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Mon 27-March-2017

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Definitions & Terms

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  • A daemon (pronounced DEE-muhn) is a program that runs continuously and exists for the purpose of handling periodic service requests that a computer system expects to receive. The daemon program forwards the requests to other programs (or processes) as appropriate. Each server of pages on the Web has an HTTPD or Hypertext Transfer Protocol daemon that continually waits for requests to come in from Web clients and their users.
  • In mythology, a daemon, according to Webster's, was "an attendant power or spirit." Daemon can be confused with demon, which has a different but similar meaning. The New Hacker's Dictionary says that a daemon is a program that runs by itself directly under the operating system whereas a demon is part of a larger application program.

DAFS - Direct Access File System

  • Dark fiber is optical fiber infrastructure (cabling and repeater s) that is currently in place but is not being used. Optical fiber conveys information in the form of light pulses so the "dark" means no light pulses are being sent. For example, some electric utilities have installed optical fiber cable where they already have power lines installed in the expectation that they can lease the infrastructure to telephone or cable TV companies or use it to interconnect their own offices. To the extent that these installations are unused, they are described as dark.
  • "Dark fiber service" is service provided by local exchange carriers (LEC s) for the maintenance of optical fiber transmission capacity between customer locations in which the light for the fiber is provided by the customer rather than the LEC.

Dark Fiber

  • See Direct Access File System.

DASD - Direct Access Storage Device

  • See Direct Access Storage Device.

Data Link Control

  • Data Link Control (DLC) is the service provided by the Data Link layer of function defined in the Open Systems Interconnection (OSI ) model for network communication. The Data Link layer is responsible for providing reliable data transfer across one physical link (or telecommunications path) within the network. Some of its primary functions include defining frames, performing error detection or ECC on those frames, and performing flow control (to prevent a fast sender from overwhelming a slow receiver).
  • Many point-to-point protocols exist at the Data Link layer including High-level Data Link Control (HDLC), Synchronous Data Link Control (SDLC ), Link Access Procedure Balanced (LAPB), and Advanced Data Communications Control Procedure (ADCCP). All of these protocols are very similar in nature and are found in older networks (such as X.25 networks). In the Internet, one of two point-to-point protocols are used at this layer: Serial Line Internet Protocol (SLIP) or Point-to-Point Protocol (PPP ) with PPP being the newer, approved standard. All of these protocols are used in point-to-point connections such as those on metropolitan area network (MAN) or wide area network (WAN) backbones or when we dial our Internet service provider (ISP) from home using a modem.
  • In local area networks (LAN s) where connections are multipoint rather than point-to-point and require more line-sharing management, the Data Link layer is divided into two sublayers: the Logical Link Control layer and the Media Access Control layer . The Logical Link Control layer protocol performs many of the same functions as the point-to-point data link control protocols described above. The Media Access Control (MAC) layer protocols support methods of sharing the line among a number of computers. Among the most widely used MAC protocols are Ethernet (IEEE 802.3), Token Bus (IEEE 802.4), and token ring (IEEE 802.5) and their derivatives.

Data Space Transfer Protocol - DSTP:

  • DSTP (Data Space Transfer Protocol) is a protocol that is used to index and retrieve data from a number of databases, file s, and other data structures using a key that can find all the related data about a particular object across all of the data. The data is typically distributed among a number of server s in a network. The servers, called DSTP servers, understand how to index and retrieve appropriate data, using the key, which is called a Universal Correlation Key (UCK). DSTP can be considered a tool for data mining.
  • DSTP is similar in concept to the NNTP, which allows Internet users to access the online discussions known as Usenet newsgroups. DSTP makes use of the Extensible Markup Language (XML). For an existing database, an XML file is created in which a UCK key or tag describes corresponding database columns.


  • DB2 is a family of relational database management system (RDBMS) products from IBM that serve a number of different operating system platforms. According to IBM, DB2 leads in terms of database market share and performance. Although DB2 products are offered for UNIX-based systems and personal computer operating systems, DB2 trails Oracle's database products in UNIX-based systems and Microsoft's Access in Windows systems.
  • In addition to its offerings for the mainframe OS/390 and VM operating systems and its mid-range AS/400 systems, IBM offers DB2 products for a cross-platform spectrum that includes UNIX-based Linux, HP-UX, Sun Solaris, and SCO UnixWare; and for its personal computer OS/2 operating system as well as for Microsoft's Windows 2000 and earlier systems. DB2 databases can be accessed from any application program by using Microsoft's Open Database Connectivity (ODBC) interface, the Java Database Connectivity (JDBC) interface, or a CORBA interface broker.

DC - Domain Controler

  • See Doamin Controller.

DCOM - Distributed Component Object Model:

  • See Distributed Component Object Model.

DDoS - Distributed Denial-of-Service Attack:

  • See Distributed Denial-of-Service Attack.

DDS - Digital Data Storage:

  • See Digital Data Storage.


  • To decompile is to convert executable (ready-to-run) program code (sometimes called object code) into some form of higher-level programming language so that it can be read by a human. Decompilation is a type of reverse engineering that does the opposite of what a compiler does. The tool that accomplishes this is called a decompiler. A similar tool, called a disassembler, translates object code into assembler language. There are a number of different reasons for decompilation or disassembly, such as understanding a program, recovering the source code for purposes of archiving or updating, finding viruses, debugging programs, and translating obsolete code. Decompilation was first used in the 1960s to facilitate the migration of a program from one platform to another.
  • Decompilation is not always successful for a number of reasons. It is not possible to decompile all programs, and data and code are difficult to separate, because both are represented similarly in most current computer systems. The meaningful names that programmers give variables and function s (to make them more easily identifiable) are not usually stored in an executable file, so they are not usually recovered in decompiling. Decompilation is sometimes used unethically, to reproduce source code for reuse or adaptation without permission of the copyright holder. Programs can be designed to be resistant to decompilation through protective means such as obfuscation.


