COURIER 1200e GLOSSARY Cross-references in the following definitions are printed in boldface. Analog Loopback A modem self-test in which data from the keyboard is sent to the modem's transmitter, modulated into analog form, looped back to the receiver, demodulated into digital form, and returned to the screen for verification. Tests either the modem's originate or answer frequency. Analog signals Continuous, varying waveforms such as the voice tones carried over phone lines. Contrast with digital signals. Answer Mode A state in which the modem transmits at the pre-defined high frequency of the communications channel and receives at the low frequency. The transmit/receive frequencies are the reverse of the calling modem which is in Originate Mode. ARQ Automatic Repeat Request. A general term for error control protocols which feature error-detection and automatic retransmission of defective blocks of data. See HST and MNP. ASCII American Standard Code for Information Interchange. A 7-bit binary code (0s, 1s) used to represent letters, numbers, and special characters such as $, !, and /. Supported by almost every computer and terminal manufacturer. See Appendix B-6 for decimal and hexadecimal code equivalents. Asynchronous Transmission Data transmission in which the length of time between transmitted characters may vary. Because the time lapses between transmitted characters are not uniform, the receiving modem must be signaled as to when the data bits of a character begin and when they end. The addition of Start and Stop bits to each character serves this purpose. Compare Synchronous Transmission. Auto Answer A feature in modems enabling them to answer incoming calls over the phone lines without the use of a telephone receiver. G-1 COURIER 1200e Auto Dial A feature in modems enabling them to dial phone numbers over the phone system without the use of a telephone transmitter. Baud Rate The number of discrete signal events per second occurring on a communications channel. Although not technically accurate, baud rate is commonly used to mean bit rate. Binary Digit A 0 or 1, reflecting the use of a binary numbering system (only two digits). Used because the computer recognizes either of two states, OFF or ON. Shortened form of binary digit is "bit." Bit Rate The number of binary digits or bits transmitted per second (bps). Communications channels using telephone channel modems are established at set bit rates, commonly 110, 300, 1200, 2400, 4800, 9600, and 144000. BPS The bits (binary digits) per second rate. Buffer A memory area used as temporary storage during input/output operations. Two examples in the Courier HST are the command buffer (last issued command) and the flow control buffer (for data flow control and storing copies of transmitted frames until they are positively acknowledged by the receiving modem). Byte A group of binary digits stored and operated upon as a unit. A byte may have a coded value equal to a character in the ASCII code (letters, numbers), for example, or have some other value meaningful to the computer. In user documentation, the term usually refers to 8-bit units or characters. 1 kilobyte = 1,024 bytes or characters; 64k = 65,536 bytes or characters. Carrier A continuous frequency capable of being either modulated or impressed with another information-carrying signal. Carriers are generated and maintained by modems via the transmission lines of the telephone companies. G-2 COURIER 1200e CCITT An international organization that defines standards for telegraphic and telephone equipment. For example, the Bell 212A standard for 1200 bps communication in North America is observed internationally as CCITT V.22. For 2400 bps communication, most U.S. manufacturers observe V.22bis. The initials CCITT represent the French name; in English it's known as the International Telegraph and Telephone Consultative Committee. Character A representation, coded in binary digits, of a letter, number, or other symbol. Characters Per Second A data transfer rate generally estimated from the bit rate and the character length. For example, at 2400 bps, 8-bit characters with Start and Stop bits (for a total of ten bits per character) will be transmitted at a rate of approximately 240 characters per second (cps). Some protocols, such as USR-HST and MNP-Service Class 3, employ advanced techniques to increase cps. When the Courier HST is set for maximum throughput, as described in Chapter 3, 140 characters in addition to the standard 960 can be transmitted for a total of 1100 cps. Cyclic Redundancy Checking (CRC) An error-detection technique consisting of a cyclic algorithm performed on each block or frame of data by both sending and receiving modems. The sending modem inserts the results of its computation in each data block in the form of a CRC code. The receiving modem compares its results with the received CRC code and responds with either a positive or negative acknowledgment. In the ARQ protocol implemented in the Courier HST, the receiving modem accepts no more data until a defective block is received correctly. Data Communications A type of communications in which computers and terminals are able to exchange data over an electronic medium. Data Compression Table A table of values assigned for each character during a call under data compression. Default values in the table are continually altered and built during each call: the longer the table, the more efficient throughput gained. G-3 COURIER 1200e If a destructive Break is sent during a call (see the &Y command), causing the modems to reset the compression tables, you can expect diminished throughput. DCE Data Communication (or Circuit-Terminating) Equipment. In this manual, the dial-up modems that establish and control the data link via the telephone network. Dedicated Line A user-installed telephone line used to connect a specified number of computers or terminals within a limited area, for example, one building. The line is a cable rather than a public- access telephone line. G-4 COURIER 1200e Default Any setting assumed, at startup or reset, by the computer's software and attached devices, and operational until changed by the user. Digital Signals Discrete, uniform signals. In this manual, the term refers to the binary digits 0 and 1. DTE Data Terminal (or Terminating) Equipment. The device, such as a personal computer, terminal, or mainframe, that generates or is the final destination of data. Duplex Indicates a communications channel capable of carrying signals in both directions. See Half Duplex, Full Duplex. Equalization A compensation circuit designed into modems to counteract certain distortions introduced by the telephone channel. Two types are used: fixed (compromise) equalizers and those that adapt to channel conditions. Error Control Various techniques which check the reliability of characters (Parity) or blocks of data. The Courier's error control protocol provides error control through error detection (CRC) and retransmission of errored frames (ARQ). Flow Control A mechanism that compensates for differences in the flow of data input to and