The Wall Street Journal's famous D conference, which stands for digital. D was cooked up by two of the Journal's star columnists ” Walt Mossberg of "Personal Technology" and Kara Swisher of "Boom Town" ” back in the heady days of 1999. The premise remains unchanged: A digital revolution continues to transform the way we work and live. Or, as Swisher claims in the event guide, "The tidal wave of technological change continues to advance upon us." But there's an added caveat: Tough times are the best times to gain an edge. D could just as easily stand for deja vue ” from the setting (the Four Seasons resort, where the last of the Industry Standard's briefly legendary digital summits was held) to the 400-person crowd that included billionaires mingling in khakis, conspiring to push the restart button for a tech community limping out of the desert of unrelentingly bad news. These comments came from a report on the August 2003 show.
Also called Channel Bank. It breaks down a T-1 circuit to its 24 channels.
A FCC designation for Personal Communications Services (PCS) license granted to a telephone company serving a Major Trading Area (MTA). This grants permission to operate at certain FCC-specified frequencies.
In an ISDN interface, the "D" channel (the Data channel) is used to carry control signals and customer call data in a packet switched mode. In the BRI (Basic Rate Interface, i.e. the lowest ISDN service) the "D" channel runs at 16,000 bits per second, part of which will carry setup, teardown , ANI and other characteristics of the call. 9,600 bps will be free for a separate "conversation" by the user . That "conversation" will typically be data. And many phone companies are now selling it as an "on the Internet all the time" channel, allowing you to receive and send email continuously. In the PRI (Primary Rate Interface, i.e. ISDN equivalent of T-1), the "D" channel runs at 64,000 bits per second. The D channel provides the signaling information for each of the 23 voice channels (referred to as "B channels"). The actual data which travels on the D channel is much like that of a common serial port. Bytes are loaded from the network and shifted out to the customer site in a serial bit stream. The customer site responds with its serial bit stream, too. An example of a data packet sent from the network to indicate a new call has the following components :
Customer Site ID
Type of Channel Required (Usually a B channel)
Call Handle (Not unlike a file handle)
ANI and DNIS information
Channel Number Requested
A Request for a Response
This packet is responded to by the customer site with a format similar to:
Channel Type is OK
Call Handle The packets change as the state of the call changes, and finally ends with one side or the other sending a disconnect notice. The important concept here is the fact the information on the D channel could actually be anything ” any kind of serial data. It could just as well be sports scores! So with that in mind, consider the Channel Number Requested packet above. This is the networks' selected channel for the customer site to use. Normally, this number is between 1 and 23, but could be a higher number if needed. This is what NFAS is all about. NFAS (Non Facility Associated signaling, pronounced N-FAST without the T) allows a D channel to carry call information regarding channels which may not even exist in the same PBX or PC system. See also ISDN and DS-0.
A type of line conditioning which controls harmonic distortion and signal-to-noise ratio so that they lie within specified limits. See also Conditioning.
A cable connecting standard housed in a shell that resembles the letter D. D connectors, especially DB-9, DB-15, and DB-25 are used to connect computers to peripheral devices such as modems.
Diagonal Link. A SS7 signaling link used to connect STP pairs that perform work at different functional levels. These links are arranged in sets of four (called quads ). Connected mated pairs of Signal Transfer Points (STPs) at different hierarchical levels. For example, from a local pair of STPs to a regional pair of STPs. Like the B-Links, D-links are assigned in a quad arrangement. Once again, the recommendation is for three way path diversity. See A, B and C Links.
That portion of the ionosphere existing approximately 50 to 90 kilometers above the surface of the Earth. Attenuation of radio waves, caused by ionospheric free- electron density generated by cosmic rays from the sun, is pronounced during daylight hours.
The standard connector used for RS-232-C, RS-423 and RS-422 communications. D-type connectors are typically seen in nine, 15 and 25 pin configurations.
Digital Advanced Mobile Phone Service. The EIA/TIA Interim Standard 136 (IS-136) which succeeded IS-54, and which addresses US digital cellular systems employing TDMA (Time Division Multiple Access). See IS-136.
Another name for channel bank. A device that multiplexes groups of 24 channels of digitized voice input at 64 Kbps into T-1 aggregate outputs of 1,544,000 bits per second.
A D-Block Carrier is a 10-MHz PCS carrier serving a Basic Trading Area (BTA) in the frequency block 1865-1970 MHz paired with 1945-1950 MHz.
Also called DBIT. The delivery confirmation bit in an X.25 packet used to indicate whether or not the DTE wishes to receive an end-to-end acknowledgment of delivery. In short, a bit in the X.25 packet header that assures data integrity between the TPAD and the HPAD.
A CLEC which specializes in delivering only data, most typically DSL services. See CLEC.
Direct-Inside Wire. Made of 24-gauge, annealed- copper conductors with color -added PVC, which allows it to be pulled in conduit without the aid of lubricants. Generally used in the horizontal subsystem.
