                                    PART I

                        Recommendations Q.140 to Q.164

                         SPECIFICATIONS OF SIGNALLING

                                  SYSTEM No 

                            SIGNALLING SYSTEM No. 5



                                 INTRODUCTION

                     PRINCIPLES OF No. 5 SIGNALLING SYSTEM


General
~~~~~~~
   System No.  5 is compatible with both TASI and non-TASI-equipped circuits
and may be applied for automatic and semi-automatic operation and both-way
working.  It requires four-wire signalling and automatic access to the
outgoing circuits.

   The signalling equipment is in two parts:

   a) line signalling - for the so-called supervisory signals

   b) register signalling - for the numerical signals.



A) LINE SIGNALLING
~~~~~~~~~~~~~~~~~~
                                         See Supplement No. 2  in this Volume.

   This is a link-by-link system using two in-band signalling frequencies 2400
Hz and 2600 Hz, two frequencies, instead of one frequency, being adopted for
the following reasons:

  i) Automatic detection of double seizing on both-way working;

 ii) Frequency discrimination between signals, no time discrimination being
     incorporated.

   Automatic detection of double seizing requires that the frequency of the
proceed-to-send signal (2600 Hz) be different from that of the seizing signal
(2400 Hz).  The detection is achieved when one end transmits the outgoing
seizing signal (2400 Hz) and at the same time receives the seizing signal
(2400 Hz) from the other end and not the 2600 Hz proceed-to-send signal
expected.

   All signal recognition times are the same (125 ms) except for the seizing
and proceed-to-send signals (40 ms).  These two signals are not subject to
signal imitation by speech and fast signalling is desired in particular to
minimize double seizings.

   To avoid, with this signalling system, relatively slow signalling in
non-TASI applications and in lightly loaded conditions (the more usual) of
TASI applications, all signals are the continuous compelled type except the
forward transfer signal.  Continuous signals ensure TASI trunk/channel
association during the actual time this function requires.  (The alternative
of TASI-prefix pulse type signals would, due to the 500-ms TASI prefix,
introduce a slight risk of failure to associate a trunk with a channel, and
would slow the signalling in terms of restoration of the transmission path
after the signalling line splits under the more usual conditions of TASI
loading and in non-TASI applications.) Only the forward-transfer signal is a
TASI-prefixed pulse since for this signal a slight risk of failure can be
accepted because it is operator-controlled and may be repeated at will.

   See for these terms footnote to Recommendation Q.151, S 3.1.1 Except for
the answer signal, all the compelled signals are normal compelled type.  For
reasons of fast speed, the answer signal is overlap-compelled at transit
points.  Fast answer signalling is desirable to minimize the risk of an
abandoned call by either the called or calling party, should the verbal answer
be lost due to the line splitting on answer signalling.

                               See for this term Recommendation Q.141, S 2.1.6
                             See for these terms Recommendation Q.141, S 2.1.7



B) REGISTER SIGNALLING
~~~~~~~~~~~~~~~~~~~~~~
   This is a link-by-link 2/6 multifrequency (m.f.) in-band en block pulse
signalling system, forward signalling only.  The alternative, continuous
compelled signalling, would be slow due to the long circuit propagation times
in certain applications.  The frequencies (700 Hz .  | | 1700 Hz) are outside
of the line signalling frequencies.  The numerical information signalling is
preceded by a KP signal (start-of-pulsing) and terminated by an ST signal
(end-of-pulsing).  En bloc non-overlap sending applies at the outgoing
international register, the seizing signal being sent, and thus the
international circuit being taken as late as possible, namely when the ST
condition is available in the outgoing international register.  When sending,
the outgoing register pulses out in a continuous sequence.  The prior GO
trunk/channel association due to the seizing signal is maintained by the TASI
speech detector hangover during the interval between cessation of the seizing
signal (on receipt of the proceed-to-send signal) and the start of the
register pulse out, and during the intervals between successive m.f.  signals.

   En bloc overlap register signalling applies at the international transit
registers and at the incoming international register to minimize the
post-dialling delay.

   Compandors affect signalling, particularly short-pulse compound signalling
(e.g.  register signalling), due to distortion and the production of
intermodulation frequencies.  By virtue of the lin-by-link signalling and the
adopted duration of the m.f.  pulses, system No.  5 functions correctly in the
presence of compandors.




                                   CHAPTER I

                      DEFINITION AND FUNCTION OF SIGNALS

                             Recommendation Q.140


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 and to seize
equipment for switching the call either to the national network of the
incoming country or 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.



 * 1.3 : Start-of-pulsing signal, also called for system No.  5 "KP signal"
         (sent in the forward direction)

   This numerical type signal is sent on receipt of a proceed-to-send signal
and may be used to prepare the incoming international register for the receipt
of the subsequent numerical signals.

   Two different KP signals are provided to discriminate between terminal and
transit calls:

   a) KP1, terminal; and

   b) KP2, transit.



 * 1.4 :  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.



 * 1.5 :  End-of-pulsing signal, also called for system No.  5 "ST signal"
          (sent in the forward direction)

   This numerical type signal is sent to show that there are no more numerical
signals to follow.  The signal is always sent in semi-automatic as well as in
automatic working.



 * 1.6 : Busy-flash signal (sent in the backward direction)

   This signal, which is sent only after the proceed-to-send 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 after register
     association, to indicate that there is congestion at that exchange or on
     the appropriate outgoing routes.

  b) An incoming international exchange must send this signal, after register
     association, if there is congestion at that exchange or on the outgoing
     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 outgoing operator or to the
   calling subscriber

 o The sending of the clear-forward by the outgoing exchange to release the
   international connection (except when otherwise arranged, for example, in
   this 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 signal is sent to the outgoing international exchange to indicate that
the called party has cleared.  In the semi-automatic...

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

   ...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 measurement of call duration if,
between 1 and 2 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.  See the corresponding Notes in
   Recommendation Q.120.



 * 1.9 :  Clear-forward signal (sent in the forward direction)

   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).

   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 Recommendation Q.118, SS 4.3.1 and 4.3.2 for automatic working
and S 4.3.1 for semi-automatic working).  This signal may also be sent after
an abnormal release of an outgoing register in the case indicated in
Recommendation Q.156 under S 3.6.2 a) 1.



 * 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.  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 :  Forward-transfer signal (sent 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 or delay operator) at the incoming
international exchange, the signal should preferably cause this operator to be
recalled.



 * 1.12 :  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 II.

   A description of the various operations corresponding to the various normal
and abnormal conditions which may arise in setting up a call are given in the
tables of Annex 2 to Part II.



                         MONTAGE:   PAGE 54 = BLANCHE

                                   CHAPTER II

                                LINE SIGNALLING

                              Recommendation Q.141

                                      2.1

                         SIGNAL CODE FOR LINE SIGNALLING

 * 2.1.1 :  General

   The line-signal coding arrangement is based on the use of two frequencies f
1 (2400 Hz) and f 2 (2600 Hz) transmitted individually or in combination as
shown in Table 1.  The use of compound signalling for the
clear-forward/release-guard sequence increases the immunity to false release
by signal imitation.

                       See the definition of assistance operator in S 1.1.6 of
                       Recommendation  Q.101.

   By taking advantage of the fixed order of occurrence of specific signals,
signals of the same frequency content are used to characterize different
functions.  For example, in the backward direction f 2 is used to indicate
proceed-to-send, busy-flash and clear-back without conflict.  The signalling
equipment must operate in a sequential manner retaining memory of the
preceding signalling states and the direction of signalling in order to
differentiate between signals of the same frequency content.  All signals
except the forward-transfer signal are acknowledged in the compelled-type
manner as indicated in Table 1.  The order of transmission of backward signals
is subject to the following restrictions:

  a) Busy-flash signal:  never after an answer signal and only after a
     proceed-to-send signal;

  b) Answer signal:  never after a busy-flash signal;

  c) Clear-back signal:  only after an answer signal.

Note - The receipt of the answer signal (f 1) permits discrimination between
       the busy-flash and the clear-back signals (both f 2).

   A clear-forward signal, which must be acknowledged by a release-guard
signal under all conditions of the equipment including the idle condition, may
be sent from an outgoing end at any time to initiate the release of the
circuit.  The clear-forward signal is completely overriding and may break into
any other signal sequence.



 * 2.1.2 :  Transit working

   In transit operation, the line equipment at the transit exchange shall be
informed (e.g.  by the register) that the condition is transit.  This will
facilitate the link-by-link transmission of line signals through the transit
exchange without bringing about consequences appropriate to the terminal
exchanges.



 * 2.1.3 :  Sending duration of line signalling

2.1.3.1 The sending durations of the line signals are shown in Table 1.
~~~~~~~ Additional requirements are:

  a) In the event of double seizing (due to both-way operation), the seizing
     signal transmitted from the end having detected double seizing should
     persist for at least 850 _ 200 ms to permit the other end to detect the
     double seizing.

  b) Should the called party flash his switch-hook at a faster rate than the
     equipment can transmit a succession of clear-back and answer signals, the
     correct indication of the final position of the switch-hook must always
     be given by the appropriate signal.

  c) Once the sending of a signal (pulse or compelled) has begun it should be
     completed (but see S 2.1.1 in regard to the clear-forward signal
     releasing the circuit at any stage and S 2.1.7 in regard to the overlap
     answer signal at transit points).  If two signals have to be sent one
     immediately after the other in the same direction, a silent interval of
     not less than 100 ms should separate the two successive signals.  The
     silent interval should not be so long as to cause unreasonable delay in
     signalling.

