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5i'
Recommendation T.30
PROCEDURES FOR DOCUMENT FACSIMILE TRANSMISSION
IN THE GENERAL SWITCHED TELEPHONE NETWORK
(former Recommendation T.4, Mar del Plata, 1968;
amended and renumbered at Geneva, 1976 and 1980,
Malaga-Torremolinos, 1984 and Melbourne, 1988)
Introduction
i) This Recommendation is intended to apply to document fac-
simile apparatus covered by CCITT Recommendations T.2, T.3 and T.4.
It describes the procedures and signals to be used where facsimile
equipments are operated over the general switched telephone net-
work. When existing equipments are operating in a non-CCITT manner,
they shall not interfere with equipments operating in accordance
with the Series T Recommendations.
ii) Arrangements for automatic calling/answering on the gen-
eral switched telephone network have been aligned as closely as
possible with those described in the Series V Recommendations for
data terminal equipment.
iii) While there are eight possible operating methods (see
Table 1/T.30) each may be described by five separate and consecu-
tive phases:
Phase A Call set up
Phase B Pre-message procedure for identifying and select-
ing the required facilities
Phase C Message transmission (includes phasing and
synchronization where appropriate)
Phase D Post-message procedure including
end-of-message and confirmation and multi-document procedures
Phase E Call release
_________________________
Facsimile apparatus referred to as Groups 1, 2 or 3 in
this Recommendation are those cnforming to
Recommendations T.2, T.3 or T.4 respectively.
iv) Two separate signalling systems are described: first a
simple system using single frequency tones and second a binary
coded system which offers a wide range of signals for more complex
operational procedures. Thus tonal signalling is restricted to
manual operation at both stations or where a manually operated sta-
tion intends to transmit to a called station equipped as an
automatic answering receiver conforming to Recommendations T.2
and T.3 will normally use the tonal signalling system although the
binary coded system may be provided in addition where complex pro-
cedures are required, e.g. comprehensive automatic functions.
v) For digital document facsimile apparatus conforming to
Recommendation T.4 it is intended that the binary coded system
shall be the standard signalling arrangement, but additionally a
tonal signalling capability may be provided when the digital fac-
simile apparatus has a fallback capability to apparatus conforming
to Recommendations T.2 and T.3. The binary coded signalling has
priority and should be tried first; if this fails to elicit a
response, tonal signalling should be attempted.
vi) The binary coded signalling system is based on a high
level data link control (HDLC) format developed for data transmis-
sion procedures. The basic HDLC structure consists of a number of
frames each of which is subdivided into a number of fields. It pro-
vides for frame labelling , error checking and confirmation of
correctly received information and the frames can be easily
extended if this should be required in the future.
vii) The transmission of the facsimile message itself (phase
C) will be according to the modulation system described in the
appropriate Recommendation for the facsimile apparatus.
CONTENTS
1 Scope
2 Explanation of terms used
3 Description of a facsimile call
4 Tonal signalling for facsimile procedure
5 Binary coded signalling for facsimile procedure
Annex A - Procedure for Group 3 document facsimile
transmission in the general switched telephone network incorporat-
ing error correction
Appendix I - Example of non-standard manual-to-manual
basic facsimile operation
Appendix II - Index of abbreviations used in
Recommendation T.30
Appendix III - List of commands and appropriate
responses
Appendix IV - Interworking between the standard mode
and the recognized optional mode for the binary coded handshaking
procedure
Appendix V - Signal sequence examples
The CCITT,
considering
(a) that facilities exist for facsimile transmission over the
general switched telephone network;
(b) that such facsimile transmission may be requested either
alternatively with telephone conversation or when either or both
stations are not attended;
(c) that for this reason the operations involved in establish-
ing and/or releasing a facsimile call should be capable of
automatic operation;
unanimously declares the view
that the facsimile apparatus should be designed and operated
according to the following standards:
1 Scope
1.1 General
1.1.1 This Recommendation is concerned with the procedures
which are necessary for document transmission between two facsimile
stations in the general switched telephone network.
