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"A" Networking Definitions & Concepts...

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AA (Auto Answer):

A modem feature in which the modem can automatically respond to a call and establish a connection.

AAL (ATM Adaptation Layer):

The topmost of three layers defined for the ATM network architecture. The AAL mediates between the ATM layer and the various communication services involved in a transmission.


Advanced backbone network developed by Qwest, Cisco, Nortel, and Indiana University to support the University Corporation for Advanced Internet Development's Internet2 project.


A UNIX operating system that runs on the 68K Macintosh platform, enhanced with some Macintosh-specific features, such as support for the Macintosh ToolBox. A/UX is based on System V Release 2 (SVR2) of AT&T's UNIX.

AARP (AppleTalk Address Resolution Protocol):

This is the protocol that reconciles addressing discrepancies in networks that support more than one set of protocols. For example, by resolving the differences between an Ethernet addressing scheme and the AppleTalk addressing scheme, AARP facilitates the transport of DDP (Datagram Delivery Protocol) packets over a highspeed EtherTalk connection. Also see ARP for similarities.

AAUI (Apple Attachment Unit Interface):

The name used for Apple's Ethernet physical interface, which uses a special connector and requires an eternal transceiver to complete a connection to the network.

Absolute Date: (Database)

An absolute date is the total number of days from some specific base date. To calculate the number of days between two absolute dates, you subtract one date from the other. For example, using a base date of January 1, 1900, August 13, 2001 has an absolute date value of 37116 and April 27, 2002 has an abslute date of 37373. If you substract the earlier date value from the later one, the result is 257 days.

Abstract Class: (UML)

A class that cannot be directly instantiated because at least one operation lacks a method (implementation). Any superclass may be abstract because the implementation can be provided by a subclass. An abstract class is identified either by italicizing the class name or by adding the property {abstract} to the operations that do not have a corresponding method, that is, the operations that prevent the class from being instantiated. Contrast: concrete class.

Abstraction: (UML)

A representation of an entity that includes only those properties that support the proposed use of the representation, and sufficient to distinguish it from all other kinds of entities. An abstraction defines a boundary relative to the perspective of the viewer. Also, the process of choosing information to include in the representation of an entity based on how and why the user plans to use the representation.

Humans think in terms of objects. We possess the marvelous ability of abstraction, which enables us to view digital images of people, planes, trees, and mountains as objects, rather than as individual dots of color. We can, if we wish, think in terms of beaches rather than grains of sand, forests rather than trees and houses rather then bricks.

Experience in building software systems indicates that significant amounts of code deal with closely related special cases. When programmers are preoccupied with special cases, the details can obscure the "big picture". With object-oriented programming, programmers focus on the commonalities among objects in the system, rather than on the special cases. This process is called abstraction.

The process of picking out (abstracting) common features of objects and procedures. In programming, an abstraction provides the API (Application Programming Interface) that all instances of a thing conform to; the abstraction defines shared features. For example, in Apple's MacOS X environment, the I/O kit, abstractions take the form of classes.

Access Grid:

The National Computational Science Alliance's hardware, software, and telecommunications capabilities for distributed research collaboration and teleconferencing.

Access Method:

The rules that manage how all the computers and other devices (i.e., 4511 or 4511A), on a network can send information through the same physical medium in an orderly fashion.

Access Modes:

A set of permissions used by AFP (AppleTalk Filing Protocol) to regulate access to a file; AFP supports four access modes: read, write, read-write, and none.

Access Network:

This is a connection between users and the locl central office (CO) switch. The switches themselves are interconnected with higher-speed communication lines through tandem switches. Multiplexing is used to combine many calls in these high-speed lines. Tandem switches, in turn, connect to toll switches that are used to provide long-distance connections.

Access Path:

The specific location of a file on a Web server in relation to the root directory, as in http://www.webmaster.com/document.html. In this case document.html is in the root directory as defined by the Web server.

Access Path (Data Warehouse):

The path chosen by a database management system to retrieve the requested data.

Access Privileges:

The privileges given to or withheld from users to open and make changes to a directory/folder and its contents. Through the setting of access privileges, you control access to the information that is stored on a file server.

Access Tandem:

An LEC (Local Exchange Carrier; LAN emulation client) switching system that provides access for the IXCs (Interexchange Carriers) to the local network. The access tandem provides the IXC with access to more than one end office within an LATA (Local Access Transport Area).

Access Time:

In hard-disk performance, the average amount of time it takes to move the read/write heads to a specified location and retrieve data at that location. The lower the value, the better the performance. Currently, hard disks access times of less than 15 milliseconds are common.

It can also be considered the period of time needed from issuance of a command to access a single sector to the time when the disk drive's head reaches the sector. Access time can be represented by the following formula:

Access Time = Seek Time + Latency + Time to Read a Sector;


A process by which network usage can be determined and charges assessed for use of network resources, such as storage, access, and services. Accounting measures include blocks read, blocks written, connect time, disk storage, and service requests.

Most network operating systems include an accounting utility or support an add-on accounting package. For example, NetWare 3.11 has an accounting option in its SYSCON utility.

Achromatic: (in Color)

Having no hue, being of the achromatic color of maximum darkness; having little or no hue owing to absorption of almost all incident light; "black leather jackets"; "as black as coal"; "rich black soil" being of the achromatic color of maximum lightness; having little or no hue owing to reflection of almost all incident light; "as white as fresh snow"; "a bride's white dress".

