DPSK: Differential phase shift keying. See modulation, phase.
DTE: Data terminal equipment. Any piece of equipment at which a communication path begins or ends, such as a terminal.
duplexing: An alternative to the process of mirroring, which occurs when a database server mirrors or backs up the database with each transaction. In mirroring, the server writes on two different hard disks through two different disk controllers. Duplexing is more redundant and therefore even safer than mirroring, because the database is written to two different hard disks on two different disk circuits. Compare with mirroring.
Dynamic Host Control Protocol (DHCP): A network-layer protocol standard used to supply TCP/IP address information using dynamic address assignment.
EBCDIC: Extended Binary Coded Decimal Interchange Code. A standard code consisting of a set of 8-bit characters used for information representation and interchange among data processing and communication systems. Very common in IBM equipment.
echo cancellation: Used in higher-speed modems to isolate and filter out (cancel) echoes when half-duplex transmissions use stop and wait ARQ (Automatic Repeat reQuest) protocols. Needed especially for satellite links.
echo suppressor: A device for use in a two-way telephone circuit (especially circuits over 900 miles long) to attenuate echo currents in one direction caused by telephone currents in the other direction. This is done by sending an appropriate disabling tone to the circuit.
Electronic Data Interchange (EDI):Electronic Data Interchange for Administration, Commerce, and Transport. Standardizes the electronic interchange of business documents for both ASCII and graphics. Endorsed by the ISO. Defines major components of the ANSI X.12 EDI standard.
Electronic Industries Association (EIA):Composed of electronic manufacturers in the United States. Recommends standards for electrical and functional characteristics of interface equipment. Belongs to ANSI. Known for the RS232 interface connector cable standard.
electronic mail (e-mail): A networking application that allows users to send and receive mail electronically.
electronic software distribution (ESD): ESD enables network managers to install software on client computers over the network without physically touching each client computer. ESD client software is installed on each client and enables an ESD server to download and install certain application packages on each client at some predefined time (e.g., at midnight on a Saturday).
emulate: Computer vendors provide software and hardware emulators that accept hardware and software from other vendors and enable them to run on their hardware or software.
encapsulation: A technique in which a frame from one network is placed within the data field of the frame in another network for transmission on the second network. For example, it enables a message initiated on a coaxial cable-based Ethernet LAN to be transmitted over an ATM fiber-optic-based network and then placed onto another Ethernet LAN at the other end.
encryption: The technique of modifying a known bit stream on a transmission circuit so that to an unauthorized observer, it appears to be a random sequence of bits.
end office: The telephone company switching office for the interconnection of calls.
envelope delay distortion: A derivative of the circuit phase shift with respect to the frequency. This distortion affects the time it takes for different frequencies to propagate the length of a communication circuit so that two signals arrive at different times.
equalization: The process of reducing frequency and phase distortion of a circuit by introducing time differences to compensate for the difference in attenuation or time delay at the various frequencies in the transmission band.
equalizer: Any combination (usually adjustable) of coils, capacitors, or resistors inserted in the transmission circuit or amplifier to improve its frequency response.
error control: An arrangement that detects the presence of errors. In some networks, refinements are added that correct the detected errors, either by operations on the received data or by retransmission from the source.
Ethernet: A LAN developed by the Xerox Corporation. It uses coaxial cable or twisted-pair wires to connect the stations. It was standardized as IEEE 802.3.
European Computer Manufacturers Association (ECMA): Recommends standards for computer components manufactured or used in Europe. Belongs to the International Organization for Standardization (ISO).
exposure: The calculated or estimated loss resulting from the occurrence of a threat, as in "The exposure from theft could be $42,000 this year." It can be either tangible and therefore measurable in dollars or intangible and therefore not directly measurable in dollars. See also comparison risk ranking.
extranet: Using the Internet to provide access to information intended for a selected set of users, not the public at large. Usually done by requiring a password to access a selected set of Web sites.
FDM: Frequency division multiplexing.
feasibility study: A study undertaken to determine the possibility or probability of improving the existing system within a reasonable cost. Determines what the problem is and what its causes are and makes recommendations for solving the problem.
Federal Communications Commission (FCC): A board of seven commissioners appointed by the U.S. president under the Communication Act of 1934, having the power to regulate all interstate and foreign electrical communication systems originating in the United States.
fiber distributed data interface (FDDI): A token ring-like LAN technology that permits transmission speeds of 100 million bps using fiber-optic cables (ANSI standard X3T9.5).
fiber-optic cable: A transmission medium that uses glass or plastic cable instead of copper wires.
fiber optics: A transmission technology in which modulated visible light wave signals containing information are sent down hair-thin plastic or glass fibers and demodulated back into electrical signals at the other end by a special light-sensitive receiver.
