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Glossary of Acronyms


A20 - Address line 20. The 80286 and higher CPUs allow addresses in real mode to extend slightly beyond the one megabyte mark, which causes an incompatibility with some older programs which expect such addresses to wrap back to the beginning of the address space. For complete compatibility with the 8088, newer machines thus contain circuitry which permits the twenty-first address line (A20) to be disabled. The CPU then effectively has only twenty address lines in real mode, just as the 8088 does, and addresses which would extend beyond the one megabyte mark wrap to the beginning of the address space.

ABI - Application Binary Interface. A software definition that describes the binary instruction encodings for a particular processor along with API compliance.

ABR - Available Bit Rate. A class of service available for ATM. ABR provides a guaranteed minimum capacity. When additional network capacity is available , you can burst data above the minimum rate without risk of cell loss. ABR is intended for applications where network delays are a concern.

An application using ABR specifies a peak cell rate (PCR) that it will use and a minimum cell rate (MCR) that it requires. The network allocates resources so that all ABR applications receive at least their MCR capacity. It then shares any unused capacity in a fair and controlled fashion among ABR sources. The ABR mechanism uses explicit feedback to the sources to assure that capacity is fairly allocated.

An example of an application using ABR is a LAN interconnection. In this case, the end systems attached to the ATM network are routers. The ABR service offers a number of benefits. First, ABR connections share available network capacity. They have access to the instantaneous capacity unused by CBR/VBR connections. Thus, ABR can increase network use without affecting the quality of service of CBR/VBR connections. Second, the share of available capacity used by a single ABR connection is dynamic and varies between an agreed MCR and PCR. Third, the network provides feedback to ABR sources so that ABR flows are limited to available capacity. The time delays inherent in providing feedback dictate the use of buffers along a connections path. Because of the large data rate and a relatively large propagation delay through a network, these buffers may be substantial, leading to large delays. Accordingly, the ABR service is appropriate for applications that can tolerate adjustments to their transmission rates and unpredictable cell delays. Finally, for ABR sources that adapt their transmission rate to the provided feedback, a low cell loss ratio is guaranteed.

ABIOS - Advanced BIOS. The IBM XT/286 and PS/2 models with 80286 or higher processors contain two separate BIOSes. The ABIOS is a protected-mode BIOS which is used by OS/2. For machines without an ABIOS, such as the IBM AT, OS/2 loads the equivalent of the ABIOS from disk.

ACE - Access Control Encryption. ACE is the system used in the Security Dynamics SecurID hardware token system. SecurID tokens, which are about the size of a credit card (although much thicker), have been used for secured access to organizational networks since the 1980s.

ACK - Acknowledgment. The transmission control protocol (TCP) requires that the recipient of data packets acknowledge successful receipt of data. Such acknowledgments (ACKs) generate additional network traffic, diminishing the rate at which data passes in favor of reliability. To reduce the impact on performance, most hosts send an acknowledgment for every other segment or when a specified time interval has passed.

ACL - Access Control List. Most network security systems operate by allowing selective use of services. An Access Control List is the usual means by which access to, and denial of, services is controlled. It is simply a list of services available, each with a list of the hosts permitted to use the service.

ACSE - Association Control Service Element. The method used in OSI for establishing a call between two applications. Checks the identities and contexts of the application entries, and could apply an authentication security check.

ACT - Automated Communications System.

AD - Administrative Domain. A collection of hosts and routers, and the interconnecting networks(s), managed by a single administrative authority.

ADC - Analog to Digital Converter.
A device that samples incoming analog voltage waveforms, rendering them as sequences of binary digital numbers. Passing waveforms through an ADC introduces quantization noise. A circuit that generates a binary (numeric) description of an analog voltage.

ADMD - Administration Management Domain. An X.400 Message Handling System public service carrier. Examples are MCImail and ATTmail in the U.S., and British Telecom Gold400mail in the U.K. The ADMD's in all countries worldwide together provide the X.400 backbone.

ADSL - Asymmetric Digital Subscriber Line. A variant of DSL, it provides better download bandwidth and is designed for video on demand, with almost all the bandwidth reserved for downstream traffic.

AFP - AppleTalk Filing Protocol. The protocol in AppleTalk used for remote access to data.

AGP - Accelerated Graphics Port. A dedicated graphics bus, developed by Intel, slated for use by the Pentium II (Klamath) processor that promises to greatly enhance the performance and quality of 3-D graphics.  AGP, supported by the Intel 440 LX chip set, is designed to improve the graphical performance of Pentium II systems by providing a direct link between the PC's graphics card processor and system RAM through the core chip set. This gets the graphics card of the slower (133 MBps) PCI bus and onto its own dedicated channel.