  • See Encryption.

Dedicated Server:

  • In the Web hosting business, a dedicated server refers to the rental and exclusive use of a computer that includes a Web server , related software, and connection to the Internet, housed in the Web hosting company's premises. A dedicated server is usually needed for a Web site (or set of related company sites) that may develop a considerable amount of traffic - for example, a site that must handle up to 35 million hit s a day. The server can usually be configured and operated remotely from the client company. Web hosting companies claim that the use of a dedicated server on their premises saves router, Internet connection, security system, and network administration costs.
  • In renting a dedicated server, the client company may be required to use a specified computer system or may be offered a choice of several systems. Some host providers allow a client company to purchase and install its own computer server at the host provider's location, a service known as colocation.
  • Typically, a dedicated server is rented that provides a stated amount of memory, hard disk space, and bandwidth ( here meaning the number of gigabytes of data that can be delivered each month). Some hosting companies allow the renter of a dedicated server to do virtual hosting, in turn renting services on the server to third parties for their Web sites. domain name system, e-mail, and File Transfer Protocol (FTP) capabilities are typically included and some companies provide an easy-to-use control interface.

Deep link:

  • A Deep Link is a hypertext link to a page on a Web site other than its home page . The "deep" refers to the depth of the page in a site's hierarchical structure of pages. Any page below the top page in the hierarchy (the home page) can thus be considered deep. The term has been given prominence in the legal suit of the Ticketmaster Corporation against Microsoft. A Microsoft-owned site (one of its Sidewalk sites) linked to a page of useful information within the Ticketmaster site, avoiding the Ticketmaster home page. Ticketmaster sued Microsoft for linking without permission, claiming that they were thus being deprived of advertising viewers for the ads on their home page. In fact, their content was appearing within a Microsoft page containing their advertising.
  • In his TBTF Newsletter, Keith Dawson refers to deep links as "rifle shots," suggesting that they get the Web surfer immediately to the information they want. Although no one has quantified the number of deep links between Web sites, it would certainly seem to be a very large number. Most major search engine and directory sites like Yahoo invite Web sites to request inclusion with the assumption that all pages will be indexed unless explicitly excluded. And, undoubtedly, most sites welcome links from anywhere to any of their pages.
  • Although the issue is still to be fully resolved, some observers think it likely that the prevalence of deep links on the Web will continue as an essential characteristic. Brad Templeton, writing at one Web site, suggests that most Web sites implicitly encourage both home page and deep links simply by being part of the Web. He suggests that those sites that want to inhibit deep links to their site could state it clearly as a way to discourage such links. Many feel that the practice of "framing" another site within a frame on your site poses some liability for the framing site, unless permission to frame is received first.

Denial of Service - DoS:

  • On the Internet, a denial of service (DoS) attack is an incident in which a user or organization is deprived of the services of a resource they would normally expect to have. Typically, the loss of service is the inability of a particular network service, such as e-mail, to be available or the temporary loss of all network connectivity and services. In the worst cases, for example, a Web site accessed by millions of people can occasionally be forced to temporarily cease operation. A denial of service attack can also destroy programming and files in a computer system. Although usually intentional and malicious, a denial of service attack can sometimes happen accidentally. A denial of service attack is a type of security breach to a computer system that does not usually result in the theft of information or other security loss. However, these attacks can cost the target person or company a great deal of time and money.
  • Common forms of denial of service attacks are: Buffer Overflow Attacks The most common kind of DoS attack is simply to send more traffic to a network address than the programmers who planned its data buffer s anticipated someone might send. The attacker may be aware that the target system has a weakness that can be exploited or the attacker may simply try the attack in case it might work. A few of the better-known attacks based on the buffer characteristics of a program or system include:
  • * Sending e-mail messages that have attachments with 256-character file names to Netscape and Microsoft mail programs
  • * Sending oversized Internet Control Message Protocol (ICMP) packets (this is also known as the Packet Internet or Inter-Network Groper (ping) of death)
  • * Sending to a user of the Pine e-mail progam a message with a "From" address larger than 256 characters
  • SYN Attack
  • When a session is initiated between the Transport Control Program (TCP ) client and server in a network, a very small buffer space exists to handle the usually rapid "hand-shaking" exchange of messages that sets up the session. The session-establishing packet s include a SYN field that identifies the sequence in the message exchange. An attacker can send a number of connection requests very rapidly and then fail to respond to the reply. This leaves the first packet in the buffer so that other, legitimate connection requests can't be accommodated. Although the packet in the buffer is dropped after a certain period of time without a reply, the effect of many of these bogus connection requests is to make it difficult for legitimate requests for a session to get established. In general, this problem depends on the operating system providing correct settings or allowing the network administrator to tune the size of the buffer and the timeout period.
  • Teardrop Attack
  • This type of denial of service attack exploits the way that the Internet Protocol (IP ) requires a packet that is too large for the next router to handle be divided into fragments. The fragment packet identifies an offset to the beginning of the first packet that enables the entire packet to be reassembled by the receiving system. In the teardrop attack, the attacker's IP puts a confusing offset value in the second or later fragment. If the receiving operating system does not have a plan for this situation, it can cause the system to crash.
  • Smurf Attack
  • In this attack, the perpetrator sends an IP ping (or "echo my message back to me") request to a receiving site The ping packet specifies that it be broadcast to a number of hosts within the receiving site's local network. The packet also indicates that the request is from another site, the target site that is to receive the denial of service. (Sending a packet with someone else's return address in it is called spoof ing the return address.) The result will be lots of ping replies flooding back to the innocent, spoofed host. If the flood is great enough, the spoofed host will no longer be able to receive or distinguish real traffic.
  • Viruses
  • Computer virus es, which replicate across a network in various ways, can be viewed as denial-of-service attacks where the victim is not usually specifically targetted but simply a host unlucky enough to get the virus. Depending on the particular virus, the denial of service can be hardly noticeable ranging all the way through disastrous. Physical Infrastructure Attacks Here, someone may simply snip a fiber optic cable. This kind of attack is usually mitigated by the fact that traffic can sometimes quickly be rerouted.
  • There are ways of preventing many forms of DoS attacks.

