PART I
Recommendations Q.120 to Q.139
SPECIFICATIONS OF SIGNALLING
SYSTEM No. 4
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SIGNALLING SYSTEM No. 4
CHAPTER I
DEFINITION AND FUNCTION OF SIGNALS
Recommendation Q.120
1. DEFINITION AND FUNCTION OF SIGNALS
1.1 seizing signal (sent in the forward direction)
This signal is transmitted at the beginning of a call to ini-
tiate circuit operation at the incoming end of an international
circuit.
The seizing signal can also perform switching functions and
two different types of seizing signal are provided for this pur-
pose, viz:
a) the terminal seizing signal, which can be used
at the incoming international exchange, to seize equipment used
exclusively for switching the call to the national network of the
incoming country;
b) the transit seizing signal, wich can be used in
the exchange at the incoming end of the international circuit to
seize equipment used exclusively for switching the call to another
international exchange.
1.2 proceed-to-send signal (sent in the backward direction)
This signal is sent from the incoming end of an international
circuit, following the receipt of a seizing signal, to indicate
that the equipment is ready to receive the numerical signals.
In System No. 4 two different proceed-to-send signals are pro-
vided:
For definitions, see Recommendations Q.10/E.160 and Q.104.
a) the terminal proceed-to-send signal, used to
invite the sending of the language digit (or the discriminating
digit plus the national (significant) number ;
b) the transit proceed-to-send signal, used to
invite the sending of only those numerical signals (beginning with
the first digit of the country code) necessary for routing the call
through the international transit exchange towards the incoming
international exchange or to another international transit
exchange.
1.3 numerical signal (sent in the forward direction)
This signal provides an element of information necessary to
effect the switching of the call in the desired direction. There is
always a succession of numerical signals sent.
1.4 end-of-pulsing signal, also called for System No. 4
code 15 (sent in the forward direction)
This numerical type signal is sent from the international out-
going exchange to show that there are no more numerical signals to
follow. In semi-automatic working, this signal is always sent. In
automatic working, this signal may be sent, viz., when, in the out-
going international exchange, it is known that there are no more
digits to follow.
1.5 number-received signal (sent in the backward direction)
1.5.1 This signal is sent from the incoming international
exchange to the outgoing international exchange when the incoming
register has recognized that all the digits required for routing
the call to the called subscriber have been received.
Purpose of the signal
1.5.2 In the semi-automatic working, the number-received sig-
nal may be used to inform the outgoing operator that the interna-
tional switching operations have been completed.
1.5.3 In automatic working, this signal is essential to show
the outgoing register at the outgoing international exchange that
it can release, and to set up speech conditions at this exchange.
Hence, it is desirable that the signal be sent as soon as possible.
Generation of the signal
1.5.4 In semi-automatic working, the incoming register (or
associated equipment) after reception of the end-of-pulsing signal
acknowledges this numerical signal with an x and then sends back
the number-received signals.
See definition in Recommendation Q.10/E.160.
1.5.5 In automatic working, the incoming register (or associated
equipment) recognizes that all the digits of a national (signifi-
cant) number have been received :
1.5.5.1 by the receipt of the end-of-pulsing signal; or
1.5.5.2 a) by checking the number of digits
received, in countries where the national (significant) number is
always made up of the same number of digits; or
b) in countries where this is not so:
i) by the receipt of the maximum number of digits,
used in the numbering plan of the country; or
_________________________
See Recommendation Q.180 for interworking between
Systems No. 4 and No.5, Q.232 for interworking between
Systems No. 4 and No. 5 | fIbis , Q.261 for interwork-
ing between Systems No. 4 and No. 6, Q.381 for inter-
working between Systems No. 4 and R2, Q.382 for inter-
working between Systems No. 5 and R2, Q.383 for inter-
working between Systems No. 5 | fIbis and R2 and Q.388
for interworking between Systems No. 6 and R2.
ii) by analyzing the first digits in the national
(significant) number to decide how many digits there are in the
subscribers' numbers in the particular national numbering zone; or
iii) by using a national end-of-selection or
national "electrical" ringing-tone signal; or
iv) exceptionally, by observing that 4 to 10 (for
new equipment 4 to 6) seconds have elapsed since the last digit was
received, and that no fresh information has been received; in such
circumstances, retransmission to the national network of the last
digit received must be prevented until the end of the waiting
period which causes the number-received signal to be sent over the
international circuit. In this way, it is ensured that no national
answer signal can arrive before the number-received signal has been
sent.
1.6 busy-flash signal (sent in the backward direction)
This signal is sent to the outgoing international exchange to
show that either the route or the called subscriber is busy. The
conditions of use of this signal are as follows:
a) An international transit exchange must send this
signal to indicate that there is congestion at that exchange or on
the appropriate outgoing routes.
b) An incoming international exchange must send
this signal if there is congestion at that exchange or on the out-
going routes directly connected to it, but sending the signal is
optional when there is congestion beyond that exchange (when there
is congestion at a point in the national network of the incoming
country or when the called subscriber's line is busy). This signal
is optional because there are several countries that do not send it
from their national networks.
Note - The receipt of the busy-flash signal at the outgoing
exchange will cause:
- an appropriate indication to be given to the out-
going operator or to the calling subscriber; and
- in automatic working, the sending of the
clear-forward by the outgoing exchange to release the international
connection (except when otherwise arranged, for example, in the
case of observations on circuits).
1.7 answer signal (sent in the backward direction)
This signal is sent to the outgoing international exchange to
show that the called party has answered the call
In semi-automatic working, the signal has a supervisory func-
tion.
In automatic working, it is used:
- to start metering the charge to the calling sub-
scriber,
- to start the measurement of call duration for
international accounting purposes.
1.8 clear-back signal (sent in the backward direction)
This is sent to the outgoing international exchange to indi-
cate that the called party has cleared. In the semi-automatic ser-
vice, it performs a supervisory function. It must not permanently
open the speech path at the outgoing international exchange.
In automatic working, arrangements must be made to clear the
international connection, stop the charging and stop the measure-
ments of call duration if, between one and two minutes after
receipt of the clear-back signal, the calling subscriber has not
cleared. Clearing of the international connection should preferably
be controlled from the point where the charging of the calling sub-
scriber is carried out.
Notes on the answer and clear-back signals
1.8.1 Note 1 - In general, the sequence of answer and
clear-back signals that will be sent when the called subscriber
depresses and releases the switch-hook of his telephone will not
always be able to follow the frequency of this operation of the
switch-hook, but correct indication of the final position of the
switch-hook must always be given:
- to the outgoing international operator in
semi-automatic operation;
- to the outgoing international equipment in
automatic operaton.
1.8.2 Note 2 - The "call party" referred to in the defini-
tions of the answer and clear-back signals may be:
- the called subscriber;
_________________________
See Recommendation Q.27 for the action to be taken to
ensure that answer signals both national and interna-
tional, are transmitted as quickly aspossible.
- in semi-automatic working, the operator who puts
the call through in her own country and who sends an answer signal
when she answers the call.
1.8.3 Note 3 - The following is a detailed description of the
various possible circumstances in which the answer and clear-back
signals are sent.
A. Called subscriber obtained automatically by the inter-
national outgoing operator
The answer and clear-back signals are sent every time the
called subscriber answer or clears.
