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\fBSeries\ U\ Recommendations\fR \v'2P'
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.OF ''' \ \ \ ^	%'
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\fBTELEGRAPH\ SWITCHING\fR 
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\fBMONTAGE:\fR \ PAGE 2 = PAGE BLANCHE
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SECTION\ 1
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\fBGENERAL\fR \v'1P'
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.sp 1P
.sp 2P
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\fBRecommendation\ U.1\fR 
.RT
.sp 2P
.ce 1000
\fBSIGNALLING\ CONDITIONS\ TO\ BE\ APPLIED\fR 
.EF '%	Fascicle\ VII.2\ \(em\ Rec.\ U.1''
.OF '''Fascicle\ VII.2\ \(em\ Rec.\ U.1	%'
.ce 0
.sp 1P
.ce 1000
\fBIN\ THE\ INTERNATIONAL\ TELEX\ SERVICE\fR 
.ce 0
.sp 1P
.ce 1000
\fI(former CCIT Recommendation E.1, Arnhem, 1953; amended at Geneva, 1956,\fR 
\fINew Delhi, 1960,\fR 
.sp 9p
.RT
.ce 0
.sp 1P
.ce 1000
\fIGeneva, 1964, Mar del Plata, 1968, Geneva, 1972, 1976, 1980,\fR 
\fIMalaga\(hyTorremolinos, 1984 and Melbourne, 1988)\fR 
.ce 0
.sp 1P
.LP
	The\ CCITT,
.sp 1P
.RT
.sp 1P
.LP
\fIconsidering\fR 
.sp 9p
.RT
.PP
(a)
that 
signalling conditions in the international telex
service
require accurate definition of the signals to be used on
international telex circuits in putting through, supervising, clearing, and
charging for international telex calls;
.PP
(b) 
that these signals must be such as to take into account that there are 
some fairly important differences in make\(hyup between the telex 
networks of different countries. In some countries, selection is done by
dialling, in others by means of start\(hystop signals. Some networks use direct
selection while others use register translators. Between some networks,
subscriber automatic selection is used whilst, in relations with other
networks, semi\(hy automatic or manual selection is still being used;
.PP
(c) 
that hence it has not been possible to lay down uniform
signals for all international telex relations. While, for certain signals, 
it has been possible to lay down rules valid for all relations, for others 
the 
choice has been left between two types of signals known as type\ A and 
type\ B, within each type it has sometimes been necessary to provide alternative 
forms for certain signals. The signals with regard to which there is a 
choice are 
described in Tables\ 1a/U.1, 1b/U.1 and\ 2/U.1 below;
.PP
(d) 
that it is intended that the signalling with which this
Recommendation deals should apply as far as possible when telex circuits 
make use of transmission devices having multiplex and signal regeneration 
facilities. In the case of operation over error\(hycorrected radio circuits,
Recommendaton\ U.20 lays down the conditions for adapting the signalling 
defined in Recommendation\ U.1. In the case of operation over channels 
using synchronous multiplex equipment in accordance with Recommendation\ 
R.44, Recommendation\ U.24 lays down the conditions for adapting the signalling 
defined in 
Recommendation\ U.1. When the signals defined in Recommendation\ U.1 are
transmitted via code and speed dependent systems in accordance with
Recommendation\ R.101, 
Recommendation\ U.25 lays down the permitted variations to the signals 
defined in Recommendation\ U.1. When the signals defined in 
Recommendation\ U.1 are transmitted via regenerative repeaters the signals
received from these transmission devices may lie outside the tolerances 
stated in this Recommendation, and the permitted variations are shown in 
Recommendation\ U.5;
.PP
(e) 
that additional signalling standards (types\ C and\ D) have been defined 
for use on international telex networks. Details of these methods of signalling 
are laid down in Recommendations\ U.11 and\ U.12; 
.PP
(f
) 
that it has been necessary to define the rules for interworking Type\ D 
signalling with Types\ A, B and C signalling in 
Recommendation\ U.15,
.bp
.sp 1P
.LP
\fIunanimously declares the view\fR 
.sp 9p
.RT
.sp 2P
.LP
\fB1\fR 	\fBSignalling types\fR 
.sp 1P
.RT
.PP
1.1
In general, as far as signalling over international telex
circuits is concerned, the outgoing country should conform to the signalling
requirements of the incoming country. Nevertheless, when in the case of 
fully automatic service this requirement would entail considerable difficulty, 
alternative arrangements may be adopted by agreement between the two
Administrations concerned.
.sp 9p
.RT
.PP
1.2
The signals shown in \(sc\(sc\ 2 to 10\ below shall be employed under
the conditions indicated.
.PP
\fINote\fR \ \(em\ Both the forward and backward path signals are described 
at the moment of their emission on the international circuit. 
.PP
1.3
The characteristics of the signals defined in \(sc\(sc\ 4, 5, 7
and 10 below can be divided into two basic groups \(em\ type\ A and type\ 
B\ \(em as 
given in Tables\ 1a/U.1, 1b/U.1 and\ 2/U.1.
.ce
\fBH.T. [T1.1]\fR 
.ce
TABLE\ 1a/U.1
.ce
\fBInternational telex circuits terminated on
.ce
\fBdistant automatic switching equipment\fR 
.ce
\fBwith semi\(hyautomatic working to subscribers\fR 
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(60p) | cw(84p) | cw(84p) .
Signal	Type A	Type B  
_
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Call\(hyconfirmation
(see \(sc\(sc\ 4 and\ 5.1 of the text)
 }	Permanent stop polarity	 {
25\(hyms pulse of stop polarity (between 17.5 and 35\ ms)
 }
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Proceed\(hyto\(hyselect
(see \(sc\ 5.1 of the text)
 }	Teleprinter signal(s)	 {
25\(hyms pulse of stop polarity (between 17.5 and 35\ ms)
 }
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Selection
(see \(sc\ 6 of the text)
 }	Teleprinter signal(s)	 {
Dial pulses, or teleprinter signals
 }
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Call\(hyconnected
(see \(sc\ 7 of the text)
 }	 {
Teleprinter signals
\fINote\fR
\ \(em\ The teleprinter signals may be preceded by a\ 150\(hyms
(\(+-\ 11\ ms) pulse of start polarity
 }	 {
Stop polarity for at least two seconds
 }
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Busy
(see \(sc\ 10.1 of the text)
 }	 {
Teleprinter signals followed by clearing signal
 }	 {
\fBii\fR
i)
165\(hy260\(hyms pulse of stop polarity followed by start polarity
for 1500\ ms (tolerance \(+-\ 30%) (see\ Note)
\fBi\fR
ii)
165\(hy260\(hyms pulse of stop polarity followed by
teleprinter signals and start polarity for 1500\ ms
(tolerance \(+-\ 20%) (see\ Note)
 }
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Out\(hyof\(hyorder, number changed, number unobtainable, etc.
(see \(sc\ 10.1 of the text)
 }	 {
Clearing signal normally preceded by teleprinter
signals
 }	 {
\fBii\fR
i)
Permanent start polarity
\fBi\fR
ii)
165\(hy260\(hyms pulse of stop polarity followed by start polarity
for 1500\ ms (tolerance \(+-\ 30%) (see\ Note)
iii)
165\(hy260\(hyms pulse of stop polarity followed by
teleprinter signals and start polarity for 1500\ ms
(tolerance \(+-\ 20%) (see\ Note)
 }
.TE
.LP

\fINote\fR
\ \(em\ This sequence of signals may be repeated until a clearing signal
is sent over the forward signalling path. However, with transmission systems
having significant propagation delay, e.g. satellite or multiplex systems,
it may be preferable to prevent such repetitions.
.nr PS 9
.RT
.LP
.ad r
\fBTableau 1a/U.1 [T1.1], p.\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.ce
\fBH.T. [T2.1]\fR 
.ce
TABLE\ 1b/U.1
.ce
\fBInternational telex circuits terminated on
.ce
\fBdistant automatic switching equipment\fR 
.ce
\fBwith fully automatic working between subscribers\fR 
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(60p) | cw(84p) | cw(84p) .