Digital to Analog conversion.
Digital to Analog converter. A device which converts digital pulses , i.e. data, into analog signals so that the signal can be used by an analog device such as amplifier , speaker, phone, or meter.
Drop and Insert. See Bit Stuffing and Bit Robbing.
T-1 framing formats developed for channel banks. All formats contain a framing bit in every 193rd bit position. The Superframe (introduced in D2 channel banks) is made up of 12 193-bit frames, with the 193rd bit sequence being repeated every 12 frames . D2 framing also introduced robbed bit signaling, where the eighth bit in frames 6 and 12 were "robbed" for signaling information (like dial pulses). D1D was introduced after D2 to allow backwards compatibility of Superframe concepts to D1 banks.
One of two European formats for analog HDTV.
24 data channels on one standard (North American standard) T-1/D3 span line. Each data channel is 8 bits wide and has a bandwidth of 8 KHz. See also DS-1.
Refers to compliance with AT&T TR (Technical Reference) 62411 definitions for coding, supervision and alarm support. D3/D4 compatibility ensures support of digital PBXs, M24 services, Megacom services and Mode 3 D3/D4 channel banks at a DS-1 level.
In T-1 digital transmission technology, D4 is the fourth-generation channel bank. A channel bank is the interface between the T-1 carrier system and an analog premises device such as an analog PBX (private branch exchange).
Refers to compliance with AT&T TR (Technical Reference) 62411 in regards to the DSI frame layout (the sequential assignment of channels and time slot numbers within the DSI).
First read T-1 FRAMING. The most popular framing format in the T-1 environment is D-4 framing. The name stems from the way framing is performed in the D- series of channel banks from AT&T. There are 12 separate 193-bit frames in a super-frame. The D-4 framing bit is used to identify both the channel and the signaling frame. In voice communications, signaling is an important function that is simulated and carried by all the equipment in the transmission path. In D-4 framing, signaling for voice channels is carried "in- band " by every channel, along with the encoded voice. "Robbed-bit-signaling" is a technique used in D-4 channel banks to convey signaling information. With this technique, the eighth bit (least significant bit) of each of the 24 8-bit time slots is "robbed" every sixth frame to convey voice related signaling information (on-hook, off-hook, etc.) for each voice channel. See also Extended Super-Frame Format.
Dedicated hand-off facility.
Doesn't answer, as in "The phone rang DA."
Discontinued Availability. Meaning a circuit that was once available is now no longer.
Destination Address, a field in FDDI, Ethernet and Token Ring packets which identifies the unique MAC (Media Access Control, the lower part of ISO layer two) address of the recipient. A six octet value uniquely identifying an endpoint and which is sent in IEEE LAN frame headers to indicate frame destination.
Desk Accessory. Standard desk accessories on the Apple Macintosh include a calculator, alarm clock and the chooser. Desk accessories are available to the user regardless of the application currently in use, networked or non-networked. Desk accessories are installed in the Apple menu and accessed from there.
Data Access Arrangement. A device required before the FCC registration program if a customer was going to hook up CPE (Customer Provided Equipment), usually modems and other data equipment, to the telephone network. Today, equipment is FCC registered (under Part 68) meaning that the device itself is approved for connection to the phone network. DAAs can still be found in old DP (data processing) installations.
Dynamically Allocable Bandwidth.
Digital Audio Broadcasting. Radio broadcasting using digital modulation and digital source coding techniques.
Digital Audio Broadcast. The international term for DARS (Digital Audio Radio System), which are proposed satellite-delivered audio/radio systems. See DARS.
Digital to Analog Converter. A device which converts digital pulses, i.e. data, into analog signals so that the signal can be used by analog device such as amplifier, speaker, phone, or meter. In the imaging field, a DAC is a chip that converts the binary numbers that represent particular colors to analog red, green and blue signals that a color monitor displays.
Dual Attachment Connector. See Dual Attachment Connector.
Digital Access Cross-Connect.
Directory Assistance Call Completion.
Digital Automatic Call Distributor. Another a central office-provided ACD supplied by a local phone company.
A packet-switched network in South Korea.
Also abbreviated as DCS, DCCS and DXC, a DACS is a Digital Access and Cross- Connect System. It is a high-capacity, non-blocking electronic cross-connect device for directing and re-directing circuits and channels in T-carrier and SONET/SDH systems. In a T-carrier system, a DACS works at the DS-0, DS-1 (T-1/E-1), and DS-3 (T-3/E-3) levels. In a SONET/SDH system, a DACS works at the STS-1 level. A DACS is a simple form of channel switch. Unlike a typical voice switch, for example, it does not switch circuits and channels "on the fly" according to dialing instructions. Unlike a packet switch, it does not direct individual packets based on packet addresses. Rather, it is pre-programmed to switch specific circuits or channels from incoming port to outgoing port. In other words and for example, you might sit at a DACS console in Los Angeles and give it specific instructions to connect this T-1 line from San Diego to that T-1 line heading to San Francisco, perhaps in support of a full-motion, broadcast quality videoconference. The next day, you can redirect the San Diego T-1 to a T-1 heading to Bakersfield, if the company president wants to set up a videoconference with the staff there. A DACS, in effect, is a programmable electronic cross- connect or patch panel.
