UKTONY@cup.portal.COM (02/21/88)
Modem Tones
I have been asked by some Americans about useing there BELL TONES modems in
the UK and by British users about using U.S. BBS's.
Here is some information on the tones used by the different breeds of modem
and just for those mystified by all those V numbers there is a list of what
they stand for.
ABOVE ALL REMEMBER:- US is 110v 60hz and the UK is 240v 50hz
So don't blow up your modem with the wrong mains!
The following table gives the standards and tones in common use. The V
standards are those used in most of the world; Bell is confined largely to
North America. V.22 and Bell 212A are more or less the same.
Service Speed Duplex Transmit Receive Answer
designator 0 1 0 1
V.21 orig 300* full 1180 980 1850 1650 -
V.21 ans 300* full 1850 1650 1180 980 2100
V.23 (1) 600 half 1700 1300 1700 1300 2100
V.23(2) 1200 f/h % 2100 1300 2100 1300 2100
V.23 back 75 f/h % 450 390 450 390 -
Bell 103 orig 300* full 1070 1270 2025 2225 -
Bell 103 ans 300* full 2025 2225 1070 1270 2225
Bell 202 1200 half 2200 1200 2200 1200 2025
V.22/212A 1200 full (see below)
V.22 bis 2400 full (see below)
* Any speed up to 300 bts/s can also include 75 and 110 bits/s services.
% Service can either be half duplex at 1200 bits/s or asymmetrical full
duplex, with 75 bit/s originate and 1200 bits/s receive (commonly used as
viewdata user) or 1200 transmit and 75 receive (viewdate host).
Transmission at higher speeds uses different signalling techniques from the
simple on-and-off keying of pairs of tones used for low-speed working.
Simple tone-detection circuits cannot switch on and off sufficiently rapidly
to be reliable so another method of detecting individual 'bits' has to be
employed. The way it is done is by using phase detection. The rate of
signalling doesn't go up - it stays at 600 baud - but each signal is
modulated at origin by phase and then demodulated in the same way at the far
end. Two channels are used, high and low, so that you can achieve
bi-directional or duplex communication.
The tones are:
originate (low channel) 1200 Hz
answer (high channel) 2400 Hz
and they are the same for the European CCITT V.22 standard and for the Bell
equivalent, Bell 212A. V.22 bis is the variant for 2400 bit/s full duplex
transmission; there is no equivalent Bell term.
The speed differences are obtained in this way:
600 bit/s (V.22) Each bit encoded as a phase change from the previous phase.
There are two possible symbols which consist of one of two phase angles; each
symbol conveys 1 bit of information.
1200 bits/s (V.22 and Bell 212A) Differential phase-shift keying is used to
give 4 possible symbols which consist of one of 4 phase angles. Each symbol
conveys 2 bits of information to enable a 600-baud rate to handle 1200 bits.
2400 bits/s (V.22 bis) Quadrature amplitude modulation is used to give 16
possible symbols which consist of 12 phase angles and 3 levels of amplitude.
Each symbol conveys 4 bits of information to enable a 600-baud signal rate to
handle 2400 bits.
International Working
Users transmit via a modem over International Telephone lines just as they
would within the UK, except when sending data to a country that cannot be
dialled direct. However some care has to be made in the initial choice of
modem, to ensure that it can cope with the extra time taken to transmit
telephone signals internationally, especially if they go by satellite. At the
same time it must be ascertained that the modem can disable echo suppressor
equipment used on the international telephone network, to prevent distortion
or interruption of data transmission.
Echo suppressor equipment is used to improve the quality of voice transmission
on international calls, but for data transmission in duplex mode has to be
disabled to prevent data dorruption. Otherwise the echo suppressor equipment
could wipe out data echoed back from the receiving terminal for error
checking, as it often is in duplex mode. Modems transmit a 2100 Hz tone
specified in the CCITT V25 recommendation as their answering tone, to ensure
that echo suppressor equipment does not interfere with data transmission. This
tone disables the echo suppressor, which would distort data if retained on the
circuit.