     Exceptionally

     1) The intervals between successive signals may be less than 100 ms.
        However, the technique of complete signals with intervals of at least
        100 ms is the preferred arrangement;

     2) the forward-transfer signal may be ceased immediately if a backward
        signal is received.  The acknowledgement of the backward signal is
        then sent.

  d) When sending a compound signal, the interval of time between the moments
     when each of the two frequencies is sent must not exceed 5 ms.  The
     interval of time between the moments when each of the two frequencies
     ceases must not exceed 5 ms.

  e) Time-out and alarm procedures

     i) Should the transmission of any size, busy-flash, answer, clear-back or
        clear-forward signal persist beyond a maximum of 10 to 20 seconds, the
        signal shall be terminated.

               Note - 10 to 20 seconds time-out for the seizing signal allows
               reasonable time for association of a register in a distant
               centre.

    ii) Should the transmission of any proceed-to-send, release-guard or other
        acknowledgement signal persist beyond a maximum of 4 to 9 seconds, the
        signal shall be terminated.

               Note 1 - The shorter time-out periods for secondary sig nals
               enable, under many conditions, detection of a fault at both
               ends of a circuit on a single call.

               Note 2 - Time-out of the answer acknowledgement signal may
               cause charging without a satisfactory transmission path to the
               called party.  If the occurrence of such time-outs should reach
               unacceptable levels, a delay in the transfer of the answer
               signal into the national network until the compelled answer
               signalling cycle is complete, may be justified.

   iii) Upon the occurrence of a time-out under the two above conditions, the
        attention of the maintenance personnel should be drawn to the fact
        that time-out has occurred.


               Note - An Administration may decide that on the time-out of an
               acknowledgement signal at the incoming end of the connection,
               when an automatic repeat clear-forward sequence is known to be
               provided at the outgoing end, no indication is given to the
               maintenance personnel, neither is the circuit taken out of
               service.

    iv) Upon the occurrence of a time-out, the circuit should automatically be
        removed from service after cleardown by the subscriber and blocked to
        outgoing calls.  However, time-out of a seize signal may be excluded
        from this provision if time-out of that signal is followed by a
        clear-forward attempt.

     v) As a test procedure, Administrations may make repeated signalling
        attempts and restore the circuit to service if it is found to perform
        in a normal manner.

    vi) Each Administration shall make appropriate arrangements to ensure that
        a single fault will not cause removal from service of more than one
        circuit or of more than one register.