DHCP - Dynamic Host Configuration Protocol:

  • See Dynamic Host Configuration Protocol

DHTML - Dynamic HTML:

  • See Dynamic HTML.

Dialed Number Identification Service - DNIS:

  • Dialed Number Identification Service (DNIS) is a telephone service that identifies for the receiver of a call the number that the caller dialed. It's a common feature of 800 and 900 lines. If you have multiple 800 or 900 numbers to the same destination, DNIS tells which number was called. DNIS works by passing the touch tone digits (dual tone multi frequency or MF digits) to the destination where a special facility can read and display them or make them available for call center programming.

DICOM - Digital Imaging and Communications in Medicine:

  • See Digital Imaging and Communications in Medicine.

DID - Direct Inward Dialing:

  • See Direct Inward Dialing.

Digital Data Storage - DDS:

  • Digital Data Storage (DDS) is a format for storing and backing up computer data on tape that evolved from the Digital Audio Tape (DAT) technology. DAT was created for CD-quality audio recording. In 1989, Sony and Hewlett Packard defined the DDS format for data storage using DAT tape cartridges. Tapes conforming to the DDS format can be played by either DAT or DDS tape drives. However, DDS tape drives cannot play DAT tapes since they can't pick up the audio on the DAT tape.
  • DDS uses a 4-mm tape. A DDS tape drive uses helical scanning for recording, the same process used by a video recorder (VCR). There are two read heads and two write heads. The read heads verify the data that has been written (recorded). If errors are present, the write heads rewrite the data. When restoring a backed-up file, the restoring software reads the directory of files located at the beginning of the tape, winds the tape to the location of the file, verifies the file, and writes the file onto the hard drive. DDS cannot update a backed-up file in the same place it was originally recorded. In general, DDS requires special software for managing the storage and retrieval of data from DDS tape drives.
  • There are four types of DDS drives:
  • * DDS-1: Stores up to 2 gigabytes of uncompressed data on a 120-minute cartridge.
  • * DDS-2: Stores up to 8 GB of data in compressed format on a 120-minute cartridge. DDS-2 is ideal for small network servers.
  • * DDS-3: Stores up to 24 GB of data on a 125-minute cartridge. The DDS-3 drive is ideal for medium-sized servers. DDS-3 uses PRML (Partial Response Maximum Likelihood). PRML eliminates electronic noise for a cleaner data recording.
  • * DDS-4: The newest DDS drive, DDS-4 stores up to 40 GB of data on a 125-minute cartridge. Small to mid-size businesses benefit from the DDS-4 drive.
  • A DDS cartridge needs to be retired after 2,000 passes or 100 full backups. You should clean your DDS tape drive every 24 hours with a cleaning cartridge and discard the cleaning cartridge after 30 cleanings. DDS tapes have an expected life of at least 10 years.

Digital Imaging and Communications in Medicine - DICOM:

  • Digital Imaging and Communications in Medicine (DICOM) is an application layer network protocol for the transmission of medical images, waveforms, and ancillary information. It was originally developed by the National Electrical Manufacturers Association (NEMA) and the American College of Radiology for CAT and MRI scan images. It is now controlled by the DICOM Standards Committee, and supports a wide range of medical images across the fields of radiology, cardiology, pathology and dentistry. DICOM uses TCP/IP as the lower-layer transport protocol.

Digital Signal X:

  • Digital Signal X is a term for the series of standard digital transmission rates or levels based on DS0, a transmission rate of 64 Kbps, the bandwidth normally used for one telephone voice channel. Both the North American T-carrier system system and the European E-carrier systems of transmission operate using the DS series as a base multiple. The digital signal is what is carried inside the carrier system.
  • DS0 is the base for the digital signal X series. DS1, used as the signal in the T-1 carrier, is 24 DS0 (64 Kbps) signals transmitted using pulse-code modulation (PCM) and time-division multiplexing (TDM). DS2 is four DS1 signals multiplexed together to produce a rate of 6.312 Mbps. DS3, the signal in the T-3 carrier, carries a multiple of 28 DS1 signals or 672 DS0s or 44.736 Mbps. Digital signal X is based on the ANSI T1.107 guidelines. The ITU-TS guidelines differ somewhat. Also see E-carrier systems.