An agent program which continuously operates on a UNIX server and which provides resources to client systems on the network. Daemon is a background process used for handling low-level operating system tasks . In Greek mythology, "Daemon" was a supernatural being acting as an intermediary between the gods and man.
Disk And Execution MONitor. A harmless UNIX program that waits in the background and runs when a request is made on the port that it is watching. It normally works out of sight of the user. On the Internet, it is most likely encountered only when e-mail is not delivered to the recipient. You'll receive your original message plus a message from a "mailer daemon."
Destination Address Field.
Decrement All Frame. Motorola definition.
I am addicted to direct mail, now Internet direct mail. My "daily fix," according to my office, is the package that arrives daily from a direct mail supplier, such as LL Bean, Eddie Bauers, Buy.com.
A method of connecting devices in a series, much as one might inter- weave daisies to make a lovely floral wreath, or so the story goes. Signals are passed through the chain from one device to the next. Jack 1 is connected to jack 2, which is connected to jack 3 and so on. The last jack in the chain is not connected to jack 1. A SCSI adapter, for instance, is a daisy chain, supporting a daisy chain of up to seven devices. Intel's Universal Serial Bus also is a daisy chain. Stackable hubs, switches and other devices are daisy-chained. While this approach yields the lovely advantage of scalability, interconnection of such devices in this manner also yields some less-than -lovely level of performance degradation, as each device in the chain becomes a point of contention and, therefore, a point of potential congestion. See also Scalable and Stackable.
Dedicated Access Line. A private tie line from you to your long distance or local phone company. The line may be analog or digital, e.g. a T-1 circuit.
Demand Assigned Multiple Access. A way of sharing a channel's capacity by assigning capacity on demand to an idle channel or an unused time slot.
A wave consisting of a series of oscillations or cycles of current gradually decreasing amplitude.
To prevent excessive route change announcements from entering a carrier's Internet network and degrading router performance. Many carriers dampen route announcements when the customer exceeds its Dampen Limit. Sprint stops dampening and renews announcing customer routes when the customer reaches its Reuse Limit. Sprint has a dampening policy to dampen Internet traffic to confine network instabilities to a localized area. Network instabilities are caused by customer route flapping. Dampening prevents network instabilities from destabilizing the Sprint Internet Network, other Sprint customer networks, and other portions of global Internet traffic. Sprint uses Cisco Router IOS BGP to dampen Internet traffic. See Dampen Limit and Reuse Limit.
Customer penalty value at which point Sprint dampens the customer route announcements. The current Dampen Limit is 2000. See Dampen and Reuse Limit.
The decreasing of the amplitude of oscillations caused by resistance in the circuit.
The progressive diminution with time of certain quantities characteristic of a phenomenon .
The progressive decay with time in the amplitude of the free oscillations in a circuit.
More generally, decreasing some dimension of a phenomenon, such as its power.
Digital Advanced Mobile Phone Service. Originally, AMPS was used as a 900 MHz frequency modulation (FM) transmission technology with bandwidth allocated according to frequency division multiple access (FDMA) schemes. To increase capability and security, digital techniques for cellular were introduced and systems are being converted from AMPS to DAMPS. The two most prevalent means of dividing frequencies in DAMPS are time division multiple access (TDMA) and code division multiple access (CDMA). These two formats are not directly compatible. See AMPS, NAMPS.
Gratuitous animated GIF files and other Web special effects that are used to impress people. "This page is kinda dull. Maybe a little dancing baloney will help?" This definition courtesy Wired Magazine.
A problem or image on your computer screen that disappears just as soon as you try to show it to someone else. The same thing seems to happen with automobiles when you take a normally troublesome car in for a checkup with the mechanic .
Directory Access Protocol. The protocol used between a Directory User Agent (DUA) and Directory System Agent (DSA) in an X.500 directory system. See X.500 and LDAP.
Optical fiber through which no light is currently being transmitted. See Dark Fiber.
The flow of electricity through the diode in a photodiode when no light is present. Photodiodes are often used as light-sensitive switches. When light hits them, they turn on. Here's a more technical explanation: Dark current is the induced current that exists in a reversed biased photodiode in the absence of incident optical power. It is better understood to be caused by the shunt resistance of the photodiode. A bias voltage across the diode (and the shunt resistance) causes current to flow in the absence of light. See also Dark Fiber.
