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"U" Networking Definitions & Concepts...

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UML (Unified Modeling Language)

The Unified Modeling Language (UML), the standard graphical notation for modeling business and software application needs, has emerged as an effective modeling tool for database design. When used as a common modeling language for the many facets of system development, the UML can serve as a unifying framework that facilitates the integration of database models with the rest of a system design.

The Unified Modeling Language (UML) is the successor to the wave of object-oriented analysis and design (OOA&D) methodes that appeared in the late '80s and early '90s. It most directly unifies the methods of Booch, Rumbaugh (OMT), and Jacobson, but its reach is wider then that. The UML went through a standardization process with the OMG (Object Management Group) and is now an OMG standard.

Just as structured analysis uses DFDs to model data and processes, systems analysts use the Unnified Modeling Language to descrobe object-oriented systems. UML is a popular technique for documenting and modeling a system. The UML uses a set of symbols to represent graphically the various components and relationships within a system.

The Unified Modeling Language is a graphical and textual language for expressing (primarily) object-oriented systems. It can be used for analysis and design of new systems as well as for documentation of existing systems. UML has its roots in the efforts of numerous contributors whoe work begain over a decade ago. UML is now a committe-run standard driven by the Object Management Group (OMG), which is made up of various academic and commercial contributors.

The major components of UML are the various specifications for document formats and diagrams that are used to describe systems. The UML class diagrams discussed in this appendix are used to describe one or more classes, their properties and methods, and their relationships with other classes in the diagram. Hierarchies of classes can be visually displayed as well as their linkage, through inheritance, aggregation, or association.

Specific symbols and text conventions, called notation, are used to describe nearly every object-oriented feature of a class structure.

The object-oriented (OO) modeling notation that is presented is the Unified Modeling Language (UML). UML is the successor to the modeling techniques found in Grady Booch's Object Technology, James Rumbaugh's Object Modeling Technique (OMT), Ivar Jacobson's Object Oriented Software Engineering, and several other methods. In 1995, a preliminary version of UML was presented to the Object-Oriented Programming, Systems, Languages, and Applications (OOPSLA) conference. By January 1997, UML had gone through several iterations and reviews, incorporating public feedback and revision by the primary authors. In January 1997, UML was presented to the Object Management Group (OMG) in response to its request for a standard modeling technique.

Elements of a Class Diagram

As might be expected with any tool used to describe a system, the nomenclature within a class diagram is very specific and intended to remove ambiguity. the elements of a class diagram that will be discussed herein are classes, relationships, packages, and notes.

UML: (Computer Desktop Encyclopedia; 1/1/2001)

(Unified Modeling Language) An object-oriented analysis and design language from the Object Management Group (OMG). Many design methodologies for describing object-oriented systems were developed in the late 1980s. UML standardizes several diagramming methods, including Grady Booch's work at Rational Software, Rumbaugh's Object Modeling Technique and Ivar Jacobson's work on use cases. There are nine kinds of diagrams that are supported under UML.

Use Case Diagram

The above diagram shows a system's functions from a user's point of view, which in this case is pretty simple. All the following diagrams were created in the Telelogic Tau UML suite by Telelogic for this same example. Five of the nine possible diagrams available in UML are shown below. The Component Diagram (software module interaction), Deployment Diagram (hardware nodes) and the Activity Diagram (tasks) are not included. The Object Diagram (instantiations of classes) is rarely used. (All diagrams courtesy of Telelogic, AB, www.telelogic.com)

Sequence Diagram

Like an MSC (Message Sequence Diagram), the Sequence Diagram depicts the message flow between entities in the system. The items between double left/right arrows are UML "stereotypes." See MSC.

Collaboration Diagram

This diagram emphasizes the structure of the relationships between entities. Note that the user is associated with the user terminal in a session, and the terminal is associated with the server in a connection. The order of messages can be read by interpreting the numbers prefixes.

Class Diagram

User, User Terminal and Server are three classes, which have attributes and operations. An actor is a UML "stereotype" that is external to the system. Lines between classes are "associations." The asterisk means "many," in this case, a many-to-one relationship between client and server.

State Chart Diagram

This describes the finite states that take place in the system. The rectangles are the states, and the lines between them are the transitions. The connection state shows three substates. The single circles are starting points, and the double circles are the ends.


As in other areas of life, electrical dangers can come from having too much or too little. Collectively, such disturbances are known as overvoltages and undervoltages, respectively. An undervoltage is a condition in which the voltage supply is below its nominal specified level.


In a packet-based network, these packets are addressed to a single destination. For instance, if the workstation Monica communicates with the web server Bill, the packets exchanged between these hosts are unicast. Monica addresses the packets to Bill, and vica versa. Unicode:

Unicode is a 16-bit character code, which supports up to 64,000 different characters. A 16-bit representation is particularly useful for languages with large alphabets or other basic units (for example, Asian languages). The Unicode specifications were developed by the Unicode Consortium. Most of the commonly used character codes (such as ASCII or EBCDIC) are encoded somewhere in Unicode's data-banks and can, therefore, be used. Character representation using Unicode is in contrast to the code-page strategy currently used in most DOS and Microsoft Windows environments. Each code page is 8 bits and has room for just 256 characters.