Signal	Type A	Type B  
_
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Call\(hyconfirmation (see \(sc\(sc\ 4 and\ 5 of the text)
 }	Permanent stop polarity	 {
25\(hyms pulse of stop polarity (between 17.5 and 35\ ms)
 }
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Proceed\(hyto\(hyselect (see \(sc\ 5.1 of the text)
 }	 {
40\(hyms (\(+-\ 8\ ms) pulse of start polarity
 }	 {
25\(hyms pulse of stop polarity (between 17.5 and 35\ ms)
 }
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Selection (see \(sc\ 6 of the text)
 }	Teleprinter signals	 {
Dial pulses, or teleprinter signals
 }
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Call\(hyconnected (see \(sc\ 7 of the text)
 }	 {
150\(hyms (\(+-\ 11\ ms) pulse of start polarity followed by stop
polarity for at least 2\ seconds and possibly by teleprinter signals
 }	 {
Stop polarity for at least 2\ seconds
 }
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Call redirection signal (see Rec.\ U.41,\ \(sc\ 2)
 }	 {
Call connected signal as defined above, followed by the
call redirection sequence defined in Rec.\ U.41,\ \(sc\ 2
 }	 {
Call connected signal as defined above, followed by the
call redirection sequence defined in Rec.\ U.41,\ \(sc\ 2
 }
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Busy\fR
(see \(sc\ 10.1 of the text)
 }	 {
Teleprinter signals followed by clearing signal
 }	 {
\fBii\fR
i)
165\(hy260\(hyms pulse of stop polarity followed by start polarity
for 1500\ ms (tolerance \(+-\ 30%) (see\ Note\ 1)
\fBi\fR
ii)
165\(hy260\(hyms pulse of stop polarity followed by
teleprinter signals and start polarity for 1500\ ms
(tolerance \(+-\ 20%) (see\ Note\ 1)
 }
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Out\(hyof\(hyorder, number changed, number unobtainable, etc.
(see \(sc\ 10.1 of the text)
 }	 {
Clearing signal normally preceded by teleprinter
signals
 }	 {
\fBii\fR
i)
Permanent start polarity (see\ Note\ 2)
\fBi\fR
ii)
165\(hy260\(hyms pulse of stop polarity followed by start polarity
for 1500\ ms (tolerance \(+-\ 30%) (see\ Note\ 1)
iii)
165\(hy260\(hyms pulse of stop polarity followed by
teleprinter signals and start polarity for 1500\ ms
(tolerance \(+-\ 20%) (see\ Note\ 1)
 }
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Changed address interception signal (see Rec.\ U.41,\ \(sc\ 1)
 }	 {
Call connected signal as defined above, followed by the
changed address interception sequence defined in Rec.\ U.41,\ \(sc\ 1
 }	 {
Call connected signal as defined above, followed by the
changed address interception sequence defined in
Rec.\ U.41,\ \(sc\ 1
 }
.TE
.LP

\fINote\ 1\fR
\ \(em\ This sequence of signals may be repeated until a clearing
signal is sent over the forward signalling path. However, with transmission
systems having significant propagation delay, e.g. satellite or multiplex
systems, it may be preferable to prevent such repetitions.
.LP
\fINote\ 2\fR
\ \(em\ The use of this signal should be avoided if possible.
.nr PS 9
.RT
.ad r
\fBTableau 1b/U.1 [T2.1], p.\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.ce
\fBH.T. [T3.1]\fR 
.ce
TABLE\ 2/U.1
.ce
\fBInternational telex circuits terminated on\fR 
.ce
\fBdistant manual switching equipment\fR 
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(60p) | cw(84p) | cw(84p) .
Signal	Type A	Type B  
_
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Call\(hyconfirmation (see \(sc\ 4 of the text)
 }	Permanent stop polarity	 {
25\(hyms pulse stop polarity (between 17.5 and 35\ ms)
 }
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Proceed\(hyto\(hytransmit (see \(sc\ 5.2 of the text)
 }	Teleprinter signals	 {
Stop polarity followed by teleprinter signals
 }
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Call\(hyconnected (see \(sc\ 7 of the text)
 }	Teleprinter signals	Teleprinter signals
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Busy, out\(hyof\(hyorder,\fR
number changed and number unobtainable (see \(sc\ 10.1 of the\ text)
 }	Teleprinter signals	Teleprinter signals
_
.TE
.nr PS 9
.RT
.ad r
\fBTableau 2/U.1 [T3.1], p.\fR 
.sp 1P
.RT
.ad b
.RT
.sp 2P
.LP
\fB2\fR 	\fBFree line condition\fR 
.sp 1P
.RT
.PP
2.1
The 
\fIfree line\fR  | is characterized by a permanent signal
corresponding to the start impulse in accordance with International Telegraph 
Alphabet\ No.\ 2 (ITA2) (see the Recommendation cited in [1]) on the forward 
and backward signalling paths. 
.sp 9p
.RT
.sp 2P
.LP
\fB3\fR 	\fBCall\fR 
.sp 1P
.RT
.PP
3.1
The \fIcall\fR  | is characterized by an inversion to STOP polarity,
received on the forward signalling path. Future designs of equipment should 
not recognize such an inversion as a valid call signal, unless it has existed 
for a period of greater than\ 50 milliseconds. 
.sp 9p
.RT
.sp 2P
.LP
\fB4\fR 
	\fBCall\(hyconfirmation signal\fR 
.sp 1P
.RT
.PP
4.1
A 
\fIcall\(hyconfirmation\fR  | signal shall be returned over
the backward signalling path following the initiation of a call to prove the
continuity of the line and the response of the distant terminal equipment.
.sp 9p
.RT
.PP
4.2
The call\(hyconfirmation signal shall be returned by the receiving
end as quickly as possible and in any event with a delay not exceeding
150\ milliseconds after the arrival of the calling signal at the receiving
end.
.sp 2P
.LP
\fB5\fR 	\fBSignals preceding selection\fR 
.sp 1P
.RT
.sp 1P
.LP
5.1
	\fIProceed\(hyto\(hyselect signal\fR 
.sp 9p
.RT
.PP
5.1.1 
In the case of international telex circuits terminated on distant automatic 
switching equipment that cannot accept the selection information 
immediately (either after the reception of the calling signal or after the
sending of the call\(hyconfirmation signal), a distinct \fIproceed\(hyto\(hyselect\fR 
signal shall be returned over the backward signalling path after the call\(hyconfirmation 
signal, to indicate that the selection information may be 
transmitted.
.sp 9p
.RT
.PP
5.1.2
For type\ A signalling, the sending duration of the stop polarity, from 
the beginning of the call confirmation signal until the moment when the 
proceed\(hyto\(hyselect signal begins to be sent, should be at least
100\ milliseconds.
.bp
.PP
5.1.3
For type\ B signalling, the time interval between the end of the
call\(hyconfirmation signal pulse and the moment when the proceed\(hyto\(hyselect 
signal begins to be sent, during which the start polarity is sent, should 
be at least 100\ milliseconds. 
.PP
5.1.4
During the busy hour, for 99\ calls in 100, the delay in the
return by the receiving system of the proceed\(hyto\(hyselect signal must 
not exceed 3\ seconds after the reception of the calling signal. (In certain 
existing 
networks, this time may be 4\ seconds.)
.PP
5.1.5
If the automatic switching equipment at the receiving end can
receive the selection information immediately after the sending of the
call\(hyconfirmation signal, the call\(hyconfirmation signal shall constitute 
the 
proceed\(hyto\(hyselect signal.
.PP
5.1.6
If the automatic switching equipment at the receiving end can
receive the selection information at the time of receiving the call signal,
there shall be no proceed\(hyto\(hyselect signal.
.sp 2P
.LP
5.2
	\fIProceed\(hyto\(hytransmit signal\fR 
.sp 1P
.RT
.PP
5.2.1 
In the case of international telex circuits terminated on a
distant manual switchboard, a 
\fIproceed\(hyto\(hytransmit\fR signal
shall be
returned over the backward signalling path following the initiation of 
a call, to indicate that the teleprinter of the distant operator has been 
connected to the international circuit. 
.sp 9p
.RT
.sp 2P
.LP
\fB6\fR 	\fBSelection signals\fR 
.sp 1P
.RT
.PP
6.1
The 
selection signals
should be in conformity with
International Telegraph Alphabet\ No.\ 2 or dial signals in conformity with
Recommendation\ U.2.
.sp 9p
.RT
.PP
6.2
In the case of dial selection into a system employing letters in the national 
numbering scheme, figures only will be used on international 
circuits, because of the difficulty in transmitting signals other than 
figures from dials. 
.PP
6.3
In the case of selection into a keyboard selection system, the
\fIprepare\(hyfor\(hydigits\fR signal
will be combination\ No.\ 30 (figure\(hyshift).
.PP
6.4
In those cases where an 
\fIend\(hyof\(hyselection\fR signal
is
required, this signal shall be combination\ No.\ 26\ (\fB+\fR ), possibly 
followed by another combination characterizing the class of traffic in 
the incoming 
country.
.PP
6.5
In systems that use keyboard selection and that require an
end\(hyof\(hyselection signal, it is preferable that the subscriber's number 
consist of a uniform number of characters. 
.PP
6.6
To avoid undue occupation of lines and equipment, Administrations should 
take all reasonable steps to ensure that the transmission of selection 
signals over international circuits is completed without undue delay. In 
particular, where excessive delays are encountered, the incoming country
may cause the connection to be cleared. When selection signals are sent by a
subscriber, or by an operator, from country\ A towards a register in country\ 
B, country\ B may disconnect itself from the call if the time interval 
between two successive selection signals (either pulse trains or teleprinter 
characters) 
exceeds 5\ seconds.