Digital Signature:

  • A Digital Signature (not to be confused with a digital certificate ) is an electronic signature that can be used to authenticate the identity of the sender of a message or the signer of a document, and possibly to ensure that the original content of the message or document that has been sent is unchanged. Digital signatures are easily transportable, cannot be imitated by someone else, and can be automatically time-stamped. The ability to ensure that the original signed message arrived means that the sender cannot easily repudiate it later.
  • A digital signature can be used with any kind of message, whether it is encrypted or not, simply so that the receiver can be sure of the sender's identity and that the message arrived intact. A digital certificate contains the digital signature of the certificate-issuing authority so that anyone can verify that the certificate is real.
  • How It Works
  • Assume you were going to send the draft of a contract to your lawyer in another town. You want to give your lawyer the assurance that it was unchanged from what you sent and that it is really from you.
  • 1. You copy-and-paste the contract (it's a short one!) into an e-mail note.
  • 2. Using special software, you obtain a message hash (mathematical summary) of the contract.
  • 3. You then use a private key that you have previously obtained from a public-private key authority to encrypt the hash.
  • 4. The encrypted hash becomes your digital signature of the message. (Note that it will be different each time you send a message.)
  • At the other end, your lawyer receives the message.
  • 1. To make sure it's intact and from you, your lawyer makes a hash of the received message.
  • 2. Your lawyer then uses your public key to decrypt the message hash or summary.
  • 3. If the hashes match, the received message is valid.
  • Also see hashing and Digital Signature Standard.

Digital Subsciber Liner - DSL:

  • Digital Subscriber Line (DSL) is a technology for bringing high-bandwidth information to homes and small businesses over ordinary copper telephone lines. xDSL refers to different variations of DSL, such as ADSL, HDSL, and RADSL. Assuming your home or small business is close enough to a telephone company central office that offers DSL service, you may be able to receive data at rates up to 6.1 megabits (millions of bits) per second (of a theoretical 8.448 megabits per second), enabling continuous transmission of motion video, audio, and even 3-D effects. More typically, individual connections will provide from 1.544 Mbps to 512 Kbps downstream and about 128 Kbps upstream. A DSL line can carry both data and voice signals and the data part of the line is continuously connected. DSL installations began in 1998 and will continue at a greatly increased pace through the next decade in a number of communities in the U.S. and elsewhere. Compaq, Intel, and Microsoft working with telephone companies have developed a standard and easier-to-install form of ADSL called G.lite that is accelerating deployment. DSL is expected to replace ISDN in many areas and to compete with the cable modem in bringing multimedia and 3-D to homes and small businesses.
  • How It Works
  • Traditional phone service (sometimes called POTS for "plain old telephone service") connects your home or small business to a telephone company office over copper wires that are wound around each other and called twisted pair . Traditional phone service was created to let you exchange voice information with other phone users and the type of signal used for this kind of transmission is called an analog signal. An input device such as a phone set takes an acoustic signal (which is a natural analog signal) and converts it into an electrical equivalent in terms of volume (signal amplitude) and pitch (frequency of wave change). Since the telephone company's signalling is already set up for this analog wave transmission, it's easier for it to use that as the way to get information back and forth between your telephone and the telephone company. That's why your computer has to have a modem - so that it can demodulate the analog signal and turn its values into the string of 0 and 1 values that is called digital information.
  • Because analog transmission only uses a small portion of the available amount of information that could be transmitted over copper wires, the maximum amount of data that you can receive using ordinary modems is about 56 Kbps (thousands of bits per second). (With ISDN , which one might think of as a limited precursor to DSL, you can receive up to 128 Kbps.) The ability of your computer to receive information is constrained by the fact that the telephone company filters information that arrives as digital data, puts it into analog form for your telephone line, and requires your modem to change it back into digital. In other words, the analog transmission between your home or business and the phone company is a bandwidth bottleneck.
  • Digital Subscriber Line is a technology that assumes digital data does not require change into analog form and back. Digital data is transmitted to your computer directly as digital data and this allows the phone company to use a much wider bandwidth for transmitting it to you. Meanwhile, if you choose, the signal can be separated so that some of the bandwidth is used to transmit an analog signal so that you can use your telephone and computer on the same line and at the same time.
  • Splitter-based vs. Splitterless DSL
  • Most DSL technologies require that a signal splitter be installed at a home or business, requiring the expense of a phone company visit and installation. However, it is possible to manage the splitting remotely from the central office. This is known as splitterless DSL, "DSL Lite," G.Lite, or Universal ADSL and has recently been made a standard.
  • Modulation Technologies
  • Several modulation technologies are used by various kinds of DSL, although these are being standardized by the International Telecommunication Union (ITU ). Different DSL modem makers are using either Discrete Multitone Technology (DMT) or Carrierless Amplitude Modulation (CAP). A third technology, known as Multiple Virtual Line (MVL), is another possibility.
  • Factors Affecting the Experienced Data Rate
  • DSL modems follow the data rate multiples established by North American and European standards. In general, the maximum range for DSL without a repeater is 5.5 km (18,000 feet). As distance decreases toward the telephone company office, the data rate increases. Another factor is the gauge of the copper wire. The heavier 24 gauge wire carries the same data rate farther than 26 gauge wire. If you live beyond the 5.5 kilometer range, you may still be able to have DSL if your phone company has extended the local loop with optical fiber cable.
  • The Digital Subscriber Line Access Multiplexer (DSLAM)
  • To interconnect multiple DSL users to a high-speed backbone network, the telephone company uses a Digital Subscriber Line Access Multiplexer (DSLAM). Typically, the DSLAM connects to an asynchronous transfer mode (ATM) network that can aggregate data transmission at gigabit data rates. At the other end of each transmission, a DSLAM demultiplexes the signals and forwards them to appropriate individual DSL connections.
  • Types of DSL
  • ADSL
  • The variation called ADSL (Asymmetric Digital Subscriber Line) is the form of DSL that will become most familiar to home and small business users. ADSL is called "asymmetric" because most of its two-way or duplex bandwidth is devoted to the downstream direction, sending data to the user. Only a small portion of bandwidth is available for upstream or user-interaction messages. However, most Internet and especially graphics- or multi-media intensive Web data need lots of downstream bandwidth, but user requests and responses are small and require little upstream bandwidth. Using ADSL, up to 6.1 megabits per second of data can be sent downstream and up to 640 Kbps upstream. The high downstream bandwidth means that your telephone line will be able to bring motion video, audio, and 3-D images to your computer or hooked-in TV set. In addition, a small portion of the downstream bandwidth can be devoted to voice rather data, and you can hold phone conversations without requiring a separate line.
  • Unlike a similar service over your cable TV line, using ADSL, you won't be competing for bandwidth with neighbors in your area. In many cases, your existing telephone lines will work with ADSL. In some areas, they may need upgrading.
  • CDSL
  • CDSL (Consumer DSL) is a version of DSL, trademarked by Rockwell Corp., that is somewhat slower than ADSL (1 Mbps downstream, probably less upstream) and has the advantage that a "splitter" does not need to be installed at the user's end. Rockwell no longer provides information about CSDL at its Web site and does not appear to be marketing it.
  • G.Lite or DSL Lite
  • G.lite (also known as DSL Lite, splitterless ADSL, and Universal ADSL) is essentially a slower ADSL that doesn't require splitting of the line at the user end but manages to split it for the user remotely at the telephone company. This saves the cost of what the phone companies call "the truck roll." G.Lite, officially ITU-T standard G-992.2, provides a data rate from 1.544 Mbps to 6 Mpbs downstream and from 128 Kbps to 384 Kbps upstream. G.Lite is expected to become the most widely installed form of DSL.
  • HDSL
  • HDSL (High bit-rate Digital Subscriber Line), one of the earliest forms of DSL, is used for wideband digital transmission within a corporate site and between the telephone company and a customer. The main characteristic of HDSL is that it is symmetrical: an equal amount of bandwidth is available in both directions. HDSL can carry as much on a single wire of twisted-pair cable as can be carried on a T1 line (up to 1.544 Mbps) in North America or an E1 line (up to 2.048 Mbps) in Europe over a somewhat longer range and is considered an alternative to a T1 or E1 connection.
  • IDSL
  • IDSL (ISDN DSL) is somewhat of a misnomer since it's really closer to ISDN data rates and service at 128 Kbps than to the much higher rates of ADSL.
  • RADSL (Rate-Adaptive DSL) is an ADSL technology from Westell in which software is able to determine the rate at which signals can be transmitted on a given customer phone line and adjust the delivery rate accordingly. Westell's FlexCap2 system uses RADSL to deliver from 640 Kbps to 2.2 Mbps downstream and from 272 Kbps to 1.088 Mbps upstream over an existing line.
  • SDSL
  • SDSL (Symmetric DSL) is similar to HDSL with a single twisted-pair line, carrying 1.544 Mbps (U.S. and Canada) or 2.048 Mbps (Europe) each direction on a duplex line. It's symmetric because the data rate is the same in both directions.
  • UDSL
  • UDSL (Unidirectional DSL) is a proposal from a European company. It's a unidirectional version of HDSL.
  • VDSL
  • VDSL (Very high data rate DSL) is a developing technology that promises much higher data rates over relatively short distances (between 51 and 55 Mbps over lines up to 1,000 feet or 300 meters in length). It's envisioned that VDSL may emerge somewhat after ADSL is widely deployed and co-exist with it. The transmission technology (CAP, DMT, or other) and its effectiveness in some environments is not yet determined. A number of standards organizations are working on it.
  • x2/DSL
  • x2/DSL is a modem from 3Com that supports 56 Kbps modem communication but is upgradeable through new software installation to ADSL when it becomes available in the user's area. 3Com calls it "the last modem you will ever need."