B. Called subscriber not obtained automatically by the
international outgoing operator
a) Only one operator involved in the incoming
country, without through-supervision via her position - (This
operator can be an incoming or a delay operator or a manual
exchange operator obtained automatically from the outgoing interna-
tional exchange.)
The answer signal is sent when the operator enters the cir-
cuit.
The clear-back signal is sent when the operator clears the
connection.
b) Only one operator involved in the incoming coun-
try, with through-supervision via her position - (The operator can
be the same as for a) above.)
Through-supervision can be effected:
- via the cord circuits, the incoming operator
intervening to clear down the connection at the end of the call;
- via cordless positions, in which case the connec-
tion is released automatically without the intervention of an
operator when the called subscriber clears and when the outgoing
operator causes the clear-forward signal to be sent.
The answer signal is sent when the operator enters the cir-
cuit.
A clear-back signal is sent when the operator goes out of cir-
cuit. This can happen, for example, when the operator hears the
ringing tone but does not wait for the called subscriber to reply.
A second answer signal is sent when the called subscriber
answers or when the incoming operator again enters the circuit.
The clear-back signal is also sent when the called subscriber
clears or when the incoming operator, by mistake, clears the con-
nection before the called subscriber has cleared.
The same signal (answer signal or clear-back signal) must not
be sent twice in succession.
c) Two operators involved in the incoming country .
These can be:
- an incoming or a delay operator at the interna-
tional exchange; and
- an operator at a national manual exchange.
c.1) There is no through-supervision via the opera-
tors' positions at the international exchange. The answer and
clear-back signals are sent as described in a) above.
c.2) The international operator's position is nor-
mally able to provide through-supervision. There are still two
cases to consider:
c.2.1) If the whole of the national chain, includ-
ing the operator's positions, gives through-supervision from the
called subscriber, the operating conditions can be as described in
b) above. An operator intervenes to send an answer signal: her
withdrawal causes the sending of a clear-back signal, an answer
signal is sent when the called subscriber answers, and a clear-back
signal is sent when the called subscriber clears. If an operator
clears down the connection in error, before the called party
clears, a clear-back signal is sent.
c.2.2) If the whole of the national chain does not
give through-supervision from the called subscriber, supervision is
extended from the point at which through-supervision cases.
In a), b) and c) above, it is recommended that the incoming or
the delay operator should have facilities to recall the outgoing
operator by sending a succession of clear-back and answer signals,
by means of a special key, for example.
If automatic service requirements necessitate the action
described under C below, it will inevitably follow that in
semi-automatic working correct supervision cannot be given, so that
the sequence of answer and clear-back signals described above can-
not be guaranteed.
C. Automatic calls
When direct access by a subscriber to an operator's position
in the incoming country cannot be barred, it is essential, to avoid
mistakes in charging, not to give the answer signal at the moment
this operator replies. Arrangements must be made to ensure that
the answer signal is sent when the called subscriber, or paid spe-
cial service, answers. The answer signal is sent:
- either by an operator (using a key); or
- automatically, by through-supervision.
1.9 clear-forward signal (sent in the forward direction)
1.9.1 This signal is sent in the forward direction at the end
of a call when:
a) in semi-automatic working, the operator at the
outgoing international exchange withdraws her plug from the jack,
or when an equivalent operation is performed;
b) in automatic working, when the calling sub-
scriber hangs up or otherwise clears (as in the case of a
subscriber's installation with extension telephones).
In automatic working, this signal is also sent after receipt
of a busy-flash signal by the outgoing international exchange, and
when there is forced release of the connection; see SS 4.3.1
and 4.3.2 in Recommendation Q.118 and Recommendation Q.131.
In semi-automatic working there may be forced release in the
case of S 4.3.1 of Recommendation Q.118.
1.9.2 At the end of the clear-forward signal, all switching
units held on the call must release at the outgoing, incoming and
transit international exchanges. (The clear-forward signal must
therefore be recognized at an international transit exchange.) Each
international circuit, however, is guarded against subsequent
seizure until the release-guard signal has been received from the
incoming end of the international circuit concerned.
1.9.3 In a transit exchange, the following arrangements must
be made on disconnection:
a) the GO channel must not be split until the
clear-forward signal has completely ceased;
b) the RETURN channel must be split as soon as
possible after recognition of the clear-forward signal;
c) a clear-forward signal received at the moment a
call is established, but before speech conditions have been set up,
must be repeated over the outgoing circuit that has been seized.
1.10 release-guard signal (sent in the backward direction)
This signal is sent in the backward direction in response to
the clear-forward signal, to indicate that the latter has been
fully effective in bringing about the release of the switching
equipment at the incoming end of an international circuit. It
serves to protect an international circuit against subsequent
seizure as long as the disconnection operations controlled by
reception of the clear-forward signal have not been completed at
its incoming end.
1.11 blocking signal (sent in the backward direction)
This signal is sent, when required, to the outgoing end of the
circuit to cause engaged conditions to be applied to the outgoing
end of the international circuit.
The design of the signalling equipment at the outgoing end of
international circuits should be such that the receipt of a block-
ing signal over a free circuit will cause that circuit to be
engaged to operators or automatic equipment which would otherwise
have access to it.
1.12 forward-transfer signal (sent in the forward direc-
tion)
This signal is sent to the incoming international exchange
when the outgoing international exchange operator wants the help of
an operator at the incoming international exchange.
The signal will normally serve to bring an assistance operator
into the circuit if the call is automatically set up at that
exchange. When a call is completed via an operator (incoming opera-
tor or delay operator) at the incoming international exchange, the
signal will cause this operator to be recalled.
1.13 Diagrams showing signal sequence
The sequence of signals in semi-automatic and automatic work-
ing is shown in Tables 1 and 2 of Annex 1 to Part I.
Tables of Annex 2 to Part II give a description of the opera-
tions corresponding to the various normal and abnormal conditions
which may arise in setting up a call.
_________________________
See the definition of assistance operator in S 1.1.6 of
Recommendation Q.101.
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CHAPTER II
SIGNAL CODE
Recommendation Q.121
2. SIGNAL CODE
2.1 General
The signals of System No. 4 are:
- signal called "line signals" for the so-called
supervisory functions;
- signals (binary code signals and their ack-
nowledgement signals) used for the transmission of numerical infor-
mation.
2.2 Transit working
In transit operation, the line equipment at the transit
exchange shall record that the condition is transit; this will
facilitate, in particular, the parallel reception of the
clear-forward signal at the transit and incoming international
exchanges. (See Recommendation Q.120, item 1.9).
2.3 Line signals
2.3.1 Line signal code
The line signal code is given in Table 1.
The use of two frequencies in this code makes it possible to
form a characteristic compound signal , in which both frequencies
are transmitted simultaneously and which can be used as a prepara-
tory signal element (called a refix ) to the control signal element
(called a suffix ) having a single frequency.
The compound signal prefix element is much less likely to be
imitated by speech currents than a single-frequency element of the
same duration and serves to prepare a switching circuit for the
reception of the suffix element which follows. The prefix signal
element also serves to bring about the splitting of the line at the
receiving end to prevent the remaining part of the signal from
passing out of the section in which it is intended to be operative.
2.3.2 Sending duration of line signal elements
The elements of each of the voice-frequency line signals shown
in Table 1 have a duration of:
P 150 | (+- | 0 ms X and Y 100 |
(+- | 0 ms XX and YY 350 | (+- | 0 ms.