.sp 2P
.LP
\fB7\fR 	\fBCall\(hyconnected signal\fR 
.sp 1P
.RT
.PP
7.1 
A 
\fIcall\(hyconnected\fR signal
shall be returned over
the backward signalling path to indicate that the call has been extended 
to the terminal of the called subscriber, which shall normally always be 
available to receive a call, as stated in Recommendation\ F.60\ [4],\ \(sc\(sc\ 
3.1.2 and\ 3.4.2.1, 
except in the cases mentioned in\ \(sc\ 10.1.1 below. In the case of fully 
automatic switching between subscribers, this signal will start the equipment 
for 
determining the charge for the call. For administrative purposes (accounting
between Administrations), the conventional start of the chargeable duration 
is fixed at 6\ \(+-\ 1\ seconds after the start of the call\(hyconnected 
signal (see 
Recommendation\ F.61\ [2]). For the same purposes, the end of the chargeable
duration will be between\ 300 and 1000\ milliseconds after the start of the
clearing signal.
.sp 9p
.RT
.PP
7.2
Switching systems not giving an automatic return of answer\(hyback
signals over the international telex circuits shall be arranged to be ready 
to respond to WRU\ signals (transmitted from the calling country) with 
a delay not exceeding two seconds from the beginning of the call\(hyconnected 
signal. To meet this requirement in the case of \fIin\(hylocal\fR working, 
the return of the 
call\(hyconnected signal has to be delayed until the moment when the teleprinter 
of the obtained subscriber has in effect been connected to line (see 
Recommendation\ S.9\ [3]).
.bp
.PP
7.3
If the incoming country automatically returns the obtained
subscriber's answerback, the interval between the start of the call\(hyconnected 
signal and the start of the answerback signals (or, if applicable, of other 
signal sequences, such as described in Recommendation\ U.41) should be 
at least two seconds to allow satisfactory reception of teleprinter signals 
by the 
calling subscriber. In order to restrict charging on unsatisfactory calls, 
the particular interval should be kept as short as possible and should 
not exceed 3\ seconds for new networks or 6\ seconds for existing networks. 
.PP
7.4
If the call has been routed via a transit centre the two\(hysecond
minimum period for the call\(hyconnected signal, which is transmitted by the
destination network, may have been reduced on signalling conversion and the
answerback signals may be received at the originating network after a minimum 
duration of 1050\ milliseconds. 
.PP
7.5
If the incoming country normally returns the obtained
subscriber's answerback code automatically, and the answerback transmission
fails to appear for some reason, the signal\ \fBDER\fR followed by the clearing
signal should be transmitted to the country of origin within 6\ seconds 
from the start of the call\(hyconnected signal. 
.PP
7.6
In the case of a call to a switchboard or service point the
call\(hyconnected signal shall be returned as soon as the call reaches the
terminal equipment even though it may be required to wait before being 
switched to the service position. 
.PP
7.7
If the answerback is preceded by a sequence of signals, such as
the RDI sequence defined in Recommendation\ U.41, this sequence should be
limited to not more than 20\ characters and it should be followed within
1100\ milliseconds by the answerback code.
.PP
7.8
If the answerback of the obtained subscriber is followed by a
sequence or sequences of signals, the interval between the end of the
answerback and the completion of the sequence (excluding the answerback of
the calling subscriber if taken automatically) should be as short as possible 
and should not exceed 4\ seconds. 
.PP
7.9
For future networks the sending of date, time and other signals
(excluding however WRU\ signals to the calling subscriber) that are additional 
to the obtained subscriber's answerback (either preceding or following 
it) 
should be avoided on international calls. In the case of call redirection, 
the service sequence RDI, as described in Recommendation\ U.41 shall precede 
the 
answerback of the connected subscriber.
.sp 2P
.LP
\fB8\fR 	\fBIdle circuit condition\fR 
.sp 1P
.RT
.PP
8.1
On an established connection, the 
\fIidle circuit\fR  | is
characterized by a permanent signal corresponding to the stop impulse, in
accordance with International Telegraph Alphabet\ No.\ 2, on the forward and
backward signalling paths.
.sp 9p
.RT
.sp 2P
.LP
\fB9\fR 	\fBClearing\fR 
.sp 1P
.RT
.sp 1P
.LP
9.1
	\fIClearing signal\fR 
.sp 9p
.RT
.PP
9.1.1 
The 
clearing signal
is characterized by a reversion
to the condition specified in \(sc\ 2.1 above on either signalling path 
maintained until the complete release of the circuit. 
.sp 9p
.RT
.PP
9.1.2
The supervisory equipment of the international connection shall
be arranged to interpret a signal of start polarity as a clearing signal
within\ 300 to 1000\ milliseconds.
.sp 2P
.LP
9.2
	\fIClear\(hyconfirmation signal\fR 
.sp 1P
.RT
.PP
9.2.1 
The 
clear\(hyconfirmation signal
is a reversion to the
condition specified in \(sc\ 2.1 above on the other signalling path in 
response to the clearing signal. When a clearing signal transmitted on 
an international 
circuit has reached the receiving end of that circuit the clear\(hyconfirmation
signal must be sent back in the other direction within\ 350 to 1500\ milliseconds 
after the initial start polarity begins. 
.sp 9p
.RT
.PP
9.2.2
The minimum period will be increased to 400\ milliseconds for
future systems.
.bp
.sp 2P
.LP
9.3
	\fIGuard delay\fR 
.sp 1P
.RT
.PP
9.3.1 
Guard arrangements at the ends of an international telex circuit should 
be such that the circuit cannot be used for a new call until the distant 
equipment is free to accept another call. 
.sp 9p
.RT
.PP
9.3.2
A guard delay of 1\ second will be maintained during which
incoming calls will not be accepted and a guard delay of 2\ seconds will be
maintained during which outgoing calls will not be offered, from the moment
when start polarity appears on both signalling paths. This start polarity 
shall be maintained throughout the guard period on both signalling paths 
of the 
international circuit.
.PP
Whilst maintaining the requirements of\ \(sc\ 9.3.1, guard delay
periods should nevertheless be generally kept to a minimum in order to 
maximize the use of the circuit. Therefore, where modern electronic switching 
equipment is used at both ends of a circuit, the above figures for incoming 
and outgoing guard delay periods may be reduced to\ 0.5 and\ 1 second respectively. 
.sp 2P
.LP
\fB10\fR 	\fBService signals\fR 
.sp 1P
.RT
.sp 1P
.LP
10.1
	\fISignals for ineffective calls\fR 
.sp 9p
.RT
.PP
10.1.1 
If a 
\fIbusy\fR , 
\fIout of order\fR , 
\fIabsent\fR 
\fIsubscriber/office closed\fR , 
\fInumber changed\fR , or 
\fInumber\fR 
\fIunobtainable\fR (i.e.\ not connected, service ceased or barred access)
condition is encountered in the distant network, this shall be indicated 
by the return of a signal to the calling end. This signal shall cause the 
connection to be cleared. 
.sp 9p
.RT
.PP
10.1.2
In printed service signal sequences the code expressions
mentioned in the Recommendation cited in\ [4] should be used. In this case, 
the signal should commence with the carriage\(hyreturn, line feed and letter\(hyshift 
signals, then the text of the code expression and ending with the carriage
return and line\(hyfeed signals and immediately followed by the clearing 
signal in all cases. Where additional information is transmitted, this 
should consist of four characters (\(*a,\ \(*b,\ \(*g,\ \(*d) and sent 
before the standard service signal at 
automatic speed. Such additional information shall be used to either
.LP
	a)
	indicate the TNIC of the network which has generated the
service signal, or
.LP
	b)
	indicate the reasons of the not\(hyready condition of the
called terminal, as shown in Table\ 4/U.1. The composition of the complete
service signal train should then be
\v'6p'
.sp 1P
.ce 1000
\(*a\ \(*b\ \(*g\ \(*d\ \(<-\ \(==\ \(da\ code
expression\ \(<-\ \(==
.ce 0
.sp 1P
.LP
.sp 1
where \(*a is always letter\(hyshift (\(da). The ability to indicate the 
international gateway exchange which has generated the service signal is 
left for further 
study.
.PP
It should be noted that some Administrations currently use their own interpretation 
of these additional information characters, for transmission over international 
circuits. The objective, however, should be always to ensure that the standard 
service signal is clearly presented to the calling subscriber or TAED with 
no reasonable possibility of confusion by the use of these 
characters and that, consequentially, the provisions of Recommendation\ 
U.40 can be unambiguously applied. 