These procedures essentially comprise the following:
- call establishment and call release,
- compatibility checking , status and control com-
mand ,
- checking and supervision of line conditions ,
- control functions and facsimile operator recall
1.1.2 Only the procedures with their corresponding signals are
specified in this Recommendation.
1.2 Classification of operating methods
1.2.1 This Recommendation regulates the operational sequence
of manually operated facsimile stations as well as of automatic
stations.
The automatic facsimile station is understood to be a station
which is capable of performing all procedures (listed in S
1.1 above) automatically. In this case, an operator is not neces-
sary.
If, however, an operator is required for any of these pro-
cedures, the station must be regarded as a manually operated sta-
tion.
1.2.2 Based upon all combinations which may result from the
fact that there are manually operated stations and automatic fac-
simile stations, the operating methods shown in Table 1/T.30 are
possible.
H.T. [T1.30]
TABLE 1/T.30
_________________________________________________________________________________
Method No. {
Description of operating method
} {
Direction of facsimile transmission
} Overall designation
_________________________________________________________________________________
{
{
1-T
{
{
1
1-R
_________________________________________________________________________________
{
{
2-T
{
{
2
2-R
_________________________________________________________________________________
{
{
3-T
{
{
3
3-R
_________________________________________________________________________________
{
{
4-T
{
{
4
4-R
_________________________________________________________________________________
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Note - There may also be operating methods which will allow mes-
sages to be received by more than one station (multipoint connec-
tion).
Table 1/T.30 [T1.30], p.
1.3 Station identification
1.3.1 For the purpose of classifying an automatic facsimile
station as a non-speech terminal, a tone must be transmitted to
line. As both automatic calling and called facsimile stations
transmit tones to line during call
establishment, a normal telephone user who becomes inadvertently
connected to one will receive tone signals for a period of suffi-
cient duration to indicate clearly to him that he is incorrectly
connected.
1.3.2 Additionally an automatic verbal announcement may be
used which can provide station identification.
1.4 General provisions
1.4.1 The control signals specified in this Recommendation
have been chosen in such a way that the telephone service is not
affected.
1.4.2 If any malfunction of the facsimile procedures described
in this Recommendation is detected, the call should be released.
1.4.3 Where the called station has automatic facsimile
apparatus which is not ready or not able to operate, the call
should not be answered automatically.
1.4.4 This Recommendation includes procedures for switching
from facsimile to speech regulations of the Administrations.
1.5 Optional provisions
1.5.1 The operator at each station may have the possibility of
calling the other station at any time during the progress of the
facsimile procedure (see S 2.2 below).
1.5.2 The procedures in this Recommendation allow a facsimile
station to transmit and/or receive several documents successively
without the aid of an operator.
1.5.3 This Recommendation includes procedures for incorporat-
ing a unique station identification command if required to prevent
unauthorized stations from demanding a message.
If enhanced security is required, this may be provided by the
use of the non-standard facilities frame.
2 Explanation of terms used
2.1 Facsimile station main functions
One or more equipments at the end of the line providing three
main functions.
2.1.1 Call establishment and call release
The establishment and release of a connection according to the
normal rules of using the general switched telephone network.
2.1.2 Procedure
To identify, to supervise and to control the facsimile
transmission according to a protocol.
2.1.3 Message transmission
To transmit and/or receive the facsimile message.
2.2 Time sequence of a facsimile call | see Figure 1/T.30)
FIGURE 1/T.30, p.
2.3 Description of phases
2.3.1 Phase A - Call establishment
Call establishment can be realized manually and/or automati-
cally.
2.3.2 Phase B - Pre-message procedure
The pre-message procedure consists of the identification of
capabilities and the commanding of the chosen conditions as well as
the confirmation of acceptable conditions.
When connection is established between apparatus operating in
accordance with this Recommendation and apparatus operating in a
non-CCITT manner, the equipments should disconnect before the
in-message procedure unless both equipments include optional, com-
patible, procedures.