TABLE [1], p.
(The durations of the signal elements P, X and Y, XX and YY
are multiples of 50 ms with a tolerance of _ | 0 ms.)
Once the sending of a signal has begun it must be sent com-
pletely. If two signals have to be sent one immediately after the
other in the same direction, a silent interval must separate the
two successive signals. The duration of this interval must not be
less than 100 milliseconds but it must not be so long as to cause
an unreasonable delay in signalling.
This 100 ms interval must also occur between the sending of a
numerical signal including the acknowledgement signal and a subse-
quent line signal.
Sending of the proceed-to-send or busy-flash signal by an
incoming or transit exchange should not take place until 50 ms
after the end of the receipt of the corresponding seizing signal.
Such a delay will normally result from the operation of equipment
(operating times of relays, time of hunting for register).
On sending, there will be no intentional interval of silence
between the prefix element and the suffix element of a signal but
where such an interval exists its duration at the sending end must
not exceed 5 ms.
It can happen, when sending the P prefix element, that the two
frequencies will not be sent simultaneously. The interval of time
between the instants when each of the two frequencies is sent must
not, in this case, exceed 1 ms. In the same way, if the suffix ele-
ment does not immediately follow the prefix but is separated from
it by an interval of silence as explained in the paragraph above,
the interval of time between the two instants when the sending of
each of the two frequencies ceases shall not exceed 1 ms.
2.3.3 Recognition time of line-signal elements at the
receiving end
At the output of the signal receiver, the duration of the
direct current signal elements produced by the line signals is
determined in terms of the sending duration of the voice-frequency
signal elements and the distortion due to the line and to the sig-
nal receiver.
This overall distortion due to the line and the signal
receiver is taken to be 10 ms maximum for a prefix-element and
15 ms for a suffix-signal element. (The distortion of the
suffix-signal element may be greater than that of the prefix-signal
element, because it depends not only on the distortion of the pulse
consisting of a single frequency which is sent as a suffix element,
but also on the moment when the other frequency used for the
prefix element ceases.)
The incoming switching equipment must recognize a signal only
after a certain time, called the recognition time, from the begin-
ning of the receipt of the direct current signal, so that risk of
recognizing false signals is reduced and so that signals of dif-
ferent length can be distinguished.
The recognition times of the line signal elements are:
P: 80 | | 0 ms X e Y: 40 | | 0 ms
XX e YY: 200 | | 0 ms.
The incoming switching equipment shall be able to recognize a
signal correctly when the prefix and the suffix of this signal are
separated by an interval of silence of 15 ms or less.
2.4 Numerical signals
2.4.1 Binary numerical signal code
The numerical signal code is given in Table 2. This code is a
binary code of four elements each separated from the next by a
short interval of silence s ; each element consists of the sending
of one or other of the signalling frequencies.
The symbols used in Table 2 and in Figure 2/Q.121 have the
following significance:
_________________________
See definition of recognition time in S 2.5 hereafter.
x short element of the single frequency x
y short element of the single frequency y .
TABLE [2], p. 2
2.4.2 Sending duration of the signal elements x and y
The sending duration of the signal elements x and y to line,
as voice-frequency signals, shall be:
35 | (+- | ms.
The sending duration of the interval of silence s between sig-
nal elements of the same digit shall have the same value of 35 |
(+- | ms.
(The maximum duration of the signal elements and intervals of
silence is not a critical factor in the design of the system but is
specified in order that the speed of signalling is not unduly
slow.)
2.4.3 Recognition time of the x, y and s elements at the
receiving end
The recognition time by the incoming switching equipment:
a) of the direct current signal elements x and y ;
b) of intervals of silence s ;
received from the output of the signal receiver is: 10 _ 5 ms.
2.4.4 Acknowledgement signals
Incoming international and international transit exchanges
shall return an acknowledgement signal to the outgoing interna-
tional exchange at the end of the reception of the 4th element of a
numerical signal.
At the outgoing international exchange a numerical signal will
_________________________
See definition of recognition time in S 2.5 | ) above.
be sent only if a signal is received from the incoming and ack-
nowledging the receipt of the preceding numerical signal. However,
to avoid this procedure delaying the transmission of numerical sig-
nals the sending of numerical signals may begin as soon as the ack-
nowledgement signal is recognized .
Two types of acknowledgement signals are provided, one consti-
tuted by the signal element x defined above and the other consti-
tuted by the signal element y defined above.
The acknowledgement signal x has two meanings:
- after a terminal proceed-to-send signal has been
received by the outgoing register: "digit received; send next
digit";
- after a transit proceed-to-send signal, but
before a terminal proceed-to-send signal has been received: "digit
received; stop the sending of digits".
The acknowledgement signal y has one meaning only, i.e. after
a transit proceed-to-send signal has been received: "digit
received; send next digit".
2.5 Signalling timing diagrams
Figures 1/Q.121 and 2/Q.121 give diagrams showing for line
signal elements (Figure 1) and for numerical signal elements x
and y (Figure 2):
a) the sending duration (transmission at
voice-frequency over the line);
b) the received duration (direct current signals at
the signal receiver output);
c) the safety margins that allow for equipment not
being in adjustment, etc.;
d) the recognition time of the receiving switching
equipment; this time assumes an operating margin is defined between
a lower limit t and an upper limit T . The switching equipment must
not recognize a signal element before t but must certainly have
recognized it at the end of time T .
2.6 General note on the operation of signalling and switch-
ing equipment
The tolerances defined in SS 2.3 and 2.4 concerning the send-
ing duration of signal and their recognition times at the receiving
end must be strictly observed in all circumstances and especially
under all conditions of battery voltage variation likely to arise
in working conditions.
FIGURE 1/Q.121, p. 3
FIGURE 2/Q.121, p. 4
CHAPTER III
SIGNAL SENDER AND SIGNAL RECEIVER
Recommendation Q.122
3.1 SIGNAL SENDER | u1)
3.1.1 Signalling frequencies
The signalling frequencies shall be:
2040 | (+- | Hz ("x " frequency); and
2400 | (+- | Hz ("y " frequency),
these frequencies being applied separately or in combination.
3.1.2 Absolute power level transmitted
The absolute power level of the unmodulated signal frequencies
at a zero relative level point shall be -9 dBm with a tolerance of
_ | dB.
These levels also apply to each signal frequency in a signal
element made up of a combination of the two frequencies (compound
signal element) but the two signalling frequencies making up such a
signal must not differ in level by more than 0.5 dB.
Note 1 - The noise as measured at the output of the line sig-
nal sender shall be as low as practicable, but in any event, at
least 40 dB below signal level. This noise includes all extraneous
power in the frequency band beteen 300 Hz and 3400 Hz including
power resulting from non-linear distortion of the signal.
Note 2 - The level of the leak current which might be
transmitted to line, for example when static modulators are used
for signal transmission, should be at least 50 dB below signal
level per frequency.
Recommendation Q.123
3.2 SIGNAL RECEIVER
3.2.1 Operating limits of the signal receiver
The signal receiver shall operate in the conditions specified
under 3.2.5 to received signals that meet the following three con-
ditions.
a) The signal frequencies shall be within the fol-
lowing limits:
"x " frequency: 2040 | (+- | 5 Hz
"y " frequency: 2400 | (+- | 5 Hz.
b) The absolute power level N of each unmodulated
received signal frequency shall be within the limits:
-18 | | fIn N _" n dBm;
where n is the relative power level at the signal receiver input.