File Transfer Protocol (FTP): FTP enables users to send and receive files over the Internet. There are two types of FTP sites: closed (which require users to have an account and a password) and anonymous (which permit anyone to use them).
firewall: A firewall is a router, gateway, or special-purpose computer that filters packets flowing into and out of a network. No access to the organization’s networks is permitted except through the firewall. Two commonly used types of firewalls are packet level and application level.
firmware: A set of software instructions set permanently or semi permanently into read-only memory (ROM).
flow control: The capability of the network nodes to manage buffering schemes that allow devices of different data transmission speeds to communicate with each other.
forward error correction (FEC): A technique that identifies errors at the received station and automatically corrects those errors without retransmitting the message.
fractional T1 (FT1): A portion of a T1 circuit. A full T1 allows transmission at 1,544,000 bps. A fractional T1 circuit allows transmission at lower speeds of 384,000, 512,000, or 768,000 bps.
fragment free switching: A type of switching that is a cross between store and cut through. Messages are stored until the header has been checked for errors and then the message is forwarded without checking for errors in the rest.
frame: Generally, a group of data bits having bits at each end to indicate the beginning and end of the frame. Frames also contain source addresses, destination addresses, frame type identifiers, and a data message.
frame check sequence (FCS): Used for error checking. FCS uses a 16-bit field with cyclical redundancy checking for error detection with retransmission.
frame relay: Frame relay is a type of packet-switching technology that transmits data faster than the X.25 standard. The key difference is that unlike X.25 networks, frame relay does not perform error correction at each computer in the network. Instead, it simply discards any messages with errors. It is up to the application software at the source and destination to perform error correction and to control for lost messages.
frequency: The rate at which a current alternates, measured in Hertz, kilohertz, megahertz, and so forth. Other units of measure are cycles, kilocycles, or megacycles; hertz and cycles per second are synonymous.
front-end processor (FEP): An auxiliary processor that is placed between a computer’s CPU and the transmission facilities. This device normally handles housekeeping functions like circuit management and code translation, which otherwise would interfere with efficient operation of the CPU.
FSK: Frequency shift keying. A modulation technique whereby 0 and 1 are represented by a different frequency and the amplitude does not vary.
full duplex: The capability of transmission in both directions at one time. Contrast with half-duplex and simplex.
gateway: A device that connects two dissimilar networks. Allows networks of different vendors to communicate by translating one vendor’s protocol into another.
Gbps: Gigabit per second; 1 Gbps is equal to 1 billion bps.
GHz: Gigahertz; 1 GHz is equal to 1 billion cycles per second in a frequency.
gigabyte: One billion bytes.
G.Lite: One de facto standard form of ADSL.
guardband: A small bandwidth of frequency that separates two voice-grade circuits. Also, the frequencies between subcircuits in FDM systems that guard against subcircuit interference.
hacker: A person who sleuths for passwords to gain illegal access to important computer files. Hackers may rummage through corporate trash cans looking for carelessly discarded printouts.
half-duplex: A circuit that permits transmission of a signal in two directions but not at the same time. Contrast with full duplex and simplex.
Hamming code: A forward error correction (FEC) technique named for its inventor.
handshaking: Exchange of predetermined signals when a connection is established between two data set devices. This is used to establish the circuit and message path.
Hertz (Hz): Same as cycles per second; for example, 3,000 Hz is 3,000 cycles per second.
high-level data link control (HDLC): A bit-oriented protocol in which control of data links is specified by series of bits rather than by control characters (bytes).
home page: A home page is the main starting point or page for a World Wide Web entry.
host computer: The computer that lies at the center of the network. It generally performs the basic centralized data processing functions for which the network was designed. The host used to be where the network communication control functions took place, but today these functions tend to take place in the front-end processor or further out in the network. Also called a central computer.
hotline: A service that provides direct connection between customers in various cities using a dedicated circuit.
HTML: Web text files or pages use a structural language called HTML (Hypertext Markup Language) to store their information. HTML enables the author to define different type styles and sizes for the text, titles, and headings, and a variety of other formatting information. HTML also permits the author to define links to other pages that may be stored on the same Web server or on any Web server anywhere on the Internet.
hub: Network hubs act as junction boxes, permitting new computers to be connected to the network as easily as plugging a power cord into an electrical socket, and provide an easy way to connect network cables. Hubs also act as repeaters or amplifiers. Hubs are sometimes also called concentrators, multistation access units, or transceivers.
idle character: A transmitted character indicating "no information" that does not manifest itself as part of a message at the destination point.