Action: (UML)

An executable statement, commonly associated with an activity, state, or transition. An action is the fundamental unit of behavior specification and represents some transformation or processing in the modeled system, be it a computer system or a real-world system. Actions are contained in an activity-expression, which provide their context. An action typically results in a change in the state of the system, and can be realized by sending a message to an object, modifying an attribute value, or by modifying a link. See: entry action, exit action, action semantics. Contrast: activity.

Action Plan (Project Management)

A detailed plan of what needs to be done and when. The set of activities, their schedules, and the resources needed to complete the project.

Action Sequence: (UML)

An expression that resolves to an ordered set of actions that occur during a transition. The series may be expressed as a textual description of a state transition, or as an icon in a State Machine diagram, that is, a rectangle containing the series of actions.

Action State: (UML)

A state of an object during which a single entry action is performed. The conpletion of entry action generates the completion event that triggers the transition out of the state. Multiple outgoing transitions are allowed if they are differentiated by guard conditions (UML 1.4). See: focus of control (UML 2.0).

Activation: (UML)

Rectangular area shown on the lifeline of an object or role in a sequence diagram to indicate when that object or role is executing an operation. In a sequence diagaram, the time during which an object is performing an action. See: focus of control

Active Class: (UML)

A class whose instances are active objects. See: active object

Active Link:

In an ARCnet network, a box used to connect two cable segments when both cable segments have high-impedance network interface cards (NICs) connected.

Active Object: (UML)

An object that may execute its own behavior without requiring method invocation. This is sometimes referred to as "the object having its own thread of control". The point at which an active object responds to communications from other objects is determined solely by the behavior of the active object and not by the invoking object. This implies that an active object is both autonomous and interactive to some degree. See: active class, thread

Active Star:

A type of star topology in which a central controller sends transmissions to each device on the network. The central controller is usually made-up of concentrator or multiport repeater located at the center of the star. All wiring runs lead to the concentrator, which is responsible for retransmitting the network signal from one wiring run to the rest of the wiring runs attached to it. It basically is a network configuration in which the central node of a star topology cleans and boosts a signal. Also see star topology.

Activity: (Project Management)

A unit of work performed to complete a project. An activity typically takes time (duration) and expends resources. Activities are often broken down into a series of individual, but related, tasks.

Activity: (UML)

A step in a workflow used to represent where work is taking place, either in the business or within the system being modelled. The activity continues until all its work is complete, or an event triggers an exit.

Activity Diagram: (UML)

A means of describing workflows, linking activities and states. Used to describe business and system workflows. A diagram that depicts behavior using a control and dataflow model. A diagram showing the flow of activities. Used to give a high-level view of a business process.

Activity Expression: (UML)

A sequence of actions associated with a state transition.

trigger-signature ['[' gaurd-constraint ']' ] '/' activiy expression.

Activity Partition (UML)

A grouping mechanism used in an Activity diagram to identify actions taht have some characteristic in common. They are often used to represent organizational units in a business model. Synonym: swimlane

Actor: (class:UML)

External user of a system or an external system, shown in a use case diagram, which communicates with a system or subsystem being modelled. A role employed in a Use Case diagram that a user or any other system or device plays when it interacts with the subject (system) under consideration. It is a type of entity that interacts with but is external to the subject. Actors may represent human users, external hardware, or other subjects. An actor does not necessarily represent a specific physical entity. For instance, a single physical entity may play the role of several different actors and conversely, a given actor may be played by multiple physical entities.

Actual Cost of Work Performed (ACWP): (Project Management)

The total costs that were incurred (direct and indirect) in accomplishing work during a given time period in the project.

Actual Finish Date: (Project Management)

The date that work was actually ended on an activity in the project.

Actual Parameter:(UML)

See: argument

Actual Start Date: (Project Management)

The date that work was actually begun on a particular activity in the project.


A scheme, determined by network protocols, for identifying the sending device and destination device for any given item of information traveling on a network. A name, set of numbers, or sequence of bits used to identify devices on a network. Each computer, printer, server, or other device on the network must have a unique address. Addresses are necessary so that information transmitted on the network will get to the right destination. The network software keeps track of the addresses.

Addressing is required to identify which network input is to be connected to which network output. Here we again see that it is natural to specify addresses according to a hierarchy: number, street, state, country. Or in dot notation number.street.state.country. Thus in the figure below the user identified by an asterisk in part (a) has an address AA.A.a.1.

Just as there is more than one way to go from Portland to Omaha, there is more than one possible way to interconnect users in a communication network. Routing involves the selecting of a path for the transfer of information among users. The use of hierarchical addresses facilitates the task of routing. For example, in routing a letter through the postal system we are concerned about getting to the right country, state, city, street, so on. Thus the hierarchical approach to addressing is in wide use because it simplifies the task of routing information across large networks.

Ad-Hoc Query: (Data Warehouse)

Any query that cannot be determined prior to the moment the query is issued. A query that consists of dynamically constructed SQL, which is usually constructed by desktop-resident query tools.

Address Mask:

A bit mask used to select bits from an IP address for subnet addressing. The mask is 32 bits long and selects the network portion of the IP address and one or more bits of the local portion.

Addresses, Network:

Every node on a network has an assigned address that other nodes use when communicating with it. For Ethernet and Token Ring network adapters, unique addresses are assigned at the factory. ARCNET networks have user-definable addresses. For example, the address of an Ethernet and Token Ring network adapter consists of a 6-byte address, half of which is a special number identifying the board's manufacturer. The last half of the address is a unique number for the board assigned at the factory. This strategy virtually guarantees that no two Ethernet or Token Ring network interface cards will ever have the same address, and prevents conflicts.