These limits give a margin of _ | dB on the nominal absolute
level of each received signal at the input to the signal receiver.
c) The absolute level of the two unmodulated signal
frequencies may differ from each other, but the received level of
the 2400 Hz signal shall not be more than 3 dB above, nor more than
6 dB below the received level of the 2040 Hz signal.
_________________________
See also Recommendation Q.112.
See 2.1.2 of Recommendation Q.112.
The tolerances given in SS a, b and c above are to allow for
variations at the sending end and for variations in line transmis-
sion.
3.2.2 Non-operate conditions for the signal receiver
a) Selectivity
The signal receiver shall not operate to a signal having an
absolute power level at the receiving end within the limits speci-
fied in S 3.2.1 when the frequency differs by more than 150 Hz from
the nominal value of 2040 Hz or of 2400 Hz.
b) Maximum sensitivity of the receiver
The signal receiver shall not operate to a signal of 2040 |
(+- | 5 Hz or 2400 | (+- | 5 Hz whose absolute power level at the
point of connection of the receiver is (-26 - 9 + n ) dBm, n being
the relative power level at this point.
This limit is 26 decibels below the nominal absolute level of
the signal current at the input to the signal receiver.
3.2.3 Efficiency of the guard circuit
The signal receiver must be protected by a guard circuit
against false operation due to speech currents, circuit noise or
other currents of miscellaneous origin circulating in the line.
The purpose of the guard circuit is to prevent:
a) signal imitation (signals are imitated if the
duration of the resulting direct current pulses at the output of
the signal receiver is long enough to be recognized as signals by
the switching equipment);
b) operation of the splitting device from interfer-
ing with speech.
To minimize signal imitation by speech currents it is advis-
able that the guard circuit be tuned.
To minimize signal interference by low frequency noise, it is
advisable that the response of the guard circuit falls off towards
the lower fequencies and that the sensitivity of the guard circuit
at 200 Hz be at least 10 dB less than that at 1000 Hz.
An indication of the efficiency of the guard circuit is given
by the following:
a) during 10 hours of speech, normal speech
currents should not, on the average, cause more than one
simultaneous operation of the receiver relays for each of the two
signalling frequencies lasting more than 55 ms (the minimum recog-
nition time of a compound signal element is 60 ms);
b) the number of false splits of the speech path
caused by speech currents should not cause an appreciable reduction
in transmission quality of the circuit.
3.2.4 Guard circuit limits
A. - Steady noise
Considering :
a) that when there is noise on a circuit an
over-sensitive guard circuit might give rise to signalling diffi-
culties and, in particular, inhibit the response of the signal
receiver;
b) that unweighted noise of a level -40 dBm0 (100
000 pW) and uniform spectrum energy may arise during end-to-end
signalling over a multilink chain of System No. 4 circuits;
it is recommended that, for either one or two signalling
currents (each being within the limits of the level specified in
S 3.2.1) the signal receiver should satisfy the conditions indi-
cated in S 3.2.5 for the distortion of signals in the presence of
noise of a level of -40 dBm0 and uniform spectrum energy over the
frequency range 300 to 3400 Hz.
B. - Surges
A guard circuit with an excessive hangover time may cause dif-
ficulties in receiving a signal, for example when it has been
immediately preceded by surges, and it is therefore recommended
that the following condition should be fulfilled:
If a disturbing current of a frequency corresponding to the
maximum sensitivity of the guard circuit and having an absolute
power level of (-10 + n ) dBm at the relative level point n where
the receiver is connected, ceases 30 ms before the application of a
signal satisfying the limits defined in S 3.2.1, the lengths of the
received signals must remain within the limits specified in
S 3.2.5.
3.2.5 Distortion of received signals
When the signal frequencies and levels are within the limits
specified in S 2.3.1, the following conditions should be met:
1. a) the delay in the start of a received
pulse consisting of one of the two signalling frequencies should be
less than 20 ms;
b) the delay in reproducing the beginning of a sig-
nal consising of a combination of the two frequencies x and y (com-
pound signal) should be less than 20 ms; this delay is defined as
the interval between the moment when the beginning of the compound
signal arrives at the signal receiver input and the moment of the
beginning the reproduction of the two frequencies x and y as a
direct current signal output of the signal receiver;
2. the change of signal length in the presence of
the noise defined in S 2.3.4 should be less than:
a) 5 ms when the signal receiver receives an iso-
lated pulse at one frequency only , with a minimum duration of
25 ms;
b) 8 ms when the signal receiver receives a com-
pound pulse of the two frequencies with a minimum duration of
50 ms; this change is defined as the difference between the simul-
taneous reception of the two received frequencies at the input of
the receiver and the simultaneous reproduction of the two com-
ponents as a direct current signal at the output of the signal
receiver;
See the definition of prefix and suffix signals under S 2.3.1 of
Recommendation Q.121.
c) 6 ms when the signal receiver receives a pulse
of current of a single frequency with a minimum duration of 80 ms,
preceded by a compound signal element (separated or not by an
interval of silence of 5 ms maximum). Consequently the change in
the duration of a signal suffix , measured from the moment when the
prefix signal mentioned under b), will be less than 6 + 8 = 14 ms.
MONTAGE: PAGE 18 = PAGE BLANCHE
CHAPTER IV
SWITCHING CONDITIONS
Recommendation Q.124
4.1 SPLITTING ARRANGEMENTS
Sending line split
4.1.1 According to Recommendation Q.25, S 2, sending split
arrangements have to be provided.
4.1.2 The exchange side of the international circuit shall be
disconnected 30 to 50 ms before a voice-frequency signal is sent
over the circuit.
4.1.3 The exchange side of the international circuit will not
be reconnected for 30 to 50 ms following the end of the sending of
a voice-frequency signal over the circuit.
Receiving line split
4.1.4 The international circuit should be split (completely
cut) at outgoing and incoming international exchanges when a com-
pound signal is received, to ensure that no fraction of the combi-
nation of the two frequencies exceeding 55 ms duration may pass out
of the international circuit.
The splitting time of 55 ms may be reduced by each Administra-
tion concerned, in order to help to protect its national network
against the effect of signals coming from the international cir-
cuit. It should be noted, however, that a shorter splitting time
can lead to an increase in the number of false operations of the
splitting device by speech currents, and impair speech transmis-
sion.
4.1.5 The split must be maintained for the duration of the
signal,
but must cease within 25 ms of the end of the direct current signal
which caused the splitting device to operate.
For the correct operation of the splitting device, it is
necessary to take into account the delay in the reproduction of the
compound signal caused by the signal receiver for which the condi-
tions are described in Recommendation Q.123, S 3.2.5.1 | ).
_________________________
See Recommendation Q.25.
4.1.6 The splitting of the line must not give rise to surges
which might cause interference with signalling over the interna-
tional circuit or with other signalling systems associated with it
for setting up an international call.
Recommendation Q.125
4.2 SPEED OF SWITCHING IN INTERNATIONAL EXCHANGES
4.2.1 It is recommended that the equipment in international
exchanges (terminal or transit) shall have a high switching speed
so that the switching time may be as short as possible.
