A B C D E F G H I J K L M N O P Q R S T U V W X Y Z.

(U)

UDP: UDP (User Datagram Protocol) is a communications method (protocol) that offers a limited amount of service when messages are exchanged between computers in a network that uses the Internet Protocol (IP). UDP is an alternative to the Transmission Control Protocol (TCP) and, together with IP, is sometimes referred to as UDP/IP. Like the Transmission Control Protocol, UDP uses the Internet Protocol to actually get a data unit (called a datagram) from one computer to another. Unlike TCP, however, UDP does not provide the service of dividing a message into packets (datagrams) and reassembling it at the other end. Specifically, UDP doesnt provide sequencing of the packets that the data arrives in. This means that the application program that uses UDP must be able to make sure that the entire message has arrived and is in the right order. Network applications that want to save processing time because they have very small data units to exchange (and therefore very little message reassembling to do) may prefer UDP to TCP. The Trivial File Transfer Protocol (TFTP) uses UDP instead of TCP.
UDP provides two services not provided by the IP layer. It provides port numbers to help distinguish different user requests and, optionally, a checksum capability to verify that the data arrived intact.
In the Open Systems Interconnection (OSI) communication model, UDP, like TCP, is in layer 4, the Transport Layer.

UENCODE: Uuencode (also called Uuencode/Uudecode) is a popular utility for encoding and decoding files exchanged between users or systems in a network. It originated for use between users of UNIX systems (its name stood for "UNIX-to-UNIX encoding"). However, its available or easily obtainable for use in all operating systems and most e-mail applications provide it as an encoding alternative, especially for e-mail attachments. If youre sending e-mail with an attachment and you suspect your recipient(s) may not have a MIME-compliant system (for example, an older PC or UNIX system), you may want to specify "Uuencode" for the attachment to an e-mail note. (In Eudora, when writing a note, look for the little box set to a default of "MIME" and change it to "Uuencode.")
Basically, what Uuencode does is to translate or convert a file or e-mail attachment (it can be an image, a text file, or a program) from its binary or bit-stream representation into the 7-bit ASCII set of text characters. Text can be handled by older systems that may not handle binary files well and larger files can be more easily divided into multi-part transmissions.

UNIX: UNIX is an operating system that originated at Bell Labs in 1969 as an interactive time-sharing system. Ken Thompson and Dennis Ritchie are considered the inventors of UNIX. The name (pronounced YEW-nihks) was a pun based on an earlier system, Multics. In 1974, UNIX became the first operating system written in the C language. UNIX has evolved as a kind of large freeware product, with many extensions and new ideas provided in a variety of versions of UNIX by different companies, universities, and individuals. Partly because it was not a proprietary operating system owned by any one of the leading computer companies and partly because it is written in a standard language and embraced many popular ideas, UNIX became the first open or standard operating system that could be improved or enhanced by anyone. A composite of the C language and shell (user command) interfaces from different versions of UNIX were standardized under the auspices of the IEEE as the Portable Operating System Interface (POSIX). In turn, the POSIX interfaces were specified in the X/Open Programming Guide 4.2 (also known as the "Single UNIX Specification" and "UNIX 95"). Version 2 of the Single UNIX Specification is also known as UNIX 98. The "official" trademarked UNIX is now owned by the The Open Group, an industry standards organization, which certifies and brands UNIX implementations.
UNIX operating systems are used in widely-sold workstation products from Sun Microsystems, Silicon Graphics, IBM, and a number of other companies. The UNIX environment and the client/server program model were important elements in the development of the Internet and the reshaping of computing as centred in networks rather than in individual computers. Linux, a UNIX derivative available in both "free software" and commercial versions, is increasing in popularity as an alternative to proprietary operating systems.

UNZIP: Unzipping is the act of extracting the files from a zip file or similar file archive. If the files in the package were also compressed (as they usually are), unzipping also uncompresses them.
When you download programs from shareware or freeware companies on the Web, they almost always send you a zipped file (the file name has a suffix of ".zip"). You can usually extract the files from a zipped file just by double-clicking on it since many archives include a "self-extracting" program. Several popular tools exist for zipping and unzipping: PKZIP in the DOS operating system, WinZip in Windows, and MacZip in Macintosh.

UPLOAD: Uploading is the transmission of a file from one computer system to another, usually larger computer system. From a network user's point-of-view, to upload a file is to send it to another computer that is set up to receive it. People who share images with others on bulletin board services (BBS's) upload files to the BBS.
Downloading is transmission in the other direction: from one, usually larger computer to another, usually smaller computer. From an Internet user's point-of-view, downloading is receiving a file from another computer.
The File Transfer Protocol (FTP) is the Internet facility for downloading and uploading files. (If you are uploading a file to another site, you must usually have permission in advance to access the site and the directory where the file is to be placed.)
When you send or receive an attached file with an e-mail note, this is just an attachment, not a download or an upload. However, in practice, many people use "upload" to mean "send" and "download" to mean receive. The term is used loosely in practice and if someone says to you "Download (or upload) such--and-such a file to me" via e-mail, they simply mean "Send it to me."
In short, from the ordinary workstation or small computer user's point-of-view, to upload is to send a file and to download is to receive a file.

UPS: A UPS (uninterruptible power supply) is a device that allows your computer to keep running for at least a short time when the primary power source is lost. It also provides protection from power surges. A UPS contains a battery that "kicks in" when the device senses a loss of power from the primary source. If you are using the computer when the UPS notifies you of the power loss, you have time to save any data you are working on and exit gracefully before the secondary power source (the battery) runs out. When all power runs out, any data in your computer's random access memory (RAM) is erased. When power surges occur, a UPS intercepts the surge so that it doesn't damage your computer.

URL: A URL (Uniform Resource Locator) (pronounced YU-AHR-EHL or, in some quarters, UHRL) is the address of a file (resource) accessible on the Internet. The type of resource depends on the Internet application protocol. Using the World Wide Webs protocol, the Hypertext Transfer Protocol (HTTP) , the resource can be an HTML page (like the one youre reading), an image file, a program such as a CGI application or Java applet, or any other file supported by HTTP. The URL contains the name of the protocol required to access the resource, a domain name that identifies a specific computer on the Internet, and a hierarchical description of a file location on the computer.
On the Web (which uses the Hypertext Transfer Protocol), an example of a URL is:
http://www.joinnet.com.jo
which describes a Web page to be accessed with an HTTP (Web browser) application that is located on a computer named www.mhrcc.org. The specific file is in the directory named /kingston and is the default page in that directory (which, on this computer, happens to be named index.html).

USENET: Usenet is a collection of notes on various subjects that are posted to servers on a worldwide network. Each subject collection of posted notes is known as a newsgroup. There are thousands of newsgroups and it is possible for you to form a new one. Most newsgroups are hosted on Internet-connected servers, but they can also be hosted from servers that are not part of the Internet. Usenets original protocol was UNIX-to-UNIX Copy (UUCP), but today the Network News Transfer Protocol (NNTP) is used.
Most browsers, such as those from Netscape and Microsoft, provide Usenet support and access to any newsgroups that you select. On the Web, Deja News and other sites provide a subject-oriented directory as well as a search approach to newsgroups and help you register to participate in them. In addition, there are other newsgroup readers, such as Knews, that run as separate programs.