D

D Channel
In ISDN the D Channel is the Data Channel. For the ISDN Basic Rate Interface the D channel is 16Kbps. In the Primary Rate Interface the D channel is 64Kbps.

Daemon
Daemon is a UNIX program similar to a DOS terminate and stay ready (TSR).

Daisy Chain
You create a daisy chain network by linking multiple devices together via a cabling system. In an AppleTalk network the daisy chain topology is created using phonenet connectors and twisted pair wiring (regular phone wire). A daisy chain configuration must be terminated at both ends using terminating resistors.

Dark Fiber
Dark fiber is fiber optic cable that does not have light traveling through it. Fiber is said to "go dark," which means light (or a signal composed of light) is no longer present.

DARPA
DARPA stands for Defense Advanced Research Projects Agency. It is the government agency that sponsored the ARPANET and the Internet.

DAS
DAS stands for Dual Access Station. An FDDI station that can attach to both FDDI rings is referred to as a DAS.

DASD
DASD stands for Direct Access Storage Device. It is an online storage device such as the IBM 3990.

Data Encryption Standard (DES)
DES is a popular single-key encryption system that uses a 56 -bit key. Triple-DES uses the DES algorithm to encrypt a message three times, using three 56-bit keys. Because hardware is faster than software, it is considered a hardware solution to encryption because of the time necessary to encrypt and decrypt a message.

Data Exchange Interface (DXI)
DXI is an ATM interface that converts variable-length network frames to fixed-length ATM cells. ATM DXI converts LAN frames to the variable-length DXI frame format. ATM CSU/DSU then converts DXI frames to fixed-length ATM cells. This two-step conversion simplifies processing on the ATM CSU/DSU, because it only needs to convert one type of frame.

Data Link Control
Data Link controls are the characters used to control the transfer of data frames (such as MAC frames) between two nodes. These control codes perform such functions as flow control and sequencing.

Data Link Layer
The Data Link Layer is the second layer of the OSI model. It resides between the Physical Layer and the Network Layer. It is responsible for point-to-point transfer of data frames.

Data Link Switching (DLSw)
DLSw is a standard (RFC 1434) that allows SNA and NetBIOS traffic to be forwarded over a TCP/IP network, using Layer 2 switching and encapsulation.

Data Set
The term data set refers to a modem (modulator/demodulator).

Data Set Ready
DSR stands for Data Set Ready. It is one of the control pins on an RS232 cable. The modems that communicate using the RS232 interface use the DSR signal to indicate that they are ready to communicate. It is located on pin 6.

Data Circuit-Terminating Equipment
DCE stands for Data Circuit-Terminating Equipment. In a network there are two broad categories of devices, DCEs and DTEs. DTE stands for data terminal equipment. The difference between the two is that one (DTE) is the end device in a network and the other (DCE) is the device or devices in the network that transmit and receive the DTE data.

Data Terminal Equipment
DTE stands for Data Terminal Equipment. The difference between the two is that one (DTE) is the end device in a network and the other (DCE) is the device or devices in the network that transmit and receive the DTE data.

Datagram
A datagram is a packet that is transferred independently of all other packets. The term datagram is usually used in referenced to an IP layer packet that uses connectionless delivery of packets (datagrams). In other words a session is not established before sending a datagram from one node to another.

DB-25
DB-25 is the standard connector used for the RS232 interface cable.

Debugger
A debugger is an application that helps find programming errors.

Decibel
A decibel is a unit of measurement that expresses the relative difference in strength of two signals (for an absolute measurement, see dBm). For example, decibels measure how much a signal has been amplified (increased) or attenuated (decreased). A positive decibel value indicates amplification, while a negative value indicates attenuation. To calculate decibels, take the logarithm of the amplification factor (A), and multiply it by 10: Gain in decibels = 10 (log A).

Decibels Over (or Under) 1 Milliwatt (dBm)
Given that the decibel is a relative measurement, dBm measures how much the strength of a signal differs from a fixed quantity of 1 milliwatt (1 milliwatt = 0 dBm). Therefore, positive dBm values indicate signals stronger than 1 milliwatt; negative dBm values indicate signals weaker than 1 milliwatt. See decibel.

DDS
DDS stands for Digital Data Service (also Dataphone Digital Service). It is a series of services provided by the telephone company providing digital facilities for data communications. DDS comes in several speeds, 2.4Kbps, 4.8Kbps, 9.6Kbps and 56Kbps.

Dedicated Circuits
When the telephone network was an analog network, electro-mechanical switches could inject a certain amount of noise into circuits. Other instruments in the transmission path, such as multiplexers, could corrupt the signal further. At lower data rates, say 300 or 1200bps, modems can correctly transmit data with few if any errors even when the line is noisy, but as data rates increase, noise causes more problems. As a practical matter, in the analog network, data could not be transmitted reliably over long distances on switched lines at rates over 4800bps. Therefore, it became common practice for the telephone companies to "lease" lines to companies for continuous, unswitched use. These leased lines are also referred to as "dedicated circuits."

