
                                    PART I

                        Recommendations Q.120 to Q.139

                         SPECIFICATIONS OF SIGNALLING

                                 SYSTEM No. 4

                            SIGNALLING SYSTEM No. 4

                                   CHAPTER I

                      DEFINITION AND FUNCTION OF SIGNALS

                             ____________________
                             Recommendation Q.120
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ

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 initiate 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 purpose, 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 provided:

   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) necessa ry 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 signal may be used to
~~~~~ inform the outgoing operator that the international 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 (significant) 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 interworking between
                  [ Systems No.  4 and No.  6, Q.381 for interworking between
                  [ Systems No.  4 and R2, Q.382 for interworking between
                  [ Systems No.  5 and R2, Q.383 for interworking 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:

             o an appropriate indication to be given to the out-going operator
               or to the calling subscriber; and

             o 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 function.

   In automatic working, it is used:

   o to start metering the charge to the calling subscriber,

   o 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 indicate that the
called party has cleared.  In the semi-automatic service, 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 measurements 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 subscriber 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:

      o to the outgoing international operator in semi-automatic operation;

      o to the outgoing international equipment in automatic operaton.


1.8.2 Note 2 - The "call party" referred to in the definitions of the answer
~~~~~ and clear-back signals may be:

      o the called subscriber;

                  [ See Recommendation Q.27 for the action to be taken to
                  [ ensure that answer signals both national and
                  [ international, are transmitted as quickly aspossible.

      o 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 international
           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 international
              exchange.)

              The answer signal is sent when the operator enters the circuit.

              The clear-back signal is sent when the operator clears the
              connection.

           b) Only one operator involved in the incoming country, with
              through-supervision via her position - (The operator can be the
              same as for a) above.)

              Through-supervision can be effected:

              o via the cord circuits, the incoming operator intervening to
                clear down the connection at the end of the call;

              o via cordless positions, in which case the connection 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 circuit.

              A clear-back signal is sent when the operator goes out of
              circuit.  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
              connection 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:

              o an incoming or a delay operator at the international exchange;
                and

              o an operator at a national manual exchange.

              c.1) There is no through-supervision via the operators'
                   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 normally able to
                   provide through-supervision.  There are still two cases to
                   consider:

                   c.2.1) If the whole of the national chain, including 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 cannot 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 special service, answers.  The
       answer signal is sent:

       o either by an operator (using a key); or

       o 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 subscriber 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 blocking 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 direction)

   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 operator 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 working 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 operations
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.





                                   CHAPTER 2

                                  SIGNAL CODE
                             ____________________
                             Recommendation Q.121
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
2 :  SIGNAL CODE
~~~~~~~~~~~~~~~~
 * 2.1 :  General

   The signals of System No.  4 are:

   o signal called "line signals" for the so-called supervisory functions;

   o signals (binary code signals and their acknowledgement signals) used for
     the transmission of numerical information.



 * 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 preparatory 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 completely.  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 subsequent 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 element 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 signal 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 beginning of the
      receipt of the direct current signal, so that risk of recognizing false
      signals is reduced and so that signals of different 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 | (ħ 1 ms)

     The sending duration of the interval of silence s between signal 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 international 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 acknowledging
      the receipt of the preceding numerical signal.  However, to avoid this
      procedure delaying the transmission of numerical signals the sending of
      numerical signals may begin as soon as the acknowledgement signal is
      recognized.

      Two types of acknowledgement signals are provided, one constituted by
      the signal element x defined above and the other constituted by the
      signal element y defined above.

      The acknowledgement signal x has two meanings:

      o after a terminal proceed-to-send signal has been received by the
        outgoing register:  "digit received; send next digit";

      o 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 switching equipment

   The tolerances defined in SS 2.3 and 2.4 concerning the sending 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
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ


 * 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 signal 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 conditions.

      a) The signal frequencies shall be within the following 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 transmission.



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 specified 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 interfering 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 recognition 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 difficulties 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 indicated 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 difficulties 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 signal consising of a
             combination of the two frequencies x and y (compound 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 isolated pulse at one
             frequency only , with a minimum duration of 25 ms;

          b) 8 ms when the signal receiver receives a compound pulse of the
             two frequencies with a minimum duration of 50 ms; this change is
             defined as the difference between the simultaneous reception of
             the two received frequencies at the input of the receiver and the
             simultaneous reproduction of the two components 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.




                                  CHAPTER IV

                             SWITCHING CONDITIONS
                             ____________________
                             Recommendation Q.124
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ

 * 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 compound signal is received,
to ensure that no fraction of the combination 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 Administration
concerned, in order to help to protect its national network against the effect
of signals coming from the international circuit.  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 transmission.



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 conditions 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 international 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 sufficient number of digits
and without waiting to receive the complete number of the called subscriber.


4.2.3 At the outgoing international exchange:
~~~~~
      o With semi-automatic operation it may be desirable for the outgoing
        register to start sending numerical signals to line

      o Without waiting to receive all the digits of the called subscriber's
        number.  However, this may depend on national conditions,

      o 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 circuits.  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:

      o A maximum delay of 5 s following recognition of a seizing signal at an
        incoming or transit exchange if a free register and/or link circuit is
        not found;

      o 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;

      o 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
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ

4.3 ANALYSIS AND TRANSFER OF DIGITAL INFORMATION
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

   (see Recommendation Q.107 | fIbis in Fascicle VI.1)


                            ____________________
                            Recommendation Q.127
                            ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 4.4 :  RELEASE OF REGISTERS

4.4.1 :  Outgoing register
~~~~~

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 signals and has
             received a local sending-finished signal from the outgoing
             operator indicating that there are no more digits to follow.

   Case 2 :  The register has received:

             o either a number-received signal from the incoming international
               exchange indicating that all the digits comprising the complete
               national number have been received;

             o 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).



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:

      o Seized, but no further digit received from the calling subscriber;

      o Not all the digits necessary to determine the routing received;

      o Correct number of digits to determine the routing received, but no
        further digit from the calling subscriber;

      o No busy-flash or a number received signal has been received although
        the complete national (significant) number or part of it has been
        sent.

      o 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 having 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:

      o 10 to 30 s following the sending of a seizing signal;
 
      o 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 indication should be
given to the operator or calling subscriber.


 
4.4.2 Transit register
~~~~~

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 circuit.

   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 immediately 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.


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
~~~~~

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 determine 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.


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 complete 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 condition 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
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 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 immediately 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 immediately the
      incoming register:

      o Has sent back the number-received signal and sent forward the
        numerical information to the national network equipment;

      o 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
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 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 condition persists for more
than about 5 minutes.

                             ____________________
                             Recommendation Q.130
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ

 * 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 promulgated.

   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 outgoing 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 recognize 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 having been recognized,
arrangements should be made at the transit exchange to:

   o Send a blocking signal in the backward direction, to busy the outgoing
     end of the incoming circuit at the transit exchange;

   o 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
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 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
      seconds;

   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 conditions is
dealt with in S 4.4.1 (2) of Recommendation Q.127.





                                    CHAPTER V



           See ATME No. 2 specification in Recommendation Q.49 (O.22).
                              TESTING ARRANGEMENTS


                             ____________________
                             Recommendation Q.133
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 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.  Combinations 61 to 63 are allocated to the automatic transmission
measuring and signalling testing equipment No.  2



                             ____________________
                             Recommendation Q.134
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 5.2 :  ROUTINE TESTING OF EQUIPMENT (LOCAL MAINTENANCE)

5.1.1 Routine testers for testing individual items of equipment such as
~~~~~ circuit equipment, connecting circuits, operator's line calling
equipment, selectors, registers, etc., must be provided 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 maintenance 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 circuit 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
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 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 circuits 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 directions.  (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 conditions.


                             ____________________
                             Recommendation Q.136
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 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 circuit 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
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 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 outgoing 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 international 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 combination 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 measured.  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 signal 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 controlled 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 provisions 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 circuit 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 control 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 outgoing end will check that the level of the received signal is
within the prescribed limits.


                             ____________________
                             Recommendation Q.138
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 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 instruments 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 limits set by
      the specifications, the reading being made with an accuracy of _ | Hz.

      Level of the signalling frequencies to be measured to be adjustable
      between the extreme limits set by the specifications, the reading being
      made with an accuracy of _ | .2 dB.


                             ____________________
                             Recommendation Q.139
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 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 verification of unsatisfactory
      signal transmission by ensuring that a seizing 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 international exchange; or

      2) to a test call signal testing and answering device, if such equipment
         is available at the distant-end international 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 signals;

      3) complete terminal and transit calls.


 
5.7.2 First method:  rapid test
~~~~~
   1.  Verification of satisfactory signal transmission:

       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 distant 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
                  [ performance or the quality of the circuit beyond the
                  [ transit 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 unsatisfactory.



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 available 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 transit calls
~~~~~
   1.  Verification of satisfactory signal transmission (frequency, level,
       duration, etc.) involved in terminal and transit calls.

       a) These tests are made in conjunction with:

          o verification and location of faults;

          o 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 acknowledged 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 signal;

       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 succession of clear-back
          and answer signals; first at a slow rate, then at a rate which is
          faster than the system is capable of following;

       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 equipment
          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 circuit releases;

       o) At the originating end, check that the release-guard signal is
          received and recognized and that the circuit 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 unblocking signal restores
          the circuit to normal;

       w) At the distant end, connect in turn a continuous 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 centre, initiate a
transit call to this centre through the international 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.