.PP
10.1.3
Ineffective telex calls
should not be charged for. With
this in view printed service signal sequences returned on ineffective calls
should never be preceded by the call\(hyconnected signal except in the case
described in Recommendation\ U.41, \(sc\ 1.2. However, under faulty
conditions that can be detected only after the call has been put through, it
may be impossible to prevent the return of the call\(hyconnected signal and
subsequent charging of the call.
.PP
10.1.4
In new networks, ineffective telex calls to a subscriber whose
number has been changed will be signalled by the return of the call connected 
signal followed, after\ 2 seconds, by the NCH service sequence described 
in 
Recommendation\ U.41 and the clearing signal.
.sp 2P
.LP
10.2
	\fIWaiting signals\fR 
.sp 1P
.RT
.PP
10.2.1 
Should a call be routed to a point in the system where it is
required to wait before connection can be made to the requested service, a
\fIwaiting signal\fR (\fBMOM\fR ) should be sent back automatically in 
accordance with Table\ 3/U.1. 
.bp
.sp 9p
.RT
.ce
\fBH.T. [T4.1]\fR 
.ce
TABLE\ 3/U.1
.ce
\fBAccess to switchboards and service points
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(60p) | cw(84p) | cw(84p) .
Signal	Type A	Type B  
_
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Call\(hyconfirmation (see \(sc\(sc\ 4 and\ 5.1 of the text)
 }	Permanent stop polarity	 {
25\(hyms pulse of stop polarity (between 17.5 and 35\ ms)
 }
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Proceed\(hyto\(hyselect (see \(sc\ 5.1 of the text)
 }	 {
40\(hyms (\(+-\ 8\ ms) pulse of start polarity
 }	 {
25\(hyms pulse of stop polarity (between 17.5 and 35\ ms)
 }
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Selection (see \(sc\ 6 of the text)
 }	Teleprinter signals	 {
Dial pulses or teleprinter signals
 }
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Call\(hyconnected (see \(sc\ 7 of the text)
 }	 {
150\(hyms (\(+-\ 11\ ms) pulse start polarity followed by stop
polarity for a period between 2 and 8\ seconds
 }	 {
Stop polarity for a period between 2 and 8\ seconds
 }
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Waiting signals (see \(sc\ 10.2 of the text)
 }	 {
Teleprinter signals which may interrupt the stop polarity period
of the call\(hyconnected signal, in which case the initial period
of stop polarity should not be less than 2\ seconds
 }	 {
Teleprinter signals which may interrupt the
call\(hyconnected signal, in which case the initial period
of stop polarity should not be less than 2\ seconds
 }
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Service\(hyconnected (see \(sc\ 10.3 of the text)
 }	 {
Teleprinter signals indicating the identification of the
switchboard or service point
 }	 {
Teleprinter signals indicating the identification of the
switchboard or service point
 }
.T&
lw(60p) | lw(84p) | lw(84p) .
 {
Busy\fR
(see \(sc\ 10.1 of the text)
 }	 {
Teleprinter signals followed by clearing signal
 }	 {
\fBii\fR
i)
165\(hy260\(hyms pulse of stop polarity followed by start polarity
for 1500\ ms (tolerance \(+-\ 30%) (see\ Note)
\fBi\fR
ii)
165\(hy260\(hyms pulse of stop polarity followed by
teleprinter signals and then by start polarity for 1500\ ms
(tolerance \(+-\ 20%) (see\ Note)
 }
.TE
.LP

\fINote\fR
\ \(em\ This sequence of signals may be repeated until a clearing signal is
sent over the forward signalling path.
.nr PS 9
.RT
.ad r
\fBTableau 3/U.1 [T4.1], p.\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.sp 1
.PP
10.2.2
The \fIwaiting signal sequence\fR  | should include the carriage\(hyreturn, 
line\(hyfeed and letter\(hyshift signals followed by the characters\ \fBMOM\fR 
. It may be useful in some instances to include characters to indicate 
the date and/or time and also characters indicating the identity of the 
switchboard or service 
point returning the signals. In some existing systems, however, the waiting
signal sequence consists only of a group of characters indicating the date
and/or time.
.PP
10.2.3
The first character of the waiting signal sequence shall be
transmitted within 8\ seconds of the commencement of the call\(hyconnected
signal.
.PP
10.2.4
The \fBMOM\fR signal sequence shall be followed by stop polarity
until the service\(hyconnected signal is returned.
.PP
10.2.5
In some systems, however, arrangements are provided so that the
transmission by the caller of suitable teleprinter characters causes the 
return of a further sequence of the \fBMOM\fR signal. Where such a facility 
is provided 
attention is drawn to the need for the Administrations returning the signal 
to make arrangements to ensure that the signal sequence can be correctly 
received without mutilation in the calling system. For this purpose it 
is acceptable to include one or two letter\(hyshift signals at the beginning 
of the \fBMOM\fR \ signal 
sequence.
.PP
10.2.6
It is desirable that when connection is established to the
requested service the service\(hyconnected signal should be returned as 
quickly as possible. 
.PP
10.2.7
The equipment must be arranged so that a caller in the waiting
condition can be released.
.bp
.sp 2P
.LP
10.3
	\fIService\(hyconnected signal\fR 
.sp 1P
.RT
.PP
10.3.1
A 
\fIservice\(hyconnected\fR signal
shall be returned
over the backward signalling path to indicate that the call has been extended 
to the teleprinter, or equivalent, of the requested service point. This 
signal may comprise the answerback code of the teleprinter or a group of 
teleprinter characters identifying the service point or switchboard position. 
The 
service\(hyconnected signal may also include characters indicating date and/or
time.
.sp 9p
.RT
.PP
10.3.2
Where waiting signals are not provided the first character of the service\(hyconnected 
signal shall be returned within 8\ seconds of the commencement of the call\(hyconnected 
signal. 
.sp 2P
.LP
10.4
	\fIBackward busying signal\fR 
.sp 1P
.RT
.PP
10.4.1 
To facilitate routine tests of the switching equipment connected at the 
incoming end of an international telex circuit, a 
backward busying   signal
might be sent on the return signalling channel to show, at the other end, 
that the circuit is occupied. 
.sp 9p
.RT
.PP
10.4.2
With fully\(hyautomatic operation, on one\(hyway circuits as well as on 
both\(hyway circuits, the signal would take the form of permanent stop 
polarity 
for not more than 5\ minutes.
.PP
10.4.3
In semi\(hyautomatic working, the signal would be either permanent
start polarity, or permanent stop polarity, lasting not more than 5\ minutes;
the particular polarity chosen would be that requested by the outgoing
country.
.PP
10.4.4
If the outgoing equipment is designed to block the outgoing end of the 
circuit in the busy position after receipt of the permanent stop polarity, 
stop polarity would be used for preference. In some instances, use of stop 
polarity could give rise to difficulties. It might, for example, cause
a call signal to appear in the outgoing manual switching equipment.
In such circumstances, recourse will have to be had to permanent start
polarity.
.PP
10.4.5
As to tests made at the outgoing end of one\(hyway circuits, there
will be no call for a forward busying signal. The blocking of these circuits 
is locally done, on the outgoing side. 
.sp 2P
.LP
10.5
	\fIRetest signal\fR 
.sp 1P
.RT
.PP
10.5.1 
When the call\(hyconfirmation signal is not received over
the backward signalling path within\ 5 to\ 10 seconds from the start of the
calling signal, Administrations may apply a 
\fIretest signal\fR , which
automatically provides for the test of the circuit in such a way that the
international circuit is marked \fIunavailable\fR for outgoing traffic 
and may be restored to service if the fault disappears in the course of 
this test. 
.sp 9p
.RT
.PP
10.5.2
This signal transmitted over the forward signalling path should
be composed of:
.LP
	\(em
	a stop polarity period of 2\ seconds duration;
.LP
	\(em
	a start polarity period of 58\ (or\ 70) seconds, 4\ minutes
58\ seconds (or 5\ minutes 58\ seconds) or 29\ minutes 58\ seconds (or
35\ minutes 58\ seconds) duration.
.PP
10.5.3
For the fault to be regarded as cleared, the return of stop
polarity should occur during the stop period of a retest.
.PP
10.5.4
The circuit should be tested up to five times at nominal
intervals of 1.0\ minute or 1.2\ minutes and a check should be made to confirm
the receipt of a call\(hyconfirmation signal in response to each test. 
If a valid call\(hyconfirmation signal has not been received at the end 
of this first group of tests, the retest will continue with a further group 
of up to five tests at either 5.0/6.0\(hyminute or 30/36\(hyminute intervals. 
If 5.0\(hy or 6.0\(hyminute 
intervals are used and a valid call\(hyconfirmation signal has not been 
received at the end of this second group of tests, a further group of up 
to nominally 
five retests will be made at 30\(hy or 36\(hyminute intervals. An alarm 
will be given at an appropriate time. However, this retest procedure may 
be discontinued at any stage at the discretion of the outgoing Administration. 
