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TUCoPS :: Phreaking Technical System Info :: 6_4_02.txt

Recommendations Q.327 to Q.412




                                  SECTION 4

                             TESTING ARRANGEMENTS
                             ____________________
                             Recommendation Q.327
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 4.1 :  GENERAL ARRANGEMENTS

   The guiding principles for the maintenance of automatic circuits as covered
in Recommendations M.700 to M.734 are in general applicable to testing of
System R1.


                             ____________________
                             Recommendation Q.328
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 4.2 :  ROUTINE TESTING OF EQUIPMENT (LOCAL MAINTENANCE)

4.2.1 Test equipment for routine testing of individual items of equipment such
~~~~~ as circuit equipment, connecting circuits, registers, etc., should be
provided in every international exchange.  Routine tests should be made in
accordance with the practice followed in each country for the local
maintenance of switching equipment and may be made with suitable
semi-automatic or automatic test equipment if available.


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 (busy) for
      the duration of the test.  Before a circuit equipment is taken for test,
      the circuit will be withdrawn from service at both international
      exchanges;

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


4.2.3 Testing of the circuit and signalling equipment should include a check
~~~~~ that the specifications of System R1 are met in regard to the following:

   a) 2600 Hz line signalling system:

      o Signal frequency;

      o Transmitted signal levels;

      o Signal frequency leak;

      o Receiving equipment operate and non-operate limits;

      o Receiving-end line split;

      o Sending-end line split;

      o Sending duration of signals.


   b) PCM line signalling equipment:

      o Receiving equipment operate and non-operate limits;

      o Sending duration of signals.


   c) Register signalling system:

      o Signal frequencies;

      o Transmitted signal levels;

      o Signal frequency leak;

      o Sending duration of signals;

      o Receiving equipment operate and non-operate limits;

      o Operation of the receiving equipment in response to a series of
        pulses;

      o Error checking features.


                             ____________________
                             Recommendation Q.329
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 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 by
verification of satisfactory signal transmission by initiating a test call to:

   a) technical personnel at the distant-end international exchange; or

   b) a test call signal testing and answering device, if such equipment is
      available at the distant-end international exchange.


4.3.2 Test calls
~~~~~
   1) Steps in the verification of satisfactory transmission of signals,
      involved in the completion of test calls (manual method):

      a) Place a call to the technical personnel at the distant international
         exchange;

      b) On completion of the connection the audible ringing tone should be
         heard and the answer signal should be received when the call is
         answered at the distant end;

      c) Request distant end to initiate a hang-up (clear-back) signal,
         followed by a re-answer signal;

      d) A hang-up (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 ring-forward (forward-transfer) signal which should be
         recognized at the distant end;

      f) Terminate the call and observe that the circuit restores to the idle
         condition.

   2) If incoming signalling testing devices are available at the distant
      international exchange, the signal verification tests should be made
      using this equipment to the extent that the applicable features
      indicated in 1) above are available.




                             ____________________
                             Recommendation Q.330
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 4.4 :  AUTOMATIC TRANSMISSION AND SIGNALLING TESTING

   Considering that automatic transmission and signalling testing of
international circuits is extremely desirable, Administrations using or
intending to use System R1 are encouraged to provide for this type of testing.
Existing automatic testing equipment presently in use in world numbering Zone
1, may be used.  When the automatic transmission measuring and signalling
testing equipment (ATME) No.  2 becomes available, it may be used as an
alternative by agreement of the Administrations concerned.



                             ____________________
                             Recommendation Q.331
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 4.5 :  TEST EQUIPMENT FOR CHECKING EQUIPMENT AND SIGNALS

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

   a) Line and register signal generators;

   b) Signal-measuring apparatus.


4.5.2 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 system
specifications.

   1) Line signal generator characteristics as follows:

      a) signal frequency should be within _ | Hz of the nominal signal
         frequency and shall not vary during the time required for testing;

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

      c) signal durations should be long enough so that the signals can be
         recognized.  See Recommendation Q.313, S 2.3.3.


   2) Register signal generator characteristics as follows:

      a) signal frequencies should be within _ | .5% of the nominal signal
         frequency or frequencies and shall not vary during the time required
         for testing;

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

      c) signal durations and intervals between signals shall be within the
         limits given in the specification in Recommendation Q.322, S 3.3.4,
         for normal operate values and in Recommendation Q.323, S 3.4.1 d) ,
         for test operate values.


4.5.3 Signal-measuring equipment
~~~~~
   Equipment capable of measuring signal frequencies, signal levels, signal
durations and other significant signal time intervals may be part of the test
equipment referred to in S 4.5.2, or separate instruments.