Optical fiber through which no light is transmitted and which, therefore, no signal is being carried. Generally speaking, a dark fiber is one of many fibers contained within a cable. Carriers commonly deploy a large number of fibers (432 is a common number) at any given time, since the incremental cost is quite modest compared to pulling them one at a time as the need arises. In fact, a carrier often has little choice, as the right of way may be granted once, and only once. The fibers that the carrier is using immediately are "lit," and those that currently are unused are left "dark." The dark fiber is available for future use. Sometimes dark fiber is sold by a carrier without the accompanying transmission electronics. The customer, which may be either an end user organization or another carrier, is expected to light it up with his own electronics. See also Dark Current, Dim Fiber and Lit Fiber.
At Apple trade shows, people who use Windows machines are known as being on the Dark Side.
Round-trip commodity trading of unused broadband ” so-called dark fiber ” among providers. The technique creates the appearance of trade activity. An unscrupulous carrier can book the swapping as revenue and thus make his financials look better to investors in the stockmarket. It happened in the late 1990s and early 2000s. And by the time you read this, some of the creative executives who thought this up should be sitting firmly in jail.
A Dense Wavelength Division Multiplexing (DWDM) term. Dark wavelength refers to a virtual channel in a fiber optic system utilizing DWDM. Each virtual channel is supported through a specific wavelength of light, with many such channels riding over the same fiber. Once the fiber system is deployed and the DWDM equipment is activated, some of the wavelengths may be activated immediately and others may be left dark for future needs. Such a fiber system is call "Dim Fiber," as it's neither completely dark, nor fully lit. When the need arises, those dark wavelengths are lit up. See also DWDM, Fiber, Optical Fiber and SONET.
The collection of networks and other technologies that enable people to illegally share copyrighted digital files with little or no fear of detection.
Defense Advanced Research Projects Agency. Formerly called ARPA, it is a US government agency that funded research and experimentation with the ARPANET and later the Internet. The group within DARPA responsible for the ARPANET is ISTO (Information Systems Techniques Office), formerly IPTO (Information Processing Techniques Office). DARPA had sponsored research in the 1960s and the 1970s to create a computer network that could survive a nuclear detonation. See also DARPA Internet, IAB, IETF and Internet.
World's largest internetwork, linking together thousands of networks around the world. Sponsored by U.S. Defense Advanced Research Projects Agency. Now called DARPANET. See next definition.
Defense ARPANET. Also known as DARPA Internet. In 1983 the ARPANET was officially split into DARPANET and MILNET. World's largest internetwork, linking together thousands of networks around world. Sponsored by U.S. Defense Advanced Research Projects Agency. DARPANET was the beginnings of the Internet. See Internet.
Digital Audio Radio System. Also known as DAB (Digital Audio Broadcasting) outside the U.S. Proposed satellite-delivered audio/radio systems, similar to DBS (Direct Broadcast System) TV systems, which have been enormously successful in competition with CATV. DARS has been debated by the FCC and the ITU-R since the initial application by CD Radio Inc. in 1990. Assuming that the FCC and ITU-R eventually agree on frequency assignments (and they now have), you may want to make room for one more satellite dish on your rooftop or on your car.
A cellular term. The magnetic tape that is used at the MTSO to record traffic statistics and call billing information. This tape is sent to a third-party 'billing-house' where the actual billing of the subscribers is done.
Direct Access Storage Device. Any on-line data storage device. Usually refers to a magnetic disk drive, because optical drives and tape are considered too slow to be direct access devices. Pronounced DAZ-dee. The term is said to have been invented by IBM.
Direct Access Secondary Storage. Same as near-line: storage on pretty-fast storage devices (e.g., rewritable optical) that are less expensive than hard drives but faster than off-line devices.
Digital Access Signaling. A British term. The original British Telecom (BT) ISDN signalling developed for both single line and multi-line Integrated Digital Access but used in the BT ISDN pilot service for single line IDA only.
Digital Access Signaling System No. 2. A British Term. A message-based signalling system following the ISO-based model developed by British Telecom to provide multi-line IDA interconnection to the BT network.
Digital Audio Tape used to identify a type of digital tape recorder and player as well as the tape cassette. DAT tape machines record music that is much crisper, and free of the hisses and pops that mar traditional analog recordings. The drawback with DAT tape machines is they require considerable tape to store music digitally. In a DAT machine, the music is recorded by sampling the music 48,000 times each second. Each of those samples is represented by a number that is written as a 16-digit string of zeros and ones. There are two such signals, once for each stereo channel, meaning that storing a single second of music requires about 1.5 million bits. On top of that, extra bits are added to allow the system to mathematically correct errors and help the machine automatically find a particular song on the tape. All together, according to Andrew Pollack writing in the New York Times, a single second of music on a digital audio tape requires 2.8 million bits. But compression techniques are cutting down the amount of information required to be recorded.
This is the old AT&T Bell Labs' definition: "A representation of facts, concepts or instructions in a formalized manner, suitable for communication, interpretation or processing." Typically anything other than voice.
