em@dce.ie (Eamonn McManus) (12/04/90)
This article lists the differences between version 0.1 of the lexicon,
posted on the 16th of November, and version 0.2, posted today.
*** /tmp/,RCSt1a10995 Mon Dec 3 18:39:35 1990
--- lexicon Mon Dec 3 18:27:43 1990
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*** 1,7 ****
! $Id: lexicon,v 0.1 90/11/16 14:10:40 em Exp $
! Comp.dcom.modems lexicon, by Eamonn McManus <em@dce.ie>. Contributions
! by Charles Bryant <ch@dce.ie>.
This lexicon is intended to provide a basic introduction to terms used in
modem technology. In the interests of brevity, some technical details and
--- 1,10 ----
! $Id: lexicon,v 0.2 90/12/03 18:27:26 em Exp $
! 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>
This lexicon is intended to provide a basic introduction to terms used in
modem technology. In the interests of brevity, some technical details and
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*** 12,22 ****
Words in angle brackets, like <this>, denote cross-references. The
following terms are defined or discussed here:
! Asynchronous, AT commands, back channel, baud, Bell standards, bps, CCITT,
! compression, DCE, DTE, DTMF, error correction, external modem, fall back,
! four wire, full duplex, half duplex, Hayes, internal modem, leased line,
! MNP, modulation, parallel, PEP, PTT, serial, speed buffering, spoofing,
! synchronous, Trailblazer, uucp, V series, window, XMODEM, ZMODEM.
---
--- 15,26 ----
Words in angle brackets, like <this>, 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.
---
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*** 24,30 ****
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>.
Back channel. A slow communications channel provided in the reverse
direction to the main channel, in <V.23> for example.
--- 28,34 ----
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. A slow communications channel provided in the reverse
direction to the main channel, in <V.23> for example.
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*** 38,48 ****
is a 300bps standard; Bell 212 is 1200bps. Not allowed in some European
countries. See <V series>.
! Bps. Bits per second.
CCITT. A French acronym for the International Telegraph and Telephone
Consultative Committee, which sets standards for telephone communication
! among other things.
Compression. Reduction of the size of data by exploiting redundancy. Many
modern modems incorporate use <MNP5> or <V.42 bis> to compress data
--- 42,62 ----
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
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*** 64,77 ****
--- 78,106 ----
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 <CD> 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.
+
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
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*** 81,87 ****
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 bis> to send data <synchronously> may make it
worthwhile to use them. See also <spoofing>.
External modem. See <internal modem>.
--- 110,116 ----
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>.
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*** 91,96 ****
--- 120,139 ----
(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.
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*** 119,128 ****
ones are these:
MNP2. <Error correction> using <asynchronous> transmission.
! MNP3. <Error correction> using <synchronous> transmission. Since each
! 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.
--- 162,172 ----
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.
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*** 150,155 ****
--- 194,203 ----
PTT. General term for the national organisation(s) in charge of telephone
and postal communications.
+ 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.
+
Serial. Sending one bit at a time. Opposite of <parallel>.
Speed buffering. Early modems had very simple hardware which modulated
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*** 175,187 ****
Turnaround time. The time taken in a <half duplex> link to reverse the
direction of communication.
! uucp. Unix-to-Unix copy. Originally the name of a Unix command, this 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.
! 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.
--- 223,237 ----
Turnaround time. The time taken in a <half duplex> 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.
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--- 266,276 ----
XMODEM. A primitive file-transfer protocol. It has the property that
files must be a multiple of 128 bytes long. It is <half duplex> 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 file-transfer protocol.