   1) Line signal measuring equipment characteristics as follows:

      a) Signal frequency between the extreme limits given in the
         specification should be measured with an accuracy of _ | Hz;

      b) Level of the signal frequency measured over the range given in the
         specification should be measured with an accuracy of _ | .2 dB;

      c) Signal durations, and other significant time intervals as given in
         the specification should be measured with an accuracy of _ | ms or _
         | % of the nominal duration, whichever yields the higher value.


   2) Register signal measuring equipment characteristics as follows:

      a) Signal frequency or frequencies between the extreme limits given in
         the specification, should be measured with an accuracy of _ | Hz;

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

      c) Signal duration and intervals between signals as given in the
         specification should be measured with an accuracy of _ | ms.


   3) In regard to measuring time intervals a recorder having a minimum of two
      input channels may be useful.  The recorder characteristic should
      conform with the accuracy requirements quoted in 1) and 2) above and be
      easily connected to the circuit under test.  The recorder input
      characteristic should be such as to have a negligible effect on circuit
      performance.



                         ANNEX A TO THE SPECIFICATIONS
                            OF SIGNALLING SYSTEM R1

                                SIGNAL SEQUENCES

                                                        Tableau [A-1], p.5

                                                      Tableau [1-A-1], p.6




                                   PART II
                             ____________________
                             Recommendation Q.332
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
                     INTERWORKING OF SIGNALLING SYSTEM R1

                       WITH OTHER STANDARDIZED SYSTEMS


                             ____________________
                             Recommendation Q.332
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
5 :  INTERWORKING
~~~~~~~~~~~~~~~~~
 * 5.1 ; General

   System R1 is capable of interworking with any of the CCITT standardized
signalling systems.

   Specifications on interworking of System R1 with other CCITT signalling
systems are not yet available.

   Typical information is found in Recommendation Q.180 of Fascicle VI.2.



                                   PART III
                        ______________________________
                        Recommendations Q.400 to Q.490
                        ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
                    SPECIFICATIONS OF SIGNALLING SYSTEM R2

                            SIGNALLING SYSTEM R 2

                                 INTRODUCTION

General
~~~~~~~
   Signalling System R2 is used as an international signalling system within
international regions (world numbering zones).  Moreover, System R2 can be
used for integrated international/national signalling if it is employed, in
conformity with the present specifications, as a signalling system in the
national networks of the region concerned.  Suitable for both automatic and
semi-automatic working it offers a high reliability in transmission of the
information necessary for setting up a call.  It allows for rapid call set-up
and provides sufficient signals in both directions to permit the transmission
of numerical and other information relating to the called and calling
subscribers' lines and to increase routing facilities.

   System R2 is specified for one-way operation on analogue and digital
transmission systems and for both-way operation on digital transmission
systems.  Distinction is made between line signalling (supervisory signals)
and interregister signalling (call set-up control signals).  Versions of line
signalling are specified for use on 4-wire carrier or PCM circuits.
Nevertheless, the interregister signalling specified can also be used on
2-wire circuits.  System R2 is suitable for use on satellite links and 3 kHz
spaced carrier circuits.  It is not designed for use on transmission systems
with speech interpolation but it may be used on digital transmission systems
with speech interpolation as long as transparency for pulsed interregister
signals is guaranteed.

   The interregister signalling is a compelled multifrequency code system.
This offers the possibility of end-to-end signalling and to take advantage of
modern switching systems by providing a sufficient number of signals in both
directions.

   System R2 is capable of normal interworking with other CCITT signalling
systems.