When separate networks are connected into an internetwork, a new addressing scheme is required. On interconnected NetWare networks, each network segment has its own address, which is used for routing purposes and to differentiate each segment from the others.

In TCP/IP networks such as the Internet, every node has a numeric address that identifies both a network and a local host or node on the network. This address is written as four numbers separated by dots, for example, ( The assignment of addresses is arbitrary within a company or organization, but if the company plans to connect with the Internet, it is important to obtain registered addresses from an outside agency to conform with international addrressing standards. Applcations running in computers also have addresses that other applications, either local or remote, use to communicate with the application. On TCP/IP networks, a socket is a combination of an Internet address plus an application address.

Address Resolution:

Conversion of an IP address into a corresponding physical address. Depending on the underlying network, resolution may require broadcasting on a local network. Basically, it is the translation of node addresses between different node-numbering schemes. See ARP (Address Resolution Protocol).

Address Spoofing:

Any enemy computer's impersonation of a trusted host's network address.

Advanced Program-to-Program Communications (APPC):

A high-level communications protocol from IBM that allows one program to interact with another across the network. It supports client-server and distributed computing by providing a common programming interface across all IBM platforms. It provides command for managing a session, sending and receiving data, and transaction management using two-phased commit (which is used by distributed databases for updates). APPC software is either part of, or optionally available, on all IBM and many non-IBM operating systems. Since APPC has only supported IBM's Systems Network Architecture, which utilizes the LU 6.2 protocol for session establishment, APPC and LU 6.2 are sometimes considered synonymous.

Advanced Technology Attachment (ATA; Harddrive):

Advanced Technology Attachment, a storage interface designed over 15 years ago and now the de facto I/O standard for desktop PCs. Though adequate for low data-demand applications, the combination of increased CPU capabilities, greater application through-put demands, and faster, more capable hard drives, severely limits the future usefulness of this interface.

Aggregation (Data Warehouse):

All data warehouses will contain pre-stored aggregates [gathered together into a mass or sum so as to constite a whole]. Once you have determined your overall design approach, you need to consider the strategy you will use to design your agregate fact and diminsion tables. The design points that require consideration now ae listed next. The other design points come into play during implementation of the design.

  • Aggregates must be stored in their own fact tables, separate from the base-level (operational) data. In addition, each distinct aggregation level must occupy its own unique fact table.
  • The dimension tables attached to the aggregate fact tables must, whenever possible, be shrunken versions of the dimension tables associated with the base fact table.

Again, use these as guidelines, but ensure that your approach will work well with your data access tool.

Aggregates can have a very significant effect on performance, in some cases speeding queries by a factor of 100 or even a 1000. No other means exist to harvest such spectacular gains. Certainly, the information system owners of a data warehouse should exhaust the potential for using aggregates before investing in hew hardware.

Some additional thoughts...

  • The lowest level of aggregation is determined by the granularity of the "fact" table.
  • Aggregations can be created on-the-fly or by the process of pre-aggregation.
  • Pre-aggregation demands more storage space but provides better query performance.
  • Aggregation is easier when facts are all additive.
Aggregate: (class, UML)

A class that represents the "whole" (the point of control in an assembly of classes) in an aggregation (whole-part) relationship. See: aggregation

Aggregate Route-Based IP Switching (ARIS)

With ARIS, connections are pre-established toward each destination. Because connections from multiple sources to a given destination typically form a tree, ARIS allows further optimization of label usage whereby multiple incoming connections can be merged into a single outgoing connection.

ARIS is IBM's scheme for switching IP datagrams. It is normally associated with ATM networks, but ARIS can be extended to work with other switching technologies. ARIS (and other IP switching technologies) takes advantage of integrated router switches. The idea is to map routing information to short fixed-length labels so that next-hop routers can be determined by direct indexing, rather than using the standard router packet evaluation and lookup process.

What ARIS does is set up a virtual circuit through a network based on the forwarding paths already established by routers that use routing protocols such as OSPF (Open Shortest Path First) and BGP (Border Gateway Protocol). IP datagrams are then switched through the network following these paths. No routing is done by any device along the virtual circuit. Instead, the datagrams are "tagged" with a label that is read by intermediate switches along the way, identified with a particular destination network, and sent along the appropriate circuit to that destination.

ARIS is very similar to MPLS (Multiprotocol Label Switching). At the time of this writing, MPLS appears to be the major focus of the industry and is an emerging Internet Engineering Task Force (IETF) standard.

Aggregation: (class:UML)

Specifies that the nature of an association is one of "whole-part". One class is part of the other class. A special form of association that specifies a whole-part relationship between the aggregate (whole) and a component part. The relationship is used to insure the integrity of a configuration of objects. The aggregation symbol is a hollow diamond on the end of the assoication (relationship) next to the class that represents the whole in the whole-part relatiionship. See: composition. A relationship between model elements indicating that one element is a "part of" another element (aggregate).

Aggregation: (class:OO)

Architecture in which one object is referenced as a property of another object; see composition.

Agile Development

Agile development methods apply timeboxed iterative and evolutionary development, adaptive planning, promote evolutionary delivery, and include other values and practices that encourage agilityrapid and flexible response to change. If agile methods have a motto, it is embrace change.[1] If agile methods have a strategic point, it is maneuverability.

