

                         Recommendations Q.140 to Q.164





                          SPECIFICATIONS OF SIGNALLING

                                  SYSTEM No. 5















                            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; and

               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  fre-
       quency  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 signal-
       ling 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 sig-
       nals  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 alter-
       native, continuous compelled signalling, would be slow due  to  the
       long  circuit  propagation  times in certain applications. The fre-
       quencies (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 outgo-
       ing 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 con-
       tinuous 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 regis-
       ter pulse out, and during the  intervals  between  successive  m.f.
       signals.

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

            Compandors affect signalling,  particularly  short-pulse  com-
       pound  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 interna-
       tional 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 sig-
       nal,  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:

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

               -          the sending of the clear-forward by the outgoing
       exchange  to release the international connection (except when oth-
       erwise arranged, for example, in this case of observations on  cir-
       cuits).


       1.7         answer signal (sent in the backward direction)


            This signal is sent to the outgoing international exchange  to
       show that the called party has answered the call

            In semi-automatic working, the signal has a supervisory  func-
       tion.

            In automatic working, it is used:

               -         to start metering the charge to the calling  sub-
       scriber;

               -          to start the measurement of  call  duration  for
       international accounting purposes.



       1.8         clear-back signal (sent in the backward direction)


            This 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 interna-
       tional, are transmitted as quickly as possible.










       service, it performs a  supervisory  function.  It  must  not  per-
       manently  open  the  speech  path  at  the  outgoing  international
       exchange.

            In automatic working, arrangements must be made to  clear  the
       international  connection,  stop the charging and stop the measure-
       ment 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  con-
       trolled 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  sub-
       scriber  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 work-
       ing). 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  work-
       ing 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 individu-
       ally 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 direc-
       tion 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 sub-
       ject 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 includ-
       ing the idle condition, may be sent from an  outgoing  end  at  any
       time to initiate the release of the circuit. The clear-forward sig-
       nal 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 condi-
       tion 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  com-
       pelled) 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 max-
       imum 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 pro-
       vided  at  the  outgoing end, no indication is given to the mainte-
       nance 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 a recognition 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  switch-
       ing 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 arrange-
       ments,    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 com-
       pelled 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 signal-
       ling 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  ack-
       nowledged  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  incom-
       ing  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  inter-
       national  exchange. The release-guard signal continues until cessa-
       tion 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  acknow
       ledgement  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 net-
       work;

               5)         time-out of an incoming international register.










               e)         Receipt of busy-flash at the  outgoing  interna-
       tional exchange will cause:

               -         after signal recognition time (125 _ 25):

               1)         the acknowledgement signal to be sent; and

               2)          an  appropriate  audible   indication   to   be
       transmitted to the operator or to the subscriber.  When the preced-
       ing circuit provides for the transmission of busy-flash, this  sig-
       nal 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; and

               2)         the busy-flash signal to be sent on the  preced-
       ing 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 inter-
       national exchange should, nevertheless, respond by sending the ack-
       nowledgement 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  Administra-
       tions  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 sig-
       nal.


                                                      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 sig-
       nal


            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 receiv-
       ing end line split of BT. The normal signal recognition of the pri-
       mary signal is still required  at  each  transit  point.  The  ack-
       nowledgement  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 com-
       pelled 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 ini-
       tiated 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 ack-
       nowledgement 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 signal-
       ling 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  ack-
       nowledgement  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 effec-
       tive 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 T3followed, without break at the  termina-
       tion  of  the time base (at time Z ), by the continuous signal con-
       trol 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 inter-
       rupted. 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  cir-
       cumstances  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) cir-
       cuits:

               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  there-
       fore 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  incom-
       ing seizing signal from the distant end. Each outgoing seizing sig-
       nal 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 termi-
       nation of the first attempt outgoing seizing signal (or the  recog-
       nition  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 sig-
       nal  sender  shall  be  as  low as practicable but in any event, at
       least 40 dB below signal level. This noise includes all  extraneous
       power  in  the  frequency band 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  sig-
       nal  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  varia-
       tions 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 speci-
       fied 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 interfer-
       ing with speech.

            To minimize signal imitation by speech currents it  is  advis-
       able that the guard circuit be tuned.

            To minimize signal interference by low-frequency noise  it  is









       advisable  that the response of the guard circuit falls off towards
       the lower 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 dif-
       ficulties 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 inter-
       national, 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  com-
       pound pulse of the two frequencies f 1 and f 2 with a minimum dura-
       tion 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 signal-
       ling 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  inter-
       national   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 interna-
       tional circuit.

            The splitting time of 35 ms may be reduced by each Administra-
       tion  concerned  in  order  to help to protect its national network
       against the effect of signals coming from  the  international  cir-
       cuit.  It  should  be noted, however, that a shorter splitting time
       can lead to an increase in the number of false  operations  of  the
       splitting device by speech currents and impair speech 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 interna-
       tional 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 regis-
       ter 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  outgo-
       ing  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 informa-
       tion 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  regis-
       ter,  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  con-
       nection,  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 subse-
       quent 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  informa-
       tion;  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 out-
       put signal transmission may be delayed until all the call  informa-
       tion  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  inter-
       national  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  mul-
       tifrequency 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 opera-
       tion;

               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 opera-
       tion; 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 ori-
       ginating  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  des-
       tination country numbering plan.

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


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

               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  frequen-
       cies 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  frequen-
       cies 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  fre-
       quencies  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 fre-
       quencies 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  sig-
       nal.  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 sig-
       nals 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  incor-
       porate 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 out-
       going 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  cir-
       cuit  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  equip-
       ment,  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  equip-
       ment  and  signalling equipment having access to the switching sys-
       tem.


       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  verifica-
       tion 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 interna-
       tional exchange.

               c)         The third method consists of complete  verifica-
       tion  of  satisfactory  line  and register signal transmission. The
       verification consists of a check of ability to:

               1.          generate and receive line and register signals;

               2.          transmit the appropriate  acknowledgement  sig-
       nals;

               3.          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  transmis-
       sion:

               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 termina-
       tion  of  the  seizing signal may result in receipt of a busy-flash
       signal provided by some Administrations from the distant-end equip-
       ment.

               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 frequen-
       cies   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 unsatis-
       factory.



       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 verifi-
       cation 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  arrange-
       ments are available.


       4.3.4         Third method: comprehensive tests; terminal and tran-
       sit test calls


               1.          Verification of satisfactory  signal  transmis-
       sion  (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 sig-
       nal   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 succes-
       sion  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 pro-
       vide 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  con-
       trolled 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 Administra-
       tion 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 inter-
       national  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  read-
       justing  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. Howev-
       er, 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 com-
       pelled 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 lev-
       els,  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 nomi-
       nal 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  negligi-
       ble 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 transmis-
       sion 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



