2.1.3.2 The duration of the forward-transfer signal is based on the
~~~~~~~ possibility that TASI may clip a signal by up to 500 ms on rare
        occasions during heavy traffic periods, and on the need for
        establishing arecognition time that minimizes signal imitation.



 * 2.1.4 :  Recognition times of line signals

   Recognition time is defined as the minimum duration a direct-current
signal, at the output of the signal receiver, must have in order to be
recognized as a valid condition by the switching equipment.  The recognition
times are given in Table 1.

   For equal immunity against signal imitation, the recognition time of
compound signals such as the clear-forward/release-guard sequence could be
less than that of the single-frequency signals liable to signal imitation.
However, for convenient design arrangements, and to improve the immunity of
the clear-forward/release-guard sequence, the recognition time of the compound
signals is the same (125 _ 25 ms) as that of the single-frequency signals
liable to signal imitation.

   After signal recognition, interruptions of up to 15 ms in the primary or
acknowledgement signals shall be ignored in the compelled signalling
sequences.  Interruptions of more than 40 ms must be recognized as the end of
the appropriate signal in the compelled signalling sequences.



 * 2.1.5 :  Line signal code of System No.  5

            The line signal code is given in Table 1.

                                                           Table [1], p.18



 * 2.1.6 :  Further specification clauses relative to the signalling code

   This type of signalling is called "continuous compelled."

   a) The seizing signal continues until acknowledged by the proceed-to-send
      signal.  The proceed-to-send signal is transmitted when an incoming
      register is associated and continues until acknowledged by the stopping
      of the seizing signal

   b) The clear-forward signal continues until acknowledged by the
      release-guard signal, which may be sent as described under 1 or 2 below:

      1) The release-guard signal is sent on recognition of the clear-forward
         signal and continues until acknowledged by the cessation of the
         clear-forward signal or until the relevant incoming equipment at the
         international exchange is released, whichever occurs later.

      2) The release-guard signal is sent in response to the clear-forward
         signal to indicate that the latter has brought about the release of
         the relevant incoming equipment at the international exchange.  The
         release-guard signal continues until cessation of the clear-forward
         signal is recognized.

   The outgoing access of the incoming end of the both-way circuit shall be
maintained busy for 200 to 300 ms after the end of the transmission of the
release-guard signal.

   c) With respect to the busy-flash, answer and clear-back signals the
      acknowledgement signal shall not be transmitted before the signal
      recognition time (125 _ 25 ms) of the primary signal has elapsed.  The
      primary signal shall not be ceased until the signal recognition time
      (125 _ 25 ms) of the acknowledgement signal has elapsed (see S 2.1.7
      with respect to the transmission of the answer signal at a transit
      point).


   d) The busy-flash will be transmitted if the call cannot be completed for
      any of the following reasons:

      1) Congestion at an incoming international exchange;

      2) Congestion at a transit international exchange;

      3) Error detected in the receipt of the register signals;

      4) busy-flash (if received) from a subsequent international system (e.g.
         system No.  4) or from the national network;

      5) Time-out of an incoming international register.


   e) Receipt of busy-flash at the outgoing international exchange will cause:
       - after signal recognition time (125 _ 25):

      1) the acknowledgement signal to be sent

      2) an appropriate audible indication to be transmitted to the operator
         or to the subscriber.  When the preceding circuit provides for the
         transmission of busy-flash, this signal should be transmitted to that
         preceding circuit;

      - after the end of the compelled sequence, i.e.  100 ms after
        termination of the acknowledgement signal (see S 2.1.3 c):

      3) A clear-forward signal to be transmitted from that exchange and the
         international circuit or chain of circuits to be released by the
         clear-forward/release-guard sequence.

  f) Receipt of busy-flash at a transit exchange will cause after signal
     recognition time:

      1) The acknowledgement signal to be sent

      2) The busy-flash signal to be sent on the preceding incoming circuit;

      3) The transit exchange and forward connection to be cleared.

Note - Where existing equipment is designed to allow clearing only from the
       outgoing international exchange, this need not be modified
       retrospectively.

  g) Upon receipt of the answer signal in the answer state or the clear-back
     signal in the clear-back state, the international exchange should,
     nevertheless, respond by sending the acknowledgement signal.

Note - This procedure will be helpful to avoid unnecessary discontinuity of
       the compelled sequence when the international exchange receives answer
       (f 1) of clear-back (f 2) signal twice within a short interval.

  h) In order to prevent irregularities Administrations may decide that the
     sending time of the release guard signal has a minimum duration of 200
     ms.  The recognition of a release guard signal without prior sending of a
     clear forward signal should be regarded as an irregularity.
     Administrations may decide to react on detection of that irregularity by
     sending the clear forward signal.

                                                      Figure 1/Q.141, p.19



 * 2.1.7 :  Backward signals on multilink connections (consider as an example
            a connection A-T-B)

  a) Normal compelled signalling for busy-flash and clear-back signals

     With normal compelled signalling (see S 2.1.6 c) above) at a transit
     point T, the transmission of the primary signal from T to A does not
     commence until the signal recognition time of the primary signal sent
     from B to T has elapsed.  This technique is applied for the transmission
     of busy-flash and clear-back signals.

  b) Overlap compelled signalling for the answer signal

     With overlap compelled signalling at a transit point T, the process of
     transmitting the primary signal from T to A is initiated as soon as the
     signal receiver response has caused at T the receiving end line split of
     BT.  The normal signal recognition of the primary signal is still
     required at each transit point.  The acknowledgement signal on a
     particular link should not be transmitted until signal recognition time
     of the primary signal has elapsed.  To speed up the transmission of the
     answer signal, the overlap compelled technique is applied for this signal
     at a transit exchange when two No.  5 circuits are switched in tandem.

     More details of the overlap compelled technique are given below:

     If the primary signal from B to T lasts less than the signal recognition
     time, transmission of a primary signal already initiated at a transit
     point T from T to A will be stopped.

     After the recognition time at T of a primary signal from B to T has
     elapsed, there shall be no control at T of the primary signal sent from T
     to A by the primary signal sent from B to T.  In this case the primary
     signal on each link is ceased by its acknowledgement signal on that link
     (as in S 2.1.6 c) above).

     Figure 1/Q.141 illustrates a typical arrangement and is included to
     illustrate the principle of overlap compelled signalling at transit
     points.  Other design arrangements may be adopted as preferred by
     Administrations.

     Transmission of the primary signal from T to A is initiated (by a "start
     to send" control condition X through the switch block at the transit
     point) as soon as the signal receiver response on the primary signal from
     B to T has caused the receiving-end line split (t1of T1).  The primary
     signal is transmitted from T to A after the sending-end line split (t3of
     T3).  Signal recognition of the primary signal is required at the transit
     point and the acknowledgement signal on a particular link should not be
     transmitted until the signal recognition time (t2of T1, t2of T4) has
     elapsed.  The primary signal is ceased after the signal recognition time
     (t2of T2, t2of T5) of the relevant acknowledgement signal.

     To prevent imitations of the primary signal on link BT lasting less than
     the signal recognition time from giving rise to an effective compelled
     signalling sequence on link TA, transmission of the primary signal on
     link TA is first under the "start to send" control X of a time base
     T3 followed, without break at the termination of the time base (at time
     Z), by the continuous signal control required for compelled signalling.
     Should the duration of the primary signal on link BT be less than the
     signal recognition time (t2of T1), the "start to send" control (X
     control) is interrupted.  This stops transmission of a primary signal on
     link TA (should this have commenced) within the period X-Z of T3and hence
     before the continuous signal control can be applied.

     After the signal recognition time of the primary signal on link BT has
     elapsed, there shall be no control of the transmission of the primary
     signal on link TA by the primary signal on link BT at the transit point.
     To achieve this, a condition is applied to the Y control to inhibit the X
     control, which should ensure that transmission of the primary signal on
     link TA cannot be stopped during the period X-Y of T3and that the
     continuous signal control of the primary signal is applied without break
     at time Y (or at time Z depending upon the particular design).  In these
     circumstances the primary signal on each link is ceased by its relevant
     acknowledgement signal.



                             Recommendation Q.142

                  2.2  DOUBLE SEIZING WITH BOTH-WAY OPERATION


 * 2.2.1 :  Unguarded interval

   Considering that on long international (intercontinental) circuits:

   a) The sending end splitting time may be 50 ms prior to signal
      transmission;

   b) TASI may occasionally clip the initial 500 ms of seizing signals;

   c) Circuit propagation time may be relatively long;

   d) The signal receiver response time must be taken into account;

   e) The recognition time of seizing signals is 40 _ 10 ms; the unguarded
      interval relative to double seizing in the extreme case approaches 600
      ms plus the circuit propagation time and the signal receiver response
      time.  The signalling system should therefore detect double seizing and
      take action as defined in S 2.2.2.



 * 2.2.2 :  Detection of double seizing

   In the event of double seizing, the same frequency (f 1) is received as is
being transmitted at each terminal.  This condition shall be detected by the
signalling equipment and shall cause stop- page of the outgoing seizing signal
at each end.  An end having detected double seizing, and terminated the
outgoing seizing signal 850 _ 200 ms after this signal has been transmitted,
will maintain the circuit in the busy condition until the stoppage of the
incoming seizing signal from the distant end.  Each outgoing seizing signal
maintained for at least 850 _ 200 ms will ensure that both ends of the circuit
will detect the double seizing.

   The signalling equipment will be released on termination of both the
outgoing and incoming seizing signals and a clear-forward shall not be sent.

   Either of the following arrangements may apply on detection of double
seizing:

   a) an automatic repeat attempt to set up the call; or

   b) a re-order indication is given to the operator or to the subscriber and
      no automatic repeat attempt is made.

   Method a) is the preferred arrangement (see Recommendation Q.108).

   Method a) does not require the repeat attempt to be limited to the circuit
used at the first attempt, but should the first circuit be seized again at the
second attempt on the second search over the circuits, a minimum time of 100
ms shall elapse between the termination of the first attempt outgoing seizing
signal (or the recognition of the cessation of the incoming seizing signal,
whichever occurs later) and the commencement of the second attempt seizing
signal.

   To minimize the probability of double seizing, the circuit selection at the
two ends should be such that, as far as possible, double seizing can occur
only when a single circuit remains (e.g.  by selection of circuits in opposite
order at the two ends).



                               Recommendation Q.143
 
                         See also Recommendation Q.112.

                             2.3 LINE SIGNAL SENDER


 * 2.3.1 :  Signalling frequencies

   2400 _ 6 Hz (f 1) and 2600 _ Hz (f 2).

   These frequencies are applied separately or in combination.



 * 2.3.2 :  Transmitted signal level

   -9 _ 1 dBm0 per frequency.

   For compound signals the difference in transmitted level between f 1 and f
2 shall not exceed 1 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 between 300 Hz and 3400 Hz including power resulting
         from non-linear distortion of the signal.

Note 2 - The level of the leak current transmitted to line should be at least
         50 dB below signal level per frequency.





                              Recommendation Q.144

                            2.4 LINE SIGNAL RECEIVER

 * 2.4.1 :  Operating limits

   The line signal receiver shall operate in the conditions specified under S
2.4.5 for the distortion of received signals that meet the following
conditions:

   a) f 1:  2400 _ 15 Hz; f 2:  2600 _ 15 Hz.

   b) The absolute power level N of each unmodulated signal received shall be
      within the limits:
                            (-16 + n )  (-2 + n ) dBm

      where n is the relative power level at the signal received input.

   These limits give a margin of _7 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 in a
      compound signal may differ from each other by not more than 5 dB.

   The tolerances given in a), b) and c) are to allow for variations at the
sending end and for variations in line transmission.



 * 2.4.2 :  Non-operate conditions of line signal receiver

   a) Selectivity

      The signal receiver shall not operate on a signal having an absolute
      power level at the receiving end within the limits specified in S 2.4.1
      when the frequency is outside:

               2400 |00 u-|50  Hz for the f 1 signal circuit or

               2600 |50 u-|00  Hz for the f 2 signal circuit.

   b) Maximum sensitivity of line signal receiver

      The signal receiver shall not operate on a signal of 2400 _ 15 Hz or
      2600 _ 15 Hz whose absolute power level at the point of connection of
      the receiver is (-17 -9 +n ) dBm, n being the relative power level at
      this point.  This limit is 17 dB below the nominal absolute level of the
      signal current at the input to the signal receiver.



 * 2.4.3 :  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 advisable 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 frequencies 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 false operation of the f 1 or the
            f 2 signal circuit lasting more than 90 ms (the minimum
            recognition time of a signal liable to imitation is 100 ms);

         b) The number of false splits of the speech path caused by speech
            currents should not cause an appreciable reduction in the
            transmission quality of the circuit.

Note - Since Signalling System No.  5 and V.22 modems are using the same
       frequency, additional tests where speech is replaced by data
       transmission should be performed so that the connection is not released
       at the start of data transmission.  The quality requirement is for
       further study.



 * 2.4.4 :  Guard circuit limits

 A.  Steady noise

     Considering :

     a) that when there is noise on a telephone 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 | 00 pW) and uniform
        spectrum energy may arise on the longest international, i.e.
        intercontinental, circuit;

        it is recommended that, for either one or two signalling currents
        (each being within the limits specified in S 2.4.1), the signal
        receiver should satisfy the conditions indicated in S 2.4.5 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 hand-over 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 2.4.1, the lengths of the received signals must
     remain within the limits specified in S 2.4.5.



 * 2.4.5 :  Distortion of received signals

   When the signal frequencies and levels are within the limits specified in S
2.4.1, the change in signal length in the presence of noise as defined in S
2.4.4, A should not exceed:

   a) 15 ms when the signal receiver receives a pulse of one frequency f 1 or
      f 2 with a minimum duration of 150 ms;

   b) 25 ms when the signal receiver receives a compound pulse of the two
      frequencies f 1 and f 2 with a minimum duration of 150 ms, the change
      being defined as the difference between the simultaneous reception of
      the two frequencies at the input to the receiver and the simultaneous
      production of the two components as a direct-current signal at the
      output of the signal receiver.

   In general, the response time of the signal receiver should be as short as
practicable to minimize the time required for signalling purposes.

   Except for the forward transfer pulse signal the above pulse distortion
requirements are of minor importance for the remaining line signals, which are
all of the continuous compelled type the limits are specified for receiver
design and test purposes.



                             Recommendation Q.145

                          2.5  SPLITTING ARRANGEMENTS

Sending line split
~~~~~~~~~~~~~~~~~~
 * 2.5.1 :  According to Recommendation Q.25, S 2, sending split arrangements
            have to be provided.

 * 2.5.2 :  The exchange side of the international circuit shall be
            disconnected 30 to 50 ms before a voice-frequency signal is sent
            over the circuit.


                      See Recommendation Q.141, S 2.1.6, explaining the  term
                      "continuous compelled."


 * 2.5.3 :  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.

 * 2.5.4 :  Exceptionally, the values quoted in SS 2.5.2 and 2.5.3 above may
            be 0 to 50 ms as the values are of minor importance with respect
            to compelled-type signals.


