From: em@dce.ie (Eamonn McManus) Newsgroups: comp.dcom.modems Subject: Re: comp.dcom.modems lexicon Date: 2 Jan 91 18:21:27 GMT Organization: Datacode Communications Ltd, Dublin, Ireland Supersedes: [Since there don't seem to have been many Frequently Asked Questions recently I'm thinking of changing the frequency of this posting to monthly instead of fortnightly. Comments are welcome.] $Id: lexicon,v 0.2 90/12/03 18:27:26 em Exp $ Comp.dcom.modems lexicon, by Eamonn McManus . Contributions by: Charles Bryant . Ronald S H Khoo David Lesher Chip Rosenthal 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 . This document is not copyrighted and may be used freely. Words in angle brackets, like , denote cross-references. The following terms are defined or discussed here: Asynchronous, AT commands, back channel, baud, Bell standards, bps, carrier, CCITT, compression, CTS, DCD, DCE, DSR, DTE, DTMF, DTR, error correction, external modem, fall back, flow control, four wire, full duplex, half duplex, Hayes, internal modem, leased line, MNP, modulation, parallel, PEP, PTT, RTS, serial, speed buffering, spoofing, synchronous, Trailblazer, uucp, V series, window, XMODEM, XON/XOFF, ZMODEM. --- Asynchronous. Used of a connection where each byte is sent independently. The start and end of a byte are indicated by separate bits so each byte takes 10 bits to transmit. See . AT commands. See . (Unrelated to the PC/AT.) Back channel. A slow communications channel provided in the reverse direction to the main channel, in for example. Baud. A unit of communication speed, defined as signalling elements per second. Not the same as bps since sometimes each signalling element carries several bits. (There is no such thing as a 9600 baud modem, for instance.) Terminals always have baud the same as bps. Bell standards. techniques used in North America. Bell 103 is a 300bps standard; Bell 212 is 1200bps. Not allowed in some European countries. See . Bps. Bits per second. See . 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 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 . Compression. Reduction of the size of data by exploiting redundancy. Many modern modems incorporate use or 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 protocols are , 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 and . CTS. Clear to send. A signal from to . Typically used for . DCD. Data carrier detect. A signal from to , indicating that a tone is being heard from the remote modem. See . DCE. Data circuit-terminating equipment. Whatever is connected to a phone line. Effectively, a modem. DSR. Data set ready. A signal from to , indicating that a connection is in progress. For connections, DSR will be on during the entire session, while 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 to . Sometimes used for , though is more usual nowadays. Typically, dropping DTR will cause the modem to hang up. 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 , error correction 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 . However, the ability of and to send data may make it worthwhile to use them. See also . External modem. See . 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. can fall back to ) or to reduce the number of errors over a link that cannot carry the fastest speed. Flow control. Ensuring that a unit, whether modem 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 and the possibilities are / and . Between two s is the only possibility. However, if a protocol such as is being run between the modems, this will contain provision for flow control. If you can guarantee that the s will always be able to accept data, you should not need flow control between the s. Four wire. A with separate connections for transmitting data in each direction. Full duplex. Able to send data in both directions at once. Half duplex. Able to send data in only one direction at a time. 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 standard for Hayes commands is in preparation. Internal modem. A modem card that slots into your computer, as opposed to an external modem, which is a separate unit. Internal modems 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. Leased line. A permanent point-to-point connection rented from the phone company. MNP. Microcom Network Protocols. A set of modem-to-modem protocols that provide and . The commonly encountered ones are these: MNP2. using transmission. MNP3. using transmission between the modems (the interface is still ). 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. 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 runs of the same character specially. Typically compresses text by 35%; with 20% for MNP4 this reduces data by almost 50%. Modulation. Converting a data stream into sounds to be sent down a phone line. The opposite is demodulation. `Modem' stands for modulator/ demodulator. Parallel. Sending several bits at a time, usually 8, each over a separate wire. Some modems have a parallel connection from to . PEP. A proprietary technique devised by Telebit and used in their Trailblazer modems. It can achieve throughput of 9600bps or better, and is reported to be more resilient than . However, it is with a long , so for file transfer it generally has to be used with protocol . Trailblazers can spoof , Kermit, and Z-modem, among other things. PTT. General term for the national organisation(s) in charge of telephone and postal communications. RTS. Request to send. A signal from to . In modern modems, this is generally used for ; when RTS is off the modem will not send data to the terminal. Serial. Sending one bit at a time. Opposite of . 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 terminal speed to the same as the line speed when a connection is established. Spoofing. Protocol spoofing is used by Trailblazers (see ). 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. Synchronous. Used of a connection where bytes are sent in a continuous stream. Some sort of protocol is needed to flag the case where no bytes are available to be sent. Trailblazer. See . Turnaround time. The time taken in a link to reverse the direction of communication. 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 series. A set of standards published by the for `Data Communication over the Telephone Network'. The following standards describe the important techniques: V.21: 300bps. V.22: 1200bps, with to 600bps. V.22 bis: 2400bps, with to V.22. V.23: 1200bps with 75bps , with to 600bps/75bps. V.29: 9600bps or (used by FAX) with to 7200bps and 4800bps. V.32: 9600bps with to 4800bps. V.32 bis: 14400bps with to 12000bps, 9600bps, 7200bps and 4800 bps. Other standards you may encounter: V.24: connection between and . 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: with to conversion. V.42 bis: data 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 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 a multiple of 128 bytes long. It is so it performs badly on fast modems. XON/XOFF. A method of . The XOFF character (ASCII 19) is sent to stop further characters, and XON (ASCII 17) is sent to resume them. This method is inferior to hardware flow control using and , since it means that XON and XOFF characters cannot be freely used in the data. ZMODEM. A file-transfer protocol.