4.2.2 It is also recommended that the incoming register at the
incoming international exchange should begin to set up the national
part of the connection as soon as the register has received a suf-
ficient number of digits and without waiting to receive the com-
plete number of the called subscriber.
4.2.3 At the outgoing international exchange:
- with semi-automatic operation it may be desirable
for the outgoing register to start sending numerical signals to
line
without waiting to receive all the digits of the called
subscriber's number. However, this may depend on national condi-
tions,
- with automatic operation, it is evident that the
sending of numerical signals must begin without waiting for the
receipt of all the digits of the called subscriber's number because
the outgoing register will not generally know how many digits there
are going to be.
4.2.4 At international exchanges, use may be made of the
advantages of continuous hunting (of circuits or common equipment),
i.e. economy in the number of outgoing circuits to be provided or
improvement in the quality of service for a given number of cir-
cuits. However, at incoming and transit exchanges, the return of a
busy-flash signal must take place within the following delay times,
specified in particular so that the release conditions of registers
can be laid down:
- a maximum delay of 5 s following recognition of a
seizing signal at an incoming or transit exchange if a free regis-
ter and/or link circuit is not found;
- a maximum delay of 10 s following receipt, at an
incoming exchange, of the information necessary for determining the
required route, if congestion is encountered;
- a maximum delay of 10 s following receipt of the
digits necessary to determine the routing at a transit exchange, if
congestion is encountered.
Recommendation Q.126
4.3 ANALYSIS AND TRANSFER OF DIGITAL INFORMATION
(see Recommendation Q.107 | fIbis in Fascicle VI.1)
Recommendation Q.127
4.4 RELEASE OF REGISTERS
4.4.1 Outgoing register
4.4.1 (1) Normal release conditions
The outgoing register shall release in either of the following
two cases:
Case 1 - The register has sent forward all the numerical sig-
nals and has received a local sending-finished signal from the out-
going operator indicating that there are no more digits to follow.
Case 2 - The register has received:
- either a number-received signal from the incoming
international exchange indicating that all the digits comprising
the complete national number have been received;
- or a busy-flash signal (this assumes that a
busy-flash signal does not initiate re-routing
_________________________
See definition of "re-routing" in
Recommendation E.170 (Recommendation Q.12).
4.4.1 (2) Abnormal release conditions
Arrangements should be made at the outgoing exchange for the
possibility of releasing the outgoing register when any one of the
following conditions arises:
1) With semi-automatic operation if, after a delay
of 10 to 20 s from the seizure of the register or the receipt of
the last digit, no further digit or local sending-finished signal
is received.
2) With automatic operation if, after a delay of 15
to 30 s from the seizure of the register or the receipt of the last
digit, the register is in one of the following conditions:
- seized, but no further digit received from the
calling subscriber;
- not all the digits necessary to determine the
routing received;
- correct number of digits to determine the routing
received, but no further digit from the calling subscriber;
- no busy-flash or a number received signal has
been received although the complete national (significant) number
or part of it has been sent.
In the first two cases, a shorter delay may nevertheless be
adopted by certain Administrations.
In the last two cases, release of the outgoing register is
made to accompany release of the international circuit by sending
the clear-forward signal.
The method of indicating the above normal conditions to the
calling subscriber will depend on the practice followed in the
various countries: a tone may be sent or, better, a recorded
announcement will ask the caller to recommence his call after hav-
ing checked the number to be dialled. (See also
Recommendations Q.116 and Q.118.)
The delay of 15 to 30 s provided for in the above conditions
is considered sufficient to cover the maximum period for receiving
a number-received signal under the most unfavourable conditions.
a) Numerical information received for which no
routing has been provided.
b) Proceed-to-send signal or busy-flash signal not
received within:
- 10 to 30 s following the sending of a seizing
signal;
- 15 to 30 s following the sending to a transit
centre of the digits necessary to determine the routing.
c) An acknowledgement signal not received with 5 to
10 s following the sending of a digit.
d) More than the appropriate number of transit
proceed-to-send signals is received (see Recommendation Q.112, S
2.1.2, for the maximum number of circuits switched in tandem).
In the various cases mentioned above, an appropriate indica-
tion should be given to the operator or calling subscriber.
4.4.2 Transit register
4.4.2 (1) Normal release conditions
The transit register shalll release as soon as it has selected
an outgoing circuit and sent forward a seizing signal on the cir-
cuit.
However, a different procedure may be used, in which release
of the register is delayed until either a proceed-to-send signal or
a busy-flash signal, is received from the next exchange. It may be
judged more convenient to make use of the transit register when it
is desired to give an alarm to show that a proceed-to-send signal
has not been received. In this case, the circuit should be switched
to the speech condition in both directions of transmission immedi-
ately following the operations mentioned above so as to allow the
proceed-to-send signal and the following numerical signals to pass
through the transit exchange.
If there is outgoing congestion from the transit exchange, the
register will release after it has returned a busy-flash signal,
and made connection to a recorded announcement.
4.4.2 (2) Abnormal release conditions
The transit register will release without returning any signal
under either of the following conditions:
a) the digits necessary for determining the routing
not received within 5 to 10 s following the sending of a
proceed-to-send signal to the outgoing exchange;
b) numerical information received for which no
routing has been provided.
On the other hand, if release of the transit register is
deferred until a proceed-to-send signal is received, in accordance
with the alternative method mentioned in S 4.4.2.1, it will release
if a proceed-to-send signal or busy-flash signal is not received
within 10 to 30 s following the sending of a seizing signal to the
next exchange.
4.4.3 Incoming register
4.4.3 (1) Normal release conditions
The incoming register will release when all the numerical
information necessary to set up the connection in the incoming
country has been sent and after a number-received signal has been
returned over the international circuit. The register will deter-
mine when the complete national (significant) number has been
received under the conditions defined in Recommendation Q.120,
S 1.5.5.
If the incoming register finds that there is congestion within
or outgoing from the incoming international exchange, it will
release after returning a busy-flash signal.
4.4.3 (2) Abnormal release conditions
The incoming register will release if any one of the following
three conditions occurs:
a) No further digit is received after a delay of 30
to 60 s from receipt of the last digit and it is not possible to
determine by one of the methods described in S 1.5 of
Recommendation Q.120 that the number which is received is a com-
plete number.
b) No digit is received within 5 to 10 s following
the return of a proceed-to-send signal.
c) A number is received for which no routing
exists, or an incomplete number is received followed by an
end-of-pulsing signal (code 15).
In cases a) and b), no signal is returned because the outgoing
register remains in circuit and can itself detect any abnormal con-
dition in the establishment of the call.
In case c), before the incoming register releases, a
number-received signal will be returned, followed, if possible, by
a recorded announcement, a number-unobtainable tone or by the
intervention of an interception operator.
Recommendation Q.128
4.5 SWITCHING TO THE SPEECH POSITION
4.5.1 Outgoing international exchange
The circuit shall be switched to the speech position when the
outgoing register release (see S 4.4.1).
4.5.2 International transit exchange
The circuit shall be switched to the speech position immedi-
ately after the transit register has sent the seizing signal (see
S 4.4.2).
4.5.3 Incoming international exchange
The circuit shall be switched to the speech condition immedi-
ately the incoming register:
- has sent back the number-received signal and sent
forward the numerical information to the national network equip-
ment;
- or has sent back the busy flash-signal;
or, if these signals are not sent, when the register releases under
abnormal conditions (see S 4.4.3.2).