Digital Subscriber Line Access Multiplexer - DSLAM:

  • A Digital Subscriber Line Access Multiplexer (DSLAM) is a network device, usually at a telephone company central office, that receives signals from multiple customer Digital Subscriber Line (DSL) connections and puts the signals on a high-speed backbone line using multiplexing techniques. Depending on the product, DSLAM multiplexers connect DSL lines with some combination of asynchronous transfer mode (ATM), frame relay, or Internet Protocol networks. DSLAM enables a phone company to offer business or homes users the fastest phone line technology (DSL) with the fastest backbone network technology (ATM).

Direct Access File System - DAFS

  • Direct Access File System (DAFS) is a network file system, similar to Network File System (NFS) and Common Internet File System (CIFS), that allows applications to transfer data while bypassing operating system control, buffering, and network protocol operations that can bottleneck throughput. DAFS uses the Virtual Interface (VI) architecture as its underlying transport mechanism. Using VI hardware, an application transfers data to and from application buffers without using the operating system, which frees up the processor and operating system for other processes and allows files to be accessed by servers using several different operating systems. DAFS is designed and optimized for clustered, shared-file network environments that are commonly used for Internet, e-commerce, and database applications. DAFS is optimized for high-bandwidth InfiniBand networks, and it works with any interconnection that supports VI including Fibre Channel and Ethernet.
  • Network Appliance and Intel formed the DAFS Collaborative as an industry group to specify and promote DAFS. Today, more than 85 companies are part of the DAFS Collaborative.