IESG: Internet Engineering Steering Group.
IETF: Internet Engineering Task Force.
in-band signaling: The transmission signaling information at some frequency or frequencies that lie within a carrier circuit normally used for information transmission.
Institute of Electrical and Electronics Engineers (IEEE): A professional organization for engineers in the United States. Issues standards and belongs to the ANSI and the ISO.
interexchange circuit (IXC): A circuit or circuit between end offices (central offices).
interLATA: Circuits that cross from one LATA (local access and transport area) into another.
intermodulation distortion: An analog line impairment whereby two frequencies create a third erroneous frequency, which in turn distorts the data signal representation..
International Telecommunications Union— Telecommunications (ITU-T): An international organization that sets worldwide communication standards. Its old name was Consultative Committee on International Telegraph and Telephone (CCITT).
Internet: The information superhighway. The network of networks that spans the world, linking more than 20 million users.
Internet Architecture Board (IAB): IAB provides strategic architectural oversight (e.g., top-level domain names, use of international character sets) that can be passed on as guidance to the IESG or turned into published statements or simply passed directly to the relevant IETF working group. The IAB does not produce polished technical proposals but rather tries to stimulate action by the IESG or the IETF that will lead to proposals that meet general consensus. The IAB appoints the IETF chair and all IESG members.
Internet Assigned Numbers Authority (IANA): IANA governs the assignment of IP numbers.
Internet Control Message Protocol (ICMP): A simple network layer protocol standard intended to exchange limited routing information between routers. Most commonly known as a ping, after the DOS and UNIX command.
Internet Engineering Steering Group (IESG): The IESG is responsible for technical management of IETF activities and the Internet standards process. It administers the process according to the rules and procedures and is directly responsible for the actions associated with entry into and movement along the Internet "standards track," including final approval of specifications as Internet standards. Each IETF working group is chaired by a member of the IESG.
Internet Engineering Task Force (IETF):IETF is a large, open international community of network designers, operators, vendors, and researchers concerned with the evolution of the Internet architecture and the smooth operation of the Internet.IETF operates through a series of working groups, which are organized by topic (e.g., routing, transport, security). The requests for comment (RFCs) that form the basis for Internet standards are developed by the IETF and its working groups.
Internet Mail Access Protocol (IMAP): An application-layer protocol standard that covers communication between an e-mail client and an e-mail server.
Internet Research Task Force (IRTF): IRTF operates much like the IETF, through small research groups focused on specific issues. Although IETF working groups focus on current issues, IRTF research groups work on long-term issues related to Internet protocols, applications, architecture, and technology. The IRTF chair is appointed by the IAB.
Internet service provider (ISP): ISPs offer connections to the Internet. Some access providers charge a flat monthly fee for unlimited access (much like the telephone company), whereas others charge per hour of use (much like a long-distance telephone call).
Internet Society (ISOC): ISOC is the closest the Internet has to an owner. ISOC is an open-membership professional society with more than 175 organizational and 8,000 individual members in over 100 countries and includes corporations, government agencies, and foundations that have created the Internet and its technologies.
internetworking: Connecting several networks together so workstations can address messages to the workstations on each of the other networks.
Internet2: There are many different organizations currently working on the next generation of the Internet, including the Abilene network, vBNS, and CA*net. Although each is working in a slightly different fashion, they join together with each other and parts of the regular Internet at gigapops (gigabit points of presence).
interoperability: The interconnection of dissimilar networks in a manner that allows them to operate as though they were similar.
IntraLATA: Circuits that are totally within one LATA (local access transport area).
intranet: Using Internet protocols on a network internal to an organization so that information is accessible using a browser, for example, but only by employees, not the public at large. Usually done by requiring a password to access a selected set of Web sites and protecting the site by a firewall so no outsiders can access it.
intrusion detection system (IDS): An IDS monitors a network segment, a server, or an application on the server for signs of unauthorized access and issues an alarm when an intrusion is detected. A misuse detection IDS compares monitored activities with signatures of known attacks, whereas an anomaly detection IDS compares monitored activities with the "normal" set of activities.
inverse multiplexer: Hardware that takes one high-speed transmission and divides it among several transmission circuits.
IPX/SPX: Internetwork packet exchange/ sequenced packet exchange (IPX/SPX), based on a routing protocol developed by Xerox in the 1970s, is the primary network protocol used by Novell NetWare. About 40 percent of all installed LANs use it.