output from a modem or other device. Frame A data communications term for a block of data with header and trailer information attached. The added information usually includes a frame number, block size data, error-check codes, and Start/End data. G-5 COURIER 1200e Full Duplex Signal flow in both directions at the same time. In microcomputer communications, may refer to the suppression of the online Local Echo. The receiving computer may provide a Remote Echo. Half Duplex Signal flow in both directions, but only one way at a time. In microcomputer communications, may refer to activation of the online Local Echo, which causes the modem to send a copy of the transmitted data to the screen of the sending computer. HST High Speed Technology, part of the trademark for USRobotics' USR- HST, a proprietary signaling scheme, design and protocol for 9600 bps modems. USR-HST incorporates trellis-coded modulation, for greater immunity from variable phone line conditions, and asym- metrical modulation for more efficient use of the phone channel. HST also represents the enhanced error control protocol at 9600 bps that is similar to and compatible with MNP error control at 2400/1200 bps. Local Echo A modem feature that enables the modem to send copies of keyboard commands and transmitted data to the screen. When the modem is in Command Mode (not online to another system) the local echo is invoked through the ATE1 command. The command causes the modem to display your typed commands. When the modem is online to another system, the local echo is invoked through the ATF0 command. This command causes the modem to display the data it transmits to the remote system. MNP Microcom Networking Protocol. An error control protocol developed by Microcom, Inc. and now in the public domain. The protocol ensures error-free transmission at 1200 and 2400 bps through error detection (CRC) and retransmission of errored frames. Modem A device that transmits/receives computer data through a communications channel such as radio or telephone lines. The Courier is a telephone channel modem that modulates, or transforms, digital signals from a computer into a form that can be carried successfully on a phone line. It also demodulates G-6 COURIER 1200e signals received from the phone line back to digital signals before passing them to the receiving computer. Intelligent modems with built-in software, such as the USRobotics modem described in this manual, simplify using phone networks for data communications. Nonvolatile Memory (NRAM) User-programmable random access memory whose data is retained when modem power is turned off. Used in the Courier HST to store a user-defined default configuration loaded into random access memory (RAM) at power on. OFF/ON Hook Modem operations which are the equivalent of manually lifting a phone receiver (taking it off hook) and replacing it (going on hook). Originate Mode A state in which the modem transmits at the pre-defined low frequency of the communications channel and receives at the high frequency. The transmit/receive frequencies are the reverse of the called modem which is in Answer Mode. Parallel Transmission The transfer of data characters using parallel electrical paths for each bit of the character, for example, 8 paths for 8-bit characters. Data is stored in computers in parallel form, but may be converted to serial form for certain operations. See Serial Transmission. Parity An error-detection method that checks the validity of a transmitted character. Character checking has been surpassed by more reliable and efficient forms of block-checking, including XMODEM, a public domain protocol used in some telecommunications software, and the ARQ protocol implemented in the Courier HST. The same type of parity must be used by two communicating computers, or both may omit parity. When parity is used, a parity bit is added to each transmitted character. The bit's value is 0 or 1, to make the total number of 1s in the character even or odd, depending on which type of parity is used. G-7 COURIER 1200e Protocol A system of rules and procedures governing communications between two or more devices. Protocols vary, but communicating devices must follow the same protocol in order to exchange data. The format of the data, readiness to receive or send, error detection, and error handling are only a few of the operations that may be defined in a protocol. RAM Random Access Memory. Memory that is available for use when the modem is turned on and clears of all information when the power is turned off. The Courier HST's RAM holds the current operational settings, a flow control buffer, and a command buffer. Remote Echo A copy of the data received by the remote system, returned to the sending system and displayed on the screen. Remote echoing is a function of the remote system. ROM Read Only Memory. Permanent memory, not user-programmable. The Courier HST's factory settings are stored in ROM and can be read (loaded) into RAM as an operational configuration. Serial Transmission The transfer of data characters one bit at a time, sequentially, using a single electrical path. See Parallel Transmission. Start/Stop Bits The signaling bits attached to a character before the character is transmitted during Asynchronous Transmission. Synchronous Transmission A form of transmission in which groups of data bits are sent at regular intervals. Because the timing is uniform, no Start or Stop bits are required. See Asynchronous Transmission. Terminal A device whose keyboard and display are used for sending and receiving data over a communications link. Differs from a microcomputer in that it has no internal processing capabilities. G-8 COURIER 1200e Used to enter data into or retrieve processed data from a system or network. Terminal Mode An operational mode required for microcomputers to transmit data. In Terminal Mode the computer acts as if it were a standard terminal such as a teletypewriter, rather than a data processor. Keyboard entries go directly to the modem, whether the entry is a modem command or data to be transmitted over the phone lines. Received data is output directly to the screen. The more popular communications software products control Terminal Mode as well as enable more complex operations, including file transmission and saving received files. Throughput The amount of actual user data transmitted per second without the overhead of protocol information such as Start and Stop bits or frame headers and trailers. Compare characters per second. Transmission Rate Same as Bit Rate. V.22bis The international CCITT standard for modem communications at 2400 bps. The standard includes an automatic data rate fallback to 1200 bps and compatibility with Bell 212A/V.22 modems. Word Length The number of bits in a character without parity, start or stop bits. XON/XOFF Standard ASCII control characters used to tell an intelligent device to stop/resume transmitting data. In most systems typing -S sends the XOFF character. Some devices, including the Courier, understand -Q as XON; others interpret the pressing of any key after -S as XON. G-9