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Here is version 1.4 of the lexicon. Comp.dcom.modems lexicon, by Eamonn McManus <em@dce.ie>. Contributions by: Charles Bryant <ch@dce.ie> Ronald S H Khoo <ronald@robobar.co.uk> David Lesher <wb8foz@mthvax.cs.miami.edu> Chip Rosenthal <chip@chinacat.unicom.com> Colin Plumb <ccplumb@rose.uwaterloo.ca> Toby Nixon <tnixon@hayes.uucp> Eric Gundrum <gundrum@sv.portal.com> Evan Gamblin <0001847804@mcimail.com> George Horosh <goshah@glas.apc.org> Maria Iakimets <masha@glas.apc.org> This lexicon is intended to provide a basic introduction to terms used in modem technology. In the interests of brevity, some technical details and qualifications are omitted. Corrections, additions, and suggestions are welcome; send them to <em@dce.ie>. This document IS NOT copyrighted and may be used freely. Words in angle brackets, like <this>, denote cross-references. The following terms are defined or discussed here: ACK, Acoustic Coupler, ASCII, Asymmetric, Asynchronous transmission, Attenuation, AT commands, Back channel, Bandwidth, Baud, BCC, Bell standards, BERT, bit, Bps, Byte, Carrier, CCITT, Command File, Common Carrier, Conditioning, Compression, CRC, CTS, C-weighting, Dataphone, dB, dBm, DCD, DCE, Delay Distortion, Dibit, DPSK, DSR, DTE, DTMF, DTR, EBCDIC, Echo, EIA232, Emulation, Error Correction, Equalazer, External modem, Fall back, FAX, Flow Control, Four wire, Frame, FSK, Full duplex, Half duplex, Handshake, Hayes, HDLC, Hertz, Hybrid Transformer, Interface, Internal modem, IPARS, ISO, Kermit, Latency, Leased line, LED, Limited distance modem, Link, Loadig coil, Local loop, LRC, Mark, MIL-STD-188-C, MNP, Modem eliminator, Modulation, Multiplexer, NAK, Network, Node, Null modem, Octet, OOK, OSI, Packet, Packet switching, Parallel, Parity bit, PBX, PEP, Phase modulation, Post processing, Protocol, Protocol converter, PTT, Public Telephone Network, QAM, Quadbit, RAM, ROM, RS-Standards, RS-232, RTS, Script file, SDLC, Serial, Space, Speed buffering, Spoofing, Start/Stop bits, Statistical multiplexer, Store and Forward, Synchronous, Synchronous transmission, Tralblazer, Turnaround time, V.standards, V.series, Window, XMODEM, X.standards, XON/XOFF, ZMODEM. ------------------------------------------------------------ ACK,Acknowledgment - A communication control character transmitted by a receiver indicating that the massage was reseived correctly. Acoustic Coupler - A device converting electrical to audio signals and vice versa, allowing the connection of a modem to the <Public Telephone Network> by means of any telephone headset. ASCII - The American Standart Code for Information Interchange is an 8-<bit> code accepted as a data communication standart in North America to achieve compatibility between data services. ASCII uses 7 <bits> for information and the 8-th <bit> for parity. Asymmetric - Faster in one direction than the other. The faster direction is called the main channel and the slower is the back channel. See <V.23> and <PEP> for examples. Both of these allow the directions of the channels to be exchanged; see <turnaround time>. Asynchronous Transmission - Transmission in which the time intervals between teansmitted charecters may be of unequal length. Transmission is controlled by the start and stop <bits> at the beginning and end of each character. Also referred to as start-stop transmission. Used of <serial> connection. See <synchronous>. AT commands - See <Hayes>. (Unrelated to the PC/AT.) Attenuation - Decrease in the signal amplitude. If the decrease is a function of frequency than the <attenuation> relative to <attenuation> at 1000 <Hz> is called <attenuation> distortion. Back channel - See <asymmetric>. Bandwidth - Range of frequencies which are subject to <attenuation> of typically less than 3 <db>. Baud - A unit of <modulation> rate. Equal to transmission speed divided by the number of <bits> per signal element.Frequently confused with <bits> per second<bps>. BCC - Block Check Character, is used for error checking and is the result of a transmission verification algorithm accumulated over a transmission block during <synchronous transmission> of data. Bell standards - <Modulation> techniques used in North America. Bell 103 is a 300<bps> standard; Bell 22 is 1200<bps>. Not allowed in some European countries. See <V series>. BERT -<Bit> error rate test set. An instrument used for checking quality of data transmission. Bit - A binary digit, can be 0 or 1, the smallest unit of information. Bps, Bits per second - Unit of transmission