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Glossary

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Term Main definition
Asynchronous

Transmission where sending and receiving devices are not synchronized (without a clock signal). Data must carry markers to indicate data division.

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Asynchronous transfer mode (ATM)

In networking terms, asynchronous transfer mode is a connection-oriented networking technology that uses a form of very fast packet switching in which data is carried in fixed-length units. These fixed-length units are called cells; each cell is 53 bytes in length, with 5 bytes used as a header in each cell. Because the cell size does not change, the cells can be switched very quickly. ATM networks can transfer data at extremely high speeds. ATM employs mechanisms that can be used to set up virtual circuits between users, in which a pair of users appear to have a dedicated circuit between them. ATM is defined in specifications from the ITU and ATM Forum. For more information, see the ATM Forum

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AT&T

Also known as 110 plug

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Attachment unit interface (AUI) port

A 15-pin connector found on older network interface cards (NICs). This port allowed connecting the NIC to different media types by using an external transceiver. The cable that connected to this port when used with older 10Base5 media was known as a transceiver cable or a drop cable.

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Attenuation

A general term indicating a decrease in power (loss of signal) from one point to another. This loss can be a loss of electrical signal or light strength. In optical fibers, it is measured in decibels per kilometer (dB/km) at a specified wavelength. The loss is measured as a ratio of input power to output power. Attenuation is caused by poor-quality connections, defects in the cable, and loss due to heat. The lower the attenuation value, the better. Attenuation is the opposite of gain. See Chapter 1 for additional information on attenuation and the use of decibels.

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Attenuation-limited operation
In a fiber-optic link, the condition when operation is limited by the power of the received signal.
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Attenuation-to-crosstalk ratio (ACR)
A copper cabling measurement, the ratio between attenuation and near-end crosstalk, measured in decibels, at a given frequency. While it is not a requirement of ANSI/TIA-568-C.2, it is used by every manufacturer as a figure of merit in denoting signal to noise ratio.
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Attenuation-to-crosstalk ratio, far end (ACRF)
The ratio between attenuation and far-end crosstalk, measured in decibels, at a given frequency; a requirement of ANSI/TIA-568-C.2 for twisted-pair cables; formerly referred to as equal-level far end crosstalk (ELFEXT) in ANSI/TIA/EIA-568-B.2.
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Attenuator
A passive device that intentionally reduces the strength of a signal by inducing loss.
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Audio
Used to describe the range of frequencies within range of human hearing; approximately 20 to 20,000Hz.
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AUI

Attachment Unit Interface

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Auxiliary AC power
A standard 110V AC power outlet found in an equipment area for operation of test equipment or computer equipment.
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Avalanche photodiode (APD)
With respect to optical fiber equipment, a specialized diode designed to use the avalanche multiplication of photocurrent. The photodiode multiplies the effect of the photons it absorbs, acting as an amplifier.
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Average picture level (APL)
A video parameter that indicates the average level of video signal, usually relative to blank and a white level.
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Average power
The energy per pulse, measured in joules, times the pulse repetition rate, measured in hertz (Hz). This product is expressed as watts.
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