Communications Systems Glossary

Access Technique.  The access technique is the method by which a number of terminals makes use of the shared bandwidth available in a network, or a radio channel.

Acknowledgement (Ack).  An acknowledgement (ack) is a message sent by a receiver to positively acknowledge the receipt of an agreed amount of data.

Acoustic coupler.  An acoustic coupler is a form of modem that does not connect directly to the phone line, but generates audible tones that are then transmitted through the telephone handset.

Adaptive delta modulation (ADM).  Adaptive delta modulation (ADM) is a form of delta modulation in which the step size is varied to improve tracking accuracy.

Adaptive differential pulse code modulation (ADPCM).  Adaptive differential pulse code modulation (ADPCM) is a more efficient form of pulse code modulation.

Adaptive routing.  Adaptive routing is a routing algorithm that evaluates the available paths across a network and chooses the one that will provide the best path for a message.

Address resolution.  Techniques by which the addressing schemes of network-layer (Layer 3) addresses are mapped onto data-link layer (Layer 2) addresses.

ADSL (Asymmetric digital subscriber line).  ADSL (Asymmetric digital subscriber line) allows broadband data services to be carried on conventional copper pair telephone cables.

Advanced Mobile Phone System (AMPS).  The Advanced Mobile Phone System (AMPS) is a US standard for first-generation analogue mobile telephones.

Aerial.  An aerial (or antenna) is a device for radiating and receiving electromagnetic waves in the radio frequency (RF) range.

Aloha.  Aloha is a multiple-access technique by which a number of terminals can access a shared transmission media.

Alternate mark inversion (AMI).  Alternate mark inversion (AMI) is a digital transmission technique in which binary values are sent by three voltage states.

Alternating current (AC).  When the voltage source varies in value with time (normally reversing its polarity sinusoidally), the current is alternating current (AC), which varies also varies sinusoidally with time

Ampere (Amp).  The ampere (A), or amp, is the measure of electrical current.

Amplification.  A mechanism by which the amplitude of a signal is increased.

Amplitude.  The amplitude is the size or magnitude of a signal, which is normally some form of voltage or current waveform.

Amplitude modulation (AM).  Amplitude modulation (AM) is the form of modulation in which information is impressed onto a carrier wave whose amplitude is caused to vary in accordance with the modulating signal.

Amplitude-shift keying (ASK).  Amplitude-shift keying (ASK) is a simple version of amplitude modulation used for digital modulation in which the two binary values (one and zero) are represented by two different amplitudes of the carrier frequency (normally, on and off).

Analogue signal.  An analogue signal is a continuously varying waveform.

Analogue-to-digital conversion (ADC).  Analogue-to-digital conversion (ADC) involves the conversion of the analogue voltage levels of an analogue signal into ones and zeros of a digital word.

Angle modulation.  Angle modulation is modulation in which the angle of a sine wave carrier is varied in accordance with the information contained in the modulating waveform.

Antenna.  An antenna (or aerial) is a device for radiating and receiving electromagnetic waves in the radio frequency (RF) range.

Application layer (Layer 7).  The application layer is layer 7 of the ISO OSI 7-layer model. The application layer determines the interface of the system with the user.

ASCII (American Standard Code for Information Interchange).  ASCII (American Standard Code for Information Interchange) is a code that uses seven bits to represent standard text characters as well as a number of terminal control characters.

Asynchronous.  Asynchronous transmission occurs without significant prior coordination between the source and the sink. Each block of data is sent by the source, which receives warning of each block by a start bit and notification of the end of the block with a stop bit (also known therefore as start-stop protocol).

Asynchronous Transfer Mode (ATM).  Asynchronous Transfer Mode (ATM) is a cell-switching and multiplexing technology designed to combine the benefits of circuit switching (constant transmission delay and guaranteed capacity) with those of packet switching (flexibility and efficiency for intermittent traffic) through the use of small, fixed-length cells (53 bytes).

ATM adaptation layer (AAL).  The ATM adaptation layer (AAL) of the ATM protocol that adapts different telecommunications service requirements to the ATM network.

Attenuation.  Attenuation is a decrease in magnitude (a weakening) of current, voltage, or power of a signal.

Audio frequency (AF).  Audio frequency describes the range of frequencies that can be detected (heard) by the human ear. The audio frequencies are normally considered to lie within the range of 15 Hz to 15,000 Hz.

Automatic gain control (AGC).  Automatic gain control (AGC) is the mechanism inside a radio receiver that automatically increases or decreases the gain of the various stages of a receiver so that, no matter how the received signal fluctuates, the operator hears a constant output.

Automatic link establishment (ALE).  Automatic link establishment (ALE) is a technique used in high frequency (HF) radio communications to choose the best available channel when a connection is established.

Automatic repeat request (ARQ).  Automatic repeat request (ARQ) is an error-control system in which a request for re-transmission is generated by the receiver when an error in transmission is detected.

Available Bit Rate (ABR).  Available bitrate is a service class for ATM in which the peak cell rate is dictated by the network.

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