Recommendation Q.129
4.6 MAXIMUM DURATION OF A BLOCKING SIGNAL
When a blocking signal is sent on a circuit, an alarm should
be given at the outgoing end of the circuit if the blocking condi-
tion persists for more than about 5 minutes.
Recommendation Q.130
4.7 SPECIAL ARRANGEMENTS IN CASE OF FAILURES
IN THE SEQUENCE OF SIGNALS
4.7.1 Blocking an outgoing circuit
Installations should provide the following facilities for
blocking outgoing circuits. These facilities will be used or not
according to the maintenance instructions which will be promul-
gated.
The alarm may be immediate or delayed depending upon the desire of
the Administration concerned.
1) If, after sending a seizing signal, a
proceed-to-send signal is not received within 10 to 30 s, the out-
going circuit should be blocked and an alarm given.
2) The outgoing circuit should be blocked and an
alarm given if a proceed-to-send signal or a busy-flash signal is
not received within 15 to 30 s of the sending to a transit exchange
of the digits necessary to determine the routing.
3) If, after sending a clear-forward signal, a
release-guard signal is not received within 5 to 10 s, the outgoing
end of the circuit should be blocked and an alarm given.
At the incoming end of the circuit, the clear-forward signal
should be recognized at any time even if the circuit is in the idle
state; the incoming line circuit must therefore be able to recog-
nize a clear-forward signal and to return a release-guard signal
even of the clear-forward signal has not been preceded by a seizing
signal.
4.7.2 Abnormal recognition of a release-guard signal at an
international transit exchange
In the case where a release-guard signal is recognized at an
international transit exchange without a clear-forward signal hav-
ing been
recognized, arrangements should be made at the transit
exchange to:
- send a blocking signal in the backward direction,
to busy the outgoing end of the incoming circuit at the transit
exchange;
- immediately release the circuit outgoing from the
transit exchange.
This prevents the receipt of the release-guard signal from
giving a wrong indication that the circuit to the transit exchange
is cleared.
Recommendation Q.131
4.8 ABNORMAL RELEASE CONDITIONS OF THE OUTGOING REGISTER
CAUSING RELEASE OF THE INTERNATIONAL CIRCUIT
In automatic operation, the international circuit should be
released when the following abnormal conditions arise:
a) if, after receiving the digits necessary to
determine the routing, the outgoing register receives no further
digit within a period of 15 to 30 s;
b) if no busy-flash or number-received signal is
received by the outgoing register within a period of 15 to 30 s
although the national (significant) number (or part of it) has been
sent.
The release of the outgoing register under these abnormal con-
ditions is dealt with in S 4.4.1 (2) of Recommendation Q.127.
MONTAGE: PAGE 24 = PAGE BLANCHE
CHAPTER V
See ATME No. 2 specification in Recommendation Q.49 (O.22).
TESTING ARRANGEMENTS
Recommendation Q.133
5.1 NUMBERING FOR ACCESS TO AUTOMATIC MEASURING
AND TESTING DEVICES
5.1.1 Automatic measuring and testing devices situated in the
ITMCs and the ISMCs of other countries will be obtainable from the
access point defined in Recommendation Q.75 by means of the
following digit sequence:
a) terminal seizing signal;
b) code 13 replacing the language digit;
c) code 12;
d) digit 0;
e) two digits which will be associated with the
type of testing or measuring device required;
f ) end-of-pulsing signal (code 15).
Note - The allocation of the digits in e) above will enable
access to be given to a number of different types of measuring or
testing equipment. Combination 51 to combination 59 are allocated
to automatic transmission measuring devices standardized by the
CCITT for ATME No. 1. Combination 00 is used for access to the
automatic testing device specified in Recommendation Q.137. Combi-
nations 61 to 63 are allocated to the automatic transmission
measuring and signalling testing equipment No. 2
Recommendation Q.134
5.2 ROUTINE TESTING OF EQUIPMENT (LOCAL MAINTENANCE)
5.1.1 Routine testers for testing individual items of equip-
ment such as circuit equipment, connecting circuits, operator's
line calling equipment, selectors, registers, etc., must be pro-
vided in every international exchange equipped for automatic
switching. These routine testers will be provided in accordance
with the practice followed in each country for the local mainte-
nance of the switching equipment.
5.2.2 The testing equipment must conform to the following
principles:
a) An item of equipment must not be taken for test
until it is free; a signal will show the exchange staff that a
piece of apparatus has not been taken for test because it was
engaged on a call; it will then be possible to test this piece of
apparatus later.
b) An item of equipment taken for test will be
marked engaged for the duration of the test. When an incoming cir-
cuit equipment is taken for test, a blocking signal will be sent to
the outgoing exchange (see Recommendation Q.129).
5.2.3 Testing of the circuit and signalling equipment should
include a check that the specifications of System No. 4 are met in
regard to the following:
Signalling frequencies;
Transmitted signal levels;
Signal frequency leak;
Receiver operate and non-operate limits;
Receiving-end line split;
Sending-end line split;
Line signal codes;
Sending duration of line signal elements;
Recognition time of line signal elements;
Sending duration of numerical signal elements;
Recognition time of numerical signal elements;
Time-out and alarm features.
Recommendation Q.135
5.3 PRINCIPLES OF RAPID TRANSMISSION TESTING EQUIPMENT
Rapid transmission tests can be made by two methods:
a) The first method consists of a loop measurement
of the GO and RETURN paths of an international circuit, these paths
being looped at the incoming end of a circuit when it is free.
b) The second method consists of sending a special
code on the international circuit to be tested so as to obtain
access to an automatic testing equipment in the incoming exchange.
The first method requires that the incoming end of all cir-
cuits should be equipped as described later in
Recommendation Q.136.
The second method assumes the existence of rapid transmission
testing equipment in all exchanges between which this method is
used. This testing equipment must be designed in accordance with
Recommendation Q.137.
Note - The first method provides overall testing on the GO
and RETURN paths without being able to differentiate between the
conditions of each of the two directions of transmission. The
second method enables separate transmission tests in the two direc-
tions. (A situation can occur, however, when it is not possible to
determine whether a transmission fault is on the GO path or on the
RETURN path of the circuit.) Since the second method requires that
for access to the incoming testing apparatus signals must be passed
over the circuit, there is some check of good signalling condi-
tions.
Recommendation Q.136
5.4 LOOP TRANSMISSION MEASUREMENTS
A permanent loop will be connected between the GO and RETURN
paths of an international circuit at its incoming end when the cir-
cuit is free, so that transmission tests can be made independently
of the signalling conditions.
The loop between the GO and RETURN paths shall be connected in
such a manner that the level diagrams of each of the two paths will
be respected when the circuit is free (loop established); the loop
may therefore include an attenuation pad of the required value.
The loop at the incoming end of the international circuit
should be disconnected when a seizing signal is received. The loop
must be disconnected
within 35 ms so as to ensure that the part of a seizing signal
which passes round the loop and which is returned to the outgoing
end cannot be recognized as a signal.