Line signalling
~~~~~~~~~~~~~~~
   The following versions of line signalling are specified:

   o line signalling for carrier systems called the analogue version;

   o line signalling for PCM systems called the digital version.

   Since multifrequency signalling permits exchange of a large amount of
information between registers, the quantity of information that has to be
transmitted in the form of line signals is small.  The versions of line
signalling for System R2 have been designed accordingly.  The analogue line
signalling version standardized originally for use on international circuits
is also suitable for national working.  An analogue (in-band) line signalling
version is only described for the specific use on 3 kHz spaced carrier
circuits of international submarine cables.

   The analogue version | s link-by-link using an out-band, low-level
continuous tone-on-idle signalling method.  Signal transfer simply involves
the transition from one signalling condition to the other, except for the
clearing sequence which is based on additional timing criteria.  It is
necessary to have a device for protection against the effect of interruption
in the signalling channel since interruption of the signalling frequency
simulates a false seizure or answer signal (interruption control).  The
signalling frequency is transmitted at a low level which avoids overloading of
the transmission system when the frequency is sent continuously in both
directions on all circuits in the idle state.

   The digital version | s link-by-link using two signalling channels in each
direction of transmission per speech circuit.  The signalling channels are two
of the four provided for channel associated signalling in a 2048 kbit/s
primary multiplex (see Recommendation G.732).  Protection against the effects
of faulty transmission is provided.

   The analogue version and the digital version of the line signalling can be
converted to each other by a transmultiplexer or other conversion equipment.
Such equipment forms a conversion point between analogue (FDM) transmission on
one hand and digital (PCM) transmission on the other hand.  The System R2 line
signalling, however, must be treated separately because for both transmission
systems different line signalling versions are specified.  Although both
versions are described in some detail, a description of the actual conversion
is given in Recommendation Q.430.  Although these versions do not include a
forward-transfer signal, forward-transfer signalling may be introduced for
international working by bilateral agreement.


Interregister signalling
~~~~~~~~~~~~~~~~~~~~~~~~
   The interregister signalling is performed end-to-end using a 2-out-of-6
in-band multifrequency code with forward and backward compelled signalling.
The signalling frequencies do not therefore overlap with the line signalling
frequency and differ according to the direction of transmission with a view to
possible use of the multifrequency part of the system on 2-wire circuits.

   System R2 is designed to use six signalling frequencies (1380, 1500, 1620,
1740, 1860 and 1980 Hz) in the forward direction and six signalling
frequencies (1140, 1020, 900, 780, 660 and 540 Hz) in the backward direction.
Nevertheless for national application less signalling frequencies may be used.

   End-to-end signalling is a method for signalling between registers over two
or more links in tandem without signal regeneration in intermediate exchanges
(see Figure 1).
                                                              Figure 1, p.

   With this signalling method, in general only the address information needed
for routing the call through an intermediate (transit) exchange is transferred
from the outgoing register to the incoming register.  In the intermediate
exchange the speech path is immediately through-connected and the incoming
register released.  Then the outgoing register can exchange information
directly with the incoming register of the next exchange.  Such end-to-end
signalling is advantageous as it reduces the interregister signalling
equipment needed and minimizes the holding time of registers in transit
exchanges.

   System R2 has been designed to allow end-to-end interregister
multifrequency signalling over several links in tandem.  However, in
circumstances where transmission conditions do not comply with the
requirements specified for System R2 and might consequently jeopardize the
exchange of the interregister signals, or in case of using System R2 via a
satellite link, the overall multi-link connection is divided into sections,
each with its individidual interregister signalling (signals being then
relayed and regenerated by a register at the point where the division is
made).

   In the case of satellite working, the register at the incoming end of the
satellite link must act as an outgoing R2 register (see also Recommendation
Q.7).

   Generally transmission conditions in a national network comply with the
requirements specified for System R2 and as such allow end-to-end signalling
over complete connections between local exchanges.

   In the case of international System R2 working transmission conditions
impose division into at least two signalling sections, when both the outgoing
and incoming countries use System R2 in the national network.  The division
must be made in an exchange in the outgoing country.  The relaying or
regenerating register situated at the point where the division is made is
called the outgoing international R2 register .  In the case when System R2 is
not used in the outgoing country the outgoing international R2 register
receives address information via a national signalling system and it controls
the call set-up over the outgoing System R2 signalling section.

   The outgoing R2 register | s generally defined as a register situated at
the outgoing end of a signalling section on which System R2 interregister
signalling, according to the present specifications, is used.  It controls the
call set-up over the whole signalling section.  It sends forward interregister
signals and receives backward interregister signals.  The outgoing R2 register
receives information via the preceding links of the connection in a form used
by the signalling system applied over the last of these links; this system may
be System R2, a decadic pulse system, or any other system.  The preceding link
may also be a subscriber's line.

   When an R2 register in a transit exchange | s working according to the
definition above it is also called an outgoing R2 register outgoing
international R2 register is the special case of the outgoing R2 register when
the outgoing signalling section comprises at least one international link.

   The incoming R2 register | s defined as a register situated at the incoming
end of a link on which System R2 interregister signalling, according to the
present specification, is used.  It receives forward interregister signals via
the preceding link(s) and sends backward interregister signals.  The
information received is used completely or in part for the control of
selection stages and may be sent in its entirety or in part to the succeeding
equipment, in which case the signalling used for retransmission is never
System R2.  Interworking then takes place between System R2 and the other
System.  Thus every register not situated at the outgoing end of a System R2
signalling section is called an incoming R2 register independently of the
exchange type.