A term in object-oriented programming. An object is sometimes referred to as an instance of an abstract data type or class. Abstract data types are constructed using the built-in data types supported by the underlying programming language, such as integer and date. The common characteristics (both attributes and methods ) of a group of similar objects are collected to create a new data type or class. Not only is this a natural way to think about the problem domain, it is a very efficient way to write programs. Instead of individually describing several dozen instances, the programmer describes the class once. Once identified, each instance is complete with the exception of its instance variables. The instance variables are associated with each instance, i.e., each object; methods exist only with the classes. See Object Oriented Programming.
DAA. Equipment that allows you to attach your data equipment to the nation's phone system. At one stage, DAAs were required by FCC "law." Now, their limited functions are built into directly attached devices, such as terminals, computers, etc.
DAP. MCI computer that holds the number translation and call- routing information for 800 and Vnet services. These computers respond to inquiries from MCI switches on how to handle these calls.
In public switched telephone networks, a single item or group of items present at the customer's premises, including all equipment that may affect the characteristics of the interface. An obsolete term. Historically, it came from the time when the phone industry insisted on an interface between its lines and equipment provided by others.
A characteristic of a data element such as length, value, or method of representation.
A collection of data in one place. The data is not necessarily logically related, nor is it necessarily consistently maintained . See Database.
See Database, which is our preferred spelling.
A method of high speed data distribution for text and graphics which uses the spare capacity in the broadcasting television, cable and satellite transmission systems.
A new organization within BellSouth to provide high-speed digital services. No one seems to know why it's called "Data Bubble," except that someone inside BellSouth clearly thinks the term is cute.
A bus that transmits and receives data signals throughout the computer or telephone system. See BUS.
DCD. A hardware signal defined by the RS-232-C standard that indicates that the device, usually a modem, is online and ready for transmission.
A centralized location where computing resources (e.g., host computers, peripherals, applications, databases, and network access) critical to an organization are maintained in a highly controlled environment. A data center is manned by highly trained professionals, many of whom are specialized in fields such as programming, security management, system repair, and technical support. The facility is highly secure with both physical and network security. All systems are provided clean power, with backup power supplies (i.e., both battery power and diesel generators) on-line in the event of the failure of the commercial power source. Application software is carefully controlled in order to ensure that software licenses are not violated, no software conflicts arise, versions are released in a controlled fashion, and software bugs are screened out through a testing phase. System backups are accomplished on a regular basis, with copies of backed -up data stored both on-site and off-site. Admittedly, this definition lists some of the characteristics of an ideal data center, and this is not a complete list. Note that even the ideal data center should have a backup in the form of a hot, warm, or cold standby. A hot standby is the ultimate, and the most expensive. It consists of an exact, up-to-the-second duplicate of the primary data center, including not only the host computers and peripherals, but also the applications and databases, and the network configuration. A hot standby is ready to take over instantaneously in the event of a catastrophic failure. A warm standby is much the same, but takes a bit longer to take over, as it must be activated, the most recent updates data must be loaded, etc. A cold standby takes longer still, as equipment might need to be reconfigured, applications loaded, entire databases loaded, etc.
Communications channels provided specifically for the exchange of data as compared to voice. See also IP Telephony.
DCTE. Also known as DCE (Data Communications Equipment). See DCE.
The capability of a circuit to transmit all data without changing its content or structure.
A process of removing redundancies and inconsistencies in operational data.
Data Competitive Local Exchange Carrier, i.e. a CLEC which only sells data services ” most of which are typically high speed access to the Internet. See CLEC.
The transfer of data between points. This includes all manual and machine operations necessary for this transfer. In short, the movement of encoded information by means of electrical transmission systems. See Data Communications Equipment.
A three-byte, 192 Kbps portion of the SONET signal that contains alarm, surveillance and performance information. It can be used for internally or externally generated messages, or for manufacturer specific messages.
DCE. Also known as Data Circuit Terminating Equipment (DCTE). A device which provides the interface between a circuit and DTE (Data Terminal Equipment. See DCE and DTE.
Reducing the size of a file of data by eliminating unnecessary information, such as blanks and redundant data. The idea of reducing the size is to save money on transmission or to save money on storing the data. The file or program which has been compressed is useless in its compressed form and must be "decompressed," i.e. brought back to normal before use. One method of data compression replaces a string of repeated characters by a character count. Another method uses fewer bits to represent the characters that occur more frequently. See also Compression.
Here's another definition, courtesy the US Department of Commerce:
The process of reducing (a) bandwidth, (b) cost, and (c) time for the generation, transmission, and storage of data by employing techniques designed to remove data redundancy.
The use of techniques such as null suppression, bit mapping, and pattern substitution for purposes of reducing the amount of space required for storage of textual files and data records. Some data compaction methods employ fixed tolerance bands, variable tolerance bands, slope- keypoints, sample changes, curve patterns, curve fitting, floating-point coding, variable precision coding, frequency analysis, and probability analysis. Simply squeezing noncompacted data into a smaller space, e.g., by transferring data on punched cards onto magnetic tape, is not considered data compression. See Data Compression Table.