The Agile Principles:

  • Our highest priority is to satisfy the customer through early and continuous delivery of valuable software.
  • Welcome changing requirements, even late in development. Agile processes harness change for the customer's competitive advantage.
  • Deliver working software frequently, from a couple of weeks to a couple of months, with a preference to the shorter time scale.
  • Business people and developers must work together daily throughout the project.
  • Build projects around motivated individuals. Give them the environment and support they need, and trust them to get the job done.
  • The most efficient and effective method of conveying information to and within a development team is face-to-face conversation.
  • Working software is the primary measure of progress.
  • Agile processes promote sustainable development.
  • The sponsors, developers, and users should be able to maintain a constant pace indefinitely.
  • Continuous attention to technical excellence and good design enhances agility.
  • Simplicitythe art of maximizing the amount of work not doneis essential.
  • The best architectures, requirements, and designs emerge from self organizing teams.
  • At regular intervals, the team reflects on how to become more effective, then tunes and adjusts its behavior accordingly.
Alias (Relational Database):

An alias (alternate name) may be used to identify the source (the table) from which the data are taken. Note that we have used the aliases "A" and "B" to label the PRODUCT and VENDOR tables in the next command sequence. Any legal table name may be used as an alias. For exaple:


An alias is especially useful when a table must be joined to itself for grouping purposes.


The difference between the maximum and minimum voltage of an electrical signal.

Amplitude Response Function:

The ability of a channel to transfer a tone of the frequency f is given by the amplitude-response function A(f), which is defined as the ratio of the amplitude of the output tone divided by the amplitude of the input tone.


A form of transmission in which the waveform is continually varied over an infinite range of voltage and time. This can also be seen as a protocol that uses wave technology for data transmission.

Information that is highly variable and has a direct, proportional relationship to the thing it describes. For example, information that describes current wind velocity or temperature or the rate of data transmission on a network.

Analog Communication:

A telecommunications system that uses analog (that is, continuous, sinusoidal) signal to represent information. An example of an anolog communication system is the clasic voice-based telephone system (which is being replaced by the newer digital systems).

Ancestor: (in UML)

Any superclass of a class regardless of the number of generalizations between the class and the superclass.

Anonymous FTP:

The ability to access and download files without being a registered user on a host computer.

ANSI (American National Standards Institute):

The principal organization in the United States dedicated to the development of voluntary standards for American industry. ANSI is a private, nonprofit membership organization that performs two functions: it coordinates the United States' voluntary consensus standards system, and it approves American National Standards. ANSI ensures that a single set of non-conflicting American National Standards are developed by ANSI-accredited standards-developers, and that all interests concerned have the opportunity to participate in the development process.

These requirements for due process have resulted in a high level of confidence and credibility and broad acceptance for American National Standards. But it is important to remember that ANSI does not develop standards. Rather, it provides the means for determining the need for standards, ensures that qualified organizations develop those standards, and coordinates standards approval. If you wish to contact ANSI, write or call:

1430 Broadway
New York, NY 10018
(212) 642-4900

Several major ANSI specifications and standards are listed here:

  • ANSI 802.1-1985 and IEEE 802.5 are specifications that define the access protocols, cabling, and interface for token ring local area networks. IBM made the standard popular.
  • ANSI/IEEE 802.3 defines coaxial cable carrier sense multiple access/collision detection (CSMA/CD) Ethernet networks. 10Base5 is the thick cable Ethernet standard and 10Base2 is the thin cable Ethernet standard. 10BaseT is the twisted-pair, star-configured Ethernet standard.
  • ANSI X3.135 is the Structured Query Language (SQL) specification that defines standardized data-base query methods for front-end clients and back-end database services.
  • ANSI X3.92 is a standard that defines an encryption algorithm that creates privacy and security in data transmissions.
  • ANSI X12 is the electronic data interchange (EDI) specification that defines how a company exchanges purchase orders bills of lading, invoices, and many other transaction forms with its vendors.
  • ANSI X3T9.5 is the Fiber Distributed Data Interface (FDDI) specification that defines 100 Mbits/sec transmissions over fiber-optic networks that have dual counter-rotating rings and a token-passing scheme. Copper wire may also be used as the medium, but distances are greatly reduced. FDDI supports up to 500 nodes over 2 kilometers. FDDI-I defines data networking while FDDI-II defines voice and transmissions.
  • The Synchronous Optical Network (SONET) is the ANSI specification for fiber-optic transmissions and defines a common global infrastructure for the transmission of synchronous and isochronous (time-sensitive data such as real-time video) information. SONET can transmit in the multimegabit and multigigabit range and provides for the insertion of lower-speed channels.
API(Application Program Interface):

A series of specifications describing the types of data and commands that can be relayed from a program to other software or a software-controlled device, such as a network interface card.

APPC(Advanced Program-to-Program Communications):

Specifications for peer-to-peer communications in an IBM SNA network. See LU6.2 and SNA.


Applets are usually small Java programs or applications that run in the framework of web browsers. It is a java program to be included in HTML pages and executed in Java-enabled browsers. Applets are also known as mini-programs that are downloaded and executed as part of a displayed Web page.


A file format specified by Apple for storing the data fork and resource fork of a Macintosh file in separate files on another computing system. Many file servers support the AppleDouble method for storing Mac files. The method allows you to share files with non-Mac users. With AppleDouble, other users simply access the data fork of a Mac file stored in the AppleDouble format.


Apple Computer's Mac-based file server software, using the AFP (AppleTalk Filing Protocol). It is a network operating system from Apple. AppleShare runs on a Macintosh network server, providing file and printer services. AppleShare uses the AppleTalk protocol suite (AFP) to carry out its tasks. When selecting an AppleShare server, it mainly involves the Name Binding Protocol (NBP), with the NBP data containing a string that directs a search for the AFP servers in the zone named MyZone (or whatever the zone name is):


The AppleShare file server provides a location where a user on the network can store and gain access to common files without disrupting other users' activities. Any Macintosh computer with one or more hard disk drives can become a dedicated file server on the network. Each hard disk attached to the AppleShare file server is called a volume.