The NetWare Directory database in Novell's NetWare 4.x uses Unicode format to store information about objects and their attributes.

Unified Messaging (UM):

Unified messaging, also known as integrated messaging, is a local-area network (LAN) based telephony service in which various kinds of messages or information can be accessed in a transparent manner. The types of information that can be handled include electronic mail (e-mail), fax, image, video, and voice transmissions.

With unified messaging, the telephony services can find and display the messages regardless of the format. This search-and-display process, known as a launch, may require certain applications. For example, the process may need an application that can display a particular type of message. Any required applications will be started up automatically.

Uniform Resource Locator (URL):

A URL provides a means of identifying a document on the Internet. The following is an example of a URL:


This URL has three main parts:

  • Information about the document type and about the protocol used to trnasport it. On the World Wide Web (WWW) the most common value is http, as in this example. This indicates that the Hypertext Transfer Protocol (HTTP) is being used -- probably to transport a hypertest document written using HTML (Hypertext Markup Language). Other possible values include FTP (File Transfer Protocol), Gopher, and file. The protocol information is almost alwoys followed by a colon and two forward slahes(://).
  • The next element is the domain name of the machine on whcih the document is found. In the exaple, this is cuiwww.unige.ch, which is a web server at the university of Geneva (unige) in Switzerland (ch).
  • The final element is the document's name. This name must be represented as an absolute path to the file. In the example, the document is named meta-index.html and is found in the root directory of the machine.

URLs are an example of the more general Universal Resource Identifiers (URI), which also encompass Univeral Resource Names (URNs).

Uninterruptible Power Supply (UPS):

Backup power for a computer system when the "main electrical power" fails or drops to an unacceptable voltage level. Small UPS systems provide battery power for only a few minutes--enough to power down the computer or server in an orderly manner. Sophisticated systems are tied to electrical generators and can provide power for days.

Universal Serial Bus (USB):

A standard interface for connecting all kinds of peripherals, such as printers, modems, hard drives, tape drives, etc., that is intended to replace the use of traditional parallel and serial ports.


UNIX is a 32-bit, multiuser, multitasking operating system. It was originally developed at AT&T's Bell Labs by Ken Thompson and Dennis Ritchie in 1969 to implement a space invaders game on some unused hardware. The operating system has since been implemented on hardware ranging from PCs to Crays; it has acquired hundreds of commands, tools, and utilities over the years.

As stated above, it is a multiuser system that supports networking and distributed file systems such as Sun Microsystems' Network File System (NFS) or the Open Software Foundation's (OSF) implementation of the Andrew File System (AFS).

UNIX development has proceeded along two major strains: the AT&T System releases (with the most recent major release being System V) and the UC Berkeley System Distribution (BSD) releases (with the most recent major release being 4). The various UNIX strains and variants were combined at the UNIX Software Operation (now UNIX Systems Group, a division of Novell). In recognition of the two UNIX strains, the most recent combined version is System V Release 4.2, known as SVR4.2.

The UNIX environment provides several types of networking resources, including the uucp (UNIX-to-UNIX copy) program and the TCP/IP (Transmission Control Protocol/Internet Protocol) suite. UNIX also makes distributed computing easier, and it forms a major part of the Internet software infrastructure. The X Window System developed at MIT (Massachusetts Institute of Technology) provides the basis of a graphical interface for UNIX.

UNIX variants, work-alikes, and extensions abound that were implemented after AT&T made the operating system available on an open basis to universities and colleges for use in research projects and computer science programs. The following is a partial lists:

  • A/UX (Macintosh)
  • AIX (IBM)
  • Coherent (Intel)
  • LINUX (Intel)
  • MACH (Macintosh, and various)
  • MINIX (various)
  • NeXTSTEP (NeXT and Intel)
  • Solaris (RISC and Intel)
  • UnixWare (Intel)
  • Xenix (Intel)
  • Yggdrasil (Intel)

Some of the major developments in the UNIX environment are listed here:

