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APPENDIX G. 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. 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 predefined 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, MNP, and V.42. ASCII American Standard Code for Information Interchange. A 7-bit binary code (0's, 1's) used to represent letters, numbers, and special characters such as $, !, and /. Supported by almost every computer and terminal manufacturer. Asymmetrical Modulation A duplex transmission technique which splits the communications channel into one high speed channel and one slower channel. During a call under asymmetrical modulation, the modem with the greatest amount of data to transmit is allocated the high speed channel. The modem with less data is allocated the slow, or back channel (450 bps). The modems dynamically reverse the channels during a call if the volume of data transfer changes. 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. Auto Answer A feature in modems enabling them to answer incoming calls over the phone lines without the use of a telephone receiver. Auto Dial A feature in modems enabling them to dial phone numbers over the phone system without the use of a telephone trans- mitter. Baud Rate The number of discrete signal events per second occurring on a communications channel. Although not technically accu- rate, baud rate is commonly used to mean bit rate. Bisync Binary Synchronous Control. An earlier protocol developed by IBM for software applications and communicating devices operating in synchronous environments. The protocol defines operations at the link level of communications, for example, the format of data frames exchanged between modems over a phone line. See Protocol, HDLC, SDLC. 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 14400. BPS The bits (binary digits) per second rate. Buffer A memory area used as temporary storage during input and output operations. An example is the modem's command buffer. Another is the Transmit Data flow control buffer used for flow control and to store 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), or have some other value meaningful to the computer. In user documentation, the term usually refers to 8-bit units or characters. 1 kilobyte (K) is equal to 1,024 bytes or characters; 64K indicates 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. 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.22 bis. 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, employ advanced techniques such as longer transmission frames and data compression to increase 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 USRobotics high speed modems, 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. 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 term applies to 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 tele- phone line. The communications channel may also be referred to as nonswitched because calls do not go through telephone company switching equipment. Default Any setting assumed, at startup or reset, by the computer's software and attached devices, and operational until changed by the user. Digital Loopback A test that checks the modem's RS-232 interface and the cable that connects the terminal or computer and the modem. The modem receives data (in the form of digital signals) from the computer or terminal, and immediately returns the data to the screen for verification. 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 that generates or is the final destination of data. In this manual, the term refers to your computer. Duplex Indicates a communications channel capable of carrying signals in both directions. See Half Duplex, Full Duplex. EIA Electronic Industries Association, which defines electronic standards in the U.S. 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. USRobotics high speed modems use adaptive equalization. Error Control Various techniques which check the reliability of characters (parity) or blocks of data. V.42, MNP and HST error control protocols use 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 indicators. Full Duplex Signal flow in both directions at the same time. In microcomputer communications, may refer to the suppression of the online Local 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. HDLC High Level Data Link Control. A standard protocol developed by the International Standards Organization for software applications and communicating devices operating in synchronous environments. The protocol defines operations at the link level of communications, for example, the format of data frames exchanged between modems over a phone line. See Bisync, Protocol, SDLC. HST High Speed Technology, USRobotics' proprietary signaling scheme, design and error control protocol for high-speed modems. HST incorporates trellis-coded modulation, for greater immunity from variable phone line conditions, and asymmetrical modulation for more efficient use of the phone channel at speeds of 4800 bps and above. HST also incorporates MNP-compatible error control procedures adapted to asymmetrical modulation. Hz Hertz, a frequency measurement unit used internationally to indicate one cycle per second. LAPM Link Access Procedure for Modems, an error control protocol incorporated in CCITT Recommendation V.42. Like the MNP and HST protocols, LAPM uses cyclic redundancy checking (CRC) and retrans- mission of corrupted data (ARQ) to ensure data reliability. 