Half Duplex

　　Half duplex is a transmission path capable of transmitting signals in both directions, but only in one direction at a time.

　　HDLC

　　high-level data link control

　　Header

　　The header is control information appended to a segment of user data for control, synchronization, routing and sequencing of a transmitted data packet or frame.

　　Hertz (Hz)

　　Hertz is the measurement that distinguishes electromagnetic waveform energy; number of cycles, or complete waves, that pass a reference point per second; measurement of frequency, by which one hertz equals one cycle per second.

　　Horizontal Wiring

　　In a premises distribution system, horizontal wiring is the connection between the telecommunications outlet in work areas and the telecommunications closet.

　　Hub

　　In local area networks (LANs), a hub is a wiring concentrator equipment used in hierarchical star physical wiring topologies. Those directly connected to terminals or other user devices are of ten referred to as local hubs or concentrators. Central hubs are those at the highest hierarchical level. Hubs often provide the means for interconnecting 10BaseT, coaxial or fiber optic cable LAN segments. Intelligent hubs may implement multiport bridging and network management functions.

　　HVAC

　　heating, ventilation and air conditioning

　　Home Page - The Web document that your browser displays when you access a site - typically the main page of a Web site.

　　Host - A computer directly connected to the Internet. Also refers to a computer on a network that provides services to other computers on the network.

　　HTML - See HyperText Markup Language

　　HyperText Link - A connection between two Web documents; usually underlined text or a graphic that you click on to display another Web document.

　　HyperText Markup Language (HTML) - The basic coding language used to create WWW documents.

　　HyperText Transport Protocol (HTTP) - The protocol for moving HyperText files across the Internet

　　HTTP - See HyperText Transport Protocol