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TUCoPS :: Phreaking Technical System Info :: 6_4_03.txt

Q.414 to Q.430





 * 2.3 :  CLAUSES FOR TRANSMISSION LINE SIGNALLING EQUIPMENT
                             ____________________
                             Recommendation Q.414
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
2.3.1 SIGNAL SENDER
~~~~~
      2.3.1.1 Signalling frequency

      The nominal value of the signalling frequency is 3825 Hz.  Measured at
      the sending point, the frequency variation from thef nominal value must
      not exceed _ | Hz.


      2.3.1.2 Send level

      The send level of the signalling frequency, measured at the group
      distribution frame or an equivalent point, must be -20 _ dBm0.


      2.3.1.3 Leaks

      The level of the signal frequency which may be transmitted to line as a
      leak current (e.g.  when static modulators are used), must be at least
      25 dB below the level of the signalling tone.


      2.3.1.4 Phase distribution of the signalling frequencies

      As the signalling frequency is sent on any circuit in idle state, the
      addition of these tones in moments of low traffic may give rise to the
      following phenomena on certain transmission systems:

      o high peak voltage on the line caused by the signalling tones and
        involving the possibility of overloading the system;

      o intelligible crosstalk due to third-order intermodulation;

      o unwanted tones coming from second-order intermodulation products and
        occurring within sound-programme circuits.

      o The following special measures must be taken to avoid these effects:

        One method recommended is to inject the signalling frequencies with
        random 0 and ~~ radian phases in the channels.  An equivalent method
        is to use carrier frequencies of which the phases are ran- domly
        distributed 0 and ~~ radians.  With these methods the probability of
        occurrence of 0 and ~~ radian phases should be 0.5

        Other methods may be used provided they give comparable results.


      2.3.1.5 Protection of the signalling channel at the sending end

      This signalling channel must be protected at the sending end against
      disturbance from the associated and the adjacent speech channel.

      When a sinewave at 0 dBm0 level is applied to the audio-frequency input
      of the associated channel, the level measured at the group distribution
      frame or at an equivalent point must not exceed the levels shown in
      Figure 6/Q.414.

      When a sinewave of frequency f is applied to the audio-frequency input
      of the adjacent channel it produces two signals that appear on the
      frequency scale of Figure 6/Q.414 as having the frequencies

      (4000 + f ) and (4000 - f ).  The level of the (4000 + f ) signal,
      measured at the group distribution frame or at an equivalent point,
      shall not be higher than -33 dBm0 when the sinewave with frequency f is
      applied to the audio-frequency input of the adjacent channel at a level
      shown in Figure 6/Q.414 for the frequency of (4000 + f ).  The level of
      the (4000 - f ) signal, measured at the group distribution frame or at
      an equivalent point,

                  [ For further details on the method of random distribution
                  [ of the phases of frequency 3825 Hz, see:  Ekholm, O.  and
                  [ Johannesson, N.O.:  "Loading Effects with Continuous Tone
                  [ Signalling", English edition of TELE , No.  2, 1969.  For
                  [ further details on a systematic method of phase
                  [ distribution, see:  Rasch, J.  and Kagelmann, H.:  "On
                  [ Measures for Reducing Voltage Peaks and Distortion Noise
                  [ on Carrier Transmission Paths with Single Channel
                  [ Supervision", Nachrichtentechnische Zeitschrift (NTZ) , 22
                  [ (1969), No.  1, pp.  24-31.

      shall not be higher than -33 dBm0 when the sinewave with frequency f is
      applied to the audio-frequency input of the adjacent channel at any
      level below the value shown in Figure 6/Q.414 for the frequency (4000 -
      f).

                                                        FIGURE 6/Q.414, p.

      When the Go path is looped to the Return path at the group distribution
      frame or an equivalent point, the signal receiver must not change
      condition when:

      o the click generator shown in Figure 7/Q.414 is connected to the
        associated speech channel or to the adjacent speech channel at the
        very point where this channel is connected to the switching equipment;

      o to take the most difficult circumstances possible, the channel level
        adjusting devices are set to such values encountered in practice which
        give rise to the worst disturbance;

      o gain is introduced in the loop at the group distribution frame or at
        the equivalent point, so that the receive level at the point in
        question is +3 dBm0.

                                                        FIGURE 7/Q.414, p.

      2.3.1.6 Response time

      The response time of the signal sender is defined as the interval
      between the instant when the change signalling condition command is
      applied to the sender and the instant at which the envelope of the
      signalling frequency, measured at the group distribution frame or at an
      equivalent point, reaches half of its value in the steady state.  For
      each of the two possible changes of signalling condition the response
      time must be less than 7 ms.


                             ____________________
                             Recommendation Q.415
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
2.3.2 SIGNAL RECEIVER
~~~~~
      2.3.2.1 Recognition of the tone-on condition

      The receiver must have assumed or assume the tone-on condition when at
      the group distribution frame or at an equivalent point:

      o the level of the received frequency has risen to -27 dBm0 or more;

      o its frequency lies between 3825 _ 6 Hz.

      The level of -27 dBm0 specified above does not preclude the use of
      individual adjustments in the channel translating equipment to
      compensate for constant level deviations.


      2.3.2.2 Recognition of the tone-off condition

      The receiver must have assumed or assume the tone-off condition when the
      level of the test frequency, at the group distribution frame or at an
      equivalent point, has dropped to the values shown in Figure 8/Q.415.

                                                        FIGURE 8/Q.415, p.