Receiving line split
~~~~~~~~~~~~~~~~~~~~
 * 2.5.5 :  The international circuit should be split at the international
            exchange when either a single-frequency or a compound-frequency
            signal is received, to ensure that no fraction of the signal
            exceeding 35 ms duration may pass out of the international
            circuit.

      The splitting time of 35 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.

 * 2.5.6 :  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.

 * 2.5.7 :  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.

 * 2.5.8 :  The splitting device may be any suitable arrangement - for
            example, physical line disconnection, high impedance electronic
            device, insertion of signalling frequency band stop filter, etc.
            The level of leak current transmitted to the subsequent circuit
            from the splitting device in the split condition should be at
            least 40 dB below the received signal level.  Exceptionally, the
            level of the leak current may be 25 dB below the received signal
            level if this causes no interference with the relevant networks.




                           Recommendation  Q.146

               2.6  SPEED OF SWITCHING IN INTERNATIONAL EXCHANGES


 * 2.6.1 :  It is recommended that the equipment in the international
            exchanges shall have a high switching speed so that the switching
            time may be as short as possible.



 * 2.6.2 :  At the outgoing international exchange the seizing of the circuit
            and the setting up of the connection should take place as soon as
            the ST end-of-pulsing condition is available (see Recommendation
            Q.152).  In automatic operation advantage should be taken of all
            cases in which the ST condition can be reasonably determined at
            once, i.e.  with avoidance of the 4-6 seconds time-out.

   At an international transit exchange the setting up of the connection on
the outgoing circuit should take place as soon as the digits necessary to
determine the routing, are received and analyzed.

   At the incoming international exchange the setting up of the national part
of the connection should start as soon as the register has received a
sufficient number of digits.



 * 2.6.3 :  At international exchanges the return of a proceed-to-send signal
            should be as fast as possible but in any case the return should
            normally be guaranteed before the time-out (minimum 10 seconds) of
            the seizing signal.

   Furthermore, in the case of congestion on the circuits outgoing from a
transit or an incoming exchange, a busy-flash signal should be returned as
soon as practicable, but in any case within a maximum delay of 10 seconds
following the receipt of the information necessary to determine the routing.





                       MONTAGE:    PAGE 64 = PAGE BLANCHE


                                   CHAPTER III
 
                               REGISTER SIGNALLING

                              Recommendation Q.151

                    3.1  SIGNAL CODE FOR REGISTER SIGNALLING


 * 3.1.1 :  General

   1) Automatic access to the international circuits must be used for outgoing
      traffic and the numerical signals from the operator or subscriber are
      stored in an outgoing international register before an international
      circuit is seized.  As soon as the ST (end-of-pulsing) condition is
      available to the outgoing register, a free international circuit is
      selected and a seizing line signal transmitted.  On receipt of a
      proceed-to-send line signal the seizing signal is terminated and a KP
      ("start of pulsing") pulse, followed by the numerical signals, is
      transmitted by the register.  The final register signal transmitted is
      an end-of-pulsing (ST) pulse.  The register signalling is not required
      to be TASI-prefixed.

      En bloc register signalling is the transmission, by a register, of all
      the call information as a whole in a regular timed sequence of signals.
      The technique requires that, in one register on the connection, all the
      relevant call information from a subscriber or operator shall be
      completely stored before output en bloc signal transmission takes place
      from that register.  At registers subsequent to the one where all the
      call information from a subscriber or operator is completely stored, the
      output signal transmission may commence before the complete reception of
      the input information; thus overlap to any desired degree of the output
      signal transmission with the input signal reception may occur and this
      may be understood as being en block overlap .  Alternatively, the output
      signal transmission may be delayed until all the call information is
      received and stored.  This may be understood as being en bloc
      non-overlap.

   2) Link-by-link register signalling applies.  The register signals are
      always sent en bloc non-overlap applies at the outgoing international
      register.  En bloc overlap applies at the transit and incoming
      international registers.

   3) On a particular link, the KP signal sent by the international register
      (outgoing or transit register) on receipt of a proceed-to-send signal
      may be used to prepare the distant international register on this link
      for the receipt of the subsequent numerical signals.  This signal may
      also serve to discriminate between terminal and transit traffic:

      a) Terminal KP (KP1).  Used to create conditions at the next exchange so
         that equipment (or techniques) used exclusively for switching the
         call to the national network of the incoming country is brought into
         circuit.

      b) Transit KP (KP2).  Used to bring into circuit, at the next exchange,
         equipment (or techniques) required to switch to call to another
         international exchange.

   4) The register signalling is a 2-out-of-6 multifrequency code, forwad
      signalling only, as shown in Table 2.


                                                           TABLE [2], p.20



 * 3.1.2 :  Sending sequence of register signals

   The sequence of the register signals shall conform to the sequence
indicated in Recommendation Q.107, noting the following:

   a) A KP start-of-pulsing signal shall precede the sequence of numerical
      signals in all the cases indicated;

   b) The ST end-of-pulsing signal will be transmitted from the register in
      automatic as well as in semi-automatic operation;

   c) Exceptionally, special numbers for giving access to incoming operators
      or delay operators may be dialled by outgoing operators and submitted by
      outgoing international registers instead of code 11 and code 12 signals.




                          Recommendation Q.152

                   3.2  END-OF-PULSING CONDITIONS  - REGISTER
                                  ARRANGEMENTS

                     CONCERNING ST (END-OF-PULSING) SIGNAL


 * 3.2.1 :  The register signalling arrangements provide for the sending of a
            ST signal for both semi-automatic and automatic operation; the
            arrangements in the outgoing international register for
            recognizing the ST end-of-pulsing condition will vary as follows:

  a) Semi-automatic operation.  The ST condition is determined by the receipt
     of the "sending-finished" signal from the operator (see Recommendation
Q.106).

  b) Automatic operation

     1) Where the ST condition is determined by the originating national
        network and an ST signal is produced and transmitted to the outgoing
        international register, no further arrangements are necessary in that
        register for this purpose.

     2) Where the ST condition is not received from the originating national
        network, the outgoing international register will be required to
        determine the ST condition.  This ST condition is determined when the
        cessation of numerical information input to the register exceeds a
        period of 4 seconds (5 | (+- | seconds) in either of the following two
        circumstances, as preferred by the Administration:

        i) After the minimum number of digits in the world numbering plan; or

       ii) After the minimum number of digits of the destination country
           numbering plan.

   In i) and ii), prolonged cessation of the numerical information input
before the minimum number of digits should result in time-out of the register
without the production of the ST condition.

   An immediate ST condition may be produced by a digit count to avoid the
4-second delay ST condition in the following circumstances:

        i) When the destination country numbering plan has a fixed number of
           digits;

       ii) When the maximum number of digits in the numbering plan of the
           destination country has been received.



 * 3.2.2 :  Under all conditions, the outgoing international circuit should
            not be seized until the ST end-of-pulsing condition is available
            in the outgoing international register.




                              Recommendation Q.153

                        3.3 MULTIFREQUENCY SIGNAL SENDER


 * 3.3.1 :  Signalling frequencies

   700, 900, 1100, 1300, 1500 and 1700 Hz.

   A signal shall consist of a combination of any two of these six
frequencies.  The frequency variation shall not exceed _ | Hz of each nominal
frequency.



 * 3.3.2 :  Transmitted signal level

   -7 | (+- |  dBmO per frequency.

   The difference in transmitted level between the two frequencies comprising
a signal shall not exceed 1 dB.

Note - The level of the leak current transmitted to line should be at least:

   a) 50 dB below the single-frequency level when a multifrequency signal is
      not being transmitted;

   b) 30 dB below the transmitted signal level of either of the two
      frequencies when a multifrequency signal is being transmitted.



 * 3.3.3 :  Signal duration

   KP1 and KP2 signals:  100 | (+- | 0 ms

   All other signals:  55 | (+- | ms

   Interval between all signals:  55 | (+- | ms

   Interval between cessation of the seizing line signal and transmission of
   the register KP signal:  80 | (+- | 0 ms.



 * 3.3.4 :  Compound signal tolerance

   The interval of time between the moments when each of the two frequencies
comprising a signal is sent must not exceed 1 ms.  The interval of time
between the moments when each of the two frequencies ceases must not exceed 1
ms.



  
                             Recommendation Q.154

                       3.4 MULTIFREQUENCY SIGNAL RECEIVER

 * 3.4.1 :  Operating limits

   The signal receiver must ensure a separate output signal for each of the
six voice-frequency signals received, and must operate satisfactorily for any
combination of two of the frequencies, received as a single pulse or in a
train of pulses, satisfying the following conditions:

    a) The frequency of the received signal is within _ | 5 Hz of the nominal
       signalling frequency;

    b) The absolute power level N of each unmodulated signal shall be within
       the limits (-14 + n 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 levels of the two unmodulated frequencies comprising a
       signal must not differ from each other by more than 4 dB;

    d) When the signal frequencies and levels are within the limits specified
       in a), b) and c) above, and in the presence of noise as defined in S
       3.4.3:

       1) at the input of a signal receiver, the minimum duration of an MF
          signal necessary to ensure correct registration of the digit shall
          not exceed 30 ms; this includes the operate time of the signal
          receiver and the two-and-two only check feature;

       2) furthermore, at the input of the signal receiver, the minimum