Default Gateway
A default gateway is a router that provides access to all hosts on remote networks. Typically, the network administrator configures a default gateway for each host on the network.

Deliverable
A deliverable is a product a project is intended to produce. Each phase of a project also includes its own deliverables, such as design documents, estimates, prototypes, or reports.

Demodulation
Modulation is the process of modifying the form of a carrier wave (electrical signal) so that it can carry intelligent information on some sort of communications medium. Digital computer signals (baseband) are converted to analog signal for transmission over analog facilities (such as the local loop). The opposite process, converting analog signal back into their original digital state is referred to as demodulation.

Destination Address
The destination address is the address of the node in a network that a message is intended for. The destination address and source addresses are sent so that the sending and receiving devices can communicate with one another and so that intermediate nodes know where to send a frame or packet. Destination addresses can be physical (i.e., the Ethernet card in a workstation) or logical (one of multiple processes in a computer). The destination node is the node that will receive the message; the source node is the node that sends the data.

Deterministic
Deterministic is a LAN characteristic that guarantees each node access to the shared physical medium at some level. Token-passing networks (Token Ring and FDDI) are deterministic, while networks that require nodes to compete for media access (Ethernet) are not.

Device Driver
A device driver is a program that controls devices attached to a computer such as a printer or a hard disk drive.

Dialup Line
A dial-up line is circuit that exists between two nodes that uses the switched telephone network to communicate. Dialup lines provide the following capabilities:

• 300 - 56Kbps transfer rates
• any-to-any connectivity (one at a time)
• requires compatible modems at each end
• before transmission can take place, call initialization must occur
• inexpensive

Digital Data
Digital data is electrical information that represents digits (i.e., 1s and 0s). 1s and 0s (bits) are combined to form bytes and characters, such as letters of the alphabet.

Digital Data Service (DDS)
DDS (also dataphone digital service) is a series of services provided by a telephone company in providing digital facilities for data communications. DDS comes in several speeds, including 2.4, 4.8, 9.6, and 56 Kilobits per second (Kbps).

Digital Signature
A digital signature is a digital code that can be embedded into a document to prove its authenticity. Digital signatures are an application of public-key encryption technology: the sender of a document uses a private encryption key to encrypt a text string, or the digest of the message. Document recipients use the sender's public encryption key to decrypt the signature and authenticate the sender.

Digital Subscriber Line (DSL)
DSL is a modem technology that converts existing twisted pair telephone lines into high-speed data lines that can also carry separate telephone communications. Variants of DSL include ADSL, RADSL, ADSL Lite (a slightly slower version), and VDSL.

Differential Manchester Encoding
Manchester code is a commonly found Physical Layer coding scheme found in local area networks such as Ethernet and Token Ring. This self-clocking code provides a signal state change in every bit transmitted. Differential Manchester encoding is a similar technique and is also used in LANs.

Direct Memory Access (DMA)
DMA is the type of memory access that does not involve (or interrupt) the main processor in a computer to transfer data directly from memory to a peripheral device, such as a hard drive or NIC.

Director, ESCON
The Director, sometimes referred to as a "dynamic switch," is a switch that acts as a communications hub for ESCON channels. It provides the capability to physically interconnect any two links that are attached to it. Such a connection between two ports provides simultaneous, two-way information transfer.

Disk Mirroring
Disk mirroring is a method of simultaneously writing information to a network hard drive while backing the same information up to a second disk. One disk controller is used to write to two hard drives.

Distributed Computing Environment
The OSF (Open Software Foundation) Distributed Computing Environment (DCE) is software that will allow software developers to write applications that work in multiple distributed client/server environments. The developers do not have to be concerned about the underlying operating system or network which the application uses when DCE services are utilized. DCE defines the following components:

• Remote Procedure Call
• Distributed File Systems
• Distributed Directory Services
• Distributed Security

Distributed Processing
Distributed processing refers to the practice of having several computer systems share the data processing load. This is in contrast to central processing where all processing takes place on a centralized host.

DIX
Digital Equipment, Intel and Xerox (DIX) jointly developed the Ethernet standard in 1980.

DNA
DNA stands for Digital Network Architecture. It is the networking architectures designed and implemented by Digital Equipment Corporation.