      2.3.2.3 Protection against near-end disturbances

      The signal receiver must not change state when any one of the following
      disturbing signals is applied at the 4-wire output of the associated
      speech channel looped at the group distribution frame or at an
      equivalent point:

      o A sinusoidal signal whose level as a function of the frequency is
        shown in Figure 9/Q.415,

      o A transient signal produced by the click generator (described in S
        2.3.1.5 above) applied at the point where the channel is connected to
        the switching equipment, all level adjusting devices being set to such
        values encountered in practice which give rise to the worst
        disturbance.
                                                        FIGURE 9/Q.415, p.


      2.3.2.4 Overall response time of signal sender and receiver

      When the modulation equipment is looped at the group distribution frame
      or at an equivalent point, the overall response time is defined as the
      interval between the instant when a change signalling condition command
      is applied to the sender and the moment when the changed signalling
      condition appears at the receiver output.  For each of the two possible
      changes of signalling condition, the overall response time must be less
      than 30 ms.


      2.3.2.5 Interference by carrier leaks

      The requirements stated in SS 2.3.2.1, 2.3.2.3 and 2.3.2.4 above must be
      fulfilled in the presence of carrier leaks.

      It is assumed that:

      o when the receive level of the signalling tone is at its nominal value
        at the group distribution frame or an equivalent point, each carrier
        leak is present at a level of - 26 dBm0;

      o the level of the carrier leak varies proportionally with any
         variations in the level of the signalling tone.


      2.3.2.6 Interference by pilots

      The specified signalling system is not intended to work in the presence
      of those pilots specified by CCITT having a frequency differing by 140
      Hz from the nearest multiple of 4 kHz (see Recommendation M.460).

      On the other hand, the requirements stated in SS 2.3.2.1, 2.3.2.2,
      2.3.2.3, 2.3.2.4 and 2.3.2.5 above must be met in the presence of any
      other pilot recommended by the CCITT.

      It is assumed that variations in level of the pilot and of the
      signalling tones are correlated.



                             ____________________
                             Recommendation Q.416
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 2.4 :  INTERRUPTION CONTROL

2.4.1 General
~~~~~
   In System R2, removal of the tone corresponds to the sending of the seizing
and answer signals.  Steps must be taken, therefore, to guard against unwanted
interruption of the signalling channels resulting in false signalling.
Special devices monitor a number of circuits and transmit an indication to
each individual equipment as soon as an interruption occurs.  The whole
protection system against the effect of interruptions is designated by the
term interruption control.

   In each case, the response time of the interruption control must be based
on the time required to recognize the signalling condition.

   The interruption control systems in the two directions of transmission
operate independently of each other.

   The interruption control specified uses the group pilot to detect
interruptions.


2.4.2 Mode of operation of interruption control
~~~~~
   For each direction of transmission of a carrier circuit connection the
equipment for interruption control comprises:

   o A group pilot generator at the outgoing end;

   o A pilot receiver and a wiring system for signalling the interruption at
     the incoming end.

   In principle, the existing pilots of the carrier system will be used.

   The receiver at one end supervises the pilot transmitted by the other end.
When a considerable fall in the level of the pilot is detected it is assumed
that an interruption has occurred on the signalling channels associated with
the carrier circuits.  The interruption control equipment then reacts to
prevent the unwanted transmission of certain signals on those circuits which
have already been seized or to ensure that idle circuits are blocked.

   Figure 10/Q.416 shows functionally an arrangement where the pilot receiver
controls the relay sets of interrupted circuits.

   To ensure proper interruption control, it is essential that the individual
transmission or switching equipments should not react to any change of
signalling state due to a fault.  The action initiated by the interruption
control must therefore be completed in less time than the sum of the response
time of the signalling receiver and the recognition time for the tone-off
condition caused by interruption of the signalling channel.  Again, to prevent
the unwanted transfer of certain signals, interruption control, during
re-establishment of the pilot, must return to alarm off after an interval long
enough for the signalling equipment to revert to normal.

   To operate independently for each direction of transmission the incoming
end interruption control supervises only the forward direction and, if
necessary, initiates an operation at the outgoing end via the line signalling
system.  Conversely, interruption control at the outgoing end supervises the
backward direction of transmission only.

   Blocking of a circuit at the outgoing end therefore takes place in two
different ways:

   o Immediate blocking by intervention of interruption control at the
     outgoing end;

   o blocking on recognition in the backward direction of the tone-off
     condition caused by interruption control intervention at the incoming
     end.

   When the transmission system is re-established, interruption control
reverts to normal and the signalling equipment must automatically revert to
normal operating.

   Since the action to be taken on the individual circuits differs according
to their state at the time the fault occurs, the different possibilities are
dealt with in detail below.

                                                      Figure 10/Q.416, p.5

      2.4.2.1 Mode of operation of interruption control at the incoming end
              (transmission interrupted in the forward direction)

              a) Circuit in idle state

                 Transition of interruption control to alarm brings about:

                 i) removal of the tone in the backward direction by locking
                    of the sending unit in the tone-off condition;

                ii) locking of the receiving unit in its position, i.e.  in
                    the tone-on condition.

                 The effect of operation i) is to block the circuit at the
                 outgoing end against possible seizing; operation ii) prevents
                 incorrect recognition of seizing of the incoming circuit.

                 Return of interruption control to normal ensures return to
                 the idle state of the circuits affected by the fault, by
                 switching sending units at the incoming end to the tone-on
                 condition.
 

              b) Circuit seized prior to answered state

                 Transition of interruption control to alarm brings about:

                 i) locking of the sending unit in its position, i.e.  in the
                    tone-on condition;

                ii) locking of the receiving unit in its position, i.e.  in
                    the tone-off condition;

               iii) start of a time-out device which after a certain interval
                    clears the chain beyond the faulty circuit; this timing
                    arrangement may be the one specified in Recommendation
                    Q.118, S 4.3.3.