All current high-speed dial-up modems also support data compression protocols. This means the sending modem will compress the data on-the-fly (as it transmits) and the receiving modem will decompress the data (as it receives it) to its original form. There are two standards for data compression protocols, MNP-5 and ITU-T V.42 bis. Some modems also use proprietary data compression protocols. A modem cannot support data compression without using an error control protocol, although it is possible to have a modem that only supports an error control protocol but not any data compression protocol. A MNP-5 modem requires MNP-4 error control protocol and a V.42 bis modem requires V.42 error control protocol. Note that although V.42 includes MNP-4, V.42 bis does not include MNP-5. However, virtually all high-speed modems that support ITU-T V.42 bis also incorporate MNP-5. The maximum compression ratio that a MNP-5 modem can achieve is 2:1. That is to say, a 9,600 bps MNP-5 modem can transfer data up to 19,200 bps. The maximum compression ratio for a V.42 bis modem is 4:1. That is why all those V.32 (9,600 bps) modem manufacturers claim that their modems provide throughput up to 38,400 bps.
Are MNP-5 and V.42 bis useful? Don't be fooled by the claim. It is extremely rare, if ever, that you will be able to transfer files at 38,400 or 57,600 bps. In fact, V.42 bis and MNP-5 are not very useful when you are downloading files from online services. Why? How well the modem compression works depends on what kind of files are being transferred. In general, you will be able to achieve twice the speed for transferring a standard text file (like the one you are reading right now). V.42 bis and MNP-5 modem cannot compress a file which is already compressed by software. In the case of MNP-5, it will even try to compress a precompressed file and actually expand it, thus slow down the file transfer! The above information courtesy modem expert, Patrick Chen.
A term from US Robotics, makers of fine modems. A data compression table is a table of values assigned for each character during a call under data compression. Default values in the table are continually altered and built during each call. The longer the table, the more efficient the throughput gained .
Also called compactors. These devices take over where high speed modems and statistical multiplexers leave off. They save phone lines by a doubling of data throughput by further compressing async or sync data streams.
A device which permits the use of a transmission media by a number of data sources greater than the number of channels currently available for transmission.
An ATM term. Data VCCs connect the LECs to each other and to the Broadcast and Unknown Server. These carry Ethernet/IEEE 802.3 or IEEE 802.5 data frames as well as flush messages.
A data block usually at the beginning of a file containing descriptive information about the file.
Converting data from one format to another. Conversion typically falls into three basic categories. 1. To convert to a form usable by the equipment you have, e.g. you convert some data from tape to disk (because you don't have a tape drive). Or you may convert from one method of encoding data to another, say from EBCDIC to ASCII, because you don't have software which can understand IBM's EBCDIC method of coding. 3. Or you may convert from one format to another, e.g. from the dBASE method of encoding databases to the Paradox method, or from WordPerfect to Word. There are many service bureaus whose job is to convert computer data from one form to another and there are now many programs out to do the conversion. Most of the leading PC programs now contain data conversion software built into them, so it's possible to open a dBASE database file in Microsoft Access or a WordPerfect file in Word. Conversion becomes part of the process of opening the file. It wasn't always that way. See also Data Compression.
A 3-digit numerical country identifier that is part of the 14- digit network terminal number plan. This prescribed numerical designation further constitutes a segment of the overall 14-digit X.121 numbering plan for a ITU-T X.25 network.
Networking as widespread as the telephone. The Internet is widely thought to contain the beginnings of Data Dialtone. See Internet, Metcalfe's Law, and World Wide Web.
A part of a database management system that provides a centralized meaning, relationship to other data, origin, usage, and format.
An inventory that describes, defines, and lists all of the data elements that are stored in a database.
Unauthorized altering of data before, during or after it is input into a computer system.
An inventory that specifies the source, location, ownership, usage, and destination of all of the data elements that are stored in a database.
A sample or whole extract of the information contained on a database. Used to develop a better picture of the data on a database.
A basic unit of information having a unique meaning and subcategories (data items) of distinct units or values. Examples of data elements are military personnel grade, sex, race, geographic location, and military unit.
DES. A 56-bit, private key, symmetric cryptographic algorithm for the protection of unclassified computer data issued as Federal Information Processing Standard Publication. DES, which was developed by IBM in 1977, was promulgated by the National Institute of Standards and Technology (NIST) ” formerly the National Bureau of Standards (NBS) ” for public and Government use. It was thought to be uncrackable until 1997, when a nationwide network of computer users broke a DES key in 140 days. Triple DES, a later version, encodes the data three times for additional security. As the cost of computer equipment has dropped and as computer power has increased, DES no longer is considered to be totally secure. It has been said that anyone who can afford a BMW can afford a DEScracker. As a result, NIST is searching for a replacement encryption standard to be known as AES (Advanced Encryption Standard). See also Encryption and AES.
Using an I/O device (input/output device), such as a keyboard on a terminal, to enter data into a computer.
