oding method used with Dataphone Digital Ser- vice, the oldest data service still in use, which uses 64Kbps channels. AMI requires the use of 8Kbps of the 64Kbps of each channel to maintain synchronization, which means that in reality only 56Kbps is available for data transmission. The signal carrying the binary value alternates between positive and negative polarities; 0 and 1 values are represented by the signal amplitude at either polarity; no-value "spaces" are at 0 amplitude. Also called bipolar. amplifier A device that boosts an attenuated signal back up to its original power level so that it can continue to make its way across the network. amplitude A measure of the height of a wave, which indicates the strength of the signal. AMPS (Advanced Mobile Phone System) A standard for analog telephony that is deployed widely in the United States. analog A signal that varies continuously (e.g., sound waves), along two parameters: ampli- tude (strength) and frequency (tone). The unit of measurement is the Hertz (Hz), or cycle per second. analog loopback A technique for testing transmission equipment and devices that isolates faults to the analog signal receiving or transmitting circuitry, and a device, such as a modem, echoes back a received (test) signal that is then compared with the original signal. ANI (automatic number identification) A feature, often associated with SS7, that passes a caller's telephone number over the network to the receiver so that the caller can be identified. Also referred to as caller ID. Glossary ANSI (American National Standards Institute) ANSI (American National Standards Institute) A standards-forming body affiliated with the ISO that develops U.S. standards for transmission codes, protocols, media, and high- level languages, among other things. ANSI X12 A U.S. standard for electronic data interchange. answer signal A supervisory signal (usually in the form of a closed loop) from the called telephone to the exchange and back to the calling telephone (usually in the form of a reverse b