ALAP - AppleTalk Link Access Protocol. Data link layer protocol Apple uses to allow devices on the network to transmit and receive frames.. It includes specifications for media access management, addressing, and data encapsulation/decapsulation, and frame transmission dialogs.

ALU - Arithmetic Logic Unit - A circuit that performs math calculations and/or logical operations.

ANSI - American National Standards Institute.
The U.S. standardization body. ANSI is a member of the International Organization for Standardization (ISO).

APA - All Points Addressable. A mode in which all points of a displayable image can be controlled by the user or a program.

API - Application Program Interface. A system call (routine) that gives programmers access to the services provided by the operating system. In IBM compatible systems, the ROM BIOS and DOS together present an API that a programmer can use to control the system hardware.

The defined set of calls which a program may make to interact with or request services of the operating system or environment under which it is running. Because the inputs and outputs of the calls are well-defined, a program using the API can continue using the identical calls even if the internal organization of the program providing the API changes.

APIC - Advanced Programmable Interrupt Controller.
A function supplied the Intel Pentium processor.

APL - A Programming Language. An interactive, mathematically oriented language which is well-suited to manipulating matrices. Originally using greek letters and numerous special symbols, thus requiring a special display, versions are now available which use keywords in place of the special symbols.

APM - Advanced Power Management.
A specification sponsored by Intel and Microsoft originally proposed to extend the life of batteries in battery powered computers. APM allows application programs, the system BIOS, and the hardware to work together to reduce power consumption. An APM compliant BIOS provides built in power management services to the operating system. The application software communicates power saving data via predefined APM interfaces.

APPC - Advanced Program to Program Communications. A component of IBM's System Network Architecture. APPC is designed to allow communications between programs running on different systems on a more or less peer-to-peer basis.

ARC - Advanced RISC Computing. ARC names area generic method of identifying devices within the ARC environment.

ARCnet - Attached Resource Computer Network. Developed by Datapoint Corporation and has been a popular reliable LAN for years. Because of its popularity, standards existed for ARCnet even before IEEE Project 802 was established. The IEEE 802.4 specification, which defines token passing on a bus using broadband technology, is the standard most similar to ARCnet. However, because ARCnet is a baseband network, it is very inexpensive and easy to install. ARCnet can have a star or bus topology. Often, however, it is considered to have a distributed star or tree topology. Manufacturers consistently follow ARCnet standards, and the products released for ARCnet networks are usually compatible with equipment from other vendors. Because it uses both passive and active hubs, ARCnet is excellent for elaborate wiring configurations.

ARLL - Advanced Run Length Limited. More complex yet powerful derivatives of the RLL (Run Length Limited) scheme. Include 1,7 and 3,9 encoding. Allows 34 or more sectors per track.

ARP - Address Resolution Protocol. The Internet protocol used to dynamically map Internet addresses to physical (hardware) addresses on local area networks. Limited to networks that support hardware broadcast. Used to dynamically discover the low level physical network hardware address that corresponds to the high level IP address for a given host. ARP is limited to physical network systems that support broadcast packets that can be heard by all hosts on the network.

ARPA - Advanced Research Project Agency. An agency of the U.S. Department of Defense responsibility for the development of new technology for use by the military. ARPA, formerly known as DARPA, was responsible for funding much of the development of the Internet we know today, including the Berkeley version of UNIX and TCP/IP.

ARPANET - A packet switched network developed in the early 1970's. The grandfather of today's Internet. ARPANET was decommissioned in June 1970.

AS - Autonomous System. A collection of routers under a single administrative authority using a common Interior Gateway Protocol for routing packets.

ASCII - American Standard Code for Information Interchange. A standard seven bit code created in 1965 by Robert W. Bemer to achieve compatibility among various types of data processing equipment. The standard ASCII character set consists of 128 decimal numbers, ranging from 0 through 127, assigned to letters, numbers, punctuation marks, and the most common special characters. In 1981 IBM introduced the extended ASCII character set with the IBM PC, extending the code to eight bits and adding characters from 128 through 155 to represent additional special mathematical, graphics, and foreign characters.

ASIC - Application Specific Integrated Circuit. A chip designed for a particular customer or system, typically from standard cells and/or gate arrays. May ultimately be sold as a standard part.