Recommendation Q.137
5.5 AUTOMATIC TESTING EQUIPMENT
The second method for rapid transmission tests consists of
extending international circuit, by means of a special code, to an
automatic testing equipment at the incoming exchange. For this
method, there must be incoming testing equipment at the incoming
international exchange and outgoing testing equipment at the outgo-
ing international exchange. This equipment should be designed in
accordance with the following conditions:
5.5.1 Incoming testing equipment
(1) Connection to incoming testing equipment:
The incoming testing equipment will normally be connected in
the four-wire part of the circuit.
Access to this equipment from an outgoing international
exchange will be obtained by sending successively on the interna-
tional circuit, according to Recommendation Q.133:
a) terminal seizing signal;
b) code 13 replacing the language digit;
c) code 12;
d) three digits 000, the last two being the combi-
nation for access to the automatic testing equipment;
e) end-of-pulsing signal (code 15).
If the incoming testing equipment is free, the answer signal
will be sent 800 to 1200 ms after it is connected.
If the incoming testing apparatus is occupied, a busy-flash
signal will be returned.
(2) Measuring condition:
When the answer signal has been sent, the incoming testing
equipment will pass to the measuring condition, in which the level
of the test signal by the outgoing testing apparatus will be meas-
ured. The passage to the
measuring condtion will be effected after a period of 600 to
900 ms calculated from the moment when the testing equipment
prompts the sending of the answer signal. This delay is necessary
to ensure that the noise which may be produced at the moment of the
passage of the circuit to the speech conditions will not influence
the measuring arrangement.
The measurement of the received signal will be made with an
accuracy of _ | dB.
To provide time for the test signal to become stabilized,
there should be a delay of 100 to 150 ms after the operation of the
detector circuit, before indications on the level of the test sig-
nal are given.
The incoming testing equipment will determine whether the
level of the test signals is within the prescribed limits; these
limits will be predetermined by an adjustment of the equipment to
specified values. These limits will provisionally be _ | dB with
respect to the nominal level at which the test tone should be
received.
(3) Passage to the sending condition:
If the received test signal is within the prescribed limits
(deviation of _ | dB from the nominal value), the incoming
testing equipment will send a test signal on the RETURN path of the
circuit.
This test signal will have a frequency of 800 Hz which is the
same as the test frequency sent on the GO path of the circuit by
the outgoing testing equipment. The frequency sent should be con-
trolled within _ | %. The test signal sent by the incoming testing
equipment will give a power of 1 mW at a zero relative level point
of the circuit. The sending level must be maintained to _ | .5 dB.
If, due to the non-reception of a clear-forward signal, the
test signal is transmitted for a period of 1 to 2 m, the incoming
testing equipment will stop transmitting this test signal and a
clear-back signal will be sent. The release of the incoming testing
equipment will then be carried out in accordance with the provi-
sions of Recommendation Q.118, S 4.3.3.
(4) Indication of unsatisfactory transmission of
the GO path of the circuit:
If the level of the received test signal is outside the
prescribed limits or if the incoming testing equipment does not
receive the signal, a
clear-back signal will be returned to the outgoing end. This
clear-back signal will be sent 5 s after passing to the measuring
position and will indicate to the testing officer at the outgoing
exchange that the transmission quality of the GO path of the cir-
cuit is not up to standard.
5.5.2 Outgoing testing equipment
(1) Connection to the outgoing testing equipment:
The outgoing testing equipment will be designed to send
automatically the numerical information mentioned under (1) in
S 5.5.1 above.
(2) Sending condition:
The receipt of an answer signal sent by the incoming testing
equipment will cause the sending of the test signal by the outgoing
testing equipment. This test signal will be sent for a period of
500 to 800 ms. To allow the incoming testing equipment to pass into
_________________________
For future equipments, the test frequency of 800 Hz
will be replaced by 1020 Hz with a tolerance of +2 Hz
and -7 Hz.
the measuring condition, this test signal should not be sent
immediately after the answer signal but should be delayed for a
period of at least 700 ms.
The test signal will be sent automatically or under the con-
trol of the officer making the tests. If the test signal is sent
automatically, the delay
in sending the test signal following the end of the receipt of
the answer signal should be between 700 and 900 ms. If the test
signal is sent under the control of the operator, the latter should
operate quickly, because the clear-back signal can be returned by
the incoming testing apparatus after a delay of 5 s.
The frequency of the test signal will be 800 Hz _ | %.
The level of the sent test signal will be adjusted to give a
power of 1 mW at a zero relative level point of the circuit. The
sent level will be accurate to _ | .5 dB.
(3) Passage to the measuring condition:
As soon as the outgoing testing equipment has sent the test
signal, it will pass automatically from the sending condition to
the measuring condition. In this condition, the level measuring
equipment will measure the level of the test signal received from
the incoming end. The operator of the automatic device at the out-
going end will check that the level of the received signal is
within the prescribed limits.
Recommendation Q.138
5.6 INSTRUMENTS FOR CHECKING EQUIPMENT
AND MEASURING SIGNALS
5.6.1 General
For local checks of correct equipment and for readjusting the
equipment, international exchanges should have available instru-
ments of the following two types:
a) calibrated signal generator;
b) signal measuring apparatus.
These instruments should have the following characteristics:
5.6.2 Calibrated signal generator
Duration of sent signals to be adjustable between the extreme
limits given in the equipment specifications, i.e. 3 to
500 ms.
The accuracy required in the duration of sent signals should
be the higher of the following two values:
_ 1 ms or _ 1% of the nominal value of the sent signal.
Frequency:
The sent frequency shall not differ by more than _ 5 Hz from
the nominal value and shall not vary during the time required for
testing.
Level of the sent signals to be variable between the extreme
limits given in the equipment specifications and able to be set to
a particular fixed value equal to the nominal value as defined in
these specifications.
Tolerances on the reading of the level of the sent signalling
frequencies to be _ | .2 dB.
5.6.3 Signal-measuring equipment
Duration of signals to be measured to be between the extreme
limits given in the equipment specifications, i.e. 3 to 500 ms.
The accuracy required in the duration of the measured signals
should be the higher of the following two values:
_ 1 ms or _ 1% of the nominal value of the received signal.
Signal frequency to be measured to be between the extreme lim-
its set by the specifications, the reading being made with an accu-
racy of _ | Hz.
Level of the signalling frequencies to be measured to be adju-
stable between the extreme limits set by the specifications, the
reading being made with an accuracy of _ | .2 dB.
Recommendation Q.139
5.7 MANAUL TESTING
5.7.1 Functional testing of signalling arrangements
Functional tests from one end of the circuit to the other can
be made in the following three ways:
a) The first method consists of a rapid verifica-
tion of unsatisfactory signal transmission by ensuring that a seiz-
ing signal is followed by the return of a proceed-to-send signal,
that a clear-forward signal is followed by the return of a
release-guard signal and that the circuit is clear.
b) The second method consists of verification of
satisfactory signal transmission by initiating a test call:
1) to technical personnel at distant-end interna-
tional exchange; or
2) to a test call signal testing and answering dev-
ice, if such equipment is available at the distant-end interna-
tional exchange.
c) The third method will consist of a complete
verification of satisfactory line and register signal transmission.
The verification consists of a check of ability to:
1) generate and receive line and register signals;
2) transmit the appropriate acknowledgement sig-
nals;
3) complete terminal and transit calls.
5.7.2 First method: rapid test
1. Verification of satisfactory signal transmis-
sion:
a) Initiate a seizing signal and verify the receipt
and recognition of the proceed-to-send signal from the distant end;
b) Initiate a clear-forward signal and verify the
receipt and recognition of the release-guard signal from the dis-
tant end.