                 Operation i) prevents the transfer of an answer signal while
                 interruption control is in action.  If the called subscriber
                 answers before the time out delay mentioned in iii) above has
                 elapsed, then the timer is stopped.  For existing equipment
                 this requirement may not apply.  If the called subscriber
                 clears while interruption control is active, the part of the
                 connection beyond the faulty circuit must be released
                 immediately.

                 Operation iii) prevents blocking of the called subscriber's
                 line if the fault persists; short breaks, on the other hand,
                 have no effect.

                 When the caller clears, operations i) and ii) block the
                 faulty circuit against any new seizure even when the backward
                 signalling channel is still intact; since the release-guard
                 signal has not been sent the outgoing circuit cannot return
                 to the idle state.

                 When interruption control reverts to normal before the called
                 subscriber has answered, the call may still mature normally,
                 provided the caller is holding.

                 If the called subscriber has answered during the time-out
                 delay and the interruption control reverts to normal with
                 both the calling and called subscribers holding, the answer
                 signal is sent immediately.

                 If at the moment when interruption control reverts to normal
                 the called subscriber has already cleared, operation ii)
                 ensures that in all cases the release-guard sequence takes
                 place as in S 2.2.2.6 a) above (either immediately if the
                 outgoing exchange has already sent the clear-forward signal
                 or when the caller clears).  If, on the other hand, the
                 called subscriber is still holding and the outgoing exchange
                 is already sending the clear-forward signal when interruption
                 control reverts to normal the circuit returns to the idle
                 state at the outgoing end as described in S 2.2.2.6 b) above.


              c) Circuit in answered state

                 Transition of interruption control to alarm brings about:

                 i) locking of the sending unit in its position, i.e.  in the
                    tone-off condition;

                ii) locking of the receiving unit in its position, i.e.  in
                    the tone-off condition.

                 When the caller clears, operation i) blocks the faulty
                 circuit against any new seizure, even when the backward
                 signalling channel is still intact; since the release-guard
                 signal has not been sent, the outgoing circuit cannot return
                 to the idle state.

                 When the called subscriber clears, the part of the connection
                 beyond the faulty circuit (including the called subscriber's
                 line) must be released immediately.

                 When interruption control reverts to normal with both
                 subscribers still on the line, the connection is maintained.

                 When the caller has already cleared by the time the
                 interruption control reverts to normal, the release-guard
                 sequence is carried out as in Recommendation Q.412, S 2.2.2.6
                 b) or c).


              d) Circuit in clear-back state

                 Transition of interruption control to alarm causes:

                 i) locking of the sending unit in its position, i.e.  in the
                    tone-on condition;

                ii) locking of the receiving unit in its position, i.e.  in
                    the tone-off condition;

               iii) immediate release of the part of the connection beyond the
                    faulty circuit (including the called subscriber's line).

                 When interruption control reverts to normal, the
                 release-guard signal is sent as in Recommendation Q.412, S
                 2.2.2.6 c) as soon as the clear-forward signal is recognized.


              e) Circuit in release

                 When interruption control functions after a clear-forward
                 signal has been recognized at the incoming end, it causes:

                 i) locking of the sending unit in the tone-off | ondition; if
                    at the instant interruption control operates, the tone-on
                    condition exists in the backward direction, it will be
                    switched to the tone-off condition following recognition
                    of the clear-forward signal and locking in the tone-off
                    condition can take place as prescribed;

                ii) locking of the receiving unit in its position, i.e.  in
                    the tone-on condition.


                 The effect of operation i) is to guard the faulty circuit
                 from a new seizure at the outgoing exchange.

                 Operation ii) ensures the release of the part of the
                 connection beyond the faulty circuit (including the called
                 subscriber's line).

                 When interruption control reverts to normal the tone-on
                 condition is established in the backward direction and causes
                 the circuit at the outgoing exchange to return to the idle
                 state.


      2.4.2.2 Mode of operation of interruption control at the outgoing end |
              transmission in the backward direction interrupted)

              a) Circuit in idle state

                 Transition of interruption control to alarm is immediately
                 followed by blocking of the outgoing circuit.

              b) Circuit seized but not in answered state (including
                 clear-back)

                 i) Transition of interruption control to alarm causes locking
                    of the receiving unit in its position, i.e.  the tone-on
                    condition.  This operation prevents recognition of an
                    answer signal or return to the answered state should the
                    called subscriber have cleared.

                ii) As soon as a clear-forward signal is sent on the part of
                    the connection preceding the faulty circuit, it must be
                    retransmitted; the tone must therefore be established in
                    the forward direction to ensure, assuming that the forward
                    signalling channel is left intact, that the part of the
                    connection beyond the faulty circuit is released.

               iii) When interruption control reverts to normal, the tone may
                    already have been sent in the forward direction as a
                    clear-forward signal.  If the forward signalling channel
                    has remained intact, recognition at the incoming end of
                    the tone-on condition will have caused generation of the
                    release-guard sequence which, because of the fault, will
                    not have been received at the outgoing end.
                    Exceptionally, therefore, return of the outgoing circuit
                    to the idle state must take place simply on recognition of
                    tone-on in the backward direction without necessarily
                    taking into account time-out T1.

              c) Circuit in answered state

                 In this case transition of interruption control to alarm does
                 not cause immediate action.  A clear-forward signal sent on
                 the part of the connection preceding the faulty circuit must
                 be repeated forward to ensure that, if the forward signalling
                 channel is left intact, the part beyond the faulty circuit is
                 cleared.