2.3.2.1 Identification section
- group identification,
- confirmation for reception,
- subscriber identification (option),
- non-standard facilities identification (option).
2.3.2.2 Command section
- group command,
- phasing / training ,
- synchronization,
- as well as the following optional commands:
- non-standard facilities command,
- subscriber identification command,
- polling (send) command,
- line conditioning ,
- echo suppressor disabling
2.3.3 Phase C1 - In-message procedure
The in-message procedure takes place at the same time as mes-
sage transmission and controls the complete signalling for
in-message procedure, e.g., in-message synchronization, error
detection and correction and line supervision.
2.3.4 Phase C2 - Message transmission
Message transmission procedure is covered by the appropriate
Recommendation for the equipment.
2.3.5 Phase D - Post-message procedure
The post-message procedure includes information regarding:
- end-of-message signalling,
- confirmation signalling,
- multipage signalling,
- end-of-facsimile procedure signalling.
2.3.6 Phase E - Call release
Call release shall be realized manually and/or automatically.
3 Description of a facsimile call
3.1 Phase A - Call establishment
The establishment of a facsimile call may be realized either
manually, if an operator is in attendance, or automatically. To
accomplish this, four operating methods have been defined.
3.1.1 Operating method 1
_________________________
See Appendix II for abbreviations used in this Recom-
mendation.
Manual operation at both the calling and called station. Fig-
ure 2/T.30 indicates the operators' actions required to establish a
call.
H.T. [T2.30]
_________________________________________________________________________________________________________
Call event No. Calling station Called station
_________________________________________________________________________________________________________
1 {
Operator hears dial tone and dials desired number
}
2 Operator hears ringing tone {
Call rings and operator answers the call
}
3 Verbal identification Verbal identification
4 {
Facsimile machine is switched to line
} {
Facsimile machine is switched to line
}
5 {
Begin facsimile procedure
(see SS 4 and/or 5 of this Recommendation)
} {
Begin facsimile procedure
(see SS 4 and/or 5 of this Recommendation)
}
_________________________________________________________________________________________________________
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Table [T2.30], p.
Figure 2/T.30, p.
3.1.2 Operating method 2
Manual operation at the calling station and automatic opera-
tion at the called station. Figure 3/T.30 indicates the operator's
and apparatus actions required to establish a call.
H.T. [T3.30]
______________________________________________________________________________________________________________
Call event No. Calling station Called station
______________________________________________________________________________________________________________
1 {
Operator hears dial tone and dials desired number
}
2 Operator hears ringing tone {
Equipment detects ring and answers the call
}
3 {
Optionally, a recorded verbal announcement may be transmitted
}
4 {
Operator hears CED and facsimile machine is switched to line
} Transmit CED
5 {
Begin facsimile procedure
(see SS 4 and/or 5 of this Recommendation)
} {
Begin facsimile procedure
(see SS 4 and/or 5 of this Recommendation)
}
______________________________________________________________________________________________________________
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Table [T3.30], p.
Figure 3/T.30, p.
3.1.3 Operating method 3
Automatic operation at the calling station and manual opera-
tion at the called station. Figure 4/T.30 indicates the operator's
and apparatus actions required to establish a call.
H.T. [T4.30]
_____________________________________________________________________________________________________________
Call event No. Calling station Called station
_____________________________________________________________________________________________________________
1 {
Equipment detects dial tone and dials desired number (see Note). To
clearly indicate to a called operator that he is connected to a
facsimile machine or to a normal telephone user that he is inadvertently
connected, CNG will be transmitted to line during the time that signals
are attempted to be detected.
Note
- An alternative procedure may be specified by
Administrations.
}
2 {
Call rings and operator answers the call
}
3 {
Operator detects CNG and switches facsimile machine to line (optionally
CED may be generated)
}
4 {
Begin facsimile procedure
(see S 5 of this Recommendation)
} {
Begin facsimile procedure
(see S 5 of this Recommendation)
}
_____________________________________________________________________________________________________________
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Table [T4.30], p.