Half duplex modems are particularly susceptible to propogation delay, which is
the other main consideration with international transmission. Telephone
signals take up to a quarter of a second to transmit on an international
circuit, which is hardly noticeable during a voice conversation. But this
propogation delay can seriously affect data communications by confusing the
timing mechanisms on some modems. Half duplex modems are most vulnerable,
because their turnround procedure to reverse the flow of data may not allow
for the delay. Most half duplex modems now have an international setting, to
make sure that they wait long enough for the appropriate signals to arrive,
without transmitting prematurely, or erroneously reporting a loss of data.
Data Tranmsission Standards
There are broadly three categories used in basic data transmission, and the
one chosen depends on where the data is being sent, and on the type of
terminal at each end of the line. CCITT V Series Recommendations are the most
universal, and widely used throughout Europe and the rest of the world. Bell
Standards are popular throughout the Americas. The third category embraces all
proprietary standards set by individual manufacturers, and usually only
applies to data transmission between their own equipment.
CCITT V SERIES RECOMMENDATIONS are set by CCITT (International Telegraph and
Telephone Consultative Committee), which is an international body of which
most telecommunications administrations are members. The recommendations
define the connection between terminals and modems, and the operating
procedure for modems of all speeds.
BELL STANDARDS originated at the Bell Laboratories in the USA, and are now in
the public domain there, with specifications defined by the USA Electronic
Industries Association (EIA). They are widely used for data transmission
within the Americas, which means that users in the UK who want to send data
across the Atlantic will need to check on modem compatibility.
PROPRIETARY STANDARDS are set by manufacturers and generally only work between
the equipment they supply. However some proprietary protocols have achieved
more widespread acceptance and may therefore be implemented by rival modem
manufacturers.
Most modems supplied in the UK conform toe CCITT recommendations and some are
also compatible with similar Bell or propietary standards. Modems made to
meet some proprietary and Bell standards which are suitable for connection to
the UK public telephone network have been approved by the British Approvals
Board for Telecommunications (BABT) in addition to modems conforming to CCITT
standards. Before buying a modem, please check that it has been approved for
connection and use over the public telecommunication networks
CCITT RECOMMENDCATIONS
CCITT V series recommendations were designed to set common data transmission
standards, making it possible to establish communication between two
terminals, whatever their manufacture. There are a number of recommendations
to cover the different possible transmission speeds in combination with the
admissable modes, such as full duplex and asynchronous. The recommendations
most commonly implemented on equipment connected to the public network are
described.
CCITT RECOMMENDATION V21
Modems built to this standard provide full duplex (both ways simultaneously)
asynchronous transmission at speeds up to 300 bit/s. Some older V21 modems
only allow 200 bit/s. It is also possible to transmit data synchronously
across V21 modems, but the synchronisation has to be established by the
terminal, not being catered for in the V21 standard.
V21 modems are generally used for low speed asynchronous transmission with
teletypes, visual display units and personal computers. V21 is not compatible
with the US Bell standard 103/113, which works to the same speed.
CCITT RECOMMENDATION V22
This provides full duplex transmission at 1200 bit/s, with an optional lower
speed of 600 bit/s. Modems built to this standard send data synchronously,
but terminal equipment can forward data to the modem in either synchronous or
asynchronous form.
V22 is used for both file transfer and two way communication between
personal computers, and is generally compatible with the parallel Bell
standard 212A.
CCITT RECOMMENDATION V22 bis
This is the same as V22 for 1200 bit/s operation, but also provides 2400
bit/s full duplex synchronous transmission. Terminals can relay data to the
modem in either synchronous or asynchronous mode.
This recommendation may supercede V22 at 1200 bit/s for applications like
bulk file transfer and communication between intelligent terminals.
CCITT RECOMMENDATION V23
V23 modems transmit data either in half duplex or asymmetrical duplex mode at
600 or 1200bit/s. In asynmmetrical duplex mode, the slower return channel
operates at 75bit/s.
This version is the more popular of the two options, being used for
applications, such as information retrieval from a central computer, where the
bulk of the data flows in one direction. Requests for information are sent to
the computer through the slow 75bit/s channel, which matches normal typing
speed, and the required data is returned from the computer at 600 or 1200
bit/s. V23 is widely used in this way on videotex services.
The slow return channel is also used sometimes for error control.