To be able to use an AppleShare file server, a user is registered on the server, given a password, and placed into one or more user groups, as appropriate. Gaining access to the file server involves a login process in which the server asks for the user's identification, consisting of a user name and a password. Once the server has examined its registered user database and validated the user, the selected server volumes' icons, much like a hard disk icon, appear on the user's Macintosh Desktop.

The logon process assures confidentiality; users must be registered and must enter a password before being able to gain access to protected portions of server volumes. Unregistered users can log in as guests (if setup); that is, they can obtain access to information that is unprotected.

Within a server volume, files are stored in folders. Folders on a Macintosh System are analogous to directories on an MS-DOS or UNIX computer; both folders and directories are named entities that hold (elements) files or other folders/directories (entities). Opening and saving files and creating folders/directories are done the same way on a file server volume as on a users local disk.


A file format specified by Apple for storing both the data fork and resource fork of a Macintosh file in a single file on another computing system.


Is a comprehensive network system that enables communication between network devices (including users' computers, file servers, and printers) which may be a mixture of Apple and non-Apple products. It was introduced in 1985 and supports Apple's proprietary LocalTalk access method as well as ethernet and token ring. Several elements make up an AppleTalk network system: AppleTalk software--which runs in each device connected to the system and observes the AppleTalk protocols--and AppleTalk hardware. With the latter including computing components and connectivity components, i.e., drop boxes.

AppleTalk Phase II is another routed protocol defined by Apple Computer, Inc., supporting both data link access methods LLC (Logical Link Control) and MAC (Medium Access Control). AppleTalk runs over the 802.2 LLC portion of the LAN data link control layer. The physical layer supports Ethernet, Token Ring, and LocalTalk hardware. The data link layer supports Ethertalk Link Access Protocol (ELAP), Token Talk Link Access Protocol (TLAP), or LocalTalk Link Access Protocol (LLAP). The network layer is strictly the Datagram Delivery Protocol (DDP), similar to IP. Routing Table Maintenance Protocol (RTMP), AppleTalk Echo Protocol (AEP), AppleTalk Transaction Protocol (ATP) and Name Binding Protocol (NBP) make up the transport layer end-to-end protocols. Zone Information Protocol (ZIP) and Appletalk Data Stream Protocol (ADSP) session layer protocols can directly interface to DDP. Other session layer protocols include AppleTalk Session Protocol (ASP) which uses ATP, and Printer Access Protocol which uses either ATP or NBP. AppleTalk Filing Protocol (AFP) and Postscript offer presentation and application layer interfaces. The AppleTalk protocol stack is similar to the OSIRM (Open Systems Interconnection Reference Model).

EtherTalk and TokenTalk are applications which run on AppleTalk for Ethernet and Token Ring networks. They both use the IEEE 802.2 Logical Link Control (LLC) protocol and Subnetwork Access Point (SNAP) protocol. The protocol stack for AppleTalk is similar to the OSIRM. AppleTalk can also be carried encapsulated within DECnet through the "tunneling technique" through the use of dedicated point-to-point VAXs.

AppleTalk Address:

A number that uniquely identifies software processes in an AppleTalk network. For example, if the networking software includes a process that sends and receives a certain type of data, that process receives its own AppleTalk address. Processes of that type are called socket clients.

The address is composed of the socket number and the identification number of the node (the node ID) containing that socket number. This combination makes the address unique for each socket. See socket.

AppleTalk Data Stream Protocol (ADSP):

The AppleTalk Data Stream Protocol allows two processes to open a virtual data "pipe" between their sockets. Either process can write data bytes into the pipe and read data bytes from it. Data bytes written into an ADSP pipe are delivered reliably at the other end in the exact same order.

Those familiar with network systems have come to expect the key reliable data-transfer service of a network system to be a connection-oriented data stream or virtual circuit. AppleTalk's heavy use of transaction protocols in lieu of stream protocols might surprise them.

Stream services are implemented on packet networks at the cost of considerable protocol overhead. However, stream protocols are a natural extension of physical connections used in most data communication applications. These virtual circuit services emulate familiar capabilities and are readily accessible to and used by programmers. These users often employ such streams, however, to implement a client-server interaction, which is of a request-response transaction nature. The programmer has to add overhead to undo the stream service, in effect, and to convert it back to a transaction service. With AppleTalk Transaction Protocol/AppleTalk Session Protocol (ATP/ASP), AppleTalk avoids the double overhead of first extracting stream service from a packet-oriented system and then converting it back to a transaction service.

Stream services of ADSP are included in the architecture for two reasons: first, as a convenience to programmers familiar with such services in other network systems; second, to provide the natural data transport service for implementing capabilities such as terminal emulation and file transfer. ADSP will also prove useful for gateways that provide end-to-end connection services between AppleTalk nodes and nodes on their network systems.

ADSP provides full-duplex byte-stream delivery, which means that a conversation between two computers can take place in both directions at the same time. ADSP also includes flow control, so that a fast sender does not overwhelm a slow receiver.

ADSP uses the concept of a connection-to-connection data exchange between two nodes. Only when both nodes have established a connection end by using ADSP can data be exchanged between them. If only one node is able to establish a connection end, the connection is called half-open and can be automatically closed by that node if a second connection end at the other node fails to be established within a present time interval.

