Modems







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:
  Asymmetric, asynchronous, AT commands, back channel, baud, Bell
  standards, bps, carrier, CCITT, compression, CTS, DCD, DCE, DSR, DTE,
  DTMF, DTR, EIA232, error correction, external modem, fall back, flow
  control, four wire, full duplex, half duplex, Hayes, internal modem,
  latency, leased line, MNP, modulation, parallel, PEP, PTT, RS232, RTS,
  serial, speed buffering, spoofing, synchronous, Trailblazer, turnaround
  time, uucp, V series, window, XMODEM, XON/XOFF, ZMODEM.


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. Used of a {serial} 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 {synchronous}. AT commands. See {Hayes}. (Unrelated to the PC/AT.) Back channel. See {asymmetric}. 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.) {RS232} terminals always have baud the same as bps. Bell standards. {Modulation} techniques used in North America. Bell 103 is a 300bps standard; Bell 212 is 1200bps. Not allowed in some European countries. See {V series}. Bps. Bits per second. See {baud}. 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}. 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}. CTS. Clear to send. A signal from {DCE} to {DTE}. Typically used for {flow control}. 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. 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. EIA232. See {RS232}. 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 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}. 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. 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 {DTE} and {DCE} the possibilities are {RTS}/{CTS} and {XON/XOFF}. Between two {DCE}s {XON/XOFF} is the only possibility. 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. 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. 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. 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. 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 permanent point-to-point connection rented from the phone company. 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 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 {DTE} to {DCE}. PEP. A proprietary {modulation} and {error correction} technique devised by Telebit and used in their Trailblazer modems. It can achieve throughput of 9600bps 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}. PTT. General term for the national organisation(s) in charge of telephone and postal communications. 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 be 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}. Serial. Sending one bit at a time. Opposite of {parallel}. 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. 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. Synchronous. Used of a {serial} 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 {PEP}. Turnaround time. The time taken in a {half duplex} or {asymmetric} 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 {CCITT} for `Data Communication over the Telephone Network'. The following standards describe the important {modulation} techniques: V.21: 300bps. V.22: 1200bps, with {fall back} to 600bps. V.22 bis: 2400bps, with {fall back} to V.22. V.23: 1200bps with 75bps {back channel}, with {fall back} to 600bps/75bps. V.29: 9600bps {half duplex} or {four wire} (used by FAX) with {fall back} to 7200bps and 4800bps. V.32: 9600bps with {fall back} to 4800bps. V.32 bis: 14400bps with {fall back} to 12000bps, 9600bps, 7200bps 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 a multiple of 128 bytes long. It is {half duplex} (has a {window} of one packet) so it performs badly on fast modems. XON/XOFF. A method of {flow control}. 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 {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}.