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Access control
Protection against unauthorized access to a computer network. A less restrictive tool than a firewall. Using access controls, a HIN will check its access control list to see if a computer requesting service from the outside world is permitted to use that service. See also encryption.

Acoustic Echo Canceller (AEC)
Full-duplex audio technology; used for the elimination of acoustically-coupled return echoes within a teleconference room. Note that all microphones connected to an AEC are active at all times. Consequently, as more microphones are added, the total transmitted noise level (caused by picking up room ambient noise) increases.

Alias, Aliasing
A false signal produced by the analog-to-digital sampling process. Often caused by failure to observe the Nyquist criterion and can create artifacts which are subjectively. A common form of aliasing is a stair-stepped appearance along diagonal and curved lines.

American College of Radiology and the National Equipment Manufacturers Association (ACR-NEMA).
They have jointly developed standards for teleradiology practice. For CT, MRI, ultrasound, nuclear medicine, digital fluoroscopy: images must be scanned at 500 pixel x 500 line resolution by 8 bit depth (256 gray scale) or better; for diagnostic X-rays: 2,000 x 2,000 ("2K by 2K") by 12 bit depth (4,096 gray scale).

American Medical Informatics Association (AMIA)

American Standard Code for Information Interchange (ASCII)
A character code for representing data.

Analog Information
that is created and transmitted as a continuous stream. Wave forms (e.g., on oscilloscopes) are analog. Compare this to digital information generated by computers. Modems are used to convert digital computer data to analog form for sending over standard POTS lines

Analog Signal
Continuous, wavelike transmission. Information is transmitted by varying the number of waves per second (frequency), the height of the wave (amplitude) or the flow of the wave (phase).

Simultaneous shared annotation of captured (or, less commonly, live video) images allows conference participants to clearly point out the areas in question on an image, and may provide significant instructional value.

Spurious effects or imperfections introduced into a signal as a result of digital signal processing. An unintended, unwanted visual aberration in a video image.

Artificial Intelligence (AI )
The concept that computers can be programmed to assume some capabilities normally thought to be like human intelligence, such as learning, adaptation, and self correction.

Asynchronous Transfer Mode (ATM)
A start/stop transmission in which each character is preceded by a start signal and followed by one or more stop signals. A variable time interval can exist between characters. ATM is becoming a preferred technology for the transfer of images.

"AT" Instruction Set
ATtention: A standard modem command-set protocol developed by modem-manufacturer Hayes and is used by most modem manufacturers for modem control. Remote control or remote diagnostic ports on video codecs can generally be connected to modems using this command set.

AudioGraphics Terminal (AGT)

Audiographic Teleconferencing
Refers to the transmission of graphic and text information over a narrowband telecommunications channel such as a telephone line or a subcarrier.

Audio only conference add-ins
The ability to add another site into a video conference via an audio connection only. This feature uses a regular phone line connected to the codec to conference in someone who is not near a video site but needs to be part of the conversation.

Audio Teleconferencing
Two-way electronic voice communication between two or more people at two or more locations.

Automatic Alternative Routing (AAR)
A system for providing continued telecommunications service in the event that a primary transmission route fails catastrophically (e.g., when a FOTS is cut).

Automatic Route Selection (ARS)
The capability of a switch, typically a private branch exchange (PBX), to automatically determine an optimal route for establishing a circuit.

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