speed is equal to quantity of phisicaly transmited <bits> per second. See <baud>. Byte - A sequence of 8 <bits>, usially corresponding to a character. Carrier - Roughly speaking, the tones a modem sends when it is not sending data. Data are then variations in these tones. When the remote modem stops emitting <carrier>, the local modem can assume it has hung up, unless the local modem is now the sending party in a <half duplex> connection. CCITT - A French acronym for the International Telegraph and Telephone Consultative Committee, which sets standards for telephone communication among other things. Their standards (or `recommendations' as they have it) are published every so often in a set of `fascicles', whose colour varies with the year. The current set is the `blue books' published in 1988. The standards are copyrighted, so they are not available electronically. You should be able to find them at any good engineering library. See <V series>. Command File -(See <Script file>) User-written program requied by the communications software to make a data phone call. Contains parameters such as the phone number, transmission speed, number of data/stop <bits>, and parity. Common Carrier - A public owner of transmission facilities which has the obligation to provide such facilities to the public on demand. Conditioning - Special teatment of transmission facilities to reduce their <attenuation> and <delay distortions>. Compression - Reduction of the size of data by exploiting redundancy. Many modern modems incorporate use <MNP5> or <V.42 bis> to compress data before they are sent over the phone line. For this to be effective, the modem must be fed data at a higher speed than the phone line speed. <compression> is most useful for interactive sessions in the modem. If you are sending files, compressing them on the computer before sending is usually more effective. In this case, make sure that the modem is not also trying to compress, because already-compressed data will become bigger if re-compressed. For <compression> to work, it is essential that the data are sent over an error-free <link>. Otherwise the modems could get out of sync and hopelessly garble the data. Since common <error correction> protocols are <synchronous>, there is usually a throughput gain there as well. Manufacturer claims that <MNP5> provides a 2:1 reduction in size, or that <V.42 bis> provides 4:1, should be taken with a grain of salt. They are only true for suitable data. See <MNP5> and <V.42 bis>. CRC - Cyclic Redundancy Check, a checking method used to detect errors, in which the numeric binary value of a block of data is divided by a constant divisor. The quotient is discarded and the remainder serves as check sequence. CTS - Clear to send. A signal from <DCE> to <DTE>. Typically used for <flow control>. C-weighting - Frequency <attenuation> shaping of certain test instruments to simulate a frequency response of human ear to various components of the audio spectrum. Dataphone - A trademark of AT&T applied to various modems. dB - Decibel - logarithmic ratio. Generally applied to power, voltages or curents. dBm - Decibel above 1 mW - unit of electrical power. DCD - Data <carrier> detect. A signal from <DCE> to <DTE>, indicating that a <carrier> tone is being heard from the remote modem. See <DSR>. DCE - Data circuit-terminating equipment. Whatever is connected to a phone line. Effectively, a modem. Delay Distortion - Characteristic of the transmission medium, affects the transmission speed of frequency components of a data signal. Results in signal distortion. Dibit - A sequence of two <bits> assosiated with one signal element. Differential Phase Shift Keying (DPSK) - <Modulation> method used by the Bell 212A modem based on detection of successive relative phase changes of the data signal. DSR - Data set ready. A signal from <DCE> to <DTE>, indicating that a connection is in progress. For <half duplex> connections,<DSR> will be on during the entire session, while <DCD> will be on only when the modem is receiving. DTE - Data terminal equipment. The computer or terminal that talks to a modem locally. DTMF - Dual tone multiple frequency. The standard method for tone dialling. DTR - Data terminal ready. A signal from <DTE> to <DCE>. Sometimes used for <flow control>, though <RTS> is more usual nowadays. Typically, dropping <DTR> will cause the modem to hang up. EBCDIC - Extended Binary Coded Decimal Interchange Code, an 8-<bit> code used mostly by IBM. Enables representation of graphics and control code characters. Echo - Data or voice reflected back to the transmitter by impedance mismatches along the transmission path. EIA232 - Electronic Industries Assosiation - a group of US manufacturers recodnized as the standards writing agency for communications equipment, See also <RS232>. Emulation - Use of the system to imitate all functions of another system. The emulating system will operate under control of any software written for the emulated system. Error correction - Communication between the modems to ensure that the data sent by one end are the same as those received by the other, even in the presence of noise on the line. Typically this is done by adding checksums to the data. If the received data don't match their checksum the receiving modem asks for them to be sent again. Like <compression>,<error correction> in the modem is most useful for interactive use. When sending files, it is generally best to let the computers at each end do the correction, using a <protocol> like <Kermit> or <uucp>. However, the ability of <MNP4> and <V.42> to send data <synchronously> may make it worthwhile to use them. See also <spoofing>. Equalizer - Device to compensate for distortion introduced by transmission facilities. External modem - See <internal modem>. Fall back - The ability of a modem to operate at a lower speed than its maximum, used either for compatibility with a different type of modem (e.g. <V.22 bis> can fall back to <V.22>) or to reduce the number of errors over a <link> that cannot carry the fastest speed. FAX - Discussions on FAX should usually be in the newsgroup comp.dcom.fax. This entry names some of the <CCITT> standards used by Group 3 FAX. Parameter negotiation and session control are done using <V.21>; the formats are defined by T.3. Images are sent using <V.27 ter> and <V.29>; the format is defined by T.4. The new <V.17> standard may be available in recent machines. FAX transmission is <half duplex>. Flow control - Ensuring that a unit, whether mode or computer, is not supplied with more data than it can cope with. The unit must have some way to signal to the data source to stop sending. Ideally, every unit on the communication path should have a way to manage <flow control> with its peers; otherwise if available buffer space is exceeded some data may be lost. Between <DTE> and <DCE> the possibilities are <RTS>/<CTS> and <XON/XOFF>. Between two <DCE>s <XON/XOFF> is the only possibiliyy. However, if a <protocol> such as <MNP> is being run between the modems, this will contain provision for <flow control>.If you can guarantee that the <DTE>s will always be able to accept data, you should not need <flow control> between the <DCE>s. Four wire - A <leased line> with separate connections for transmitting data in each direction. Frame - Transmission data segment. In <synchronous> protocols data is formatted in blocks or <frames> for transmission. Each <frame> consists of a start flag followed by an adress field, control field, data field, <frame> check sequence and a stop flag. Frequency Shift Keying (FSK) - Method of <modulation>, where a binary "0" generates one frequency and a binary "1" generates another frequency. Full duplex - Able to send data in both directions at once. Half duplex - Able to send data in only one direction at a time. Some <protocol> is usually used to switch the direction as needed. Handshake - Exchange of predetermined signals occuring when the data connection is first established. Hayes - Modem manufacturer. The `Hayes command set' is supported by most modern modems. Hayes commands look like ATD1234 (dial 1234) or ATA (answer the phone). The commands for simple things like this are fairly standard, but more complex things tend to be manufacturer-specific. A <CCITT> standard for Hayes commands is in preparation. HDLC - High Level Data Link Control. A <link> level <bit> oriented <protocol> used in <synchronous transmission>. Hertz (Hz) - Unit of frequency, equal to cycles per second. Hybrid Transformer - A special kind of transformer used to wire transmission systems to separate the two directions of transmission. Interface - A shared boundary between two communications devices, e.g. a computer, a modem, a printer or a communication channel. Internal modem - A modem card that slots into your computer, as opposed to an <external modem>, which is a separate unit. <Internal modem>s are usually cheaper but they have some disadvantages. An <external modem> can be used with any computer; it will have diagnostic lights to see what is going on; and it means the phone line is