          duration of an interval necessary to ensure the correct functioning
          of the registration device shall not exceed 30 ms; this includes the
          release time of the signal receiver and the restoration time of the
          two-and-two only check feature.

Note 1 - The tolerances given in a), b) and c) are to allow for variations at
         the sending end and in line transmission.

Note 2 - The test values indicated in d) are less than the working values.
         The difference between the test and working values will allow for
         pulse distortion, difference in time of the receipt of the two
         frequencies comprising a signal, etc.



 * 3.4.2 :  Non-operating conditions

  a) Maximum sensitivity

     The signal receiver shall not operate under the effect of a signal as
     indicated in S 3.4.1 | ) whose absolute power level at the point of
     connection of the receiver is (-17 -7 + n ) dBm, n being the relative
     power level at this point.

     This limit is 17 dB below the nominal absolute power level of the signal
     current at the input to the signal receiver.

  b) Transient response

     Operation of the signal receiver shall be delayed for a minimum period
     necessary to guard against false operation due to spurious signals
     generated within the receiver on reception of any signal.

  c) Short signal response

     The signal receiver should not operate to a pulse signal of 10 ms or
     less.  This signal may be of single frequency or two frequencies received
     simultaneously.

     Likewise the signal receiver should ignore short intervals.



 * 3.4.3 :  Steady noise

   Considering that unweighted noise of a level -40 dBm0 (100 000 pW) and
uniform spectrum energy may arise on the longest international circuit, the
multifrequency receiver should satisfy the condition indicated in S 3.4.1 | )
for minimum signal and interval durations in the presence of noise of level
-40 dBm0 and uniform spectrum energy over the frequency range 300 to 3400 Hz.



 * 3.4.4:  Input impedance

   The input impedance should be such that the return loss over a frequency
range 300 to 3400 Hz against a 600 ohm non-inductive resistor is greater than
20 dB.





                               Recommendation Q.155

                3.5 ANALYSIS OF DIGITAL INFORMATION FOR ROUTING

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






                               Recommendation Q.156

                    3.6  RELEASE OF INTERNATIONAL REGISTERS

 * 3.6.1 :  Normal release conditions

  a) An outgoing international register shall be released when it has
     transmitted the ST signal.

  b) An incoming international register shall be released in either one of the
     following two cases:

     1) Depending on the arrangements adopted by the Administration concerned
        at the incoming international exchange.  For example:  release on
        transmission of the ST signal, release on receipt of a number-received
        condition from the national network, etc.

     2) When the busy-flash signal is returned.  The return of the busy-flash
        signal in the case of congestion at the incoming exchange should take
        place as soon as practicable, but in any case within a maximum delay
        of 10 seconds following the receipt, at the incoming exchange, of the
        digits necessary to determine the routing.

  c) A transit international register shall be released in either one of the
     following two cases:

     1.  When it has transmitted the ST signal.

     2.  When the busy-flash signal is returned.  The return of the busy-flash
         signal in the case of congestion at the transit exchange should take
         place as soon as possible, but in any case within a maximum delay of
         10 seconds following the receipt, at the transit exchange, of the
         digits necessary to determine the routing.



 * 3.6.2 :  Abnormal release conditions

  a) An outgoing international register shall be released in either one of the
     following two cases:

     1.  Proceed-to-send signal not received.

     To release after the maximum delay of 10 to 20 seconds indicated by the
     time-out of the seizing signal.  Register release after this delay will
     depend upon the arrangements preferred by the Administrations concerned,
     but release should preferably take place as quickly as possible after the
     time-out of the seizing line signal.  On the affected incoming circuit,
     the preferred action is to return a congestion signal.

     2.  Proceed-to-send signal received.

     This case assumes that the proceed-to-send signal has ceased at the
     incoming end in the normal way but owing to a fault condition the
     outgoing register has not pulsed out.  The outgoing register will be
     released by the clear-forward/release-guard sequence prompted by the
     busy-flash signal sent from the incoming end on non-receipt of register
     signals within the appropriate time.  This assumes that the busy-flash
     signal is received at the outgoing end before the termination of any
     forced release delay that Administrations may wish to incorporate in the
     outgoing register.

  b) An incoming international register shall be released in either one of the
following two cases:

     1.  The ST signal not received within a certain time after commencement
         of the transmission of the proceed-to-send signal from the incoming
         end.

     2.  On return of the busy-flash signal, transmitted from the incoming end
         when an error is detected in the receipt of the register
         multifrequency signals.

  c) A transit international register shall be released in any one of the
     cases stated for the release of the outgoing and incoming registers in SS
     a) and b) above.




                             Recommendation Q.157

                     3.7  SWITCHING TO THE SPEECH POSITION


   At the outgoing and transit international exchanges, the circuit shall be
switched to the speech position when the register (outgoing or transit) is
released after sending the ST signal.

   At the incoming international exchange, the circuit will be switched to the
speech position when the register is released (see S 3.6.1 of Recommendation
Q.156).

       MONTAGE:    PAGE 70 = PAGE BLANCHE








                                   CHAPTER IV

            MANUAL TESTING ARRANGEMENTS FOR SIGNALLING SYSTEM No. 5

                              Recommendation Q.161

                  4.1  GENERAL ARRANGEMENTS FOR MANUAL TESTING

           See Recommendation Q.49/O.22: "Specifications  for  the









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

                              Recommendation Q.162

              4.2 ROUTING TESTING OF EQUIPMENT (LOCAL MAINTENANCE)

 * 4.2.1 :  Routine tests for testing individual items of equipment such as
            circuit equipment, connecting circuits, operator's line calling
            equipment, selectors, registers, etc., must be provided for in
            every international exchange equipped for automatic switching.
            These routine tests will be made in accordance with the practice
            followed in each country for the local maintenance of the
            switching equipment.


 
 * 4.2.2 :  The testing equipment must conform to the following principles:

  a) an item of equipment must not be taken for test until it is free;

  b) an item of equipment taken for test will be marked "engaged" for the
     duration of the test.  Before a circuit equipment is taken for test, the
     circuit will be withdrawn from service at both international exchanges;

  c) as an alternative to b), a like item of equipment, known to be properly
     adjusted, may be switched in, and the item of equipment to be tested is
     switched out during the test.



 * 4.2.3 :  Testing of the circuit and signalling equipment should include a
            check that the specifications of System No.  5 are met in regard
            to the following:

  a) Line signalling system

     Signalling frequencies

     Transmitted signal levels

     Signal frequency leak

     Receiver operate and non-operate limits

                     CCITT  automatic  transmission measuring and signalling
                     testing equipment ATME No. 2."

     Receiving-end line split

     Sending-end line split

     Line signal codes

     Sending duration of signals

     Recognition time of signals

     Overlap transmission of answer signal on transit calls

     Double seizing

     Time-out and alarm features


  b) Register signalling system

     Signalling frequencies

     Transmitted signal levels

     Signal frequency leak

     Sending duration of signals

     Receiver operate and non-operate limits

     Operation of the receiver to a series of pulses

     Error-checking features



 * 4.2.4 :  Simulated end-to-end tests

   It is desirable that a means be provided whereby end-to-end testing can be
simulated on a local basis.  A local loop-around arrangement permitting an
outgoing test call to be routed directly on a four-wire basis into incoming
equipment should be provided.  The loop-around arrangement replaces the
international line and is connected to the circuit equipment under test on the
one side and on the other side to similar working spare both-way circuit
equipment and signalling equipment having access to the switching system.






                              Recommendation Q.163

                               4.3 MANUAL TESTING

 * 4.3.1 :  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 satisfactory signal
      transmission by ensuring that a seizing signal is followed by the return
      of a proceed-to-end signal, that a clear-forward signal is followed by
      the return of a release-guard signal.

   b) The second method consists of verification of satisfactory signal
      transmission by initiating a test call:

      1.  to technical personnel at the 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 consists of 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.  provide required duration and spacing of MF signals;

      4.  complete terminal and transit calls.



 * 4.3.2 :  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.

Note - Absence of numerical information following termination of the seizing
       signal may result in receipt of a busy-flash signal provided by some
       Administrations from the distant-end equipment.

       b) Initiate a clear-forward signal and verify the receipt and
          recognition of the release-guard signal from the...

                                                   See the note to S 4.3.4.3.

          ...  distant end.

   2.  Failure to complete the seizing/proceed-to-end signalling sequence or
       the clear-forward/release-guard signalling sequence should result in
       the automatic termination of the frequencies being transmitted within
       10-20 seconds/4-9 seconds (see Recommendation Q.141, S 2.1.3.1 | ).

   3.  In the event of a failure, appropriate steps should be taken to locate
       and correct the trouble.

   4.  The above tests are short, simple, and should be performed at least
       monthly from each end of the circuit as appropriate.  This minimum
       periodicity should be increased to as often as daily if the incidence
       of trouble encountered is unsatisfactory.



 * 4.3.3 :  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:

          i) The audible ringing tone should be heard;

         ii) 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
          in 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 tests should be
       made using this equipment to the extent that the applicable features
       indicated in 1 above are available.

   3.  The above tests should be made from each end of the circuit.  They
       should be made monthly when the manual testing methods prescribed in 1
