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Glossary Section "H" ( Definitions )

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Half –Duplex Data flows only in one direction at a time.

Harmonic Distortion An additional signal generated by the nonlinear characteristics of the transmission line which distorts the primary signal

header The portion of a packet, preceding the actual data, containing source and destination addresses and error-checking fields. The word can also be used to describe the part of an electronic mail message (or USENET news article) that precedes the body, although one usually talks about headers (plural) in that case.

HDLC High level Data Link Control. A protocol used at the Data Link Layer which provides point-to-point communications over a physical transmission medium by creating and recognizing frame boundaries.

HDSL High Data rate Digital Subscriber Line  1.54 Mbps T1 or 2.048 Mbps E1 symmetrical

HFC Hybrid-fiber Coaxial. A technology being developed by the cable TV industry to provide two-way, high-speed data access using a combination of fiber optics and traditional coaxial cable.

hierarchical routing The complex problem of routing on large networks can be simplified by reducing the size of the networks. This is accomplished by breaking a network into a hierarchy of networks, where each level is responsible for its own routing. The Internet has, basically, three levels: the backbones, the mid-levels, and the stub networks. The backbones know how to route between the mid-levels, the mid-levels know how to route between the sites, and each site knows how to route internally.

HIPPI High Performance Parallel Interface. An ANSI standard which extends the computer bus over fairly short distances at speeds of 800 and 1600 Mb/s. HIPPI is often used in a computer room to connect a supercomputer to routers, frame buffers, mass-storage peripherals, and other computers.

hop A term used in routing. A hop is one data link. A path from source to destination in a network is a series of hops. Often used to measure the number of routers that a packet must traverse.

hop count A routing metric used to measure the distance between a source and a destination. Particularly used by RIP.

HSSI High Speed Serial Interface

HTML Hyper Text Markup Language. The language used in the World-Wide Web to create Web pages with links to other documents, rich text enhancements (bold, italic, etc.) and so on. The "source" file for what you see on a Web page is written in HTML.

HTTP Hyper Text Transfer Protocol. The protocol most commonly used in the World-Wide Web to transfer information from Web servers to Web browsers.  HTTP defines how messages are formatted and transmitted, and what actions Web servers and browsers should take in response to various commands. 

host A computer that allows users to communicate with other host computers on a network. Individual users communicate by using application programs, such as electronic mail, Telnet and FTP.

hostname The name given to a machine. See Fully Qualified Domain Name.

hub (1)A device connected to several other devices. In ARCnet, a hub is used to connect several computers together. In a message handling service, a hub is used for the transfer of messages across the network. An Ethernet hub is basically a "collapsed network-in-a-box" with a number of ports for the connected devices.(2) Communications center, (3) Major routing station for connecting channels, (4) DDS connecting center

hyperlink A pointer within a hypertext document which points (links) to another document, which may or may not also be a hypertext document.

hypertext A document, written in HTML, which contains hyperlinks to other documents, which may or may not also be hypertext documents. Hypertext documents are usually retrieved using WWW.

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