Direct Access Storage Device - DASD

  • Direct Access Storage Device (DASD), pronounced DAZ-dee, is a general term for magnetic disk storage devices that has historically been used in the mainframe and minicomputer (mid-range computer) environments. When used, it may also include hard disk drives for personal computers. A recent form of DASD is the redundant array of independent disks (RAID). The "direct access" means that all data can be accessed directly in about the same amount of time rather than having to progress sequentially through the data.

Direct Inward Dialing - DID:

  • Direct Inward Dialing (DID) is a service of a local phone company (or local exchange carrier) that provides a block of telephone numbers for calling into a company's private branch exchange (PBX) system. Using DID, a company can offer its customers individual phone numbers for each person or workstation within the company without requiring a physical line into the PBX for each possible connection.
  • For example, a company might rent 100 phone numbers from the phone company that could be called over eight physical telephone lines (these are called "trunk lines"). This would allow up to eight ongoing calls at a time; additional inbound calls would get a busy signal until one of the calls completed or be able to leave a voice mail message. The PBX automatically switches a call for a given phone number to the appropriate workstation in the company. A PBX switchboard operator is not involved.
  • A DID system can be used for fax and voice mail as well as for live voice connections. Compared to regular PBX service, DID saves the cost of a switchboard operator, calls go through faster, and callers feel they are calling a person rather than a company.

Disk Cache:

  • Disk caching works under the same principle as memory caching, but instead of using high-speed SRAM, a disk cache uses conventional main memory. The most recently accessed data from the disk (as well as adjacent sectors) is stored in a memory buffer. When a program needs to access data from the disk, it first checks the disk cache to see if the data is there. Disk caching can dramatically improve the performance of applications, because accessing a byte of data in RAM can be thousands of times faster than accessing a byte on a hard disk. Also see Cache.

Distributed Component Object Model - DCOM:

  • Distributed Component Object Model (DCOM) is a set of Microsoft concepts and program interfaces in which client program objects can request services from server program objects on other computers in a network. DCOM is based on the Component Object Model (COM), which provides a set of interfaces allowing clients and servers to communicate within the same computer (that is running Windows 95 or a later version).
  • For example, you can create a page for a Web site that contains a script or program that can be processed (before being sent to a requesting user) not on the Web site server but on another, more specialized server in the network. Using DCOM interfaces, the Web server site program (now acting as a client object) can forward a Remote Procedure Call (RPC ) to the specialized server object, which provides the necessary processing and returns the result to the Web server site. It passes the result on to the Web page viewer.
  • DCOM can also work on a network within an enterprise or on other networks besides the public Internet. It uses TCP/IP and Hypertext Transfer Protocol . DCOM comes as part of the Windows operating systems. DCOM is or soon will be available on all major UNIX platforms and on IBM's large server products. DCOM replaces OLE Remote Automation.
  • DCOM is generally equivalent to the Common Object Request Broker Architecture (CORBA ) in terms of providing a set of distributed services. DCOM is Microsoft's approach to a network-wide environment for program and data objects. CORBA is sponsored by the rest of the information technology industry under the auspices of the Object Management Group (OMG).

Distributed Denial-of-Service Attack - DoS:

  • On the Internet, a distributed denial-of-service (DDoS) attack is one in which a multitude of compromised systems attack a single target, thereby causing denial of service for users of the targeted system. The flood of incoming messages to the target system essentially forces it to shut down, thereby denying service to the system to legitimate users.
  • A hacker (or, if you prefer, cracker ) begins a DDoS attack by exploiting a vulnerability in one computer system and making it the DDoS "master." It is from the master system that the intruder identifies and communicates with other systems that can be compromised. The intruder loads cracking tools available on the Internet on multiple -- sometimes thousands of -- compromised systems. With a single command, the intruder instructs the controlled machines to launch one of many flood attacks against a specified target. The inundation of packets to the target causes a denial of service.
  • While the press tends to focus on the target of DDoS attacks as the victim, in reality there are many victims in a DDoS attack -- the final target and as well the systems controlled by the intruder.

DLC - Digital loop carrier:

  • Digital loop carrier (DLC) is equipment that bundles a number of individual phone line signals into a single multiplexed digital signal for local traffic between a telephone company central office and a business complex or other outlying service area. Typically, up to 24 analog voice calls are combined into a single signal and transmitted over a single copper T-carrier system or E-carrier line, an optical fiber cable, or a wireless connection. In a home, business, or other installation using digital loop carrier, the analog phone lines of individual users are connected to a local DLC box which then converts the analog signals into digital and combines (multiplexes) them into one signal that it sent to the phone company's central office on the single line. At the central office, the combined signal is separated back into the original signals. An estimated 20% of today's telephone users are being served by digital loop carriers.
  • Digital loop carrier can carry traffic for regular phone calls (plain old telephone service) and Integrated Services Digital Network (ISDN) service. More recently, approaches have been developed for using DLC to handle the higher bandwidth of Digital Subscriber Line (DSL) service. Digital loop carrier is typically used as an efficient way to provide service to an office building or complex and to extend service to new areas outside the current local loop . DLC is also used to set up telephone service in emergency situations. Customers can easily migrate from a T-1 or E-1 line to fiber optic when it becomes needed and is available.