       are used.

       They may be made daily when semi-automatic test arrangements are
       available.



 * 4.3.4 :  Third method:  comprehensive tests; terminal and transit test
            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:

          - 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 4.2.3
          should have been completed at both terminals.  New circuits assigned
          to Time Assignment Speech Interpolation (TASI) equipment should be
          patched as non-TASI for the duration of these tests.


   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.  Check the following:

       a) At the originating end check that a seizing signal is following by
          the receipt and recognition of the proceed-to-send signal from the
          distant end.  Check that the proceed-to-send signal persists until
          the seizing signal ceases.

       b) At the distant end check the following:  Duration of transmitted
          signal

          1.  Interval between termination of seizing signal and start of KP
              signal 80 _ 20 ms
          2.  KP signal duration 100 _ 10 ms
          3.  Digital and ST signal duration 55 _ 5 ms
          4.  Interval between all signals 55 _ 5 ms

       c) Check that the audible ringing tone is heard at the originating end.

       d) At the originating end check that the answer signal is received,
          recognized and acknowledged.  Check that the acknowledgement signal
          persists until the answer signal ceases.

       e) At the distant end initiate a clear-back signal.

       f) At the originating end check that a clear-back signal is received,
          recognized and acknowledged.  Check that the acknowledgement signal
          persists until the clear-back signal ceases.

       g) At the originating end initiate a forward-transfer signal.

       h) At the distant end check the receipt of the forward-transfer signal.
          The transmitted duration of this signal should be 850 | (+- | 00 ms.
          This signal may be subject to TASI clipping.

       i) 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.

       j) 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 fast
          transmission of switch-hook flashes the equipment indicates the
          final position of the switch-hook.

       k) At the originating end release the circuit and check that the
          clear-forward signal is followed by the receipt and recognition of
          the release-guard signal from the distant end.  Check that the
          release-guard signal ceases after the clear-forward signal ceases.
          Check that the circuit restores to the idle condition.

       l) 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.

       m) At the originating end check that the busy-flash signal is received,
          recognized and acknowledged.  Check that the acknowledgement ceases
          after the busy-flash signal ceases.  (Some Administrations at the
          incoming end may find it convenient to provide a test call device
          which prompts the return of a busy-flash signal.)

   In normal service the receipt of a busy-flash signal causes (after the
acknowledgement) a clear-forward signal to be sent automatically from the
international exchange originating the call.  On a test call procedure some
Administrations may prefer to avoid this process.  In this case, the release
of the connection is controlled by the personnel at the terminal originating
the test call.


   Note on items a) to m) - As part of the comprehensive tests it may, in
certain circumstances such as fault localization, be desirable to test the
frequency, level, and duration of received signals.  Normally, however, it may
be assumed that each Administration has verified the accuracy of its signal
transmission locally as covered in S 4.2.3.


   3.  Transit calls

       a) After securing the cooperation of a third international exchange
          initiate a transit call to this exchange through the international
          exchange covered in 2 above.

       b) With the assistance of technical personnel at the third
          international exchange repeat steps 2 | ) to 2 | ) except that in
          step 2 | ) measurement of the duration of the forward-transfer
          signal need not be made.

   Note - Detailed tests of certain transit features such as that of the
transmission of the answer signal on an overlap basis at the transit point
should be performed locally.






                             Recommendation Q.164


             4.4 TEST EQUIPMENT FOR CHECKING EQUIPMENT AND SIGNALS

 * 4.4.1 :  General

   For local checks of correct equipment operation and for re-adjusting the
equipment, international exchanges should have test equipment available which
includes:

   a) Line and register signal generators.

   b) Signal-measuring apparatus.

   c) Loop-around equipment (see 4.4.4).



 * 4.4.2 :  Signal generators

   The signal generators should be able to simulate all line and register
signals.  The generators may be part of test equipment which cycles the
equipment to be tested through actual signalling sequences, in a manner which
enables rapid complete testing to determine whether the equipment meets the
system specifications.

   When making transit test calls it is not the intention to check the
performance or the quality of the circuit beyond the transit exchange, this
being completely the responsibility of the Administration concerned.  However,
it is important that in principle the transit operations can be checked.

   The generators should have the following characteristics:

   a) Line signal generator

      1) Signal frequencies should be within _ | Hz of the nominal signalling
         frequency or frequencies and shall not vary during the time required
         for testing.

      2) Signal levels should be variable between the limit given in the
         specification and be able to be set within _ | .2 dB.

      3) Signal duration should be long enough so that the signals can be
         recognized and long enough in the case of compelled signals to
         complete the acknowledgement process.

   b) Register signal generator

      1.  Signal frequencies should be within _ | Hz of the nominal signalling
          frequency or frequencies and shall not vary during the time required
          for testing.

      2.  Signal levels should be variable between the limits given in the
          specification and be able to be set within _ | .2 dB.

      3.  Signal durations and intervals between signals shall be within the
          limits given in the specification in Recommendation Q.153, S 3.3.3,
          for normal operate values and in Recommendation Q.154, S 3.4.1 | ),
          for test operate values.



 * 4.4.3 :  Signal-measuring equipment

   Equipment capable of measuring signal frequencies, signal levels, signal
durations and other significant signal time intervals may be part of the test
equipment referred to in S 4.4.2, or separate instruments.  In either case the
characteristics of the measuring equipment should be as follows:

   a) Line signal-measuring equipment

      1.  Signal frequency or frequencies to be measured to be between the
          extreme limits given in the specification, the reading being made
          with an accuracy of _ | Hz.

      2.  Level of the signal frequency or frequencies measured over the range
          given in the specification to be measured with an accuracy of _ | .2
          dB.

      3.  Signal durations, signal recognition times and other significant
          time intervals as given in the specification should be measured
          within an accuracy of 1 ms or _ | % of the nominal duration,
          whichever yields the higher value.  The range of time intervals to
          be measured is approximately 5 to 1050 ms.  Time-out intervals of 10
          to 20 seconds and of 4 to 9 seconds should be determinable within an
          accuracy of _ | second.

   b) Register signal-measuring equipment

      1.  Signal frequency or frequencies to be measured to be between the
          extreme limits given in the specification, the reading being made
          with an accuracy of _ | Hz.

      2.  Level of the signal frequency or frequencies measured over the range
          given in the specification to be measured within an accuracy of _ |
          .2 dB.

      3.  Signal duration and intervals between signals as given in the
          specification should be measured with an accuracy within 1 ms.

   c) In regard to measuring time intervals a recorder having a minimum of two
      input channels may be useful.  The recorded characteristic should
      conform with the accuracy quoted in a) and b) above and be easily
      connected to the circuit under test.  The recorder input characteristic
      should be such as to have a negligible effect on circuit performance.



 * 4.4.4 Loop-around equipment


   Local four-wire loop-around equipment should simulate line facilities
without introducing signalling degradation.  The gain of the loop-around
equipment should be set to provide proper transmission levels.  Alternatively,
if the testing of the individual items of equipment is on a limit test basis
it would not be essential to set the gain of the loop to provide the exact
transmission levels.  In this event a straight patch would be adequate.


                    MONTAGE:    PAGE 76 = PAGE BLANCHE











                       ANNEXES TO SIGNALLING SYSTEM No. 5

                                 SPECIFICATIONS



                                     ANNEX 1


                              Signalling sequences



  Table 1 - Semi-automatic (SA) and automatic (A) terminal traffic.

  Table 2 - Semi-automatic (SA) and automatic (A) 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
                  Description 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. 21





                             ANNEX 1 TABLE [1 CONT],(a l'italienne), p. 22





                       ANNEX 1 TABLE [1 CONCLUDED], (a l'italienne), p. 23





                                 ANNEX 1 TABLE [2], (a l'italienne), p. 24





                            ANNEX 1 TABLE [2 CONT], (a l'italienne), p. 25





                            ANNEX 1 TABLE [2 CONT], (a l'italienne), p. 21





                            ANNEX 1 TABLE [2 CONT], (a l'italienne), p. 21





                       ANNEX 1 TABLE [2 CONCLUDED], (a l'italienne), p. 21







                                                     ANNEX 2 TABLE [1], p.
















                                                 Table [2] (annex 2), p.30







                                                 Table [3] (annex 2), p.31







                                                 Table [4] (annex 2), p.32







                                                 Table [5] (annex 2), p.33







                                                 Table [6] (annex 2), p.34






       MONTAGE:    PAGE 92 = PAGE BLANCHE

















                                    PART III






                              Recommendation Q.180





                           INTERWORKING OF SIGNALLING

                            SYSTEMS No. 4 AND No. 5














       MONTAGE:    PAGE 94 = PAGE BLANCHE








                    INTERWORKING OF SYSTEMS No. 4 AND No. 5



        Recommendation Q.180


                    INTERWORKING OF SYSTEMS No. 4 AND No. 5




               General


   It is possible to ensure normal operation for both semi-automatic and
automatic service when interworking takes place between Signalling Systems No.
4 and No.  5, in either the "4 to 5" of "5 to 4" direction.

   The interworking is possible because:

   - The line signals (i.e.  the supervisory signals) generally have the same
     meaning and the same function in both systems;