further from your PC and so less likely to conduct lightning strikes into it. IPARS - International Passenger Airline Reservation System. Originally developed by IBM for its nominal purpose, but fast becoming a generic term for any airline <protocol>. ISO - Internetional Standards Organizattion responsible for development of general <network> <protocol>. Kermit - A file-transfer <protocol>, available for a wide variety of machines. It contains provisions for transferring text and binary files over 7- and 8-<bit> connections. Latency - The delay between data being sent and being received by the far end. A combination of transmission delays and <modulation> properties. Leased Line - A telecommunication channel leased from the <Common Carrier> between two or more fixed locations. Charges are ussualy a fixed sum per month. Light Emmitting Diode (LED) - A semiconductor low current, low voltage lamp. Special LEDs are used as generators of light energy for lasers and for fider optics transmission. Limited Dictance Modem - A device which permits data communications over short distances. Link - A transmission path between two data sets, channels or switching offices. Loading Coil - Inductance added to a telephone line to improve its frequency distortion at low frequencies. Not erecommended for data transmission. Local Loop - A pair of wires connecting the telephone subscriber to the local telephone office. LRC - Longitudinal Redundancy Check, a simple parity check of a block of data. Mark - A signal on a data circuit inditicating a binary "one". MIL-STD-188-C - A US military standard similar to RS-232-C, except that the voltage levels are different. MNP - Microcom Network Protocols. A set of modem-to-modem protocols that provide <error correction> and <compression>. The commonly encountered ones are these: MNP2. <Error correction> using <asynchronous transmission>. MNP3. <Error correction> using <synchronous transmission> between the modems (the <DTE> <interface> is still <asynchronous>). Since each eight-<bit> <byte> takes eight rather than ten <bits> to transmit there is scope for a 20% increase in throughput. Unfortunately the MNP3 <protocol> overhead is rather high so this increase is not realised. MNP4. Introduces `data phase optimisation', which improves on the rather inefficient <protocol> design of MNP2 and MNP3.<Synchronous> MNP4 comes closer to achieving the 20% throughput increase mentioned above. MNP5. Simple data <compression>. Dynamically arranges for commonly occurring characters to be transmitted with fewer <bits> than rare characters. It takes account of changing character frequencies as it runs. Also encodes long rnns of the same character specially. Typically compresses text by 35%; with 20% for MNP4 this reduces data by almost 50%. Modem Eliminator - Sometimes used to describe a limited distance modem, sometimes to describe a "null" modem. Modulation - Converting a data stream into sounds to be sent down a phone line. The opposite is demodulation. `Mddem' stands for modulator/ demodulator. Multiplexer - A device to combine several signals into a composite data stream for economic transmission. Techniques employed are frequensy division (FDM) and time division (TDM). NAK - Negative Acknowledment, a communication control character indicating that some information was received incorrectly. Network - A series of point interconnected by communication channels, often on switched basis. Networks are either common to all users or are privately leased from common carriers for exclusive use by a customer. Node - A point in a transmission system, where lines or trunks from many system meet. Also the point in a <network>, where switching occurs. Null Modem - A cable or dual-sided plug which allows the connection of two <DCE>s or two <DTE>s. Octet - Standard <CCITT> term for a group of eight <bits>, i.e., what the rest of us think of as a <byte>. They avoid `byte' because some strange machines have <bytes> with more or less than eight <bits>. OOK - On/Off Keying, the simplest form of <modulation> consists of turning the signal on or off. OSI - Open System Interconnection.Standardized procedures for exchange of information among terminal devices, computers, people, <networks>, processes, etc., that are "open" to one another for this purpose by virtue of their mutual use of these procedures. OSI is being developed by <ISO> and <CCITT>. It will provide a common basis for the coordination of standards development for the purpose of system interconnection whithin a seven layer model. Packet - Data drouped for transmission through a public data <network> such as PSTN and X.25 <network>. Packet Switching - Transfer of data by means of addressable <packets>, where a channel is only occupied for the duration of transmission of the <packet>. The channel is then avaible for transmission of other <packets>. In contrast with circuit switching, the data <network> determines routing during, rather then prior to, the transfer of a <packet>. Parallel - Sending several <bits> at a time, usually 8, each over a separate wire. Some modems have a parallel connection from <DTE> to <DCE>. Parity <bit> - An eighth <bit> added to the seven <bits> representing a character, so that the total "one" or "<mark>" <bits> in a character including the parity <bit>, will be either even, for even parity, or odd, for odd parity. PBX or PABX - Private Branch Exchange, telephone switching service usually at a customer's premises. PEP - A proprietary <modulation> and <error correction> technique devised by Telebit and used in their Trailblazer modems. It can achieve throughput of 9600<bps> or better, and is reported to be able to sustain noisy connections better than <V.32>. However, it is <asymmetric> a with a very slow <back channel> and a long <turnaround time> and <latency>. <Protocols> with small <windows> work very poorly unless <spoofing> is used. Trailblazers can spoof <uucp>, <Kermit>, and <XMODEM>. Phase Modulation - Mithod of <modulation>, where a binary "0" generates one phase value and the binary "1" generates another frequency. Phase values are frequently assigned to <dibits> or <quadbits>, rather than to single <bits>. Post Processing - The ability of <protocol> analyzer to perform data analisis on data contained in a data capture buffer just as if the data were arriving in real time. Protocol - A formal set of conventions governing the format and control of inputs and outputs between two communications devices or processes. Protocol Converter - A device for translating output of one computer or terminal into formats which can be interpreted by another computer or terminal. PTT - General term for the national organisation(s) in charge of telephone and postal communications. Public Telephone Network - Switched telephone <network> operated by the <Common Carrier>. Quadrature Amplitude Modulation (QAM) - <Modulation> method used by high speed modems combining amplitude and <phase modulation> of the data signal. Quadbit - A sequence of four <bits> associated with one signal element. RAM - Random Access Memory, used to store temporary data in the computer. ROM - Read Only Memory, used to store permanent data which can not be changed. RS Standards - US Industry standards for interfacing computers to data communications equipment developed by EIA. The most popular <interface> standard in the US is the RS-232-C. RS232 - The usual connection between <DCE> and <DTE>, now properly called <EIA232>. It allows for 25 signals, most of which are not used on typical connections. The minimum needed signals are ground, TxD (transmit data), and RxD (receive data). Typically modem control lines <DCD> and <DTR> and flow control lines <RTS> and <CTS> will also b used. See <V.24>. RTS,Request to send - A signal from <DTE> to <DCE>. In modern modems, this is generally used for <flow control>; when <RTS> is off the modem will not send data to the terminal. In <half duplex> connections, <RTS> may mean that the <DTE> has data to send, so the <DCE> should stop sending to it and assert <CTS>. Script File -(See <Command file>) User-written program required by the communications software to guide the data phone call after a connection has been established. Contains parameters such as the logon sequence, password, and menu selection. SDLC - Synchronous Data Link Control, a <synchronous> data transmission <protocol>. Serial - Sending one <bit> at a time. Opposite of <parallel>. Space - A binary "zero", opposite of "<mark>". Speed buffering - Early modems had very simple hardware which modulated data from the terminal directly onto the phone line. This meant that the terminal speed and the line speed had to be the same. Modern modems allow them to be different (speed buffering), but unfortunately the old assumption is wired into many communications programs, so modems also have to provide the ability to change the erminal speed to the same as the line speed when a connection is established. If the terminal speed is faster than the line speed, <flow control> to the terminal will usually be needed. Spoofing - <Protocol> spoofing