After a connection is established, ADSP uses a 32-bit sequence number to ensure the sequential flow of data between connection ends (compare this ATP's 16-bit transaction ID). ADSP also uses packet sequencing to make certain that packets are received in the correct order. Every ADSP data packet contains a number identifying the packet's sequence in the data stream -- the receiving node compares this number with its own counter to track the next expected packet. If the two numbers coincide, the packet is accepted; the packet is discarded.

ADSP is also capable of controlling the rate at which data is sent from one node to another, a process known as flow control. Flow control keeps a data transmitter from overwhelming the buffer space of a slower receiver. To accomplish this controlled rate, the receiving node periodically updates the transmitting node by reporting the amount of available buffer space. As a side benefit of this mechanism, two nodes participating in a connection can negotiate a suitably sized window to take advantage of larger bandwidth networks (which is something ATP cannot do).

ADSP uses two types of packets to set up and maintain connections: control packets and data packets. Control packets can be used to probe or acknowledge a connection, open or close a connection, or negotiate a retransmission of a series of data packets. The second type of packets, data packets, are just what the name suggests: These are packets designed to exchange data using DDP (Datagram Delivery Protocol). Up to 572 bytes of data can be stuffed into a packet. The DDP type field equals 7 for ADSP packets.

AppleTalk Echo Protocol (AEP):

An AppleTalk transport layer protocol used to determine whether two nodes are connected and both available. In general, echo protocols are used to determine whether a particular node is available. They can also be used to get an estimate of the roundtrip time on the network.

The AppleTalk Echo Protocol (AEP) is used to send a datagram from one node to another and cause the destination node to return, or echo, the datagram to the sender. To implement this process, each AppleTalk node has an echoer socket.

Because the function of the AEP is comparatively uncomplicated, this protocol has only two packet types: the Echo Request and the Echo Reply.

Yet despite its simplicity, the AEP can prove to be useful on a network. First, this protocol can determine whether a node is accessible before any sessions are started. Second, a programmer can use the AEP to estimate the round-trip delay time for a data transmission between two nodes. (Apple now includes other mechanisms for timing round-trip delay times with AppleTalk version 56 and above, released with System 7.0.

AppleTalk Phase 2:

An enhancement of the original AppleTalk network system. AppleTalk Phase 2 was introduced in June 1989. Phase 2 enables users to build networks with thousands of AppleTalk devices, supports industry standards such as the Token Ring environment, and provides more efficient routing techniques to improve performance in large networks.

When Apple first created AppleTalk, its designers defined the network protocols for small networks tat would be easy to install and maintain. But AppleTalk quickly grew popular, and its easy installation encouraged networks that outgrew the expectations of AppleTalk's architects. A variety of problems began to crop up on the larger networks that used multiple routers and corporate backbones, notably on those using Ethernet. To resolve the inadequacies of the original AppleTalk release, AppleTalk Phase 2 was released in 1989 with a redesigned suite of protocols.

Prior to the development of Phase 2, AppleTalk networks were restricted to 254 nodes per physical cable. For users with a large Ethernet backbone and Macs scattered throughout the plant, this arrangement quickly became unsatisfactory. Now, with Phase 2, AppleTalk uses the concept of extended addressing. Rather than relying on the limited 8-bit node identification number, Phase 2 networks using extended addressing employ a network address that consists of the 8-bit node number plus a 16-bit network number. This expands the network node limits to 2(^24) addresses, or over 16 million nodes on a single network. Note that each workstation can be assigned to a different network number, even though all workstations are on the same cable.

But not all networks running Phase 2 are extended networks. LocalTalk does not implement extended addressing. EtherTalk and TokenTalk networks, however, do implement extended addressing. This means that LocalTalk networks are still limited to 254 nodes per cable.

AppleTalk Transaction Protocol (ATP):

In Macintosh-based AppleTalk networks, a transport layer protocol that can provide reliable packet transmission. Packets are transported within the framework of a transaction, which is an interaction between a requesting and a responding entity (program or node).

The AppleTalk Transaction Protocol (ATP) uses three types of transactions the ATP Transaction Request (TReq), the ATP Transaction Response (TResp), and the ATP Transaction Release (TRel). The ATP is one of the methods that AppleTalk uses to ensure that DDP (Datagram Delivery Protocol) packets are delivered to a destination without any losses (the other is the AppleTalk Data Stream Protocol). ATP accomplishes this by requiring a reply to every ATP transaction. In other words, every time that ATP is requested to send a packet, the receiver socket must report the outcome of the transfer. The first action is referred to as a transaction request, and the report of the action is a transaction response.

Apple Remote Access (ARA):

Software written by Apple to allow remote computers to log into a Mac and AppleTalk network via a modem connection. This software will also allow remote communication and control of a host Macintosh from a remote Macintosh.


Software (usually application programs) that a user interacts with, or uses. Examples of applications are word processing programs, database programs, spreadsheet programs, and electronic mail programs.

Application Gateways:

Application gateways live at the opposite end of the protocol stack. Each application being relayed requires a specialized program at the firewall. This program understands the peculiarities of the application, such as data channels for FTP, and does the proper translations as needed.

It is generally acknowledged that application gateways are the safest form of firewall. Unlike packet filters, they do not pass raw data; rather, individual applications, invoked from the inside, make the necessary calls. The risk of passing an inappropriate packet is thus eliminated.

This safety comes at a price, though. Apart from the need to build new gateway programs, for many protocols a change in user behavior is needed. For example, a user wishing to telnet to the outside generally needs to contact the firewall explicitly and then redial to the actual destination. For some protocols, though, there is no user visible change; these protocols have their own builtin redirection or proxy mechanisms. Mail and the World Wide Web are two good examples.