   - The numerical (address) information is sent in the same sequence in both
     systems;

   - all conditions for the use of the language digit in the semi-automatic
     service and the discriminating digit in the automatic service are the
     same in both systems.

   In general, the interworking of the two systems merely requires that a
signal received in the code of one of the systems be converted into the
corresponding signal of the code used by the other system.  Nevertheless, in a
transit exchange where there is interworking of Systems No.  4 and No.  5,
special precautions are necessary with regard to signals which are used
differently in the two systems.  These differenes are as follows:

   a) System No.  5 always uses a forward end-of-pulsing signal (the ST
      signal), whereas the end-of-pulsing signal (code 15) is not always given
      in System No.  4;

   b) System No.  4 uses a backward number-received signal which is not
      provided in System No.  5.




 * 2 Calls from System No.  4 to System No.  5


 * 2.1 :  Semi-automatic calls from System No.  4 to System No.  5

                                                                Figure, p.

   1.  In semi-automatic operation, the outgoing exchange A of System No.  4
       sends an end-of-pulsing signal over link AT and the outgoing register
       at A is released.

   2.  The end-of-pulsing signal of System No.  4, which is a numerical type
       signal (code 15), is acknowledged.

   3.  On receipt of an end-of-pulsing signal from A, the incoming part of
       System No.  4 at transit exchange T sends to A an acknowledgement of
       the end-of-pulsing signal and then sends to A a number-received signal.

   4.  At T, an end-of-pulsing signal ST is sent over link TB on System No.5;
       this ST signal is obtained by converting the end-of-pulsing signal
       (code 15) of Sysem No.  4.

       These register functions may be combined in a single register.

   5.  At T, the outgoing register of System No.  5 is released as soon as the
       signal ST has been sent over link TB on System No.  5.  At T, the
       incoming register of System No.  4 is released after the
       number-received signal of System No.  4 has been sent back to A.

       Note - The number-received signal is sent from T over link TA in order
       to conform to the specifications of System No.  4.  Since the outgoing
       register at A will be released as soon as A has sent the end-of-pulsing
       signal (in accordance with the specifications of System No.  4), the
       only possible role of the number-received signal at A is to indicate to
       the operator that the selection procedure has been effected.  However,
       since the number-received signal relates only to operations on the link
       AT on System No.  4, this signal provides no information about the
       entire selection process from A to B; the indication to the operator is
       hence of little value.


 * 2.2 :  Automatic calls from System No.  4 to System No.  5

                                                                Figure, p.

   1.  The System No.  4 link AT does not provide for an end-of-pulsing signal
       from exchange A in automatic operation; the transit exchange T will
       therefore have to recognize that all the digits have been received in
       order to:

       a) Send a forward ST signal to B in System No.  5; and

       b) Send a backward number-received signal to A in System No.  4.

       In this situation, the System No.  5 register signalling at T will be
       en bloc non-overlap taken by the System No.  5 register at T to
       recognize that all digits have been received.)

   2.  At A, release of the outgoing register of System No.  4 depends upon
       reception of the number-received signal.

       At the incoming end of System No.  4 at T, the incoming register of
       System No.  4 is released as soon as the number-received signal is sent
       backward and all the numerical information necessary for setting up the
       call is sent to B.

       At T, an ST end-of-pulsing signal is sent to B by System No.  5 after
       the numerical information has been sent, and the outgoing register of
       System No.  5 at T is then released.




   3.  Calls from System No.  5 to System No.  4

  * 3.1 :  Semi-automatic calls from System No.  5 to System No.  4

                                                                Figure, p.

   1.  In semi-automatic operation at transit exchange T the incoming register
       part of System No.  5 receives an ST end-of-pulsing signal 55 ms after
       reception of the last numerical signal.

   2.  At T, the ST end-of-pulsing signal of System No.  5 is converted into
       an end-of-pulsing signal of System No.  4, which is sent to the
       incoming end B of this system.

   3.  The end-of-pulsing signal of System No.  4, which is a numerical type
       signal (code 15), is acknowledged.

   4.  At A, the outgoing register of System No.  5 is...

                       See for this term the footnote to Recommendation Q.151.

       ...released after the ST signal has been sent.

   5.  At T, the outgoing register of System No.  4 is released when the
       end-of-pulsing signal is sent.

   6.  At B, the incoming register of System No.  4 is released as soon as the
       number-received signal is sent backward to T on System No.4 and all the
       numerical information necessary for setting up the call in the incoming
       country has been sent forward.

   7.  The number-received signal is sent over link BT upon reception at B of
       the end-of-pulsing signal.  It will be noted that number-received
       signal is sent over link BT solely in order to conform to the
       specifications of System No.  4, even though this signal is superfluous
       in this case as:

       a) The number-received signal is not needed to release the outgoing
          register of System No.  4 at T since it is released when it sends
          the end-of-pulsing signal;

       b) This signal cannot be used to give any information to the operator
          at A since it cannot be passed by System No.  5 on link AT.



 * 3.2 :  Automatic calls from System No.  5 to System No.  4

                                                                Figure, p.

   1.  This situation raises no difficulties since System No.  5 posses the ST
       signal which, by determining the end-of-pulsing at T, places the
       outgoing register of System No.  4 a T in conditions that are
       comparable to those existing with semi-automatic operation in System
       No.  4.

   2.  At T, the ST signal of System No.  5 is converted into the
       end-of-pulsing signal (code 15) of System No.  4.

   3.  The specifications of System No.  4 require that the incoming exchange
       B of System No.  4 must send the number-received signal as soon as:

       a) An end-of-pulsing signal has been received, or

       b) It has recognized that all the digits have been received.


       In this case of interworking, condition a) is generally fulfilled
       first.  It may happen, however, that a complete national number is
       recognized before the end-of-pulsing signal is received (for example,
       when the number of digits in the national number of the incoming
       country is constant).  The transit exchange T must hence be capable of
       receiving the number-received signal not only after emission of the
       end-of-pulsing signal but also when the last digit preceding that
       signal has been sent.

   4.  Exchange B must be capable of receiving the end-of-pulsing signal (code
       15) on automatic calls.



 * 4.  Overflow from System No.  5 to System No.  4

 * 4.1 :  In an exchange equipped with System No.  4 and System No.  5, it may
          be desirable to provide for overflow from a group of circuits
          operated by System No.  5 to a group operated by System No.  4.
          This may be the case for a call outgoing from an exchange A (Figure
          1/Q.180) or for a call from an outgoing exchange K (Figure 2/Q.180)
          and arriving via a group of circuits in System No.  4 at transit
          exchange T where a choice must be made between a first-choice route
          operated by System No.  5 and an overflow route operated by System
          No.  4.


 * 4.2 :  There are two possible ways of arranging for the overflow, in
          particular with respect to the moment at which the decision is taken
          to use the overflow route:

          - Single exploration;

          - Double exploration.


                                       FIGURES 1/Q.180 + 2/Q.180, pp.39-40



       4.2.1         Single exploration


            With single exploration, the state of occupation,  or  availa-
       bility,  of  the  System No. 5 group at exchange A or exchange T is
       considered  only  when  the  condition ST  is  determined  at  this
       exchange in the System No. 5 outgoing register.



               a)         If the exploration at A or at T  shows  that  no
       free  circuit  in  the System No. 5 group is available, overflow to
       the System No. 4 group takes place.


               For this overflow the register has all the numerical infor-
       mation  (even  in  the  case of a transit register like that of the
       T exchange mentioned above), and the end-of-pulsing indication  ST.
       The  register  at the transit exchange T will be regarded, for suc-
       cessive selection operations, as an outgoing System No. 4 register.
       Thus,  in  this  case,  signalling  between  the three System No. 4









       registers involved will be link-by-link and not end-to-end.

               In the case of Figure 2, as soon as  the  ST  condition  is
       available  (at  the latest immediately after the 5 | (+- |  seconds
       delay provided for in  the  specifications  for  System No. 5)  the
       number-received   signal   will   be  sent  back  from  T  to K  in
       System No. 4.

               The  ST  condition  will  also  be  used   to   cause   the
       end-of-pulsing  signal  (code 15)  to  be sent forward from T to L,
       even in automatic  working.  The  code 15  signal  will  cause  the
       number-received  signal  to be sent back from L to T, so that there
       will be no need to wait for five or ten seconds at L before  it  is
       known that a complete number has been received.

               The number-received signal sent by T will be received at  K
       and  will,  in  the case of an automatic call, release the outgoing
       register at that exchange. The second number-received signal, which
       will be sent by L, will be received at T to release the register at
       that exchange, despite the fact that the register  has  transmitted
       an  end-of-pulsing  signal which could have been used for releasing
       the register. The establishment of the speech path at T takes place
       immediately after the release of the register.

               If in the case of Figure 1/Q.180 the ST condition is recog-
       nized in the outgoing exchange A, even in automatic operation. This
       end-of-pulsing  signal   will   in   the   same   way   cause   the
       number-received signal to be sent back from L to A.



               b)         If the exploration at A or at  T  shows  that  a
       free  circuit  in  the System No. 5 group is available, the en bloc
       numerical information has to be transmited over this circuit,  fol-
       lowed  by  the  end-of-pulsing  signal ST,  in  accordance with the
       specifications for System No. 5.


               In the case of  Figure  2/Q.180,  the  conditions  for  the
       number-received signal and the release of the outgoing register are
       the same as under 4.2.1 | ).


       4.2.2         Double exploration