  • In the early 1980's, the Portable Operating System Interface for UNIX (POSIX) standard was defined by the Institute of Electrical and Electronics Engineers (IEEE) and the American National Standards Institute (ANSI) to create an industry-standard UNIX-like system and a set of application program interfaces for developing portable applications. Various working groups within the IEEE are still defining the standard. The original specifications were adopted by the International Orgainization for Standardization (ISO) in 1988.
  • The X/Open group was founded in 1984 by Bull, Nixdorf, Philips, Siemens, and other companies to promote open UNIX standards by testing for conformity among products.
  • The Open Software Foundation was formed in 1988 by IBM, DEC, Hewlett-Packard, Nixdorf, Siemens, and hundreds of other members to develop the OSF/1 UNIX distributed operating system as well as the Distributed Computing Environment (DCE).
  • UNIX International was formed in 1988 by AT&T's UNIX System Laboratory (USL), Control Data, Data General, Informix, Intel, Motorola, NCR, Olivetti, Texas Instruments, and hundreds of other companies to promote open UNIX through publicly available documentation. After fulfilling goals, the organization disbanded in late 1993.
  • The Common Open Software Environment (COSE) was founded in 1993 by IBM, Hewlett-Packard, SunSoft, Novell, and others, with the goal of cooperating to deliver a common desktop environment (graphical user interface) for UNIX that can rival Microsoft Windows.
  • In 1991, Novell and AT&T's UNIX System Laboratories (USL) joined forces to create Univell, a company with goals of developing UnixWare, a desktop UNIX system with built-in Novell NetWare support. In 1993, Novell purchased USL and formed the UNIX Systems Group (USG) to manage UnixWare. Univel will eventually become nonexistent.

With the purchase of USL, Novell gained control of UNIX SVR4 to the dismay of other UNIX vendors. However, in an attempt to consolidate the industry on a common UNIX operating system, Novell gave the UNIX trademark to the X/Open organization. X/Open will grant the UNIX trademark to UNIX implementations that are compatible with a set of specifications defined by the COSE (Common Open Software Environment) group. This set of specifications, called the COSE Spec 1170 APIs, defines programming interfaces that promote the portability of applications between operating systems.

One of the main advantages of UNIX is its widespread use as a development platform and disktop operating system. As mentioned earlier, AT&T made the code available in the academic environment, spurring its proliferation onto many platforms and the development of unique applications. Most of this work was done at the University of California at Berkeley, which produced versions called the Berkeley Software Distributions (BSDs). UC Berkeley is responsible for adding the Transimission Control Protocol/Internet Protocol (TCP/IP) networking protocols and porting UNIX to the DEC VAX. A number of UNIX variations exist as given above.

The operating system initially became popular in the engineering, computer-aided design, and scientific environments, where it was implemented on powerful workstations. It eventually found its way into business, medical, and many other environments. Another contributing factor to UNIX's popularity is that it is written in the C programming language. Thus, UNIX is highly portable and contains system components written in a common, well-known programming language that are easily recompiled to work on a variety of systems.

The major growth and development of UNIX today are taking place on smaller systems such as those based on Intel x86 and motorola 68000 series processors and low-priced workstations based on the Reduced Instruction Set Computer (RISC) chip. The number of UNIX versioins has stalled its industry-wide acceptance, but the growing use of Windows NT has provided incentive for UNIX vendors to create compatible products and common graphical user interfaces to promote the UNIX environment.

Use Case (UML):

A complete flow of actions initiated by an actor that the system performs, which provide value to that actor.

Use Case Analysis(UML):

An object-oriented method for designing information systems by breaking down requirements into user functions. Each use case is a transaction or sequence of events performed by the user. Use cases are studied to determine what objects are required to accomplish them and how they interact with other objects.

Use Case Diagram (UML):

A diagram that shows use cases and their relationships with actors and other use cases. The use case model is a model of the system's intended functions and its environment, which supports the business processes. This model serves as a contract between the customer and the developers.

Use Case Model (UML):

A model that describes a system's functional requirements in terms of use cases, use case narratives, and scenarios.

User Authentication Method (UAM):

Any procedure used by a server and workstation by which the server is convinced of the user's identity.

User Datagram Protocol (UDP):

-- The TCP/IP standard protocol that allows an application program on one machine to send a datagram to an application program on another machine. UDP uses the Internet Protocol (IP) to deliver datagrams. Conceptually, the important difference between UDP datagrams and IP datagrams is that UDP includes a protocol port number, allowing the sender to distinguish among multiple destinations (application programs) on the remote machine. In practice, UDP also includes a checksum over the data being sent.

User-Friendly (UF):

A characteristic of human-operated equipment and systems which makes them safe, comfortable, and easy to use.

User-Generated Traffic (UGT):

Network activity that is generated by user requests and that constitutes the majority of network traffic. Examples include accessing the file server, using the Chooser, and printing documents. Compare with overhead traffic.

User Name (UN):

A string of characters that uniquely identifies a user for login purposes; the user name is entered by the user and confirmed in a user-authentication database before the user is permitted to gain access to the network resource.

User Node IDs (UNID):

One of two classes of node Id numbers; user node IDs fall within the range 1-127 ($01-$7F) and are generally used by workstations.

User-to-Network Interface (UNI):

In ATM networks, one of three levels of interface. The other two are network-to-network (NNI) and user-to-user (UUI).


The <> stereotype groups global variables and procedures in the form of a class declaration. The utility attributes and operation become global variables and global procedures, respectively. A utility is not a fundamental modeling construct, but a programming convenience.


An application layer protocol for transferring files between UNIX systems. The uucp (for UNIX-to-UNIX copy program) protocol is dial-up and store-and-forward, so that its services are limited. uucp is available for just about every operating environment. It is commonly used as a low-end access protocol for the Internet.

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