ASN.1 - Abstract Syntax Notation One. The OSI language for describing abstract syntax.

ASVD - Analog Simultaneous Voice/Data. A modem technology that comes with video phone capabilities and is compatible with the v.80 standard that supports ITU's H.324 parameter standard allowing for video, voice, and to be shared simultaneously over high speed modem connections.

AT - Advanced Technology. A computer model introduced by IBM in 1984 where the CPU, memory, and I/O bus all shared a common 8 MHz clock. Known today as the ISA bus. Replaced the IBM XT with a speed nearly five times as fast. Contained the 80286 CPU and had 16 bit card slots.

ATA - Advanced Technology Attachment. Same as IDE (Integrated Disk Electronics). A disk drive implementation designed to integrate the controller onto the drive itself, thereby reducing interface costs, and making firmware implementations easier. Introduced in March 1989, it defines a compatible register set and a 40 pin connector and its associated signals.

ATA-2 - Advanced Technology Attachment-2. A compatible extension of ATA (IDE) The most important additions are performance enhancing features such as fast PIO and DMA modes. Another important novelty is the souped up Identify Drive command allowing a drive to tell the software exactly what its characteristics are. This is essential for both Plug and Play and compatibility with future revisions of the standard. Also called Fast ATA, Fast ATA-2, and Enhanced IDE (EIDE).

ATAPI - Advanced Technology Attachment Packet Interface. A standard designed for devices such as CD ROMS and tape drives that plug into an ordinary ATA (IDE) port. The principal advantage of ATAPI hardware is that it is cheap and works on every PC with an IDE adapter. For CD ROMS, it has somewhat lower CPU usage compared to proprietary adapters but there is no performance gain otherwise. For tape drives, ATAPI has potential for superior performance and reliability compared to the popular QIC117 floppy tape drives.

ATM - Asynchronous Transfer Mode. A versatile broadband network architecture capable of delivering in varying speeds. The original specification for ATM came from the International Telecommunications Union (ITU) , the organization whose standards address the worldwide telecommunications infrastructure. In the early 1980's the ITU defined Integrated Services Digital Network (ISDN), which is now called N-ISDN for narrowband ISDN. N-ISDN had two access interfaces, or transfer rates (Basic 144 kbps and Primary 1.544 Mbps). In the late 1980's, the ITU further enhanced N-ISDN by bringing out the specifications for B-ISDN, or broadband ISDN. Unlike N-ISDN, B-ISDN offered much higher transmission rates, up to 622 Mbps. The signals generated by B-ISDN are carried by ATM. ATM transmits in what is known as a cell stream. A cell is a term for ATM broadband transmission that can be thought of as a predefined data packet. The data packet, or cell, is 48 bytes long with a 5 byte header for addressing.

Asynchronous Transfer Mode (ATM) is a cell-based fast-packet communication technique that supports data-transfer rates ranging from sub-Tl speeds (less than 1.544 Mbps) up to 1 0 Gbps. Like other packet-switching services (Frame Relay, SMDS), ATM achieves its high speeds in part by transmitting data in fixed-size cells, and dispensing with error-correction protocols. Instead, it relies on the inherent integrity of digital lines to ensure data integrity. ATM networks are extremely versatile. An ATM network can be treated as a single network, whether it connects points in a building or across the country. Its fixed-length cell-relay operation, the signaling technology of the future, offers more predictable performance than variable-length frames. And it can be integrated into an existing network as needed, without having to upgrade the entire LAN.

ATP - AppleTalk Transaction Protocol. A transport layer protocol that provides reliable guaranteed delivery of packets from a source socket to a destination socket.

ATPS - AppleTalk Print Services. The software utility within the AppleTalk Networking Operating System that contains Macintosh computer printer on an Apple Network.

AUI - Attached Unit Interface Cable. A four twisted pair interface cable that connects an Ethernet device to an Ethernet external transceiver.

AUP - Acceptable Use Policy. Many transit networks have policies which restrict the use to which the network may be put. For example, some networks may only be used for non-commercial purposes. Some AUP's limit the type of material which can be made available to the public. Enforcement of AUP's varies with the network.

AVI - Audio Video Interleave. A storage technique developed by Microsoft for their Video for Windows product that combines audio and video into a single frame or track, saving valuable disk space and keeping audio in synchronization with the corresponding video.


Compiled by Scott McArdle, MagnaCom Limited. I hope this list has helped you and if there is an item that should be on this list, please let me know. Thanks. PS, I've spent 100's of hours maintaining this list, please don't be a LAMER.

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