                 Once the interruption control reverts to normal the
                 connection is maintained provided the caller and the called
                 subscriber are still holding.  On the other hand, by the time
                 the interruption con- trol reverts to normal the
                 clear-forward signal may already have been sent and the
                 situation will be the one described in S 2.4.2.2 b), iii).

              d) Circuit in release

                 [See S 2.4.2.2 b), iii).]


2.4.3 Clauses on interruption control equipment
~~~~~
   Adoption of thresholds with widely differing levels makes for economy in
the design of interruption control equipment.  Against this must be set the
fact that the device cannot cope with the effects of certain slow drops in
level.  However, the probability of these occurring in practice is very small.


      2.4.3.1 Pilots

      Interruption control uses the 84.08 kHz group pilot or by bilateral
      agreement and, at the request of the receiving end country, the 104.08
      kHz group pilot.

      However, if the ends of the supergroup link coincide with the end of the
      five group links it is carrying, the supergroup pilot may also be used.


      2.4.3.2 Alarm-on threshold

      Interruption control must pass to alarm-on | hen the pilot level,
      measured at the group distribution frame or at an equivalent point,
      drops to -29 dBm0.


      2.4.3.3 Alarm-off threshold

      Interruption control must revert to alarm-off | i.e.  normal when the
      pilot level, measured at the group distribution frame or at an
      equivalent point, rises to -24 dBm0.


      2.4.3.4 Response time for a drop in level

      Interruption control must pass from normal to alarm-on within an
      interval t v such that:

                            5 ms  t v  tr\ds m\di\dn
                                    + 13 ms


      when the pilot level, measured at the group distribution frame or at an
      equivalent point, suddenly drops from its nominal level to -33 dBm0.

      In the above formula, tr\dsm\di\dnis the minimum response time of the
      signalling receiver for a drop in level, taking into account a possible
      variation of _ | dB in the signalling tone level from its nominal value,
      the level being measured on the receiving side of the group distribution
      frame or at an equivalent point.

      If the value (40 _ 10) ms is exclusively applied, it is possible to use
      the minimum value of 30 ms instead of 13 ms for the interruption control
      device.  The figure of 13 ms in the above formula is derived on the
      assumption that the output of the interruption control equipment acts
      upon the input of the device which regulates the recognition time for
      the tone-on and tone-off conditions (20 _ 7 ms) , i.e.  absence of a
      direct current signal at this input for a period of up to 13 ms has no
      relevance.


      2.4.3.5 Response time for rise in level

      Interruption control must revert from the alarm-on to normal in an
      interval t ^ such that:

                            tr\ds m\da\dx - 13 ms  t
                                   ^  500 ms

      when the pilot level, measured at the group distribution frame or at an
      equivalent point, suddenly rises from its nominal level to -33 dBm0.

      In the above formula, tr\dsm\da\dxis the maximum response time of the
      signalling receiver for a rise in level, taking into account a possible
      variation of _ | dB in the signalling tone level from its nominal value,
      the level being measured on the receiving side of the group distribution
      frame or at an equivalent point.

      The figure of 13 ms in the above formula is derived on the assumption
      that the output of the interruption control equipment acts upon the
      input of the device which regulates the recognition time for tone-on and
      tone-off condition (20 _ 7 ms) i.e.  absence of a direct current signal
      at this input for a period of up to 13 ms has no relevance.


      2.4.3.6 Precautions against noise

      An interruption may produce increased noise on the group link.
      Interruption control must be capable of distinguishing between the pilot
      itself and a high level noise simulating the pilot.

      Interruption control must not revert to normal in the presence of white
      noise having a spectral power density of not more than -47 dBm0 per Hz.

      To facilitate the design of interruption control equipment operating
      satisfactorily at high noise levels, the upper limit of 500 ms for t ^
      has been specified.




                                    SECTION 3

                        LINE SIGNALLING, DIGITAL VERSION
                             ____________________
                             Recommendation Q.421
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 3.1 :  DIGITAL LINE SIGNALLING CODE

3.1.1 General
~~~~~
   Primary PCM multiplexes (see Recommendations G.732 and G.734) economically
provide more than one signalling channel per speech circuit in each direction
of transmission.  By making use of the increased signalling capacity,
simplification of the outgoing and incoming switching equipment can be
achieved since the timing conditions necessary for the System R2 line
signalling, analogue version, are not required.  For this reason the digital
version of System R2 line signalling is recommended for use on PCM systems in
national and international public switched networks and is specified below.

Note - The continuous line signalling scheme specified for FDM systems may
       also be used on PCM systems by utilizing one signalling channel only in
       each direction.  In this case relay sets designed for the continuous
       line signalling system on FDM channels can be used provided that the
       functions specified for the interruption control on FDM circuits (see
       Recommendation Q.416) are performed by use of the local alarm facility
       provided by PCM equipment.  This method of line signalling on PCM
       systems is not recommended for use on international circuits.

       The digital version of System R2 line signalling uses two signalling
channels in each direction of transmission per speech circuit.  These
signalling channels are referred to as afand bffor the forward direction (i.e.
the direction of call set-up) and aband bbfor the backward direction.

   Under normal conditions:

   o The afchannel identifies the operating condition of the outgoing
     switching equipment and reflects the condition of the calling
     subscriber's line.

   o The bfchannel provides a means for indicating a failure in the forward
     direction to the incoming switching equipment.

   o The abchannel reflects the condition of the called subscriber's line (on
     hook or off hook).

   o The bbchannel indicates the idle or seized state of the incoming
     switching equipment.

   The line signals are transmitted link-by-link.

   The digital version of System R2 line signalling also specifies a means for
appropriate action in the case of faulty transmission conditions on the PCM
multiplex, see Recommendation Q.424.

   The signalling system is specified for one-way operation, but both-way
operation is also possible (see S 3.2.7 below).