FIGURE 4/T.30, p.
3.1.4 Operating method 4
Automatic operation at both the calling and called stations.
Figure 5/T.30 indicates the actions required by the apparatus to
establish a call.
H.T. [T5.30]
____________________________________________________________________________________________________________
Call event No. Calling station Called station
____________________________________________________________________________________________________________
1 {
Equipment detects dial tone and dials desired number (see Note). To
clearly indicate to a normal telephone user that he is inadvertently
connected, CNG will be transmitted to line during the time that signals
are attempted to be detected.
Note
- An alternative procedure may be specified by
Administrations.
}
2 {
Equipment detects ring and answers the call
}
3 {
Optionally, a recorded verbal announcement may be transmitted
}
4 Transmit CED
5 {
Begin facsimile procedure
(see S 5 of this Recommendation)
} {
Begin facsimile procedure
(see S 5 of this Recommendation)
}
____________________________________________________________________________________________________________
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Table [T5.30], p.
FIGURE 5/T.30, p.
3.2 Phases B, C and D - Facsimile procedure
When entering phase B, the following rules should be adhered
to:
All manual receivers and all auto-answering units must enter
phase B by identifying their capabilities (i.e. Node R of the flow
diagram in S 4.2 or 5.2). All manual transmitters and all
auto-calling units must enter phase B prepared to detect the capa-
bilities and issue the appropriate mode setting command
(i.e. Node T of the flow diagram in S 5.2). To allow for operating
method 2-R, the delay between the transmission of the digital iden-
tification signals shall be 4.5 seconds _ | 5% when sent from a
manual facsimile receiver.
The detailed information pertaining to the tonal and binary
coded facsimile procedures is contained in SS 4 and 5 below. The
relationship between these two procedures and an overview regarding
the total system operation is given in the following:
3.2.1 The interaction between tonal and binary coded pro-
cedures
Facsimile procedures, as described in this Recommendation, may
be realized in two different ways:
- tonally, with a limited number of tones for sim-
ple procedures (see S 4 below) and
- binary coded, for more comprehensive procedures
(see S 5 below).
Binary coded signalling is especially desirable for machines
which use:
- comprehensive automatic functions;
- digital concepts internally (e.g. redundancy
reduction techniques);
- fast transmission rates (in order to keep pre-
and post-message time short compared to total transmission time);
- special security features.
Recommendations concerning the interaction between tonal and
binary coded signalling recognize the principle of the priority of
coded procedures such that, when available, binary coded signalling
shall be tried first. The interaction steps are as follows:
- The unattended called station shall answer a call
with the CED signal.
- The unattended calling station shall indicate a
call with the CNG signal.
- Whenever it is capable of binary coded signal-
ling, the called station will start with binary coded signalling.
- Facsimile stations being capable of tonal signal-
ling only will start tonally.
- Facsimile stations being capable of both binary
coded and tonal signalling will send a sequence of signals, the
first being a binary coded signal and the second and all following
signals being a composite of tonal and binary coded information.
- If the calling station reacts binary coded then
the binary coded signalling goes on through all control procedures.
- If the calling station reacts tonally, then the
tonal signalling goes on through all procedures.
An example of a station having both binary-coded and tonal
capabilities is shown in Figure 6/T.30 for further clarification.
3.2.2 Signal sequences
The recommended system utilizes the interchange of signals
between the two equipments to verify compatibility and assure
operation. To do this,
the called station identifies its capabilities tonally (in the
simplest configuration) and/or binary coded. The calling station
responds to this accordingly with a command tonally or binary
coded. Now the transmitter continues phase B.
Following the transmission of the message, the transmitter
sends an end-of-message signal and the receiver confirms reception.
Multiple documents can then be transmitted by the repetition of
this procedure.
The flow of signals is shown in Figure 7/T.30 for the confi-
guration where the calling station is transmitting. These signals
may be tonal or binary coded, subject to the conditions of
S 3.2.1 above.
