em@dce.ie (Eamonn McManus) (11/16/90)
Here's the first draft of the lexicon. Comments are very welcome. It is
my intention to post this on the 1st and 16th of every month, with a
suitable Expires header.
--
Eamonn McManus <em@dce.ie>
`A buffalo with an aqualung.'
---
$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
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:
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.
---
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>.
Back channel. A slow communications channel provided in the reverse
direction to the main channel, in <V.23> 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. <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.
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
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>.
DCE. Data circuit-terminating equipment. Whatever is connected to a phone
line. Effectively, a modem.
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.
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 bis> 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.
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.
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.
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. 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.
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> 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 <V.32>. However, it is
<half duplex> with a long <turnaround time>, so for file transfer it
generally has to be used with protocol <spoofing>. Trailblazers can
spoof <uucp>, Kermit, and Z-modem, among other things.
PTT. General term for the national organisation(s) in charge of telephone
and postal communications.
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.
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> 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.
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> so it
performs badly on fast modems.
ZMODEM. A file-transfer protocol.ronald@robobar.co.uk (Ronald S H Khoo) (11/17/90)
In article <whoissylvia@dce.ie> em@dce.ie (Eamonn McManus) writes: > CCITT. A French acronym for the International Telegraph and Telephone > Consultative Committee, which sets standards for telephone communication > among other things. It would be *very* nice to have a definition of "Recommendation" as used by the CCITT in the lexicon. Toby ? :-) -- ronald@robobar.co.uk +44 81 991 1142 (O) +44 71 229 7741 (H)
tnixon@hayes.uucp (Toby Nixon) (11/20/90)
In article <1990Nov17.003626.7340@robobar.co.uk>, ronald@robobar.co.uk (Ronald S H Khoo) writes: > It would be *very* nice to have a definition of "Recommendation" as used > by the CCITT in the lexicon. Toby ? :-) A CCITT Recommendation is a document, agreed to by technical experts from Postal, Telegraph, and Telephone Administrations, Recognized Private Operating Agencies, and Scientific and Industrial Organizations around the world, indicating their recommendation on how some aspect of the international telegraph or telephone network should operate. Compliance is voluntary; however, many nations incorporate CCITT recommendations into their national regulations, in which case the provisions become mandatory for use of equipment within that country. Is that what you had in mind? -- Toby Nixon, Principal Engineer | Voice +1-404-449-8791 Telex 151243420 Hayes Microcomputer Products Inc. | Fax +1-404-447-0178 CIS 70271,404 P.O. Box 105203 | UUCP uunet!hayes!tnixon AT&T !tnixon Atlanta, Georgia 30348 USA | Internet hayes!tnixon@uunet.uu.net