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Glossary

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Term Main definition
H3G

Hutchison 3G (Three), 3UK

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Half-duplex
A system in which signals may be sent in two directions, but not at the same time. In a half-duplex system, one end of the link must finish transmitting before the other end may begin.
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Half-duplex transmission
Data transmission over a circuit capable of transmitting in either direction. Transmission can be bidirectional but not simultaneously.
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Halogen
One of the following elements: chlorine, fluorine, bromine, astatine, or iodine.
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Handset Lead

May be referred to as curly cord or curly handset lead.

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Hard-clad silica fiber
An optical fiber with a hard plastic cladding surrounding a step index silica core.
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Hardware address
The address is represented by six sections of two hexadecimal addresses; for example, 00-03-fe-e7-18-54. This number is hard-coded into networking hardware by manufacturers. Also called the physical address; see also MAC address.
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Hardware loopback
Connects the transmission pins directly to the receiving pins, allowing diagnostic software to test whether a device can successfully transmit and receive.
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HD

High Density also known as HDC or HD

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HDC

High Density also known as HDC or HD

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Head end
(1) The central facility where signals are combined and distributed in a cable television system or a public telephone system. See central office.
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Header
The section of a packet (usually the first part of the packet) where the layer-specific information resides.
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Headroom
The number of decibels by which a system exceeds the minimum defined requirements. The benefit of more headroom is that it reduces the bit-error rate (BER) and provides a performance safety net to help ensure that current and future high-speed applications will run at peak accuracy, efficiency, and throughput. Also called overhead or margin.
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Hertz (Hz)
A measurement of frequency defined as cycles per second.
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Heterojunction structure
An LED design in which the pn (P-type and N-type semiconductor) junction is formed from similar materials that have different refractive indices. This design is used to guide the light for directional output.
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