Application Layer, OSI Model:

The layer of the OSI Reference Model that defines protocols for user or application programs. See OSI Reference Model.

The application layer is part of the Open Systems Interconnection (OSI) model. The OSI model guides software developers and hardware vendors in the design of networking communications products. When two systems need to communicate, they must use the same network protocols. The OSI models divide protocols in seven layers, with the lowest layer defining the physical connection of equipment and electrical signalling. The highest layer defines how an application running on one system can communicate with an application on another system. Middle layers define protocols that set up communication sessions, keep sessions alive, provide reliable delivery, and perform error checking to ensure that information is transmitted correctly. See "Open Systems Interconnection Model" for more information on the complete OSI stack.

The application layer is the top layer in the OSI protocol stack. Applications that provide network features reside at this layer and access underlying communication protocols. Examples include file access and transfer over the network, resource sharing, and print services. The OSI model specifies that applications must provide their own layer 7 protocols. The OSI File Transfer, Access, and Management (FTAM) utility and the X.400 electronic mail standard provide services at the application layer.

Application Network:

For environments where content is dynamically generated when requested, an application server is required. It's common to place application servers on the content network; however, we recommend that you place application servers on their own network. This configuration is essential if an ISP wants to offer application service provider (ASP) services in the near future.

For smaller ISPs, you can omit the application network if web content is static and application servers are not required. This design approach applies to smaller ISPs offering basic services with personal home web page hosting. Another option for smaller ISPs is to combine the application network with the content network.

Application Service Element (ASE):

As pertains to the OSI Reference Model, an ASE (Application Service Element) is any of several elements that provide the communications and other services at the application layer. An application process (AP) or application entity (AE) requests these services through predefined interfaces, such as those provided by Application program Interfaces (APIs).

ASEs are grouped into common application service elements (CASEs) and specific application service elements (SASEs). The CASEs provide services for many types of applications; the SASEs represent or provide services for specific applications or genres (a particular sort, kind, or category, characterized by a certain form, style, or subject matter).


The following CASEs are commonly used:

  • ACSE (Association Control Service Element): This element establishes the appropriate relationship between two applications (AEs) to enable the applications to cooperate and communicate on a task. Since all associations or relationships must be established through the ACSE, and since applications must establish a relationship to communicate, the ACSE is needed by all applications.
  • CCRSE (Commitment, Concurrency, and Recovery Service Element): This element is used to implement distributed transactions which may require multiple applications. The CCRSE helps ensure that distributed data remains consistent by making sure that applications do not interfere with each other when doing their work and that actions are performed completely or not at all.
  • ROSE (Remote Operations Service Element): This element supports interactive cooperation between two applications, such as between a client and a server. ROSE provides the services needed for the reliable execution of requested operations and transfer of data.
  • RTSE (Reliable Transfer Service Element): This element helps ensure that PDUs (Protocol Data Units), or packets, are transferred reliably between applications. RTSE services can sometimes survive an equipment failure because they use transport-layer services.


The following SASEs are commonly used:

  • DS (Directory Service): This element makes it possible to use a global directory, which is a distributed database with information about all accessible network entities in a communications system.
  • FTAM (File Transfer Access and Management): This element enables an application to read, write, or otherwise manage files on a remote machine.
  • JTM (Job Transfer and Manipulation): This element enables an application to do batch data processing on a remote machine. With JTM, a node could, for example, start a computation on a supercomputer at a remote location and retrieve the results when the computation was complete.
  • MHS (Message Handling System): This element enables applications to exchange messages; for example, when using electronic mail
  • MMS (Manufacturing Message Service): This service element enables an application on a control computer to communicate with an application on a slave machine in a production line or other automated operation.
  • VT (Virtual Terminal): This programming element makes it possible to emulate the behavior of a particular terminal, which enables an application to communicate with a remote system without considering the type of hardware sending or receiving the communications.

The entire set of ASEs required for a particular application is known as the application context (AC) for that application.


Archie is a process that allows use of the File Transfer Protocol (FTP) without an account at the host offering the files. Archie is a database that servers as an index to the holdings of some of the major anonymous FTP sites. Thousands of computers offer more than 2 million files for this kind of access. This is an important avenue for obtaining public domain or sharing software on the Internet.

The Archie service allows a user to locate files without knowing the file attribute. Archie servers index information based on filename. There are two database indexes: FILENAMES and WHATIS. The FILENAMES database indexes the names of files available from hundreds of Internet FTP sites. Archie allows queries based on specified patterns, archive sites, or filename match. The WHATIS database contains the names and descriptions of software packages and documents. Entries include text strings consisting of keywords and associated descriptions. Archie automatically updates entries in the FILENAMES, but the WHATIS database is manually maintained by a systems administrator.


As it relates to networks, it is the design plan that determines how the network's components function together.

ARCnet (Attached Resource Computer Network):

First developed by Datapoint Corporation, ARCnet is a local area network for IBM personal computers and compatibles. ARCnet uses the token-passing method.

Areal Density (Disk Storage):

Areal density is the number of bits that a disk drive can hold in a given space on the disk platter. The first generation of drives used in microcomputers had a areal density of about 15 megabits per squar inch; current drives have about 5000 to 10000 megabits per square inch (i.e., 5 to 10 Gigabits).

Areal density is also called bit density. The amount of data that can be packed onto a storage medium. Areal densities are generally measured in gigabits per square inch. The term is useful for comparing different types of media, such as magnetic disks and optical disks. Current magnetic and optical disks have areal densities of several gigabits per square inch.