2. In the event of a failure, appropriate steps
should be taken to locate and correct the trouble.
3. The above tests are short, simple, and should
be performed at least monthly from each end of the circuit as
_________________________
Transit test calls are not intended to check the per-
formance or the quality of the circuit beyond the tran-
sit exchange; this being entirely the responsibility of
the Administration concerned. However, it is important
that in principle the transit operations can be
checked.
appropriate. This minimum periodicity should be increased to as
often as daily if the incidence of trouble encountered is unsatis-
factory.
5.7.3 Second method: test calls
1. Verification of satisfactory transmission of
signals involved in completion of test calls (manual method):
a) Place a call to the technical personnel at the
distant international exchange.
b) On completion of connection:
1. the audible ringing tone should be heard;
2. the answer signal should be received when the
call is answered at the distant end.
c) Request distant end to initiate a clear-back
signal, followed by an answer signal.
d) A clear-back signal should be received and
recognized when the distant end hangs up and a second answer signal
should be received and recognized when the distant end re-answers
the call.
e) Initiate a forward-transfer signal which should
result in bringing the assistance operator at the distant end.
f ) Terminate the call and observe that the circuit
restores to the idle condition.
2. Verification of satisfactory transmission of
signals involved in completion of test calls (semi-automatic
method).
If test call signal testing and answering devices are avail-
able at the distant international exchange, the signal verification
test should be made using this equipment to the extent that the
applicable features indicated in 1 above are available.
3. The tests should be made monthly when the
manual testing methods prescribed in S 5.7.3.1 are used.
They may be made daily when semi-automatic test arrangements
are available.
5.7.4 Third method: comprehensive tests; terminal and tran-
sit calls
1. Verification of satisfactory signal
transmission (frequency, level, duration, etc.) involved in termi-
nal and transit calls.
a) These tests are made in conjunction with:
- verification and location of faults;
- ensuring that new circuits are satisfactory in
operation before being brought into service.
b) When establishing new circuits, all of the tests
outlined in S 5.2.3 should have been completed at both terminals.
2. Terminal calls
Initiate a call to the distant end test centre. Coordinate
this test with the distant end so that appropriate test equipment
is connected prior to establishing the call. The tests shall
proceed as follows:
a) at the originating end, check that a terminal
seizing signal is followed by the receipt of a terminal
proceed-to-send signal from the distant end;
b) at the distant end, check that the individual
signal elements are correctly received and that each digit is ack-
nowledged correctly;
c) at the originating end, check that the number
received signal is received;
d) check that the audible ringing tone is heard at
the originating end:
e) at the distant end, initiate an answer signal;
f ) at the originating end, check that the answer
signal is received and recognized;
g) at the distant end, initiate a clear-back sig-
nal;
h) at the originating end, check that the
clear-back signal is received and recognized;
i) at the originating end, initiate a
forward-transfer signal;
j) at the distant end, check the receipt of the
forward-transfer signal;
k) at the distant end, arrange to transmit a suc-
cession of clear-back and answer signals; first at a slow rate,
then at a rate which is faster than the system is capable of fol-
lowing;
l) at the originating end, check during the slow
transmission of the switch-hook flashes that each clear-back and
answer signal is received and properly recognized. Verify that
after the transmission of the fast switch-hook flashes, the equip-
ment indicates the final position of the switch-hook;
m) at the originating end, initiate the release of
the circuit;
n) at the distant end, check that the
clear-forward signal is received and recognized and that the cir-
cuit releases;
o) at the originating end, check that the
release-guard signal is received and recognized and that the cir-
cuit releases;
p) at the originating end, set up a call to a busy
line or to a test call device which provokes the return of a
busy-flash signal and check that the busy-flash signal is received
and recognized;
q) at the originating end, after receipt of the
busy-flash signal, initiate a release of the connection and check
that the equipment releases correctly;
r) at the distant end, after sending the busy-flash
signal, check that the clear-forward signal releases the equipment;
s) at the distant end, initiate the transmission of
a blocking signal;
t at the originating end, check that the blocking
signal busies the circuit;
u) at the distant end, initiate the transmission of
an unblocking signal;
v) at the originating end, check that the unblock-
ing signal restores the circuit to normal;
w) at the distant end, connect in turn a continu-
ous x tone, a continuous y tone, a continuous x + y tone, with the
circuit in the idle state in each case;
x) at the originating end, check that the receipt
of a continuous x tone, or a continuous y tone, or a continuous x
+ y tone busies the circuit;
y) at the originating end, check that the
clear-forward signal sent to the incoming equipment in the idle
condition results in the return of the release-guard signal and
that the equipment restores to the idle condition;
z) at the originating end, check the presence of a
transmission test loop with the circuit in an idle condition and
then check that within 35 ms of receipt of a seizure signal, the
loop is removed.
3. Transit calls (System No. 4 to System No. 4)
After securing the cooperation of a third international cen-
tre, initiate a transit call to this centre through the interna-
tional centre, covered in S 2 above, which thus becomes the transit
centre. Check the following sequence:
a) at the originating end, check that a transit
seizure signal is followed by the receipt of a transit
proceed-to-send signal from the transit centre;
b) at the transit centre, check that the necessary
routing digits are received and acknowledged correctly and that a
circuit to the terminal centre is selected;
c) at the originating end, check that a terminal
proceed-to-send signal is received and that the correct digital
information is sent to the terminal centre;
d) with the assistance of technical personnel at
the terminal centre, check that the number received, answer,
clear-back, forward transfer, busy-flash, clear-forward and
release-guard are correctly interpreted.
MONTAGE: PAGE 32 = PAGE BLANCHE
ANNEXES TO SIGNALLING SYSTEM No. 4
SPECIFICATIONS
ANNEX 1
Signalling sequences
Table 1 - Signalling sequences in terminal traffic
Table 2 - Signalling sequences in transit traffic
In these tables, the arrows have the following meanings:
transmission of a signalling frequency (permanent or pulse
emission)
end of transmission of the signalling frequency in the
case of its permanent transmission.
transmission of an audible tone.
ANNEX 2
Descriptions of the operations corresponding
to the various normal and abnormal conditions
which may arise in setting up a call
Table 1 - Outgoing exchange - Normal conditions
Table 2 - Outgoing exchange - Abnormal conditions
Table 3 - Incoming exchange - Normal conditions
Table 4 - Incoming exchange - Abnormal conditions
Table 5 - Transit exchange - Normal conditions
Table 6 - Transit exchange - Abnormal conditions
Annex 1, Table [1] (a l'italienne), p. 5
Annex 1 Table [1 (cont.)] (a l'italienne), p. 6
Annex 1 Table [1 (conc.)], (a l'italienne), p. 7
Annex 1, Table [2], (a l'italienne), p. 8
Annex 1, Table [2 (cont.)], (a l'italienne), p. 9
Annex 1, Table [2 (cont.)], (a l'italienne), p. 10
Annex 1, Table [2 (conc.)], (a l'italienne), p. 11
Annex 2, Table [1], p. 12
Annex 2 Table [2], p. 13
Annex 2 Table [3], p. 14
Annex 2 Table [4], p. 15
Annex 2 Table [5], p. 16
Annex 2 Table [6], p. 17