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 {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
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

      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
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
Modulation. Converting a data stream into sounds to be sent down a phone
      line. The opposite is demodulation. `Modem' stands for modulator/
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
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}.

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