ARP (Address Resolution Protocol):

--The TCP/IP protocol used to dynamically bind a high level IP Address to a low-level physical hardware address. ARP is only across a single physical network and is limited to networks that support hardware broadcast. Basically ARP is responsible for translating an IP address into a physical address, such as an Ethernet address.

ARPAnet (Advanced Research Projects Agency):


ASCII (American Standard Code for Information Interchange):

The ASCII code represents keyboard characters, control characters, and some graphics elements as an on/off pattern of 7 bits plus one more bit for an error-checking process known as parity checking. For example, the ASCII code for the letter "a" is 0110001 in binary, 61 in hexadecimal, and 97 in decimal.

Although initially developed as the code for representing text in a file on a single computer, the ASCII code is now also one of the most common codes used for transmitting data on networks of personal computers or between pieces of data processing equipment such as a computer and a printer. The ASCII code can represent 128 characters, however, there are extensions to this code for a total of 256 characters, using 8 bits.

Asynchronous Communication:

A method for transmitting data that sends one character at a time. Asynchronous also refers to commands, as in a windowing environment that may be sent without waiting for a response from the previous command. Also see synchronous communication.

Asynchronous Transfer Mode (ATM):

ATM (Asynchronous Transfer Mode) is a packet-switched, broadband network architecture that is expected to become an established standard by the late 1990s. It forms the core of a broadband ISDN (BISDN) architecture, which extends the digital transmission capabilities defined by ISDN to allow data, voice, and multimedia transmissions on the same lines. It is also known as cell relay, to distinguish it from frame relay.

ATM is a real-time architecture that will be able to provide very high bandwidth as needed. The initial implementations will operate at 155.52 megabits per second (Mbps), then at 622.08 Mbps. Speeds up to 2.488 gigabits per second (Gbps) are planned and have been demonstrated in limited tests.

The very high bandwidth and the ability to transmit multiple media make ATM an attractive, high-speed architecture for both local-area networks (LANs) and wide-area networks (WANs). It is useful for enterprise networks, which often connect LANs over wide areas and may need to transport large amounts of data over very long distances.

Long-haul, high-bandwidth capabilities are particularly attractive for WANs, which have until now been shackled by the relatively low bandwidths over long-distance lines. FDDI (Fiber Distributed data Interface) is a good architecture for LANs, and frame relay has possibilities for WANs, but neither of these architectures is suitable for both LANs and WANs. But note that ATM is still quite expensive.


The loss of signal strength that occurs as a signal is transmitted through a cable. It is measured in decibels (dB) per kilometer (expressed as dB/km) or per 100 feet. In the logarithmic decibel scale, a 3 dB loss means a 50 percent loss in power, as computed in the following equation. Specifically, the formula for power loss is:

dB = 10 log (base 10) * Power (out)/Power (in)

In this equation, a 50 percent loss would actually yield a result of -3 dB. Under certain conditions, the coefficient in the equation will be 20, in which case a result of -6 dB would indicate a 50 percent loss. When describing losses, however, the negative sign is dropped, so that a result of -6 dB is expressed as a 6 dB loss.

Attenuation depends on several factors, including:

  • Wire composition and size,
  • Shielding, and
  • Frequency range of the signal.

For copper cable, attenuation increases with signal frequency; for optical fiber, attenuation is relatively constant over a large frequency range.

Fiber-optic cable has the least attenuation, usually fractions of a decibel per kilometer. Unshielded untwisted-pair cable (such as the silver, flat-satin cables used in short-distance telephone and modem lines) has the most attenuation of any cable types used in telecommunications. This type of cable is not used directly in networks.

Attenuation Factor:

A value that expresses the amount of a signal lost over a given distance, such as decibel loss per kilometer (expressed as dB/km).

AWG (American Wire Gauge):

AWG (American Wire Gauge) is a classification system for copper wire. The system is based on the gauge, or diameter, of the conducting wire. The lower the gauge, the thicker the wire and the lower the resistance per unit length. The table below "Diameter and Resistance Values for Selected Wire Gauges" shows some gauge values and corresponding diameters.

Diameter and Resistance Values for Selected Wire Gauges


In mathematics, axiomatization is the process of defining the basic axiomatic systems from which mathematical theories can be derived.

Every mathematical theory is based on a set of axioms. Usually these axioms are not mentioned when a mathematical equation is presented. Mathematicians know from their education on which axioms mathematical theories are based. Indeed, mathematical theories usually are based on very few axioms. Some of them are mentioned in the example below.

Example: The axiomatization of natural numbers

The mathematical system of natural numbers 1, 2, 3, 4, ... is based on an axiomatic system that was first written down by the mathematician Peano in 1901. He defined the axioms (see Peano axioms) for the set N of natural numbers as being:

  • There is a natural number 0.
  • Every natural number a has a successor, denoted by a + 1.
  • There is no natural number whose successor is 0.
  • Distinct natural numbers have distinct successors: if a <> b, then a + 1 <> b + 1.
  • If a property is possessed by 0 and also by the successor of every natural number it is possessed by, then it is possessed by all natural numbers.

Any more-or-less arbitrarily chosen system of axioms is the basis of some mathematical theory, but such an arbitrary axiomatic system will not necessarily be free of contradictions, and even if it is, it is not likely to shed light on anything. Philosophers of mathematics sometimes assert that mathematicians choose axioms "arbitrarily", but the truth is that although they may appear arbitrary when viewed only from the point of view of the canons of deductive logic, that is merely a limitation on the purposesthat deductive logic serves.

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