DLT - Digital Linear Tape:

  • DLT (digital linear tape) is a form of magnetic tape and drive system used for computer data storage and archiving. A special compression algorithm, known as Digital Lempel Ziv 1 (DLZ1), facilitates storage and retrieval of data at high speeds and in large quantities. In the DLT drive, data is written on the tape in dozens of straight-line (linear) tracks, usually 128 or 208. Some cartridges can hold 70 gigabytes (GB ) of data when compression is used. A variant of DLT technology, called SuperDLT, makes it possible to store upwards of 100 GB on a single cartridge. The SuperDLT drive can transfer data at speeds of up to 10 megabytes per second (Mbps).
  • DLT is one of several technologies developed in recent years to increase the data-transfer rates and storage capacities of computer tape drives. Some examples of competing devices include the linear tape open (LTO) drive, the advanced intelligent tape (AIT) drive, and the Mammoth drive.

DNS - Domain Name System:

  • See Domain Name System.

DNIS - Dialed Number Identification Service:

  • See Dialed Number Identification Service.

DOM - Domain:

  • See Domain Name


  • See Domain Name

Domain Controller - DC

  • Primary domain controller (PDC) and backup domain controller (BDC) are roles that can be assigned to a server in a network of computers that use the Windows NT operating system. Windows NT uses the idea of a domain to manage access to a set of network resources (applications, printers, and so forth) for a group of users. The user need only to log in to the domain to gain access to the resources, which may be located on a number of different servers in the network. One server, known as the primary domain controller, manages the master user database for the domain. One or more other servers are designated as backup domain controllers. The primary domain controller periodically sends copies of the database to the backup domain controllers. A backup domain controller can step in as primary domain controller if the PDC server fails and can also help balance the workload if the network is busy enough.
  • In Windows NT, a domain combines some of the advantages of a workgroup (a group of users who exchange access to each others' resources on different computers) and a directory (a group of users who are managed centrally by an administrator). The domain concept not only allows a user to have access to resources that may be on different servers, but it also allows one domain to be given access to another domain in a trust relationship. In this arrangement, the user need only log in to the first domain to also have access to the second domain's resources as well.
  • In a Windows NT network, not all servers need to be a PDC or BDC. A server can be designated as a member server whose resources become part of a domain without having a role in the logon process. Setting up and maintaining PDCs and BDCs and domain information is a major activity for the administrator of a Windows NT network. In Windows 2000, the domain controller concept is retained but the PDC and BDC server roles are generally replaced by the Active Directory.

Domain Name:

  • A domain name locates an organization or other entity on the Internet. For example, the domain name locates an Internet address for "" at Internet point and a particular host server named "www". The "com" part of the domain name reflects the purpose of the organization or entity (in this example, "commercial") and is called the top-level domain name. The "totalbaseball" part of the domain name defines the organization or entity and together with the top-level is called the second-level domain name. The second-level domain name maps to and can be thought of as the "readable" version of the Internet address.
  • A third level can be defined to identify a particular host server at the Internet address. In our example, "www" is the name of the server that handles Internet requests. (A second server might be called "www2".) A third level of domain name is not required. For example, the fully-qualified domain name could have been "" and the server assumed.
  • Subdomain levels can be used. For example, you could have "". Together, "" constitutes a fully-qualified domain name. Second-level domain names must be unique on the Internet and registered with one of the ICANN -accredited registrars for the COM, NET, and ORG top-level domains. Where appropriate, a top-level domain name can be geographic. (Currently, most non-U.S. domain names use a top-level domain name based on the country the server is in.) To register a U. S. geographic domain name or a domain name under a country code, see an appropriate registrar.
  • On the Web, the domain name is that part of the Uniform Resource Locator(URL) that tells a domain name server using the domain name system (DNS ) whether and where to forward a request for a Web page. The domain name is mapped to an IP address (which represents a physical point on the Internet). More than one domain name can be mapped to the same Internet address. This allows multiple individuals, businesses, and organizations to have separate Internet identities while sharing the same Internet server. To see the IP address for a domain name, (ping).
  • It may be worth noting that the domain name system contains an even higher level of domain than the top-level domain. The highest level is the root domain, which would be represented by a single dot (just as in many hierarchical file systems, a root directory is represented by a "/" ) if it were ever used. If the dot for the root domain were shown in the URL, it would be to the right of the top-level domain name. However, the dot is assumed to be there, but never shown.

Domain Name System - DNS:

  • The Domain Name System (DNS) is the way that Internet domain names are located and translated into Internet Protocol addresses. A domain name is a meaningful and easy-to-remember "handle" for an Internet address.Because maintaining a central list of domain name/IP address correspondences would be impractical, the lists of domain names and IP addresses are distributed throughout the Internet in a hierarchy of authority. There is probably a DNS server within close geographic proximity to your access provider that maps the domain names in your Internet requests or forwards them to other servers in the Internet.

Domain Server:

DoS - Denial of Service:

  • See Denial of Service


  • See Digital Signal X.

DSL - Digital Subsciber Liner:

  • See Digital Subscriber Line.

DSLAM - Digital Subscriber Line Access Multiplexer:

  • See Digital Subscriber Line Access Multiplexer.

DSTP - Data Space Transfer Protocol:

  • See Data Space Transfer Protocol.

