as call barring and call forwarding.
DTMF penetration in the mass-market the UK is small but growing,
but high in the business community.
Duplex Simultaneous,
two-way independent transmission of data.
Dynamic node address An
Apple-patented feature of AppleTalk under which each node assigns
itself a unique address code each time it is initialized. Conventionally,
nodes are assigned fixed addresses that do not change.
Dynamic routing A process for selecting the most appropriate
path or route for a packet or datagram to travel around a network.
At the end of each leg of the journey of the packet across the
network the router decides on the most appropriate path for the
packet or datagram to follow if there are multiple routes available.
This is done using network status information gathered from around
the Internet and passed from router to router through the use
of routing information protocols.