            With double  exploration  the  state  of  availabiliy  of  the
       System No. 5 circuit group is examined twice, namely:

               -         as soon as the direction to be chosen  is  deter-
       mined;

               -         after receipt of the complete numerical  informa-
       tion.

            With the double exploration, advantage is taken  of  the  fact
       that,  without  awaiting condition ST, exchange A or exchange T can









       know that the direct route by System No. 5 is occupied as  soon  as
       the direction to be chosen is determined.

               a)         If the first exploration at A or at T shows that
       no free circuit in the System No. 5 group is available, overflow to
       the System No. 4 group should take place immediately;  the  use  of
       the  signals  on  the System No. 4 circuit(s) must be in accordance
       with the normal procedure of that system:

               -         in the  case  of  Figure  2,  end-to-end  working
       (K-T-L) for the numerical signals and the number-received signal;

               -         the end-of-pulsing  signal  (code  15)  only  for
       semi-automatic calls.


            The procedure of overflow at this  first  exploration  reduces
       post-dialling  delay in automatic working since there is no need to
       wait until all the digital information is assembled en bloc  before
       proceeding  with  the  setting  up  of the connection. On the other
       hand, there is a slight reduction in the efficiency  of  the  first
       choice System No. 5 group.

               b)         After the first exploration has shown no conges-
       tion  of  the  System No. 5  group,  it may happen, during or after
       reception in the outgoing register at A (or  transit  register,  in
       the  case  of T)  of  the  digits following the digits necessary to
       determine the routing, that the System No. 5 group gets busy,  more
       particularly  because  such  a  circuit  group  is  a  first-choice
       high-usage route with consequently  a  high  probability  of  loss.
       When  this hapens, after noting by this second exploration that all
       circuits in the  System No. 5  group  are  busy,  overflow  to  the
       System No. 4 group takes place.

       For this case of overflow, the conditions can be considered  to  be
       the same as in 4.2.1 | ).



               c)         If also the second exploration shows no  conges-
       tion  of  System No. 5 group, the conditions mentioned in 4.2.1 | )
       fully apply.


            4.3 Line signalling for calls set up in overflow  through  two
       successive   No. 4 systems   will   be   done  normally,  that  is,
       end-to-end. The number-received signal, however, will  be  sent  as
       stated in 4.2.1 | ), 4.2.2 | ) or 4.2.2 | ).


       5.          Interworking line signalling conditions



       5.1         Forward-transfer signal











            The forward-transfer signal, in the event of  a  transit  call
       going  from  System No. 4  to System No. 5, or from System No. 5 to
       System No.  4, should cause the assistance operator to intervene in
       the country of arrival, and not at the transit exchange.

            The incoming line relay set of the first system at the transit
       exchange  is  informed, e.g. by the transit register, that the call
       is a transit call. Hence, when a forward-ransfer signal arrives  on
       the  first  system,  it  causes  the  forward-transfer signal to be
       transferred to the outgoing line relay set  of  the  second  system
       without intervention by the operator at the transit exchange.



       5.2         Answer signal



       5.2.1         System No. 4 to System No. 5 (traffic direction)


            The answer signal on the System No. 4 link should be sent only
       after  complete  recognition of the answer signal received from the
       System  No. 5 link, i.e. overlap transmission should not be used.


            The considerations for this requirement are:

               -         the overlap technique could give rise to troubles
       in signalling system No. 4 in case of imitation of the P signal;

               -          in  the  transfer  of  the  answer  signal  from
       System No. 5  to  System No. 4  the  sending end line split (silent
       period) before starting P is a necessity due to the pulse type sig-
       nals  of  System No. 4.  The  requirement for this sending-end line
       split period (40 | (+- | 0 ms) would have meant little speed advan-
       tage  of  overlap  operation in transferring the answer signal from
       System No. 5 to System No. 4 (about 40 ms);

               -         overlap operation would be contrary to the design
       characeristic  of System No. 4 in that once the sending of a signal
       has begun it must be sent completely.



       5.2.2         System No. 5 to System No. 4 (traffic direction)


            In the interworking  arrangements  in  a  transit  centre  for
       transferring  the  answer  signal  backward  from  System No. 4  to
       System No. 5, overlap transmission should not be used.

            In System No. 4, overlap operation is  incompatible  with  the
       use of time measurement for recognition of the suffix signal (short
       suffix or long suffix). The  overlap  operation  would  not  permit
       waiting  for  the  end of a signal PY (answer signal), to determine
       that it is not a PYY signal release-guard signal).









       5.3         Busy-flash signal


            In  the  case  of  interworking  at  a  transit   point   from
       System No. 4  to  System   No. 5  or vice visa, a busy-flash signal
       received at that transit point from the outgoing circuit is  to  be
       converted to a busy-flash signal on the incoming circuit.

            In the case of interwoking from System No. 5 to System No.  4,
       the  busy-flash  signal will cause the release of the international
       connection initiated from the outgoing exchange.

            In the case of interworking from System No. 4 to System No. 5,
       the System No. 5 equipment at this transit exchange should function
       as  an  outgoing  System No. 5  equipment  on  the  receipt  of   a
       busy-flash  signal  and  release  the System No. 5 circuit from the
       transit point. It should be noted that the System No. 4 circuit  is
       also released in the case of automatic calls.

            Note  - In the case of interworking from System No. 5 to  Sys-
       tem  No. 4 it has been determined that no advantage is to be gained
       by the release of the international connection being initiated only
       by  the outgoing exchange. Therefore in both cases of interworking,
       the transit exchange and the forward  connection  may  be  released
       immediately  on receipt of the busy-flash signal. However, there is
       no need to modify existing equipment.


       5.4         Time-out delays to clear a connection in the  event  of
       signal failures



       5.4.1         Non-reception  of  a  clear-forward  signal  after  a
       clear-back signal has been sent


            In the event of transit working System No. 4 to System  No.  5
       at  an  exchange   T,  this  latter  represents  the  terminal  for
       System No. 4.

            The action to be taken at an international  incoming  exchange
       for  System No. 4  holds  good for the exchange T. After a time-out
       of  2 to 3 minutes, the System No. 4 incoming equipment at T should
       produce  an effect forward on the circuit of System No. 5, so as to
       release the international circuit (for  example,  should  there  be
       some interruption in the System No. 4 circuit). This release should
       proceed in the same way as the release of the national part of  the
       connection,  when  the  incoming  exchange  is  indeed the incoming
       international exchange of the international connection.

            For symmetry's sake, the action at T to release the connection
       should  also  be  undertaken  when  there  is  transit working from
       System No. 5 to System No. 4, since a time-out  of 2  to  3 minutes
       exists in System  No. 5 to release the connection forward.











       5.4.2         Delay  in  clearing  by  the  calling  subscriber  in
       automatic working


            In the case of automatic calls with interworking  from  System
       No.  4  to  System   No. 5,  or  from System No. 5 to System No. 4,
       release of the international connection as  brought  about  by  the
       time-out  of 1  to  2   minutes  must  take  place  at the outgoing
       exchange only , and not at the exchange  T, the point of connection
       of  the two systems. In exchange T, the outgoing line relay sets of
       the second system in the connection must accordingly be marked that

            they are acting, not as relay sets for the  terminal  outgoing
       end of the system in question, but as transit exchange relay sets.



       5.4.3          Non-reception of an answer-signal  at  the  outgoing
       exchange  after reception of a number-received signal or generation
       of the ST condition


            When a connection passes through System No. 4  towards  System
       No.  5,  or  vice versa, release must be undertaken at the outgoing
       exchange only Hence nothing must be done at the transit exchange T,
       the point at which Systems Nos. 4 and 5 are connected.

            In the case of System No. 4 towards System No.  5,  exchange T
       represents  the  connection  transit  exchange  for  both  systems.
       Non-reception at T of an answer-signal within 2 to 4 minutes  after
       condition ST   has  been  determined  must  produce  no  effect  at
       exchange T. It will be for the outgoing exchange to  cause  release
       (by  sending  the  clear-forward  signal)  on  the 2  to 4 minutes'
       time-out  after  reception  of  the  number-reeived   signal   from
       exchange T.

            In the case of System No. 5 towards System No.  4  exchange  T
       represents  the  connecting  transit  exchange  for  both  systems.
       Non-reception at  T of an answer-signal within 2 to 4 minutes after
       reception  of the number-received signal from the incoming exchange
       must not affect exchange T. It will be for the outgoing exchange to
       cause  release of the connection (by sending the clear-forward sig-
       nal) after the delay of 2 to 4 minutes following the generation  of
       the ST condition at that exchange.





       MONTAGE:    PAGE 102 = PAGE BLANCHE



                     ANNEXES TO INTERWORKING SPECIFICATIONS
                           OF SYSTEMS No. 4 AND No. 5

                                     ANNEX 1









                      Signalling sequences in interworking

                       from System No. 4 to System No. 5
                                     ANNEX 2
                      Signalling sequences in interworking

                       from System No. 5 to System No. 4


       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 1 (table) (a l'italienne) p.41








                              annex 1 (table) (suite) (a l'italienne) p.42





                              annex 1 (table) (suite) (a l'italienne) p.43





                              annex 1 (table) (suite) (a l'italienne) p.44





                                annex 1 (table) (fin) (a l'italienne) p.45











                                      annex 2 (table) (a l'italienne) p.46








                              annex 2 (table) (suite) (a l'italienne) p.47





                              annex 2 (table) (suite) (a l'italienne) p.48





                                annex 2 (table) (fin) (a l'italienne) p.49