A database typically contains multiple files of information. Each file contains multiple records. Each record is made up of one or more fields. Each field contains one or more bytes of data. The terms file, record and field find their roots in manual office filing systems.
One name for the specifications of your ISDN phone lines. Ask for your ISDN Data Fill. It will give you useful information, such as how your lines are set up ” voice, data, data/voice, etc.
Grouping of traffic, identified by a combination of source address/mask, destination address/mask, IP next protocol field, and source and destination ports, where the protocol and port fields can have the values of any. In effect, all traffic matching a specific combination of these values is grouped logically together into a data flow. A data flow can represent a single TCP connection between two hosts , or it can represent all the traffic between two subnets. IPSec protection (A collection of IP security measures) is applied to data flows.
An SCSA term. A set of time slots which are grouped together for synchronization purposes. The number of time slots in each frame depends on the SCbus or SCxbus Data Bus data rate. Each frame has a fixed period of 125us. Frames are delineated by the timing signal FSYNC.
The long-haul transport of bits in bulk. Any corporation with cross-country rights-of-way can get into the datafreight biz by laying fat fiber. Qwest is in the datafreight business.
A circuit which is suitable for transmitting data. High speed data needs better quality phone lines than normal dial-up phone circuits. You can acquire such circuits from many telephone companies. To upgrade voice phone lines to high-speed data circuits, you must sometimes "condition" the phone line. See Conditioning.
A number of data lines providing access to the same resource.
An AT&T Merlin term. A group of analog or digital data stations that share a common extension number. Calls are connected in a round- robin fashion to the first available data station in the group.
The data you receive is exactly what was sent you. Typically the concept of data integrity relates to data transmission. Data integrity is also a performance measure based on the rate of undetected errors. A measure of how consistent and accurate computer data is. In data transmission, error correcting protocols, such as LAP-M, MNP and X-modem ” provide methods of ensuring that data arrives at the destination in its full integrity. This is done in many ways, including retransmitting messed-up blocks.
Data integrity can be threatened by hardware problems, power failures and disk crashes, but most often by application software. In a database system, data integrity can be threatened if two users are allowed to update the same item or record of data at the same time. Record locking, where only a single user at a time is allowed access to a given data record, is a method of insuring data integrity.
The point at which a data line is connected to a telephone system.
A measuring device that bridges a data line and looks at how clean the data is, whether the addressing is accurate, the protocol, etc. Being only a monitor, it does not in any way affect the information traveling on the line. See Data Monitor.
Prohibits activities which would insert tones on a data station line used by a facsimile machine, a computer terminal or some other device sensitive to extraneous noise.
A term used to describe the communications link used for data transmission from a source to a destination. In short, a phone line for data transmission. Or, a fiber optic transmitter, cable, and receiver that transmits digital data between two points.
DLC. Characters used in data communications that control transmission by performing various error checking and housekeeping functions ” connect, initiate, terminate, etc.
A Micosoft driver used to connect Windows 95 and NT workstations to IBM mainframes when TCP/IP is not available. However, the protocol is not routable and it is not designed for peer-to-peer communications between workstations. It is designed only for connectivity to mainframes and minicomputers.
The first control character of a two-character sequence used exclusively to provide supplementary line-control signals.
The second layer of the Open Systems Interconnection data communications model of the International Standards Organization. It puts messages together and coordinates their flow. Also used to refer to a connection between two computers over a phone line. See OSI Model.
DLEC. A DLEC is a CLEC (Competitive Local Exchange Carrier) that specializes in delivering only data, most typically DSL services.
A small, single-subject warehouse used by individual departments or groups of users.
A message included in the GPS (Global Positioning System) signal which reports the satellite's location, clock corrections and health. Included is rough information on the other satellites in the constellation.
DMH. A method used in the cellular industry for exchanging non-signaling messages between service providers on a near real-time basis. DMH originally was used to facilitate call hand-offs between carriers. It was extended to serve a number of other purposes. The DMH extension for fraud detection and prevention in known as NSDP-F (Non-Signaling Data Protocol for Fraud), and the extension for the transfer of billing and settlement information is known as NSDP-B&S. DMH has been standardized by the TIA as IS-124.
Data mining refers to using sophisticated data search capabilities that use statistical algorithms to discover patterns and correlations in data. A comparison to traditional gold or coal mining, data mining is defined as a way to find buried knowledge ("data nuggets" ” no kidding!) in a corporate data warehouse (or information that visitors have dropped on a Web site) and to improve business users' understanding of this data. The data mining approach is complementary to other data analysis techniques such as statistics, on-line analytical processing (OLAP), spreadsheets, and basic data access. In a nutshell , data mining is another way to find meaning in data. On September 22, 1999, Gile Felton described data mining in the New York Times: "Every click tells a story. See Data Warehouse.
A device used to look at a bit stream as it travels on a circuit. It will show the user what is going down both sides of a data channel. It will show what the user at his terminal is typing, and what the computer is responding with. Extremely useful for troubleshooting data communications problems. Also called a Data Scope.