CCITT RECOMMENDATION V26 bis
Like V23 this provides a choice between half duplex and asymmetrical duplex
transmission, but doubling the speed options to 1200 and 2400 bit/s. So
once again a 75 bit/s asynchronous return channel can be provided, conforming
to the V23 Recommendation, although within the UK 150bit/s return channel
may be used.
Some V26 modems have a coding selection switch can be set to A or B. The B
option should be tried initially, because the distant modem is more likely
to have this setting operation in the Alternative B coding scheme.
V26 modems are generally used on applications involving bulk transfer
of data, or where large amounts of information periodically need to be
retrieved from a remote system.
CCITT RECOMENDATION V26 ter
This standard provides full duplex synchronous or asynchronous transmission
at 1200bit/s or 2400bit/s, and therefore fits the same applications as V22
bis, including bulk file transfer, and interactive data communication
between personal computers or intelligent terminals. However V22 bis modems
are most widely used of the two for these applications.
CCITT RECOMMENDATION V27 ter
This gives either half duplex or asymmetrical duplex transmission at 4800
bit/s, with fall back to 2400 bit/s if there are problems at the higher
speed. In the asymmetrical duplex mode a 75 bit/s asynchronous return channel
is provided, conforming to the V23 Recommendation.
V27 ter modems are used for bulk file transfer, and the higher operating
speed makes them suitable for connecting to the telephone network as a back up
for leased circuits.
CCITT RECOMMENDATION V29
This standard moves up to 9600 bit/s, the top speed currently possible over
the telephone network, with 4800 bit/s transmission also provided. Data is
transmitted in synchronous form in either half duplex mode over a single
telephone call, or full duplex mode over double dial up, which is two
separately set up simultaneous calls.
Like V27 ter, V29 is used either for bulk file transfer or backing up leased
circuits.
CCITT RECOMMENDATION V32
This standard is likely to supercede V29, because it provides full duplex
synchronous or asynchronous transmission at 9600 bit/s over a single telephone
call connection, with 4800 bit/s as an option. However it is a new standard,
and you are recommended to discuss its international use with your supplier.
As development of V32 modems progresses, versions suitable for use on
international routes are likely to become more readily available.
OTHER CCITT V SERIES RECOMMENDATIONS
CCITT RECOMMENDATION V24 defines how data terminal equipment, such as printers
and personal computers, should connect to the data circuit terminating
equipment, ie modems, covered by the recommendations already described. The
standard applies to the serial data port on the terminal equipment to which
modems are connected by appropriate cable. For this reason such ports are
sometimes called V24, although they are more often referred to by the
equivalent US standard, RS232C.
CCITT RECOMMENDATIONS V25 & V25 bis define the procedures for automatic
answering and calling on the public network. They cover the disabling of echo
suppressors on both manually established and automatic international calls in
the process described in section 7.
BELL STANDARDS
Where there is a measure of compatibility between CCITT V Series and US Bell
standards the provision of a guard tone is required to protect the telephone
network from interference and disruption of the data call.
BELL 103/113
The Bell standard for transmitting asynchronous data at 300 bit/s full duplex.
It is not compatible with CCITT V21.
BELL 201C
The Bell standard for 2400 bit/s data transmission at half duplex. Modems
based on this standard are usually compatible with those based on the
equivalent CCITT V26 bis standard.
BELL 202C
The Bell standard for 1200 bit/s half duplex transmission, which is generally
not compatible with the equivalent CCITT V23.
BELL 212A
This is similar to CCITT V22 and generally compatible with it for 1200 bit/s
synchronous transmission at full duplex, and also for asynchronous data when
the character length is 9 or 10 bits. However the 300 bit/s full duplex
fall-back facility with which Bell 212A modems are equipped conforms to Bell
103/113 standards, which are not compatible with V21, the CCITT equivalent at
this speed.
BELL 208A
The Bell standard for 4800 bit/s half duplex working, not compatible with the
parallel CCITT standard, V27 ter.
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The biggest problem I have found to be is LOW LEVEL TONES from the number you
are calling.
As mentioned above, the need to tell your modem that it may take a very long
time to get an answer tone and not to give up on a call too soon is needed.
The modems we have at work do this and the timing is beyond my control.
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Uk Tony
#6675