.PP
10.5.5
If, however, during the above sequence of retests, a valid
call\(hyconfirmation signal is received, a clearing signal shall be transmitted 
in the place of the retest signal. Following a valid clear\(hyconfirmation 
signal, 
the incoming and the outgoing sides of the trunk circuit should not be 
returned to service until after expiry of the appropriate guard delay time. 
.PP
10.5.6
In order to cater for the possibility that a faulty circuit
may be seized at both ends, the automatic retest equipment should be arranged 
to allow an incoming call to be received during the start polarity period 
of 
the automatic retest signals. Administrations may, however, ignore such 
calls which occur during the incoming guard delay period. 
.bp
.PP
10.5.7
Where an exchange has knowledge of a transmission system failure, it is 
desirable that retest signals shall not be applied to the circuits 
affected.
.PP
10.5.8
In order to avoid simultaneous seizure of too many registers
at the distant centre, it is advisable that the retest signals, which might 
be sent simultaneously on various circuits subjected to this test, should 
be out of phase with one another. 
.PP
10.5.9
The intervals between the tests at the two ends of the trunk
route should be made different to be sure that successive retests do not
overlap at both ends. In general, the international/intercontinental transit
centre having the higher F.69\ [5] telex destination code should take the 
longer interval (i.e.\ 1.2, 6\ and\ 36 minutes). The tolerance on all above 
time 
intervals is \(+- | 0%. Nevertheless, when this requirement would entail
considerable difficulty, alternative arrangements may be adopted by agreement 
between the two Administrations concerned. 
.sp 2P
.LP
\fB11\fR 	\fBSetting\(hyup time\fR 
.sp 1P
.RT
.PP
11.1
The setting\(hyup time is defined as the period of time from the
initiation of the call on the international circuit until the initiation 
of the return of either the call\(hyconnected signal or a service signal 
indicating that the call has been unsuccessful, provided the selection 
signals have been 
transmitted at the maximum speed.
.sp 9p
.RT
.PP
11.2
For new networks, the objectives are as follows:
.LP
	\(em
	an average of 8\ seconds;
.LP
	\(em
	a maximum of 15\ seconds with a probability of\ 1% exceeding this value.
.sp 2P
.LP
\fB12\fR 	\fBBoth\(hyway working\fR 
.sp 1P
.RT
.PP
12.1
For both\(hyway cable circuits used in the fully automatic telex
service, the following action to minimize the incidence of head\(hyon collision 
is recommended: 
.sp 9p
.RT
.LP
	a)
	that inverse order testing, or a close approximation to it
by testing the route in small groups of circuits in fixed order,
always starting the search from the same initial positions,
should be adopted at opposite ends of a group of both\(hyway trunk circuits;
.LP
	b)
	that calls should be offered in such a way that each
circuit is treated once only for the minimum period of time
necessary to ascertain whether it is free or busy, and the
outgoing selectors should not have facilities for delayed
hunting.
.PP
12.2
The absence of the proceed\(hyto\(hyselect signal in type\ A signalling 
or the substitution of call signal for the call\(hyconfirmation signal 
in type\ B signalling will serve respectively to detect a head\(hyon collision 
when the group of circuits is totally occupied or very nearly totally occupied. 
The two calls will then be cleared down unless there are still free circuits 
in the 
route.
.sp 2P
.LP
\fB13\fR 	\fBTransit working\fR 
.sp 1P
.RT
.PP
13.1
It is noted that a number of Administrations use signalling
systems in accordance with Recommendation\ U.1 to provide international 
transit facilities. Whilst Recommendations\ U.11 and U.12 (types\ C and\ 
D) are intended for signalling between telex transit centres, nevertheless 
transit operation 
using type\ A or\ B signalling is feasible. To provide guidance for this 
specific application, the following general rules should apply. 
.sp 9p
.RT
.PP
13.2
Circuits provided for terminal calls will normally also be used
to carry transit calls.
.PP
13.3
The signalling conditions for transit calls between the
originating centre and the transit centre should, as far as possible, be the
same as those used for terminal calls to subscribers in the transit
network.
.PP
13.4
The signalling conditions for transit calls between the transit
centre and the terminating centre should, as far as possible, be the same as
those used for terminal calls to subscribers in the terminating network.
.bp
.PP
13.5
Any signal conversion to meet the requirements of the distant
terminal network is the responsibility of the transit centre.
.PP
13.6
An appropriate numbering scheme should either:
.LP
	a)
	include\ F.69 [5] destination codes on both terminal and
transit calls; or
.LP
	b)
	 use\ 0 as a standard transit prefix. Should\ 0 be precluded by the national 
numbering plan in the transit network, another 
digit might be agreed with the transit Administration.
.PP
Either way the originating centre should bar irregular routing, by discriminating 
the digits transmitted by calling subscribers. 
.PP
13.7
A single stage of selection in which all the selection digits are transmitted 
as a single block should be employed over the circuit from the 
outgoing centre to the transit centre.
.ce
\fBH.T. [T5.1]\fR 
.ce
TABLE\ 4/U.1
.ce
\fBStandardization of additional information characters\fR 
.ce
\fBin printed service signals\fR 
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(36p) | cw(60p) | cw(60p) .
Character	Indication of TNIC (Note)	 {
Indication of
not\(hyready condition
 }
_
.T&
cw(36p) | cw(60p) | cw(60p) .
\(*a	LS	LS
_
.T&
cw(36p) | cw(60p) | cw(60p) , c | c | ^ , c | c | ^ .
\(*b	Z	 {
See
Recommendation\ U.45
 }
.TE
.LP

\fINote\fR
\ \(em\ TNICs are listed in Annex\ A to
Recommendation\ F.69.
\(*g	First letter of TNIC
\(*d	 {
Second letter of TNIC
(or L/S)
.nr PS 9
.RT
.ad r
\fBTableau 4/U.1 [T5.1], p.\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.sp 6
.sp 2P
.LP
	\fBReferences\fR 
.sp 1P
.RT
.LP
[1]
	CCITT Recommendation \fIOperational provisions for the international\fR 
\fIpublic telegram service\fR , Rec.\ F.1, Division\ C, No.\ 8.
.LP
[2]
	CCITT Recommendation \fIThe chargeable duration of a telex call\fR ,
Rec.\ F.61.
.LP
[3]
	CCITT Recommendation \fISwitching equipment of start\(hystop apparatus\fR ,
Rec.\ S.9.
.LP
[4]
	CCITT Recommendation \fIOperational provisions for the international\fR 
\fItelex service\fR , Rec.\ F.60, \(sc\ 4.1.
.LP
[5]
	CCITT Recommendation \fIPlan for telex destination codes\fR ,
Rec.\ F.69.
.bp
.sp 2P
.LP
\fBRecommendation\ U.2\fR 
.RT
.sp 2P
.ce 1000
\fBSTANDARDIZATION\ OF\ \fR \fBDIALS\fR \ \fBAND\ \fR \fBDIAL\ PULSE\ GENERATORS\fR 
.EF '%	Fascicle\ VII.2\ \(em\ Rec.\ U.2''
.OF '''Fascicle\ VII.2\ \(em\ Rec.\ U.2	%'
.ce 0
.sp 1P
.ce 1000
\fBFOR\ THE\ INTERNATIONAL\ TELEX\ SERVICE\fR 
.ce 0
.sp 1P
.ce 1000
\fI(former CCIT Recommendation E.2, 1951;\fR 
.sp 9p
.RT
.ce 0
.sp 1P
.ce 1000
\fIamended at Arnhem, 1953 and Geneva, 1956)\fR 
.ce 0
.sp 1P
.LP
	The\ CCITT,
.sp 1P
.RT
.sp 1P
.LP
\fIconsidering\fR 
.sp 9p
.RT
.PP
(a)
that, when dials and dial pulse generators are used for
the process of automatic selection by subscribers to the international telex
network, it is advantageous to standardize as far as possible the
characteristics of such dials and dial pulse generators;
.PP
(b)
that the standardization of the 
dialling speed
and
lost motion periods of dials
presents no technical difficulty;
.PP
(c)
that, for the satisfactory working of certain automatic
systems, the time between successive pulse trains should not be less than
500\ milliseconds, but that experience has shown that the minimum time 
taken by an experienced operator to rotate the dial is of the order of 
300\ milliseconds;
.PP
(d)
that pulse ratios from 1.2:1 to\ 1.9:1 will ensure the
satisfactory working of existing automatic switching systems;
.PP
(e)
that these pulse ratios can be usefully adopted with a
view to simplifying direct calling between subscribers,
.sp 1P
.LP
\fIunanimously declares the view\fR 
.sp 9p
.RT
.PP
(1)
that in the international telex service, when dials or
dial pulse generators are used for the automatic selection of
subscribers:
.LP
	a)
	the dialling speed shall be standardized at 10\ pulses per
second with a tolerance of \(+- | 0%;
.LP
	b)
	the lost motion period of dials shall be not less than
200\ milliseconds nominal value;
.LP
	c)
	 the inter\(hydigit pause of dial pulse trains generated by dial pulse 
generators shall not be less than 600\ milliseconds; 
.LP
	(2)
	a)
	 that the pulse ratio must be between\ 1.2:1 and\ 1.9:1, the nominal ratio 
may be chosen as lying between\ 1.5:1 or\ 1.6:1; 
.LP
	b)
	that when the selection signals pass through a
regenerative repeater it may be advantageous to use a nominal ratio
of\ 1.5:1.