Figure 6/T.30, p.
Figure 7/T.30, p.
The condition where the calling station is to receive docu-
ments is shown in Figure 8/T.30. The simple tonal systems do not
provide this capability.
FIGURE 8/T.30, p.
3.3 Phase E - Call release
Call release occurs after the last post-message signal of the
procedure or under certain conditions, e.g.:
3.3.1 Time out
When a signal as specified by the facsimile procedure is not
received within the specified time-out period, the apparatus may
signal to the operator (if one is in attendance) or disconnect the
telephone connection. The appropriate time-out periods are speci-
fied in SS 4 and 5 below.
3.3.2 Procedural interrupt
The facsimile procedure may be interrupted by sending a pro-
cedural interrupt signal, by notifying the attending operator or by
disconnecting the connection. The signal is defined in SS 4 and
5 below.
3.3.3 Command
In the case where binary coded procedures are utilized, the
call may be immediately terminated by the binary coded system com-
mands, as specified in S 5 below.
4 Tonal signalling for facsimile procedure
This signalling system covers operating methods 1-T and 2-T
and has to be implemented for apparatus operating according to
Recommendations T.2 and T.3.
4.1 Description
Phases B and C
H.T. [T6.30]
_______________________________________________
Transmitter Receiver
_______________________________________________
1. Transmit GI
2. GI detected
3. Select appropriate group
4. Transmit GC
5. Transmit phasing
{
6.
Detect GC and phasing
.ta 672u 1104u
Select group and phase
}
7. Transmit CFR
8. Detect CFR
9. Transmit message
_______________________________________________
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Table [T6.30], p.
Phase D
H.T. [T7.30]
__________________________________________________________________________
{
Single-document transmitter
} Multi-document receiver
__________________________________________________________________________
1. Transmit EOM
2. Detect EOM
3. Transmit MCF
{
4.
Prepare for next document
}
{
5.
Detect MCF
.ta 1128u 1752u
Switch back to telephone
.ta 1128u 1752u
Operator loads document
}
{
6.
When ready to receive transmit GI
}
{
7.
Operator hears GI and switches machine to line
}
8. Detect GI
9. Transmit GC
Continue phases B and C
__________________________________________________________________________
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TABLEAU ETAPE D, (Recup.), [T7.30], p.
H.T. [T8.30]
____________________________________________________________________________
Multi-document transmitter Single-document receiver
____________________________________________________________________________
1. Transmit EOM
2. Detect EOM
3. Transmit MCF
{
4.
Switch back to telephone
Operator loads paper
}
{
5.
Detect MCF and prepare for next document
}
{
6.
When ready to transmit, transmit CNG (optional)
}
{
7.
Operator hears CNG and switches machine to line
}
8. Transmit GI
9. Detect GI
10. Transmit GC
Continue phases B and C
____________________________________________________________________________
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TABLEAU ETAPE D, (Recup.), [T8.30], p.
Multi-document transmitter to multi-document receiver and sin-
gle document facsimile apparatus operate accordingly.
Note - It is acknowledged that there are existing equipments
in the field that may not conform in all aspects to this Recommen-
dation. Therefore, the decision may be made to go to a mode of
operation other than specified herein. The diagram of Appendix I
describes, as an example, one of these conditions. Other methods
may be possible as long as they do not interfere with the recom-
mended operation.
4.2 Flow diagram | Figure 9/T.30)
FIGURE 9/T.30, p.
4.3 Tonal signal functions and formats
The signals used are single frequencies to line. The equipment
used to detect the signal should be capable of functioning
correctly with the frequency tolerances quoted plus an additional
tolerance of _ | Hz due to the line.
4.3.1 Facsimile receiver signals (signals transmitted by
the receiver)
4.3.1.1 Group identification (GI) signals
4.3.1.1.1 GI 1 (Group 1)
Format | Figure 10/T.30)
FIGURE 10/T.30, p.