ISDN: Integrated services digital network. A hierarchy of digital switching and transmission systems. The ISDN provides voice, data, and image in a unified manner. It is synchronized so all digital elements speak the same "language" at the same speed. See also basic rate interface (BRI) and primary rate interface (PRI).
ISO: International Organization for Standardization, in Geneva, Switzerland. The initials ISO stand for its French name. This international standards-making body is best known in data communications for developing the internationally recognized seven-layer network model called the Open Systems Interconnection (OSI) Reference model. See also OSI model.
jack: The physical connecting device at the interface that mates with a compatible receptacle—a plug.
jitter: Type of analog communication line distortion caused by the variation of a signal from its reference timing positions, which can cause data transmission errors, particularly at high speeds. This variation can be in amplitude, time, frequency, or phase.
jumper: 1. A small connector that fits over a set of pins on a microcomputer circuit card. 2. A patch cable or wire used to establish a circuit for testing or diagnostics.
K: A standard quantity measurement of computer storage. A K is defined loosely as 1,000 bytes. In fact, it is 1,024 bytes, which is the equivalent of 210.
Kbps: Kilobits per second. A data rate equal to 103 bps (1,000 bps).
Kermit: Kermit is a very popular asynchronous file transfer protocol named after Kermit the Frog. The Kermit protocol was developed by Columbia University, which released it as a free software communications package. Various versions of Kermit can be found on public bulletin board systems for downloading to a microcomputer.
key management: The process of controlling the secret keys used in encryption.
KHz: Kilohertz; 1 KHz is equal to 1,000 cycles per second in a frequency..
kilometer: A metric measurement equal to 0.621 mile or 3,280.8 feet.
laser: Light amplification by stimulated emission of radiation. A device that transmits an extremely narrow and coherent beam of electromagnetic energy in the visible light spectrum. (Coherent means that the separate waves are in phase with one another rather than jumbled as in normal light.)
LATA: Local access transport area. One of approximately 200 local telephone service areas in the United States roughly paralleling major metropolitan areas. The LATA subdivisions were established as a result of the AT&T/Bell divestiture to distinguish local from long-distance service. Circuits with both end points within the LATA (intraLATAs) generally are the sole responsibility of the local telephone company. Circuits that cross outside the LATA (interLATAs) are passed on to an interexchange carrier like AT&T, MCI, or US Sprint.
latency: The delay between when the first bits of a message arrive at a device and when it begins transmitting them.
leased circuit: A leased communication circuit that goes from your site to some other location. It is a clear, unbroken communication path that is yours to use 24 hours per day, 7 days per week. Also called private circuit or dedicated circuit.
line: A circuit, channel, or link. It carries the data communication signals. An early telephone technology term that may imply a physical connection, such as with a copper wire. Compare with channel, circuit, and link.
link: An unbroken circuit path between two points. Sometimes called a line, channel, or circuit.
Listserv: A listserver (or Listserv) is a mailing list. One part, the Listserv processor, processes commands such as requests to subscribe, unsubscribe, or to provide more information about the Listserv. The second part is the Listserv mailer. Any message sent to the Listserv mailer is resent to everyone on the mailing list.
LLC: The logical link control, or LLC, sublayer is just an interface between the MAC sublayer and software in layer 3 (the network layer) that enables the software and hardware in the MAC sublayer to be separated from the logical functions in the LLC sublayer. By separating the LLC sublayer from the MAC sublayer, it is simpler to change the MAC hardware and software without affecting the software in layer 3. The most commonly used LLC protocol is IEEE 802.2.
local area network (LAN): A network that is located in a small geographic area, such as an office, a building, a complex of buildings, or a campus, and whose communication technology provides a high-bandwidth, low-cost medium to which many nodes can be connected. These networks typically do not use common carrier circuits, and their circuits do not cross public thoroughfares or property owned by others. LANs are not regulated by the FCC or state public utilities commissions.
local exchange carrier: The local telephone company, such as one of the seven regional Bell operating companies (RBOCs).
local loop: The part of a communication circuit between the subscriber’s equipment and the equipment in the local central office.
log: 1. A record of everything pertinent to a system function. 2. A collection of messages that provides a history of message traffic.
longitudinal redundancy check (LRC): A system of error control based on the formation of a block check following preset rules. The check formation rule is applied in the same manner to each character. In a simple case, the LRC is created by forming a parity check on each bit position of all characters in the block. (That is, the first bit of the LRC character creates odd parity among the 1-bit positions of the characters in the block.)