\v'6p'
.sp 2P
.LP
\fBRecommendation\ U.3\fR 
.RT
.sp 2P
.ce 1000
\fBARRANGEMENTS\ IN\ SWITCHING\ EQUIPMENT\fR 
.EF '%	Fascicle\ VII.2\ \(em\ Rec.\ U.3''
.OF '''Fascicle\ VII.2\ \(em\ Rec.\ U.3	%'
.ce 0
.sp 1P
.ce 1000
\fBTO\ MINIMIZE\ THE\ EFFECTS\ OF\ FALSE\ CALLING\ SIGNALS\fR 
.ce 0
.sp 1P
.ce 1000
\fI(former CCIT Recommendation E.3, Geneva, 1956)\fR 
.sp 9p
.RT
.ce 0
.sp 1P
.LP
	The\ CCITT,
.sp 1P
.RT
.sp 1P
.LP
\fIconsidering\fR 
.sp 9p
.RT
.PP
(a)
that transmission systems at present in use for
international telex trunks are liable to generate false calling
signals;
.PP
(b)
that such false calling signals can seize and engage
switching equipment, thereby reducing the grade of service. This is of
particular importance with systems in which common equipment normally used 
only to set up calls is seized by false calling signals; 
.PP
(c)
that the ill effects of false calling signals can be
minimized by delaying the operation of the calling relay at the termination 
of the international telex trunk circuit; 
.bp
.PP
(d)
that, however, when direct dial selection is employed over
an international trunk line, unless it is a manually selected circuit not
preceded by a stage of automatic selection, there is normally insufficient 
time available between successive digits to permit the use of slow operating 
relays;
.PP
(e)
that, nevertheless, Administrations may agree among one
another to use digit storage at the outgoing end of the circuit so that the
inter\(hytrain pause can be increased to permit the calling relays to be 
made slow to operate, 
.sp 1P
.LP
\fIunanimously declares the view\fR 
.sp 9p
.RT
.PP
(1)
that the design and maintenance of transmission systems
should be such as to reduce to a minimum the number and duration of false
calling signals. In this connection attention is drawn to the merits of
frequency\(hymodulated voice\(hyfrequency telegraph systems, particularly 
with long overhead lines; 
.PP
(2)
that, wherever possible, 
calling relays
on
international telex trunk circuits should have an operation lag of at least
100\ milliseconds. Administrations using circuits on lines prone to
long\(hyduration false calling signals may agree to use calling relays 
with longer operation lags. 
\v'1P'
.sp 2P
.LP
\fBRecommendation\ U.4\fR 
.RT
.sp 2P
.ce 1000
\fBEXCHANGE\ OF\ INFORMATION\ REGARDING\ SIGNALS\fR 
.EF '%	Fascicle\ VII.2\ \(em\ Rec.\ U.4''
.OF '''Fascicle\ VII.2\ \(em\ Rec.\ U.4	%'
.ce 0
.ce 1000
\fBDESTINED\ TO\ BE\ USED\ OVER\ INTERNATIONAL\ CIRCUITS\fR 
.ce 0
.sp 1P
.ce 1000
\fBCONCERNED\ WITH\ SWITCHED\ TELEPRINTER\ NETWORKS\fR 
.ce 0
.sp 1P
.ce 1000
\fI(former CCIT Recommendation E.4, Geneva, 1956;\fR 
.sp 9p
.RT
.ce 0
.sp 1P
.ce 1000
\fImodified at New Delhi, 1960 and Geneva, 1972)\fR 
.ce 0
.sp 1P
.LP
	The\ CCITT,
.sp 1P
.RT
.sp 1P
.LP
\fIconsidering\fR 
.sp 9p
.RT
.PP
(a)
that certain signals and certain characteristics of
signals used in the international telex service have been standardized in
Recommendation\ U.1;
.PP
(b)
that certain Administrations have introduced automatic telex transit switching 
facilities based upon the signalling standards shown in Recommendation\ 
U.1; 
.PP
(c)
that standardized signals for the European switched
network for the public telegram service (gentex network) are advocated in
Recommendation\ U.30;
.PP
(d)
that in view of the foregoing an exchange of information regarding the 
precise nature of the signals proposed to be used in the 
above\(hymentioned services by interested Administrations would be very useful;
.PP
(e)
that certain Administrations have already supplied such details regarding 
their telex services in a useful form (see supplements to the documents 
of the VIIIth Plenary Assembly of the CCIT, and subsequent Plenary 
Assemblies of the CCITT),
.sp 1P
.LP
\fIunanimously recommends\fR 
.sp 9p
.RT
.PP
that Administrations concerned in the international telex service
and gentex network be invited to supply to the CCITT time\(hysequence diagrams
showing in each case the signals at present transmitted or proposed to be
transmitted over the international circuit for incoming calls. The diagrams
should show not only the sequence and characteristics of the signals, but 
also the timing tolerances to be expected. The diagrams should show the 
signalling conditions applicable to transit as well as to terminal calls, 
including any 
conversion of the signals that are received from the destination
network.
.bp
.sp 2P
.LP
\fBRecommendation\ U.5\fR 
.RT
.sp 2P
.ce 1000
\fBREQUIREMENTS\ TO\ BE\ MET\ BY\ REGENERATIVE\ REPEATERS\fR 
.EF '%	Fascicle\ VII.2\ \(em\ Rec.\ U.5''
.OF '''Fascicle\ VII.2\ \(em\ Rec.\ U.5	%'
.ce 0
.sp 1P
.ce 1000
\fBIN\ INTERNATIONAL\ CONNECTIONS\fR 
.ce 0
.sp 1P
.ce 1000
\fI(former CCIT Recommendation E.5, Geneva, 1956;\fR 
.sp 9p
.RT
.ce 0
.sp 1P
.ce 1000
\fIamended at Geneva, 1964, Mar del Plata, 1968 and Geneva, 1976)\fR 
.ce 0
.sp 1P
.LP
	The\ CCITT,
.sp 1P
.RT
.sp 1P
.LP
\fIconsidering\fR 
.sp 9p
.RT
.PP
(a)
that it may be desirable to include regenerative repeaters in teleprinter 
switching networks; 
.PP
(b)
that the only signals other than teleprinter signals that
must be transmitted by a regenerative repeater are the clearing signal 
and the call\(hyconnected signal (see \(sc\ 3.1.3\ below), since all other 
signals can be 
bypassed;
.PP
(c)
that other signals may be transmitted by regenerative
repeaters,
.sp 1P
.LP
\fIunanimously declares the view\fR 
.sp 9p
.RT
.PP
\fB1\fR that, when regenerative repeaters are used in switching
systems, the clearing signal should be retransmitted with a minimum of 
delay. This delay is of course the same as for the transmission of teleprinter 
signals;
.sp 9p
.RT
.PP
\fB2\fR 
that to ensure the correct retransmission of the
call\(hyconnected signal (see \(sc\ 3.1.3\ below) and the clearing signal, the
regenerative repeater
must not automatically insert the stop element in either of these signals;
.sp 9p
.RT
.PP
\fB3\fR that for other signals that may pass through regenerative
repeaters, the tolerances at the origin and after retransmission through
the regenerative repeaters are as stated below.
.sp 9p
.RT
.PP
\fINote\fR \ \(em\ The characteristics and tolerances quoted are for the
signals at the origin. The tolerances at the input to the regenerative
repeater will depend on the degree of distortion in the transmission path 
from the origin to the input of the regenerative repeater. The tolerances 
at the 
output will depend on the normal tolerances for the regenerative
repeater.
.sp 2P
.LP
3.1
	\fIPulse signals\fR 
.sp 1P
.RT
.sp 1P
.LP
3.1.1
	\fICall\(hyconfirmation (proceed\(hyto\(hyselect) signal\fR . Type\ B
signalling
.sp 9p
.RT
.PP
A pulse of stop polarity of duration from\ 17.5 to 35\ milliseconds.
