A

Address
A label associated with a quantity of data that indicates to where it should be sent.

ADPCM
Adaptive Differential Pulse Code Modulation. A method for compressing and encoding speech for transmission over digital networks.

Alphanumeric character.
A letter or number

Aloha
The basis of the access technique used for TDMA systems.

AM
Amplitude Modulation. A system for modifying the magnitude of a carrier wave so that it will convey information.

Amplifier
A device that increased the magnitude, or amplitude, of an electronic signal.

Analog
A method of representing real world values with an electrical signal that mimics the value as it smoothly changes.

AOR
Atlantic Ocean Region. A group of orbital positions for satellite serving Europe, Africa and the Americas.

Aperture
The surface area of an antenna. cf. effective aperture, the surface area of an antenna multiplied by the antenna's efficiency.

Apogee
The point in a satellite's orbit at which it is furthest from the Earth.

Application Software, or a combination of software and hardware that performs some function in an organisation.

ARPA
US Department of defense Advanced Research Projects Agency, responsible for initiating the Internet.

ARP
Address Resolution Protocol. A method by which an application can discover the network address of the application with which it needs to communicate. cf. RARP a reverse ARP whereby an application discovers its own address.

ASK
Amplitude Shift Keying. A form of Amplitude Modulation whereby digital data causes a carrier magnitude to be switched from one value to another.

Asymetric
Satellite systems can easily support asymetric connections, these are services where the data speeds are different in the different directions. This is typically used for Internet access where the ‘request’ traffic is less than the information data return traffic.

ATM
Asynchronous Transfer Mode. A data communications protocol that uses fixed sized "cells" of information in a fast packet switched system.

Asynchronous
Any system where timing for the transmitter and receiver is derived from more than one source rather than a single reference clock.

Attenuation
The reduction in magnitude of an electronic signal as it passes through a device or through space.

AUI
Autonomous Unit Interface or Attachment Unit Interface. A 15 pin connector for Ethernet.

Authentication
A system requiring legitimate users to identify themselves before using a system, often by way of a login and password.

AFC
Automatic Frequency Control, the control of a system's frequency using some form of feedback to ensure it remains within limits. This process helps to stabilise the output from low cost components.

AGC
Automatic Gain Control, the control of a signal's amplitude by way of a variable gain amplifier responding to slow changes in the signal's amplitude. The purpose of this system is to ensure that the signal level remains stable where it is critical to do so.

Availability
The percentage of time for which a device or system is considered to be functioning normally.

AZ/EL Mount
The portion of an antenna's mount that allows the antenna to be rotated vertically and horizontally to enable it to be pointed at the satellite.

Azimuth
The horizontal angle between the direction the antenna points and true North.