M: Mega. The designation for 1 million, as in 3 megabits per second (3 Mbit/s).
management information base (MIB): The extent of information that can be retrieved from a user microcomputer when using the Simple Network Management Protocol (SNMP) for network management. MIBs are sets of attributes and definitions that pertain to specific network devices.
Manchester encoding: The digital transmission technique used in the physical layer of Ethernet LANs.
Mbps: A data rate equal to 106 bps. Sometimes called megabits per second (1,000,000 bps).
media access control (MAC): A data link layer protocol that defines how packets are transmitted on a local area network. See also CSMA/CD, token bus, and token ring.
medium: The matter or substance that carries the voice or data transmission. For example, the medium can be copper (wires), glass (fiber-optic cables), or air (microwave or satellite).
megabit: One million bits.
megabyte: One million bytes.
mesh network: A network topology in which there are direct point-to-point connections among the computers.
message: A communication of information from a source to one or more destinations. A message usually is composed of three parts: (1) a heading, containing a suitable indicator of the beginning of the message together with some of the following information: source, destination, date, time, routing; (2) a body containing the information to be communicated; (3) an ending containing a suitable indicator of the end of the message.
message switching: An operation in which the entire message being transmitted is switched to the other location without regard to whether the circuits actually are interconnected at the time of your call. This usually involves a message store and forward facility.
meter: A metric measurement equal to 39.37 inches.
metropolitan area network (MAN): A network that usually covers a citywide area. Because MANs use LAN and fiber-optic technologies, transmission speeds can vary between 2 million and 100 million bps.
MHz: megahertz; 1 MHz is equal to 1 million cycles per second in a frequency. Extension (MIME).
MIPS: One million instructions per second. Used to describe a computer’s processing power.
mirroring: A process in which the database server automatically backs up the disk during each database transaction. During this process, the computer writes on two different hard disks on the same disk circuit every time the hard disk is updated. This creates two mirror images of the database data. Disk mirroring can be accomplished only when the database server contains two physical disk drives, because the records or data structures are written to both disks simultaneously. Should a problem develop with one disk, the second disk is available instantly with identical information on it. Compare with duplexing.
mnemonic: A group of characters used to assist the human memory. A mnemonic frequently is an acronym.
modem: A contraction of the words modulator-demodulator. A modem is a device for performing necessary signal transformation between terminal devices and communication circuits. Modems are used in pairs, one at either end of the communication circuit.
modulation, amplitude: The form of modulation in which the amplitude of the carrier is varied in accordance with the instantaneous value of the modulating signal.
modulation, frequency: A form of modulation in which the frequency of the carrier is varied in accordance with the instantaneous value of the modulating signal.
modulation, phase: A form of modulation in which the phase of the carrier is varied in accordance with the instantaneous value of the modulating signal. Phase modulation has two related techniques. Phase shift keying (PSK) uses a 180° change in phase to indicate a change in the binary value (0 or 1), Differential phase shift keying (DPSK) uses a 180° change in phase every time a bit is transmitted; otherwise, the phase remains the same.
modulation, pulse code: See pulse code modulation (PCM).
MTBF: mean time between failures. The statistic developed by vendors to show the reliability of their equipment. It can be an actual calculated figure that generally is more accurate, or it can be a practical (theoretical) figure.
MTTD: Mean time to diagnose. The time it takes the network testing and problem management staff to diagnose a network problem.
MTTF: Mean time to fix. The time it takes vendors to remedy a network problem once they arrive on the premises.
MTTR: 1. Mean time to repair—the combination of mean time to diagnose, mean time to respond, and mean time to fix, indicating the entire length of time it takes to fix a fault in equipment. 2. Mean time to respond—the time it takes the vendor to respond when a network problem is reported.
multidrop (multipoint): A line or circuit interconnecting several stations/nodes in a sequential fashion. multiplexer: A device that combines data traffic from several low-speed communication circuits onto a single high-speed circuit. The two popular types of multiplexing are FDM (frequency division multiplexing) and TDM (time division multiplexing). In FDM, the voice-grade link is divided into subcircuits, each covering a different frequency range in such a manner that each subcircuit can be employed as though it were an individual circuit. In TDM, separate time segments are assigned to each terminal. During these time segments, data may be sent without conflicting with data sent from another terminal.
multiplexing (MUX): The subdivision of a transmission circuit into two or more separate circuits. This can be achieved by splitting the frequency range of the circuit into narrow frequency bands (frequency division multiplexing) or by assigning a given circuit successively to several different users at different times (time division multiplexing).
