


       5i'

















                                     PART I






                         Recommendations Q.120 to Q.139





                          SPECIFICATIONS OF SIGNALLING

                                  SYSTEM No. 4






       MONTAGE: PAGE ... = PAGE BLANCHE








                            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.





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                                   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






























