The nominal duration of the pulse after retransmission through the regenerative 
repeater should not be less than 20\ milliseconds or more than 
40\ milli
seconds.
.PP
\fINote\fR \ \(em\ This signal will be transmitted over only one international
trunk circuit and should thus normally pass through not more than one
regenerative repeater.
.RT
.sp 1P
.LP
3.1.2
	\fIDial selection signals\fR . Type\ B signalling
.sp 9p
.RT
.PP
These signals have been standardized (Recommendation\ U.2) at a dial speed 
of 10\ pulses per second\ \(+- | 0%, and a pulse ratio (start/stop) between 
the tolerance of\ 1.2:1 and\ 1.9:1 with a nominal ratio lying between\ 
1.5:1 
and\ 1.6:1. Such signals after retransmission through several regenerative
repeaters should not fall outside the tolerances stated above.
.RT
.sp 1P
.LP
3.1.3
	\fICall\(hyconnected signal\fR . Type\ A signalling
.sp 9p
.RT
.PP
A pulse of start polarity lasting 150\ \(+-\ 11\ milliseconds. The nominal 
duration of the pulse after retransmission through several regenerative 
repeaters should be within the limits of\ 140 to 160\ milliseconds.
.bp
.RT
.sp 1P
.LP
3.1.4
	\fIBusy signal\fR . Type\ B signalling
.sp 9p
.RT
.PP
Pulses of stop polarity lasting 165\(hy260\ milliseconds, separated by
intervals of start polarity lasting 1.5\ seconds \(+- | 0%. After retransmission 
through several regenerative repeaters neither the pulses nor the intervals
should be shortened by more than\ 10%.
.RT
.sp 2P
.LP
3.2
	\fISequence signals\fR \fI(involving a single change of\fR 
\fIpolarity)\fR 
.sp 1P
.RT
.sp 1P
.LP
3.2.1
	\fICalling signal\fR . Types\ A and\ B signalling
.sp 9p
.RT
.sp 1P
.LP
3.2.2
	\fICall\(hyconnected signal.\fR Type\ B signalling
.sp 9p
.RT
.PP
These signals (inversion from start to stop polarity) have no timing tolerances 
as such. It is, however, essential that they should be retransmitted by 
a regenerative repeater with a minimum of delay which in no case should 
exceed 20\ milliseconds.
\v'6p'
.RT
.sp 2P
.LP
\fBRecommendation\ U.6\fR 
.RT
.sp 2P
.ce 1000
\fBPREVENTION\ OF\ \fR \fBFRAUDULENT\ TRANSIT\ TRAFFIC\fR 
.EF '%	Fascicle\ VII.2\ \(em\ Rec.\ U.6''
.OF '''Fascicle\ VII.2\ \(em\ Rec.\ U.6	%'
.ce 0
.sp 1P
.ce 1000
\fBIN\ THE\ FULLY\ AUTOMATIC\ INTERNATIONAL\ TELEX\ SERVICE\fR 
.ce 0
.sp 1P
.ce 1000
\fI(New Delhi, 1960; amended at Geneva, 1964)\fR 
.sp 9p
.RT
.ce 0
.sp 1P
.LP
	The\ CCITT,
.sp 1P
.RT
.sp 1P
.LP
\fIconsidering\fR 
.sp 9p
.RT
.PP
(a) 
that, with fully automatic working in the international
telex service, the possibility of fraudulent routing by subscribers of
international calls involving tandem connection of international telex 
trunks might arise whenever subscribers are given automatic access to international 
telex trunk circuits that have, at their incoming ends, automatically switched 
access to other international telex trunk circuits; 
.PP
(b)
that, by the adoption of a systematic plan, such traffic can be barred 
without involving either expensive or elaborate equipment 
arrangements;
.PP
(c)
that, to be effective, such a plan would need to be adopted by all Administrations 
and recognized private operating agencies since failure to provide barring 
facilities on the traffic between two countries could open the way for 
irregular routings at the expense of a third country, 
.sp 1P
.LP
\fIunanimously declares the view\fR 
.sp 9p
.RT
.PP
(1)
that national telex systems shall be so arranged that the first digit of 
the selection signals transmitted over incoming international 
telex trunks will indicate whether an automatic transit call is concerned;
.PP
\fINote\fR \ \(em\ The use of a common first digit to indicate access to 
both international telex trunk circuits and manual switchboards leads to 
complication in the barring arrangements and should therefore be avoided 
as far as possible. 
.PP
(2)
that where an international telex trunk carrying fully
automatic traffic also carries traffic requiring access at the incoming 
end to outlets selected by means of the digit characterizing an automatic 
transit 
call, the country of origin will bar irregular routings by discriminating on
the digits transmitted by calling subscribers;
.PP
(3)
that where an international telex trunk carrying fully
automatic traffic does not carry traffic requiring access at the incoming 
end by means of the digit characterizing an automatic transit call, the 
incoming 
equipment shall be so arranged that the corresponding outlets are not
accessible and that when access is attempted the \fInumber unobtainable\fR 
signal is returned; 
.PP
(4)
that it is not admitted that two Administrations can agree
to omit the provision of barring facilities on traffic between their respective 
countries. However, where the incoming country has an existing network 
in which considerable difficulty would be experienced in barring in accordance 
with 
\(sc\ 3 above, the responsibility for barring may, by agreement, be assumed 
by the country of origin in the manner specified in\ \(sc\ 2. 
.bp
.sp 2P
.LP
\fBRecommendation\ U.7\fR 
.RT
.sp 2P
.sp 1P
.ce 1000
\fBNUMBERING\ SCHEMES\ FOR\ AUTOMATIC\ SWITCHING\ NETWORKS\fR 
.EF '%	Fascicle\ VII.2\ \(em\ Rec.\ U.7''
.OF '''Fascicle\ VII.2\ \(em\ Rec.\ U.7	%'
.ce 0
.sp 1P
.ce 1000
\fI(former CCIT Recommendation\ E.7, Geneva, 1956)\fR 
.sp 9p
.RT
.ce 0
.sp 1P
.LP
	The\ CCITT,
.sp 1P
.RT
.sp 1P
.LP
\fIconsidering\fR 
.sp 9p
.RT
.PP
that with fully automatic working between subscribers in the
international telex service it is desirable to envisage the
possibility:
.RT
.LP
	a)
	of routing traffic over the appropriate international trunk
route where more than one such route exists between two countries;
.LP
	b)
	of enabling the appropriate tariff to be determined
automatically (in the originating country), even if the
destination country is divided into several tariff zones,
.sp 1P
.LP
\fIunanimously declares the view\fR 
.sp 9p
.RT
.PP
(1)
that subscribers' national numbering plans should be
systematically arranged;
.PP
(2)
that, where more than one international trunk route exist
between two countries, the corresponding geographical division and hence the
appropriate point of entry should be identifiable by examination of the 
initial digits of the called subscriber's national number; 
.PP
(3)
that, where a multiple tariff scale exists, the different
tariff zones should be identifiable in the originating country by the initial 
digits of the called subscriber's national number; 
.PP
(4)
that the number of initial digits to be examined should be
limited, preferably to one, but in any case should not exceed two. When a
single digit provides the discrimination it will usually be the first digit,
but, where the subscribers' national numbers have a uniform initial digit
(usually\ 0) to permit discrimination on internal calls, the following 
(second) digit should be used. 
.PP
\fINote\fR \ \(em\ The attention of Administrations (and recognized private
operating agencies) is drawn to the considerable technical advantage that 
would result from the adoption of a single tariff between two countries. 
\v'6p'
.sp 2P
.LP
\fBRecommendation\ U.8\fR 
.RT
.sp 2P
.ce 1000
\fBHYPOTHETICAL\ REFERENCE\ CONNECTIONS\ FOR\ TELEX\fR 
.EF '%	Fascicle\ VII.2\ \(em\ Rec.\ U.8''
.OF '''Fascicle\ VII.2\ \(em\ Rec.\ U.8	%'
.ce 0
.sp 1P
.ce 1000
\fBAND\ GENTEX\ NETWORKS\fR 
.ce 0
.sp 1P
.ce 1000
\fI(Malaga\(hyTorremolinos, 1984)\fR 
.sp 9p
.RT
.ce 0
.sp 1P
.LP
	The\ CCITT,
.sp 1P
.RT
.sp 1P
.LP
\fIconsidering\fR 
.sp 9p
.RT
.PP
(a)
the operational provisions for the telex service and the
gentex network indicated in Recommendations\ F.60 and\ F.20;
.PP
(b)
the overall subscriber\(hyto\(hysubscriber performance
objectives;
.PP
(c)
the technical provisions in Recommendations\ R.57 and
R.58 concerning standard limits of transmission quality;
.PP
(d)
the need to standardize the signalling functions in
international/intercontinental transit exchanges;
.PP
(e)
the telex signalling specified in Recommendations\ U.1
(types\ A and\ B), U.11 (type\ C) and U.12 (type\ D);
.PP
(f
)
the level differences existing among the type\ A, B, C and\ D signalling 
functions, 
.bp
.sp 1P
.LP
\fIunanimously recommends\fR 
.sp 9p
.RT
.PP
the use of the hypothetical reference connections contained in
this Recommendation.