Function
1) To indicate the apparatus is in the receive
mode and capable of receiving at least one page in the Group 1
mode.
2) The signal is repeated until detection of GC or
time T1 elapses.
4.3.1.1.2 GI 2 (Group 2)
Format | Figure 11/T.30)
FIGURE 11/T.30, p.
Function
1) To indicate the apparatus is in the receive mode
and is capable of receiving at least one page in the Group 2 mode.
2) The signal is repeated until detection of GC or
time T1 elapses.
4.3.1.1.3 GI 1/2 (Group 1/2)
Format | Figure 12/T.30)
FIGURE 12/T.30, p.
Function
1) To indicate the apparatus is in the receive mode
and is capable of receiving at least one page in the Group 1 or
Group 2 mode. The apparatus is capable of adjusting automatically
to the speed of the transmitting.
2) The signal is repeated until detection of GC or
time T1 elapses.
Note - To prevent confusing the repeating GI signal with the
busy tone, it may be required by certain Administrations that a
delay be incorporated prior to answering the call.
4.3.1.2 Confirmation to receive (CFR) signals
4.3.1.2.1 CFR 1 (Group 1)
Format | Figure 13/T.30)
FIGURE 13/T.30, p.
Function
To indicate that the receiver has phased and is ready to
receive at least one page in the Group 1 mode. The signal must
start after the completion of the phasing signal at the receiver
with a maximum delay of one second.
4.3.1.2.2 CFR 2 (Group 2)
Format | Figure 14/T.30)
FIGURE 14/T.30, p.
Function
To indicate that the receiver has phased and is ready to
receive at least one page in the Group 2 mode. The signal must
start after the completion of the phasing signal at the receiver
with a maximum delay of one second.
4.3.1.3 Message confirmation (MCF) signal
4.3.1.3.1 MCF 1 (Group 1)
Format
The same frequency and duration as for CFR 1.
Tolerances: timing _ | 5%; frequency _ | Hz.
Function
To indicate that the receiver has received one page in
Group 1 mode.
4.3.1.3.2 MCF 2 (Group 2)
Format
The same frequency and duration as for CFR 2.
Tolerances: timing _ | 5%; frequency _ | Hz.
Function
To indicate that the receiver has received one page in the
Group 2 mode.
Note - The MCF signal must start a maximum of 0.5 second
after the completion of the EOM signal (see S 4.3.2.4) at the
receiver.
4.3.2 Facsimile transmitter signals (signals transmitted
by the transmitter)
4.3.2.1 Group command (GC) signal
Format
GC1 = 1300 Hz _ 32 Hz for a duration of more than
1.5 seconds and less than 10 seconds.
GC2 = 2100 Hz _ 10 Hz for a duration of more than
1.5 seconds and less than 10 seconds.
Function
To indicate to the receiver the Group that the transmitter
has chosen. The GC signal starts at the end of the capabilities
identification signal with a maximum delay of 1 second as measured
on the line at the transmitter.
Note - It should be noted that the capabilities identifica-
tion of a combined Group 1 or 2 and Group 3 machine may consist of
the tonal GI signal concatenated with the binary coded identifica-
tion signal. Some equipment exists which sends the GC signal at the
end of the GI signal and not at the end of the capabilities iden-
tification signal. This should be avoided in new designs. However,
manufacturers of combined group equipments should take account of
this anomaly.
4.3.2.2 Line conditioning signals (LCS)
Format
As in Recommendation T.3.
Function
1) To enable a receiver to equalize the line.
2) This is an optional signal and non-transmission
should not affect compatibility.
4.3.2.3 Phasing
Format and function
As defined by Recommendations T.2 and T.3.
4.3.2.4 End-of-message (EOM) signal
Format
1100 Hz _ 38 Hz. Timing: 3 seconds _ 15% immediately fol-
lowing the message.
Function
To indicate phase C has been completed.