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. MI/MIC Mode Indicate/Mode Indicate Common, also called Forced or Manual Originate. Provided for installations where other equipment, rather than the modem, does the dialing. In such installations, the modem operates in Dumb mode (no Auto Dial capability), yet must go off hook in Originate mode to connect with answering modems. See MI/MIC Closure in Chapter 2. MNP Microcom Networking Protocol, an asynchronous error control protocol developed by Microcom, Inc. and now in the public domain. The protocol ensures error-free transmission through error detection (CRC) and retransmission of errored frames. USRobotics modems use MNP Levels 1+4 and Level 5 data com- pression. MNP Levels 1+4 have been incorporated into CCITT Recommendation V.42. Compare HST. 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 trans- forms, digital signals from a computer into the analog form that can be carried successfully on a phone line. It also demodulates signals received from the phone line back to digital signals before passing them to the receiving computer. Nonvolatile Memory (NVRAM) User-programmable random access memory whose data is retained when modem power is turned off. Used in Courier modems to store a user-defined default configu- ration 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). Online Fallback A feature that allows high speed error- control modems to monitor line quality and fall back to the next lower speed if line quality degrades. The modems fall forward as line quality improves. Originate Mode A state in which the modem transmits at the predefined 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-type protocols and the ARQ protocol implemented in Courier modems. 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 1's in the character even or odd, depending on which type of parity is used. 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 correction are some of the opera- tions that may be defined in protocols. RAM Random Access Memory. Memory that is available for use when the modem is turned on, but that clears of all infor- mation when the power is turned off. The modem's RAM holds the current operational settings, a flow control buffer, and a command buffer. Remote Digital Loopback A test that checks the phone link and a remote modem's transmitter and receiver. Data entered from the keyboard is trans- mitted from the initiating modem, received by the remote modem's receiver, looped through its transmitter, and returned to the local screen for verification. 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's factory settings are stored in ROM and can be read (loaded) into RAM as an operational configuration if DIP switch S10 is ON at power on. 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. SDLC Synchronous Data Link Control. A protocol developed by IBM for software applications and communicating devices operating in IBM's Systems Network Architecture (SNA). The protocol defines operations at the link level of communications, for example, the format of data frames exchanged between modems over a phone line. See Bisync, Protocol, HDLC. Synchronous Transmission A form of transmission in which blocks of data are sent at strictly timed intervals. Because the timing is uniform, no Start or Stop bits are required. Compare Asynchronous Transmission. Some mainframes only support synchronous communications unless their owners have installed a synchronous adapter and appropriate software. 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. 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.21 A CCITT standard for modem communications at 300 bps. Modems made in the U.S. or Canada follow the Bell 103 standard. However, the modem can be set to answer V.21 calls from overseas. See Register S27 in Chapter 6. V.22 A CCITT standard for modem communications at 1200 bps, compatible with the Bell 212A standard observed in the U.S. and Canada. V.22 bis A CCITT standard for modem communications at 2400 bps. The standard includes an automatic link negotiation fallback to 1200 bps and compatibility with Bell 212A/V.22 modems. V.23 A CCITT standard for modem communications at 1200 bps with a 75 bps back channel. Used in the U.K. V.25 A CCITT standard for modem communications that specifies an answer tone different from the Bell answer tone used in the U.S. and Canada. All USRobotics modems can be set with the B0 command so that they use the V.25 2100 Hz tone when answering overseas calls. V.32 A CCITT standard for modem communications at 9600 bps and 4800 bps. V.32 modems fall back to 4800 bps when line quality is impaired, and fall forward again to 9600 bps when line quality improves. V.32 bis A CCITT standard that extends the V.32 connection range: 4800, 7200, 9600, 12K and 14.4K bps. V.32 bis modems fall back to the next lower speed when line quality is impaired, and fall back further as necessary. They fall forward to the next higher speed when line quality improves. V.42 A CCITT standard for modem communications that defines a two-stage process of detection and negotiation for LAPM error control. V.42 also supports the MNP error control protocol, levels 1+4. V.42 bis An extension of CCITT V.42 that defines a specific data compression scheme for use with V.42 and MNP error control. Word Length The number of bits in a data character without parity, start or stop bits. Xmodem The first of a family of error control software protocols used to transfer files between modems. These protocols are in the public domain and are available from many bulletin board services. XON/XOFF Standard ASCII control characters used to tell an intelligent device to stop/resume transmitting data. In most systems typing <Ctrl>-S sends the XOFF character. Some devices, including the Courier, understand <Ctrl>-Q as XON; others interpret the pressing of any key after <Ctrl>-S as XON.