3.1.2 Signalling code
~~~~~
   Table 2/Q.421, shows the signalling code on the PCM line under normal
conditions.

                                 H.T. [2/Q.421]
                                  TABLE 2/Q.421

               __________________________________________________
                                            Signalling code
                State of the circuit   Forward    Backward   a f
               __________________________________________________
                Idle/Released          1     0       1        0
                Seized                 0     0       1        0
                Seizure acknowledged   0     0       1        1
                Answered               0     0       0        1
                Clear-back             0     0       1        1
                Clear-forward       |
                                    |
                                    |
                                    |
                                    |
                                    |
                                    |
                                    |
                                    |







                                       1  |
                                          |
                                          |
                                          |
                                          |
                                          |
                                          |





                                             0 |
                                               |
                                               |
                                               |
                                               |
                                               |
                                               |
                                               |






                                                     0    |
                                                          |
                                                          |
                                                          |
                                                          |
                                                          |
                                                          |





                                                              1


                         or
                                                     1        1
                Blocked                1     0       1        1
               __________________________________________________

                                                       Table [2/Q.421], p.











                             ____________________
                             Recommendation Q.422
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 3.2 :  CLAUSES FOR EXCHANGE LINE SIGNALLING EQUIPMENT

3.2.1 Recognition of a change of signalling code
~~~~
      3.2.1.1 Signalling channel transitions

      The recognition time for a transition from 0 to 1 or vice versa on a
      signalling channel is 20 _ 10 ms.  This value presupposes the existence
      of protection against the effects of faulty transmission conditions on
      the PCM multiplex.

      The recognition time is defined as the duration that the signals
      representing 0 or 1 must have at the output of the terminal equipment of
      a signalling channel in order to be recognized by the exchange
      equipment.


      3.2.1.2 Change of signalling code

      Recognition of a change of signalling code is thus defined as either of
      the following:

      a) Recognition of a transition detected on one signalling channel with
         no transition detected on the second signalling channel during the
         recognition period.

      b) Recognition of a transition detected on the second signalling channel
         during the recognition period already being applied to the first
         signalling channel.  In this case, a change of signalling code is
         recognized only when both recognition timing periods have elapsed.


3.2.2 Sent signal time tolerance
~~~~~
   The time difference between application of transitions intended to be
simultaneous on two signalling channels in the same direction of transmission
must not exceed 2 ms.


3.2.3 States and procedures under normal conditions | see Table 2/Q.421)
~~~~~
      In the forward direction bf= 0 is established permanently.

      3.2.3.1 Idle state

      In the idle state the outgoing end sends af= 1, bf= 0.  At the incoming
      end this results in sending ab = 1, bb= 0 in the backward direction,
      provided that the switching equipment at the incoming end of the circuit
      is idle.


      3.2.3.2 Seizing procedure

              i) Seizure

                 Seizing should occur only if ab = 1, bb = 0 is recognized.
                 The outgoing end changes af = 1 into af = 0.  The code af =
                 0, bf = 0 must be maintained until the seizing
                 acknowledgement signal is recognized.  In this way the
                 outgoing switching equipment will only be able to send the
                 clear-forward signal after recognition off the seizing
                 acknowledgement signal.

             ii) Seizure acknowledgement

                 After having recognized the seizing signal, the incoming end
                 sends ab = 1, bb = 1 as an acknowledgement.

      3.2.3.3 Answering

      The off-hook condition of the called subscriber's line provokes the
      incoming switching equipment to send ab = 0, bb = 1.

      The answered state must be established on the preceding link immediately
      after it is recognized:  see also S 3.2.3.6 below.


      3.2.3.4 Clear-back

      The on-hook condition of the called subscriber's line provokes the
      incoming switching equipment to send ab = 1, bb = 1.  The clear-back
      state must be established on the preceding link immediately after it is
      recognized:  see also S 3.2.3.6 below.


      3.2.3.5 Clear-forward procedure

      The cleared condition of the calling subscriber's line or the release of
      the outgoing switching equipment will normally result in sending af = 1,
      bf = 0.  The outgoing switching equipment will not be restored to the
      idle state until recognition of the code ab = 1, bb = 0:  see also SS
      3.2.3.2, 3.2.3.6 and Table 3/Q.422.


      3.2.3.6 Release procedure

      Recognition of the clear-forward signal in the incoming switching
      equipment initiates the release of the succeeding link even though
      answering or clearing by the called party has occurred.  Upon complete
      release of the incoming switching equipment, the code ab = 1, bb = 0 is
      established on the circuit.  This will cause the circuit to be restored
      to the idle state and the outgoing switching equipment to become
      available for another call.


      3.2.3.7 Blocking and unblocking procedure

      Blocking of an idle circuit to new calls at the outgoing end must occur
      as soon as ab = 1 and bb = 1 is recognized:  see also Tables 3/Q.422 and
      4/Q.422.

      The recognition of ab = 1, bb = 0 restores the circuit to the idle
      state.


