Glossary of Terms

AC (Alternating Current): A power source whose signal crosses a reference voltage (usually called ground or zero). Alternating between a maximum and minimum voltage, AC may also be referred to as a bipolar signal. Contrast with DC.

ACK (ACKnowledgement code) (pronounced "ack"): A communications code sent from a receiving modem to a transmitting modem to indicate that it is ready to accept data. It is also used to acknowledge the error-free receipt of transmitted data. Contrast with NAK.

Address: A numbered location inside a computer. It's how the computer accesses its resources, like a video card, serial ports, memory, etc.

Alphanumeric: The basic character set which includes the letters A to Z (and a to z) and the digits 0 to 9.

Amplitude: The difference between the maximum and minimum voltages of a waveform expressed as a "peak-to-peak" voltage.

Amplifier: An active device within a circuit which increases the voltage level of all signals (desirable and undesirable).

Analog loopback: A modem diagnostic used to test either the local analog loop (the modem's internal circuitry) or the remote analog loop (the telephone line). The local analog loop test is accomplished by activating the self-test mode and tying the modem's modulator to its demodulator and examining the return stream of data at the PC or terminal it services. The remote analog loopback can only be activated on four-wire leased line connections with a remote modem capable of performing the same test.

Analog signal: A waveform which has amplitude, frequency and phase, and which takes on a range of values between its maximum and minimum points. Analog implies continuous movement from point A to point B, as opposed to discrete jumps. For example, sound is continuously varying air vibrations and is converted into analogous electrical signals to be carried on a telephone line.

Analog Transmission: One of two types of telecommunications which uses an analog signal as a carrier of voice, data, video, etc. An analog signal becomes a carrier when it is modulated by altering its phase, amplitude and frequency to correspond with the source signal. Compare with digital transmission.

ANSI (American National Standards Institute) (pronounced "ansy"): A U.S. standards organization supported by over 1000 companies and trade organizations. It is a non-profit,non-government group that is the U.S. member of the ISO (International Stan- dards Organization).

ANSI character set: An 8-bit character set that contains 256 characters. The first 128 characters are alphanumeric punctuation and the second 128 contain math and foreign language symbols.

ASCII (American Standard Code for Information Interchange) (pronounced "askey"): A binary code for data that is used in communications and in many computers and terminals. The code is used to represent numbers, letters, punctuation and control characters. The basic ASCII code is a 7-bit character set which defines 128 possible characters. The extended ASCII file provides 255 characters.

Asynchronous Transmission: The transmission of data in which each character is a self-contained unit with its own start and stop bits. This is a common method of transmission between a computer and a modem. One character at a time, encoded into a series of electrical pulses, is transmitted or received. This is the oldest method of data transfer. When it is used with error correcting software and data compression algorithms, along with the increase in maximum attainable speeds, it continues to be a viable alternative to synchronous transmission..

Auto Answer: A modem feature which enables the modem to go "off-hook" when it detects an incoming call.

Auto Dial: Some modems provide this feature for asynchronous dialing. This feature is a predefined macro that allows the user to enter the location of a phone number (i.e., N0 through N9) and have the modem go off hook, dial and establish the connection. With the auto dial feature, an asynchronous terminal can establish a dialing directory without running a communication software package.

Baud: Baud is rate, the signalling rate of a line, the switching speed, or the number of transitions (voltage or frequency changes) that are made per second. Transmission speeds are often expressed in baud, though bits per second is more accurate. The speed at which your computer talks to your modem.

BCC (Block Check Character): An error control method used in character-oriented or byte-synchronous protocols. Two 8-bit BCC's are used to create the CRC (Cyclic Redundancy Check) field of a synchronous data packet.

Bell 103: The U.S. modulation standard for 300 bps full-duplex transmission over dial-up lines.

Bell 212A: The U.S. modulation standard for 1200 bps full-duplex transmission over dial-up lines.

Binary: A numbering system based on two digits, 1 and 0 which is conducive to the two-state digital electronics used within computers. All input to a computer is encoded as a binary value. Binary also refers to a file format that uses 8-bit characters, to allow for control characters (i.e., all non-ASCII files).

BSC (Binary Synchronous Communications): Also called "bisync", this communications protocol was the first synchronous data format used by IBM. It is still in use, but is rapidly being replaced by IBM's newer Synchronous Data Link Control (SDLC). Bisync is a byte-synchronous protocol that has longer delays and more overhead than the bit-synchronous SDLC. It uses two synchronization characters to head every packet.

Bit (Binary digIT): A bit is the basis of the binary number system. It can take the value of 1 or 0. Bits are generally recognized as the electrical charge generated or stored by a computer that represent some portion of usable information.