DNS (Domain Name Server)
In a TCP/IP network, a user can communicate with another user by specifying a domain name, such as johnd@engr.company.com. TCP and IP require Internet addresses for messages, so one must be translated to the other. That's the job of the name server-given a domain name, they will return an Internet address (but not the reverse). Recall that domain names are hierarchical. There is a corresponding hierarchy of name servers. Given the domain name kdr@mcdata.com, DNS will resolve this from right to left. DNS will first locate a server on the Internet that knows all of the locations for companies. That DNS will then locate the company McDATA and ask for the address of kdr.

Document Object Model (DOM)
A DOM is a system that assigns an object name to each piece of a document or Web page. Each object may be manipulated by name, and objects may be deleted, added, or changed. DOM is working its way toward becoming a standard supported by W3C. Unfortunately, Microsoft and Netscape (AOL) support different DOMs. They are leaving the final decisions to W3C.

Domain Name System
The Domain Name System (DNS) is a hierarchical, distributed method of organizing the name space of the Internet. The DNS administratively groups hosts into a hierarchy of authority that allows addressing and other information to be widely distributed and maintained. A big advantage to the DNS is that using it eliminates dependence on a centrally maintained file that maps host names to addresses.

DOS
DOS stands for Disk Operating System. MS-DOS stands for Microsoft Disk Operating System. It is the low-level software that resides on many PCs that control the operation of a computer and its peripheral devices.

DOS Requester
A DOS requester is a set of software modules that allow DOS workstations to access resources available on NetWare networks. NetWare services are accessed via a module called IPXODI, which encapsulates the data for delivery over the network. The components of the DOS requester are:

• LSL: Link Support Layer is a software driver which interfaces higher layer protocols such as IP or IPX with the driver software located on the Network Interface Card (NICs).
• IPXODI: Internetwork Packet Exchange Open Data-link Interface is a driver that loads the IPX protocol stack and communicates with the LSL.
• VLM: Virtual Loadable Module is an executable file that loads the requester software.

Dotted Decimal Notation
This is a concise format for writing a 32-bit Internet address. Each 32-bit Internet address is divided into four 8-bit octets. The value of each octet is expressed independently as a decimal number, with the numbers separated by dots.

Downlink
Downlink refers to the signal path that extends from a satellite down to the earth. Uplink is just the opposite; it is the path from the earth up to a satellite.

Download
Download refers to the process of transferring files from one computer to another, typically from a host computer or network server to a PC or workstation. Upload would be just the opposite. One would upload a file from a PC to a host via a host file transfer program.

Downstream
Downstream refers to the relative position of a Token Ring LAN device. A station that receives the token after another device is said to be downstream of that device. A downstream node is a node on a LAN that is downstream from the host in the network.

DQDB
DQDB stands for Distributed Queue Dual Bus. It is the IEEE 802.6 standard for metropolitan area networks (MANs).

DSU
A DSU is another device needed to connect customer premise equipment to digital transmission facilities. It is normally used in conjunction with a CSU. A DSU takes information from a LAN device and creates digital information suitable for public transmission facilities.

Dual Access Station (DAS)
A FDDI node can communicate on one or two of the channels (the standard does not require communication on both channels, so lower-cost, single-channel machines can be built). If the node communicates on both channels and one channel fails, the node often will still be able to communicate on the other. The two types of FDDI stations are referred to as Dual-Access Station (DAS) and Single-Access Station (SAS).

Dual-Homed Gateway
A dual-homed gateway is a type of firewall. Software applications running on a highly secure host that has two network interfaces (one to the internal network and one to the external network). See proxy server and firewall.

Dumb Terminal
A dumb terminal is a terminal that is solely dependent on a host computer for processing capabilities. Dumb terminals typically do not have a processor, hard drive or floppy drives, only a keyboard, monitor and a method of communicating to a host (usually through some type of controller). Intelligent terminals on the other hand contain their own processor and storage devices as well as software programs.

Duplex
Duplex refers to the process of transmitting data in two directions at the same time. This is also referred to as duplex transmission or duplex operation. There are three modes of transmission: simplex, half duplex and duplex. Simplex transmission is when a signal can only be transmitted in one direction. With half duplex transmission the signal can travel in both directions, but not simultaneously.

Dynamic Bandwidth Allocation
Dynamic bandwidth allocation is the process used by some multiplexers which allows many slower speed devices (circuits) to share a common high speed circuit, even when the sum of all of the slower speed devices is greater than the bandwidth of the high speed circuit.

Dynamic Host Configuration Protocol (DHCP)
DHCP is a proprietary approach from Microsoft that simplifies IP network management by dynamically assigning IP addresses to logical end stations for fixed periods of time.

Dynamic Hypertext Markup Language (DHTML)
DHTML is an extended set of HTML tags that add interactive features to a Web page without sending additional requests to the server. W3C is in the process of creating the official DHTML specification.

Dynamic Routing
Dynamic routing is a process where routing is adjusted automatically when the topology of a network changes or the conditions within a network change (such as congestion).