3.2.4 Actions appropriate to various signalling conditions
~~~~~
   In addition to normal conditions described in Table 2/Q.421 other
conditions due to faults may be encountered.  Tables 3/Q.422 and 4/Q.422
indicate the states appropriate to each signalling code recognized and the
actions to be taken at the outgoing and incoming end respectively of a circuit
operated with the digital version of System R2 line signalling.

      3.2.4.1 Outgoing end

                                 H.T. [3/Q.422]
                                  TABLE 3/Q.422

       _____________________________________________________________________________________________________________________________________


                                                      Received code

                  {
                                                    a b = 0, b b = 0        a b = 0, b b = 1         = 1, b b = 0        a b = 1, b b = 1
       _____________________________________________________________________________________________________________________________________
        Idle/Released          a f = 1, b f = 0   Abnormal,  see Note 1   Abnormal,  see Note 1        Idle                  Blocked
       _____________________________________________________________________________________________________________________________________
        Seized                 a f = 0, b f = 0   Abnormal,  see Note 2   Abnormal,  see Note 2    Sed  see Note 2     Seizure acknowledged
       _____________________________________________________________________________________________________________________________________
        Seizure acknowledged   a f = 0, b f = 0   Abnormal,  see Note 3         Answered          Abmal,  see Note 3   Seizure acknowledged
       _____________________________________________________________________________________________________________________________________
        Answered               a f = 0, b f = 0   Abnormal,  see Note 4         Answered          Abmal,  see Note 4        Clear-back
       _____________________________________________________________________________________________________________________________________
        Clear-back             a f = 0, b f = 0   Abnormal,  see Note 4         Answered          Abmal,  see Note 4        Clear-back
       _____________________________________________________________________________________________________________________________________









        Clear-forward          a f = 1, b f = 0   Abnormal,  see Note 1       Clear-forward         leased = Idle         Clear-forward
       _____________________________________________________________________________________________________________________________________
        Blocked                a f = 1, b f = 0   Abnormal,  see Note 1   Abnormal,  see Note 1        Idle                  Blocked
       _____________________________________________________________________________________________________________________________________


Note 1 - In these abnormal conditions the outgoing end must prevent a new
         seizure of the circuit.  A delayed alarm should also be given (see S
         3.2.6).

Note 2 - Non-recognition of the seizing acknowledgement signal 100 ms-200 ms
         after sending the seizing signal on a terrestrial link or 1-2 seconds
         after sending the seizing signal on a satellite link results in an
         alarm and either congestion information being sent backward or a
         repeat attempt being made to set up the call.  The outgoing end must
         prevent a new seizure of the circuit.  When the seizing
         acknowledgement signal is recognized after the time-out period has
         elapsed, the clear-forward signal must be sent.

Note 3 - Receipt of b b = 0 by the outgoing switching equipment for 1-2
         seconds after recognition of the seizing acknowledgement signal and
         prior to recognition of the answer signal, results in an alarm and
         either congestion information being sent backward or a repeat attempt
         being made to set up the call.  The outgoing end must prevent new
         seizures of the circuit.  When b b reverts to 1 after the 1-2 seconds
         timeout period has elapsed, the clear-forward signal must be sent.

Note 4 - In the case of recognition of b b = 0 whilst in the answered or
         clear-back state, immediate action is not necessary.  On receipt of
         clearing from the preceding link, the clear-forward signal (a f = 1,
         b f = 0) must not be sent until b b is restored to 1.  A delayed
         alarm should also be given.  Tableau [3/Q.422] + Remarques, p.7


         3.2.4.2 Incoming end

                                 H.T. [4/Q.422]
                                  TABLE 4/Q.422

       _____________________________________________________________________________________________________________________________________________________


                                                                    Received code

                  {
                                                                   a f = 0, b f = 0              a f0, b f = 1        a f = 1, b f = 0    a f = 1, b f = 1
       _____________________________________________________________________________________________________________________________________________________
        Idle/Released               a b = 1, b b = 0                    Seized                  Faulsee Note 1              Idle          Fault  see Note 1
       _____________________________________________________________________________________________________________________________________________________
        Seizure acknowledged        a b = 1, b b = 1            Seizure  acknowledged           FaulSee Note 2         Clear-  forward    Fault  See Note 2
       _____________________________________________________________________________________________________________________________________________________
        Answered                    a b = 0, b b = 1                   Answered                 Faulsee Note 3         Clear-  forward    Fault  see Note 3
       _____________________________________________________________________________________________________________________________________________________
        Clear-back                  a b = 1, b b = 1                  Clear-back                Faulsee Note 4         Clear-  forward    Fault  see Note 4
       _____________________________________________________________________________________________________________________________________________________
        Clear-forward                       {
        a
        b = 0, b
        b = 1
        or
        a
        b = 1, b
        b = 1
         }                     Abnormal  seized see Note 7        Fault  see Note 7         Clear-  ward see Note 7   Fault  see Note 7
       _____________________________________________________________________________________________________________________________________________________
        Blocked                     a b = 1, b b = 1         Abnormal  seized  see Note 5       Faulsee Note 6             Blocked        Fault see Note 6
       _____________________________________________________________________________________________________________________________________________________


Note 1 - When in the idle/released state b f changes to 1, b b must be changed
         to 1.

Note 2 - In these cases a time-out device is started, which after a certain
         interval clears the connection beyond the faulty circuit:  this
         timing arrangement may be the one specified in Recommendation Q.118,
         Section 4.3.3.  If the answer signal is recognized during the
         time-out delay, the timer is stopped but the answer signal is not
         sent on the preceding link until recognition of a f = 0, b f = 0.  If
         the clear-back signal is recognized while the fault persists, the
         connection beyond the faulty circuit must be released immediately.
         Additionally, when the incoming register has not started to send the
         last backward signal, the rapid release procedure described in Note 5
         may be used.

Note 3 - In these cases no action is taken until the clear-back signal is
         recognized, at which stage the connection beyond the faulty circuit
         is immediately released.

Note 4 - Under these conditions the succeeding link must be released
         immediately.

Note 5 - In this case immediate action is not necessary.  However, rapid
         release of the circuit should occur if the incoming end simulates
         answer by sending a b = 0, b b = 1.

Note 6 - Under these conditions no action is taken.

Note 7 - After clear-forward signal is recognized and until the code a b = 1,
         b b = 0 is sent, all transitions in the forward direction shall be
         ignored.

                                       Tableau [4/Q.422] + Remarques,  p.8

3.2.5 Abnormal conditions
~~~~~
      3.2.5.1 Special release arrangements

      a) If an exchange where an outgoing R2 register is situated recognizes
         ab = 0, bb = 1 (premature answer) before an address-complete signal
         A-6 or a Group B signal is received, the connection must be released.
         Congestion information is then sent backward or a repeat attempt is
         made to set up the call.

      b) In the cases of non-receipt of the answer signal, of delay in
         clearing by the calling subscriber in automatic working and of
         non-receipt of the clear-forward signal by the incoming exchange
         after the clear-back signal has been sent, the provisions of
         Recommendation Q.118 apply.