is used by Trailblazers (see <PEP>). The modem knows what <protocol> you are using to transfer files to the far end. It pretends to be the remote computer and acknowledges the local data as soon as it gets them. The two Trailblazers then conspire to get the data safely to the far end. Start/Stop Bits - Special character-delimiting <bits used in <asynchronous transmission>. Statistical Multiplexer - Multiplexer equipment, which dynamically allocates transmission capacity to only active channels, thus allowing better channel utilization than with regular <multiplexer>. Store and Forward - A method of transmission in which messages received from one user are stored at intermediate points and retransmitted to other users whenever a transmission path is available. Used in <packet> switching <networks>. Synchronous - Used of a <serial> connection where <bytes> are sent in a continuous stream. Some sort of <protocol> is needd to flag the case where no <bytes> are available to be sent. Synchronous Transmission - Method of communication in which synchronization between the transmitter and receiver is achieved by sending timing signal independent of the transmitted characters. Timing can be common to all <network> stations or can be included in each block of data. Trailblazer - See <PEP>. Turnaround time - The time taken in a <half duplex> or <asymmetric> <link> to reverse the direction of communicaton uucp. Unix-to-Unix copy. This is the name of a Unix command, but it is now also used to refer to the <protocols> used by it to transfer files between Unix machines. There are a number of such <protocols>, and the two machines choose between the ones supported by each. Free implementations also exist for VMS and MS-DOS. The newsgroup comp.mail.uucp may be more appropriate for discussions. V. stsndard - A series of voice grade recommendations (standards) developed by <CCITT> for interfacing and modem standards. V series - A set of standards published by the <CCITT> for `Data Communication over the Telephone Network'. The following standards describe the important <modulation> techniques: V.17: 14400<bps> <half duplex> with <fall back> to 12000<bps>, 9600<bps> and 7200<bps>. V.21: 300<bps>. V.22: 1200<bps>, with <fall back> to 600<bps>. V.22 bis: 2400<bps>, with <fall back> to V.22. V.23: 1200<bps> with 75<bps> <back channel>, with <fall back> to 600<bps>/75<bps>. V.27 ter: 4800<bps> with <fall back> to 2400<bps>, used by <FAX>. V.29: 9600<bps> <half duplex> or <four wire> (used by <FAX>) with <fall back> to 7200<bps> and 4800<bps>. V.32: 9600<bps> with <fall back> to 4800<bps>. V.32 bis: 14400<bps> with <fall back> to 12000<bps>, 9600<bps>, 7200<bps> and 4800 <bps>. Other standards you may encounter: V.24: connection between <DCE> and <DTE>. Effectively the same as <RS232>, though V.24 only specifies the meaning of the signals, not the connector nor the voltages used. V.25 bis: a cryptic command language for modems. V.42: <error correction> with <asynchronous> to <synchronous> conversion. V.42 bis: data <compression> using a Lempel-Ziv related technique, which detects frequently occurring character strings and replaces them with tokens. This is similar to Unix compress. Typical <compression> for text is 50% or better; with nearly 20% gain from <synchronous> conversion this gives reduces transmission time by almost 60%. Window - Most <protocols> divide the data to be sent into `<packets>'. To eliminate delays several <packets> may be sent before any one is acknowledged. If this is allowed by a <protocol>, the <window> is the number of <packets> that can be sent before an <acknowledgement> is received. XMODEM - A primitive file-transfer <protocol>. It has the property that files must be padded to a multiple of 128 <bytes> long. It is <half duplex> (has a <window> of one <packet>) so it performs badly on fast modems. X. Standards - A series of <CCITT> recommendations for transmission of data over public data <network>. The best known of these standards is X.25. XON/XOFF - A method of <flow control>. The XOFF character (<ASCII> 19) is sett to stop further characters, and XON (<ASCII> 17) is sent to resume them. This method is inferior to hardware <flow control> using <RTS> and <CTS>, since it means that XON and XOFF characters cannot be freely used in the data. ZMODEM - A fast file-transfer <protocol> with <windows>. It has been carefully optimised for a variety of conditions, and has useful features such as the ability to resume an aborted transfer where it left off. ------------------------------