A device allowing several data sources to simultaneously use a common transmission medium while guaranteeing each source its own independent channel. See Multiplexer.
Jim Seymour. His writing style was folksy and irreverent. Mr. Seymour regarded the PC mainly as a tool that put computational power in the hands of individual workers, an important departure from the centralized, mainframe era of computing. He derided the mainframe diehards as "numbskulls" and "data Nazis."
DNIC. In the ITU-T International X.121 format, the first four digits of the 14-digit international data number; the set of digits that may comprise the three digits of the data country code (DCC) and the 1-digit network code (which is called the "network digit"). See DNIC.
DNPA. In the U.S. implementation of the ITU-T X.25 network, the first three digits of a network terminal number (NTN). The 10-digit NTN is the specified addressing information for an end-point terminal in an X.25 network.
An individually addressable unit of information, specified by a data template and its content, that can persist independently of the invocation of a service.
DOV. See DOV.
Also called UART Overrun. This definition from Derrick Moore of JDR Microdevices (1-800-538-5000). Data overrun occurs when your PC is unable to accept interrupts as fast as needed from your serial port chip. Because your PC may be busy reading from a disk or refreshing the screen, it may not handle a serial interrupt before the next byte of data overfills the receiver buffer on the chip. Early PCs used the 8250 which only had a one byte buffer. It was enough because modems were slow and PCs only did one thing at a time. Later, when modems became faster, the serial chips like the 16550 were upgraded to 16 byte buffers. Startech's 16C650 chip has a 32 byte buffer and allows more time for the operating system to service interrupts as well as reduce the number of interrupts that occur. If you run Windows 3.1, 95, NT, or OS/2, and use a high speed modem, you have probably experienced data overrun. Since most communications programs merely request that data be re-sent, the problem is masked. In short, make sure your PC has at a least a 16-byte UART. See UART and UART Overrun.
Although a computer and modem can send data one character at a time, when you're surfing the Internet, downloading files, or sending email, it's much more efficient to send information in larger blocks called data packets. Modems generally send packets of around 64 characters along with some extras for error checking. When downloading files using a protocol like Xmodem, however, the packets are larger. And when using Internet protocols such as TCP/IP, the packets are larger still ” around 1,500 characters. Such packets of data contain the information you're sending or receiving, an address (i.e. where it's going) and some start and stop information. See also asynchronous communication, Ethernet, TCP/IP and Xmodem.
All telephone switches use the same three general types of signals:
Event Signaling initiates an event, such as ringing.
Call Progress Signaling denotes the progress (or state) of a call, such as a busy tone, a ringback tone, or an error tone.
Data Packet Signaling communicates certain information about a call, for example, the identify of the calling extension, or the identity of the extension being called.
System-common equipment that electronically distributes information among data terminal equipment connected to a data transmission network. The switch distributes information by means of information packets addressed to specific terminal devices.
A PBX for switching lots of low-speed asynchronous data. A switch that allows a user on an attached circuit to select from among other circuits, usually one at a time and on a contention basis, for the purpose of establishing a through connection. Distinguished from a PBX in that only digital transmissions, and not analog voice, are supported. Like a telecommunications PBX that makes and breaks phone connections, a data PBX makes and breaks connections between computers and peripherals. In response to dynamic demand, it establishes communications paths between devices attached to its input/output ports by receiving, transmitting and processing electrical signals. Usually, data PBXs work off PCs' serial ports rather than through cable attached to a network interface card. For that reason, they are restricted to serial speeds, topping out at about 19.2K bits per second. For switching lots of low-speed asynchronous data, a data PBX (also called a line selector) can be better than a LAN. Total throughput can actually be higher. See also Line Selector.
A phase of a data call during which data signals may be transferred between DTEs that are interconnected via the network.
Point of access to a computer that uses trunks or lines for transmitting or receiving data.
A means of ensuring that data on the network is safe. Novell's NetWare protects data primarily by maintaining duplicate file directories and by redirecting data from bad blocks to reliable blocks on the NetWare server's hard disk. A hard disk's Directory Entry Table (DET) and File Allocation Table (FAT) contain address information that tells the operating system where data can be stored or retrieved from. If the blocks containing these tables are damaged, some or all of the data may be irretrievable. NetWare reduces the possibility of losing this information by maintaining duplicate copies of the DET and FAT on separate areas of the disk. If one of the blocks in the original tables is damaged, the operating system switches to the duplicate tables to get the location data it needs. Data protection within standard NetWare also involves such features as read-after- write verification, Hot Fix, and disk mirroring or duplexing . See Disk Mirroring and Hot Fix.
The rate at which a channel carries data, measured in bits per second, also known as data signaling rate. If there are restrictions on the pattern of bits, the information capacity of the channel could be less than the data rate. In short, data rate is the measurement of how quickly data is transmitted ” but it may be very different (i.e. less) than what the channel is theoretically capable of.