4.3.3 Common signals
4.3.3.1 Procedure interrupt signal (PIS) (applicable in
both directions)
Format
462 Hz _ 1.5 Hz for 3 seconds minimum.
Function
1) To stop a distant machine.
2) May be used as operator recall.
Note 1 - This is an optional signal.
Note 2 - Some Administrations have in use national telephone
signalling systems which may interpret this signal as a clearing
signal. This may cause clear down of the connection.
Note 3 - Some machines use this signal as a disconnect sig-
nal only when the receiver detects this signal immediately after
transmitting MCF or transmitting MCF/GI and, in either case, before
a subsequent GI.
Note 4 - The satisfactory operation of the PIS signal cannot
be guaranteed in the presence of, for example, echo suppressors
4.3.3.2 Called station identification (CED)
At 1.8 to 2.5 seconds after the called station is connected to
the line, it sends a continuous 2100 Hz _ 15 Hz tone for a duration
of not less than 2.6 seconds and not more than 4.0 seconds.
The called station delays for a period of 75 _ 20 milliseconds
after terminating the CED tone before transmitting further signals.
Function
To indicate a called non-speech terminal.
4.3.3.3 Calling tone (CNG)
Format | Figure 15/T.30)
FIGURE 15/T.30, p.
Function
1) To indicate a calling non-speech terminal. This
signal is mandatory for automatic calling units and optional for
manual units.
2) To indicate that the apparatus is in the
transmit mode and is ready to transmit on receipt of the appropri-
ate GI.
3) Where an apparatus is capable of sending more
than one document without the necessity of operator assistance,
this signal may be transmitted between documents whilst the
transmitter is waiting for the appropriate GI. It would indicate to
an operator that the transmitter was still connected to line.
Note - It should generally be assumed that for Group 1 and
Group 2 transmissions, echo suppressors may be in the circuit.
5 Binary coded signalling for facsimile procedure
For Group 1 and Group 2 machines that require additional
facilities to those provided by the procedures described in
S 4 above, the binary coded control procedures should be transmit-
ted in a synchronous mode at 300 bits per second.
For Group 3 machines, 300 bits per second is the standard data
signalling rate for the transmission of binary coded procedural
data. Additionally, signalling of the binary coded procedural data
at 2400 bits per second is allowed as a recognized option.
For Group 3 machines, an error correction capability is util-
ized as a recognized option. This procedure is defined in Annex A.
Except as otherwise noted, the binary coded control procedures
should be transmitted in a synchronous mode on the general switched
telephone network at 300 bits per second _ | .01% utilizing the
characteristics of the Recommendation V.21 channel No. 2 modulation
system. (For the tolerances, see S 3 of Recommendation V.21.) Sig-
nal generators should have a distortion not exceeding 1% and the
control signal receivers should accept signals with a distortion
not exceeding 40%.
Note 1 - For Group 3 machines, the transmission of training ,
TCF, and all in-message signals, shall be at the data rate of the
high-speed message channel.
Note 2 - It is acknowledged that existing equipments may not
conform in all aspects to this Recommendation. Other methods may be
possible as long as they do not interfere with the recommended
operation.
Note 3 - Transmission of signals utilizing the modulation
system of Recommendation V.21 channel No. 2 should be followed by a
delay of 75 _ 20 milliseconds before the signalling, utilizing a
different modulation system commences, (e.g. the delay between DCS
and the Recommendation V.27 | fIter or V.29 training sequence ).
Note 4 - The transmission of signalling utilizing the modula-
tion systems of Recommendation V.27 | fIter or Recommendation V.29
should be followed by a delay of 75 _ 20 milliseconds before the
signalling, utilizing a different modulation system, commences
(e.g. the delay between RTC and MPS).
5.1 Description
Phases B, C and D
Case 1 : Calling station wishes to transmit (see
Figure 7/T.30).
H.T. [T9.30]
____________________________________________________________________________________________
Calling station Called station
____________________________________________________________________________________________
1. Transmit DIS
2. DIS detected
3. Transmit DCS
4. DCS detected