      3.2.5.2 Safeguard against failures

      The PCM equipment and the exchange line signalling equipment should be
      designed in such a way that at least those faults which are most likely
      to occur in this equipment or in the interconnecting cables, result in
      blocking of the circuit at the outgoing end and in the ultimate clearing
      of the connection beyond the incoming switching equipment.  This can be
      achieved, as far as possible, by ensuring that a = 1, b = 1 is sent on
      line upon:

      o removal of PCM or switching equipment by maintenance personnel;

      o occurrences of abnormal conditions (e.g.  open wire, low voltage) in
        switching equipment.


3.2.6 Alarms for technical staff
~~~~~
   According to Recommendation Q.117, an alarm must in principle be given to
the technical staff upon recognition of abnormal conditions.

   Arrangements for these alarms are to be specified by the Administrations.

  It is recommended that a delayed alarm be given at the outgoing end for the
procedure described under S 3.2.3.7 above (blocking) and for the following
reasons:

   o when the abnormal conditions covered by Note 1 to Table 3/Q.422 are
     applicable;

   o when the seizing acknowledgement signal is not recognized within the time
     specified in Note 2 to Table 3/Q.422 after sending the seizing signal;

   o when, after recognition of the seizing acknowledgement signal and prior
     to recognition of the answer signal, bb = 0 is received for 1-2 seconds;

   o when the abnormal conditions covered by Note 4 to Table 3/Q.422 are
     applicable.

   It is also recommended that a delayed alarm be given under PCM failure
conditions specified in Recommendations G.732 and G.734.


3.2.7 Both-way working
~~~~~
   System R2 is specified for one-way working, but in principle the line
signalling code detailed in Recommendation Q.421 is also suitable for use on
both-way circuits.  Where Administrations have undertaken, by bilateral
agreement, to use both-way working, the clauses and additional specifications
for exchange signalling equipment detailed in SS 3.2.7.1 and 3.2.7.2 below,
must be observed.

      3.2.7.1 Procedures under normal conditions

      a) Double seizure

         Double seizure is assumed if the outgoing equipment is in a seized
         state and the signalling code ab = 0, bb = 0 is recognized instead of
         ab = 1, bb = 1 (seizure acknowledgement).  In such a situation the
         connection must be released at both ends and congestion information
         sent to the calling subscriber or a repeat attempt must be made.  On
         recognition of double seizure the line signalling equipment at both
         ends must maintain the seized state for a minimum of 100 ms after
         which the clear-forward signal af = 1, bf = 0 must be sent.

         100 ms after sending the clear-forward signal and on recognition of
         ab = 1, bb = 0 each end may assume the idle state.

         The clear-forward state af = 1, bf = 0 must be maintained for at
         least 100 ms to ensure that it is recognized at the other end.

         In the sense of preventive action it is recommended that an opposite
         order of circuit selection be used by each exchange of a both-way
         circuit group to minimize double seizure.


      b) Requirements for circuit release

         When a both-way circuit is released, the end which acted as the
         incoming end must maintain the signalling code ab = 1, bb = 0 for at
         least 100 ms to ensure that the signal is recognized at the other end
         after which the circuit becomes idle.


      c) Blocking and unblocking procedure

         When a both-way circuit is blocked manually in its idle state at one
         end (e.g.  end B) the blocking signal must be sent to the other end
         (A).  The circuit must then be kept blocked locally (at end A)
         against all calls in the A to B traffic direction as long as the
         blocked state persists in the B to A direction.

         In order to avoid permanent blocking, end A should maintain the
         signalling code a = 1, b = 0 in the direction A to B.

         When the blocked state is removed, end B must send the clear-forward
         signal and must maintain that state for at least 100 ms, before
         assuming the idle state.


      3.2.7.2 Special arrangement

      The physical realization of signalling equipment terminating a bothway
      circuit may allow that part of the equipment concerned with outgoing
      calls to be removed without preventing the remaining equipment from
      being used for incoming calls.  In this case it is only necessary to
      block the circuit locally against outgoing calls and a blocking signal
      need not be sent to the other end.




                             ____________________
                             Recommendation Q.424
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 3.3 :  PROTECTION AGAINST THE EFFECTS OF FAULTY TRANSMISSION

   Faulty transmission conditions in PCM systems can lead to degradation of
the speech channels and erroneous signalling.  In the case of primary PCM
multiplex equipment operating at 2048 kbit/s failures due to loss of frame or
multi-frame alignment and/or failure of any other important function results
in both PCM terminals going into alarm condition in accordance with


                       ________________________________
                       Recommendations G.732 and G.734.
                       ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
   Thus both PCM terminals apply the state corresponding to state 1 on the PCM
line on each "receive" signalling channel at the interfaces with the switching
equipment, as indicated in Table 4 of Recommendation G.732.  In this way, the
incoming switching equipment receives the equivalent of af = 1, bf = 1 on the
PCM line and the outgoing switching equipment receives the equivalent of ab =
1, bb = 1.