Dynamic HTML:

  • Dynamic HTML is a collective term for a combination of new Hypertext Markup Language (HTML ) tags and options, that will let you create Web pages more animated and more responsive to user interaction than previous versions of HTML. Much of dynamic HTML is specified in HTML 4.0. Simple examples of dynamic HTML pages would include (1) having the color of a text heading change when a user passes a mouse over it or (2) allowing a user to "drag and drop" an image to another place on a Web page. Dynamic HTML can allow Web documents to look and act like desktop applications or multimedia productions.
  • The features that constitute dynamic HTML are included in Netscape Communications' latest Web browser , Navigator 4.0 (part of Netscape's Communicator suite), and by Microsoft's browser, Internet Explorer 4.0. While HTML 4.0 is supported by both Netscape and Microsoft browsers, some additional capabilities are supported by only one of the browsers. The biggest obstacle to the use of dynamic HTML is that, since many users are still using older browsers, a Web site must create two versions of each site and serve the pages appropriate to each user's browser version.
  • The Concepts and Features in Dynamic HTML Both Netscape and Microsoft support:
  • * An object-oriented view of a Web page and its elements
  • * Cascading style sheets and the layering of content
  • * Programming that can address all or most page elements
  • * Dynamic fonts
  • An Object-Oriented View of Page Elements
  • Each page element (division or section, heading, paragraph, image, list, and so forth) is viewed as an "object." (Microsoft calls this the "Dynamic HTML Object Model." Netscape calls it the "HTML Object Model." W3C calls it the "Document Object Model.") For example, each heading on a page can be named, given attributes of text style and color, and addressed by name in a small progam or "script" included on the page. This heading or any other element on the page can be changed as the result of a specified event such a mouse passing over or being clicked or a time elapsing. Or an image can be moved from one place to another by "dragging and dropping" the image object with the mouse. (These event possibilities can be viewed as the reaction capabilities of the element or object.) Any change takes place immediately (since all variations of all elements or objects have been sent as part of the same page from the Web server that sent the page). Thus, variations can be thought of as different properties of the object.
  • Not only can element variations change text wording or color, but everything contained within a heading object can be replaced with new content that includes different or additional HTML as well as different text. Microsoft calls this the "Text Range technology." Style Sheets and Layering A describes the default style characteristics (including the page layout and font type style and size for text elements such as headings and body text) of a document or a portion of a document. For Web pages, a style sheet also describes the default background color or image, hypertext link colors, and possibly the content of page. Style sheets help ensure consistency across all or a group of pages in a document or a Web site.
  • Dynamic HTML includes the capability to specify style sheets in a "cascading style sheet " fashion (that is, linking to or specifying different style sheets or style statements with predefined levels of precedence within the same or a set of related pages). As the result of user interaction, a new style sheet can be made applicable and result in a change of appearance of the Web page. You can have multiple layers of style sheet within a page, a style sheet within a style sheet within a style sheet. A new style sheet may only vary one element from the style sheet above it.
  • Layering is the use of alternate style sheets or other approaches to vary the content of a page by providing content layers that can overlay (and replace or superimpose on) existing content sections. Layers can be programmed to appear as part of a timed presentation or as the result of user interaction. In Internet Explorer 4.0, Microsoft implements layers through style sheets. Netscape supports the style sheet approach but also offers a new HTML tag set (that Microsoft does not support). Both approaches are being considered by the W3C Working Committee and both companies say they will support whatever W3C decides will be the recommended approach. Programming
  • Although JavaScript, Java applet, and ActiveX controls were present in previous levels of Web pages, dynamic HTML implies an increased amount of programming in Web pages since more elements of a page can be addressed by a program. Dynamic Fonts Netscape includes dynamic fonts as part of dynamic HTML. This feature of Netscape's Navigator browser in its Communicator suite lets Web page designers include font files containing specific font styles, sizes, and colors as part of a Web page and to have the fonts downloaded with the page. That is, the font choice no longer is dependent on what the browser provides.

Dynamic Host Configuration Protocol - DHCP:

  • Dynamic Host Configuration Protocol (DHCP) is a communications protocol that lets network administrators manage centrally and automate the assignment of Internet Protocol (IP ) addresses in an organization's network. Using the Internet Protocol, each machine that can connect to the Internet needs a unique IP address . When an organization sets up its computer users with a connection to the Internet, an IP address must be assigned to each machine. Without DHCP, the IP address must be entered manually at each computer and, if computers move to another location in another part of the network, a new IP address must be entered. DHCP lets a network administrator supervise and distribute IP addresses from a central point and automatically sends a new IP address when a computer is plugged into a different place in the network.
  • DHCP uses the concept of a "lease" or amount of time that a given IP address will be valid for a computer. The lease time can vary depending on how long a user is likely to require the Internet connection at a particular location. It's especially useful in education and other environments where users change frequently. Using very short leases, DHCP can dynamically reconfigure networks in which there are more computers than there are available IP addresses.
  • DHCP supports static addresses for computers containing Web servers that need a permanent IP address. DHCP is an alternative to another network IP management protocol, Bootstrap Protocol (BOOTP ). DHCP is a more advanced protocol, but both configuration management protocols are commonly used. Some organizations use both protocols, but understanding how and when to use them in the same organization is important. Some operating systems, including Windows NT/2000, come with DHCP servers. A DHCP or BOOTP client is a program that is located in (and perhaps downloaded to) each computer so that it can be configured.

Dynamic Port Numbers:

  • The dynamic port numbers (also known as the private port numbers) are the port number s that are available for use by any application to use in communicating with any other application, using the Internet's Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP). When one application communicates with another application at another host computer on the Internet, it specifies that application in each data transmission by using its port number. The port numbers range from 0 through 65535. However, certain port numbers - the well-known port numbers and the registered port numbers - are registered and administered by the Internet Corporation for Assigned Names and Numbers (ICANN) for use by certain classes of applications. The dynamic port numbers are in the highest range, from 49152 through 65535. Before the arrival of ICANN, the port numbers were administered by the Internet Internet Assigned Numbers Authority (IANA).


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