.RT
.sp 2P
.LP
\fB1\fR 	\fBGeneral\fR 
.sp 1P
.RT
.PP
1.1
The hypothetical reference connections set down in the present
Recommendation (see Figure\ 1/U.8) are intended for assessing the overall
subscriber\(hyto\(hysubscriber performance, for determining answerback 
return delay, signal transfer delay and other characteristics and setting\(hyup 
delays related to the hypothetical reference circuit. 
.sp 9p
.RT
.PP
1.2
The hypothetical reference connections concerning signalling
aspects set down in the present Recommendation (see Figure\ 2/U.8 and
Tables\ 1/U.8 to\ 3/U.8) are intended for specifying the transit environment
where the signalling functions should be considered.
.sp 2P
.LP
\fB2\fR 	\fBSignalling levels\fR 
.sp 1P
.RT
.PP
2.1
There will be two levels of signalling:
.sp 9p
.RT
.LP
	a)
	low level (type A or B);
.LP
	b)
	high level (type C or D). High level in this case indicates
the ability of the signalling system to signal additional
customer facilities and/or additional network facilities,
such as alternative routing.
.PP
2.2
Only high level signalling will be used in a transit connection
where alternative routing is possible because of the need to indicate changes 
of routing for accounting purposes. 
.PP
2.3
Routing may be on the basis of all high level, all low level or
one transition from low to high and then back from high to low if
required.
.PP
2.4
To restrict the call set\(hyup delay to a reasonable period,
.LP
	a)
	low level signalling types, because of their slower
compelled nature, will not be used for transit switching on
routes with long propagation delays, e.g.,\ satellite links;
.LP
	b)
	dial selection will not be used for transit
switching.
.PP
2.5
Only Recommendation\ F.69 codes will be used for routing
purposes in transit switching.
.PP
2.6
It is noted that as an interim solution, transit traffic is at
present being switched on a fixed routing basis using only low level
signalling.
.PP
2.7
Connections using ARQ radio circuits and signalling according
to Recommendation\ U.20 have been excluded.
.LP
.rs
.sp 16P
.ad r
Blanc
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 31P
.ad r
\fBFigure 1/U.8, (M), p. 6\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 16P
.ad r
\fBFigure 2/U.8, (M), p. 7\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.ce
\fBH.T. [T1.8]\fR 
.T&
lw(48p) | lw(24p) | lw(24p) | lw(48p) .
			
.T&
lw(48p) | lw(24p) | lw(24p) | lw(48p) .
 {

C
\ 3 
.

D
\ 4 
.

A
\ 5
B
B
\ 6
 }			
.T&
lw(48p) | lw(24p) | lw(24p) | lw(48p) .
 {

C
\ 7 
.

D
\ 8 
.

A
\ 9
C
B
10
 }			
.T&
lw(48p) | lw(24p) | lw(24p) | lw(48p) .
C 11  .   D 12  .   A 13			
.T&
lw(48p) | lw(24p) | lw(24p) | lw(48p) .
B 14 D C 15 .   D			
.TE
.nr PS 9
.RT
.ad r
\fBTableau 1/U.8 [T1.8], p. 8\fR 
.sp 1P
.RT
.ad b
.RT
.ce
\fBH.T. [T2.8]\fR 
.T&
lw(36p) | lw(12p) | lw(12p) | lw(12p) | lw(30p) | lw(36p) | lw(12p) | lw(12p) 
| lw(12p) | lw(30p) . 
									
.T&
lw(36p) | lw(12p) | lw(12p) | lw(12p) | lw(30p) | lw(36p) | lw(12p) | lw(12p) 
| lw(12p) | lw(30p) . 
C B	17	C B  \ 2							
.T&
lw(36p) | lw(12p) | lw(12p) | lw(12p) | lw(30p) | lw(36p) | lw(12p) | lw(12p) 
| lw(12p) | lw(30p) . 
	C	18		 {
C 
\ 3
A
.
D

.
19
C
.
D
\ 4
.
A
.
20
.
A
\ 5
 }					
.T&
lw(36p) | lw(12p) | lw(12p) | lw(12p) | lw(30p) | lw(36p) | lw(12p) | lw(12p) 
| lw(12p) | lw(30p) . 
D B	21	D B \ 6							
.T&
lw(36p) | lw(12p) | lw(12p) | lw(12p) | lw(30p) | lw(36p) | lw(12p) | lw(12p) 
| lw(12p) | lw(30p) . 
	C	22		 {
C
\ 7
.
D
.
23
.
D
\ 8
.
A
.
24
.
A
\ 9
 }					
.T&
lw(36p) | lw(12p) | lw(12p) | lw(12p) | lw(30p) | lw(36p) | lw(12p) | lw(12p) 
| lw(12p) | lw(30p) . 
C B	25	C B 10							
.T&
lw(36p) | lw(12p) | lw(12p) | lw(12p) | lw(30p) | lw(36p) | lw(12p) | lw(12p) 
| lw(12p) | lw(30p) . 
	C	26		 {
C
11
B
.
D

.
27
D
.
D\fR
X
12
.
A
.
28
.
A
13
 }					
.T&
lw(36p) | lw(12p) | lw(12p) | lw(12p) | lw(30p) | lw(36p) | lw(12p) | lw(12p) 
| lw(12p) | lw(30p) . 
	B	29		B 14					
.T&
lw(36p) | lw(12p) | lw(12p) | lw(12p) | lw(30p) | lw(36p) | lw(12p) | lw(12p) 
| lw(12p) | lw(30p) . 
D C	30	D C 15 . . D . 31 . . D							
.TE
.nr PS 9
.RT
.ad r
\fBTableau 2/U.8 [T2.8], p. 9\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.ce
\fBH.T. [T3.8]\fR 
Unable to convert table
.TE
.nr PS 9
.RT
.ad r
\fBTableau 3/U.8 [T3.8], p. 10 \ \ \ A L'ITALIENNE\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 2P
.LP
\fBRecommendation\ U.10\fR 
.RT
.sp 2P
.sp 1P
.ce 1000
\fBEQUIPMENT\ OF\ AN\ INTERNATIONAL\ TELEX\ POSITION\fR 
.EF '%	Fascicle\ VII.2\ \(em\ Rec.\ U.10''
.OF '''Fascicle\ VII.2\ \(em\ Rec.\ U.10	%'
.ce 0
.sp 1P
.ce 1000
\fI(former CCIT Recommendation F.60; modified at New Delhi, 1960)\fR 
.sp 9p
.RT
.ce 0
.sp 1P
.LP
	The\ CCITT,
.sp 1P
.RT
.sp 1P
.LP
\fIconsidering\fR 
.sp 9p
.RT
.PP
that an 
international telex position
that is a manual
position in an international telex exchange and is used to set up international 
telex calls should be so equipped as to permit satisfactory operation in 
conformity with Recommendation\ F.60 [1],
.RT
.sp 1P
.LP
\fIunanimously declares the following view\fR 
.sp 9p
.RT
.PP
(1)
An international telex position must be equipped in such a way as to receive 
the clearing signal from both sides. 
.PP
(2)
It should not be possible to recall the operator of that position by a 
signal to an established connection, except if 
Recommendation\ U.21 is applied.
.PP
(3)
Precaution must be taken that, in the event of the
operator of the international telex position's delaying to remove the plug 
on reception of the clearing signals, a new call from a subscriber on one 
network cannot pass to the other network. 
.PP
(4)
When the call has been established, the answer\(hyback
signals of equipment used at the intermediate telex positions must not 
be sent to line when figure\(hyshift\ D is received. 
.PP
(5)
The international telex position must be provided with
equipment to determine the chargeable time of calls controlled by these
positions, this timing equipment to be brought into operation in accordance
with the Recommendation cited in\ [2] but to be stopped on receipt of the 
first clearing signal. 
.sp 2P
.LP
	\fBReferences\fR 
.sp 1P
.RT
.LP
[1]
	CCITT Recommendation \fIOperational provisions for the international\fR 
\fItelex service\fR , Rec.\ F.60.
.LP
[2]
	\fIIbid.\fR , \(sc\ 3.3.
.LP
.rs
.sp 20P
.ad r
Blanc
.ad b
.RT
.LP
.bp
.LP
\fBMONTAGE: PAGE 24 = PAGE BLANCHE\fR 
.sp 1P
.RT
.LP
.bp