   These characteristics are taken into account in the present specifications
(see S 3.2.4) so that:

   o at the outgoing end (see Table 3/Q.422) a PCM fault results in a blocked
     state, seizure acknowledged state or clear-back state.  This means that
     all circuits in the idle state of a faulty PCM multiplex will be blocked
     against seizure and that seized circuits will go to or remain in the
     seizure acknowledged or clear-back state;

   o at the incoming end (see Table 4/Q.422) a PCM fault can be identified and
     appropriate actions can be taken.

   When the signalling equipment is a part of a digital exchange, it may
receive alarm indications in a form other than both signalling bits in state
one.  The failure may be detected by the signalling equipment or an indication
be received from the PCM terminal according to Recommendation G.734.

   When the signalling equipment recognizes a failure it must:

   o block the detection of signalling transitions to avoid recognition of
     erroneous signalling codes caused by the failure.  This action must be
     taken as soon as possible and at least within 3 ms as specified in
     Recommendation G.734 for a PCM termi nal,

   o react as specified in Tables 3/Q.422 and 4/Q.422 when a signalling code a
     = 1, b = 1 is detected at the input of signalling equipment situated at
     the analogue access of a PCM terminal equipment complying with
     Recommendation G.732.



                             ____________________
                             Recommendation Q.430
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 3.5 :  CONVERSION BETWEEN ANALOGUE AND DIGITAL VERSIONS OF SYSTEM R2 LINE
          SIGNALLING

  This Recommendation is applicable to a conversion equipment placed on the
circuit between two switching exchanges each using one of the two versions
specified for System R2 line signalling.  Owing to this particular use, all
the specifications for the two versions of the line signalling may not be
fully observed.  Nevertheless, the diagrams which follow are based on the
principles of CCITT Recommendations Q.411, Q.412 and Q.416 for the analogue
version and on Recommendations Q.421, Q.422 and Q.424, for the digital
version.  The only time conditions taken into account in this Recommendation
are those set out in the aforementioned Recommendations.  The operation of the
interruption control device is also, wherever possible, that specified in
Recommendation Q.416.  As stated in that Recommendation, the receiver is
blocked  immediately  whenever  pilot  tone  off is detected:  since this is a
routine operation, it has not been represented explicitly in the diagrams.

   The conversion diagrams have been divided into four parts:

   o for conversion between the analogue version at the incoming end and the
     digital version at the outgoing end, into:

   o incoming analogue,

   o outgoing digital;

   o for conversion between the digital version at the incoming end and the
     analogue version at the outgoing end, into:

   o incoming digital,

   o outgoing analogue.

   It should be noted, however, that this Recommendation can be simplified
with respect to alarm processing when the conversion equipment is connected
directly at the input or output of a switch- ing exchange; the processes
necessary for this application are shown on the diagrams with thickened lines.


1.  Drawing conventions
~~~~~~~~~~~~~~~~~~~~~~~

               -         tone

               tf         = 1 tone on forward

               tf         = 0 tone off forward

               tb         = 1 tone on backward

               tb         = 0 tone off backward


   When recognized


               p         = 1 pilot tone on

               p         = 0 pilot tone off

               -         signalling bits

   The conventions are those of Recommendation Q.421.


2.  Conversion incoming analogue version to outgoing digital version
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 * 2.1 :  Incoming analogue

                                                    Figure CCITT-55980, p.

            List of timers:

               T2:         Recommendation Q.412 (S 2.2.2.7)

               T3: 2 to 3 mn         Recommendation Q.118 (S 4.3.3)

   In sheets 2 and 4 the diagram for group pilot supervision is given.

   In sheets 3 and 5 the diagram for supergroup pilot supervision is given.

   Sheet 1 is for group and supergroup pilot supervision.


                                     FIGURE CCITT-60881 (feuillet 1), p.10

                                     FIGURE CCITT-60891 (feuillet 2), p.11

                                       FIGURE T1115760-88 (feuillet 3), p.

                                     FIGURE CCITT-60901 (feuillet 4), p.12

                                       FIGURE T1115770-88 (feuillet 5), p.


 * 2.2 :  Outgoing digital

                                                    Figure CCITT-55990, p.

            List of timers:

               T6: Recommendation Q.422 (S 3.2.4.1, Note 2 to Table 3)

                                     Figure CCITT-60911 (Sheet 1 of 3), p.

                                     Figure CCITT-60920 (Sheet 2 of 3), p.

                                     Figure CCITT-60930 (Sheet 3 of 3), p.


3.  Conversion incoming digital version to outgoing analogue version
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 * 3.1 :  Incoming digital

                                                    Figure CCITT-56000, p.
            List of timers:

               T3: 2 to 3 mn Recommendation Q.118 (S 4.3.3)


                                     Figure CCITT-60940 (Sheet 1 of 3), p.

                                     Figure CCITT-60950 (Sheet 2 of 3), p.

                                     Figure CCITT-60960 (Sheet 3 of 3), p.


 * 3.2 :  Outgoing analogue

                                                    Figure CCITT-50610, p.


            List of timers:

               T1:         Recommendation Q.412 (S 2.2.2.7)

               T5: 100 ms         Recommendation Q.412 (S 2.2.2.1)

            Automatic  restoration  of  an  abnormally   blocked   circuit
       Recommendation Q.490 (S 6.6)

               n:         number of attempts made

               T4:         30 s to 2 mn

               T7:         2 to 3 s




                                     Figure CCITT-60971 (Sheet 1 of 5), p.

                                     Figure CCITT-60981 (Sheet 2 of 5), p.

                                     Figure CCITT-60991 (Sheet 3 of 5), p.

                                     Figure CCITT-70001 (Sheet 4 of 5), p.

                                                                Figure, p.




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