   The following operational features are provided by using all the 15 forward
and 15 backward combinations of the multifrequency code:

   o Capability of transferring address information for automatic,
     semi-automatic and maintenance calls;

   o Indicators in order to identify transit and maintenance (test) calls;

   o Language digits or discriminating digit(s) for international calls;

   o Transfer of the next digit or repetition of the last but one, the last
     but two, or the last but three digit on request by the register at the
     incoming end;

   o Forward and backward signals for control of echo suppressors;

   o Information concerning the nature and the origin of the call (calling
     party's category) can be transferred to the register at the incoming end,
     i.e.  whether national or international, whether from an operator or from
     a subscriber, whether from data-transmission, maintenance or other
     equipment, etc.;

   o Information concerning the nature of the circuit can be requested by the
     register at the incoming end, i.e.  whether a satellite link is already
     included or not;

   o Information on congestion, unallocated number and on the condition of the
     called subscriber's line can be transferred to the outgoing register,
     i.e.  whether free, busy or out of service, etc.;

   o A restricted number of signals left to the discretion of Administrations
     for allocation of national meanings.

   The interregister signalling specified for System R2 can also be used
together with any non-standardized line-signalling system.  The resulting
combination is not considered to be System R2.

   System R2 gives potential for a short post-dialling delay using overlap
operation with dialling and end-to-end signalling.  The outgoing R2 register
starts call set-up as soon as it has received the minimum requisite
information.  Therefore signal transfer starts before the complete address
information is received, i.e.  before the caller finishes dialling.  This
practice particularly applies at an outgoing R2 register where the complete
address information from a subscriber or operator is stored (e.g.  local
registers).  This is in contrast to en bloc register signalling i.e.  the
transmission of all the address information as a whole in one sequence
starting only after complete reception of the address information.

   Unused signalling capacity provides potential for development and allows
for future requirements yet undefined.  This spare capauity may be used for
increasing the number of signals and signalling procedures e.g.  for new
services to be offered to subscribers.





                                    SECTION 1

                      DEFINITIONS AND FUNCTIONS OF SIGNALS
                             ____________________
                             Recommendation Q.400
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 1.1 :  FORWARD LINE SIGNALS

1.1.1 Seizing signal
~~~~~
   A signal sent at the beginning of the call to initiate transition of the
circuit at the incoming end from the idle state to seized state.  At the
incoming exchange it causes the association of equipment capable of receiving
register signals.


1.1.2 Clear-forward signal
~~~~~
   A signal sent to terminate the call or call attempt and to release in the
incoming exchange and beyond it all switching units held on the call.

   The signal is sent when:

   a) In semi-automatic working the operator of the outgoing international
      exchange takes the plug out or performs an equivalent operation;

   b) in automatic working, the calling subscriber clears or performs an
      equivalent operation.

   This signal is also sent by the outgoing international exchange upon
receiving a backward register signal requesting the outgoing international R2
register to clear the connection, or in the case of forced release of the
connection as mentioned in Recommendation Q.118.  This signal may also be sent
as the result of abnormal release of the outgoing international R2 register.


1.1.3 Forward-transfer signal
~~~~~
   A signal sent on semi-automatic calls when the outgoing international
exchange operator wants the help of an operator at the incoming international
exchange.  The signal will usually bring an assistance operator (see
Recommendation Q.101) into the circuit.  If the call is completed via an
incoming or delay operator at the incoming international exchange, the signal
indicates that recall of this operator is wanted.



 * 1.2 :  BACKWARD LINE SIGNALS

1.2.1 Seizing-acknowledgement signal
~~~~~

                  [ This signal is not provided in either the analogue or
                  [ digital version of System R2 line signalling.  Information
                  [ about possible arrangements for such a signal and
                  [ signalling procedures involved are contained in Annex A to
                  [ the present Specifications.  This signal is only used in
                  [ the digital version of System R2 line signalling.

   A signal sent to the outgoing exchange to indicate the transition of the
equipment at the incoming end from the idle state to seized state.
Recognition of the seizing acknowledgement signal at the outgoing end causes
the state of the circuit to change from seized to seizure acknowledged.


1.2.2 Answer signal
~~~~~
   A signal sent to the outgoing international exchange to indicate that the
called party has answered the call (see Recommendation Q.27).  In
semi-automatic working this signal has a supervisory function.

   In automatic working this signal is used:

   o To start metering the charge to the calling subscriber, unless the
     register signal indicating no charge has been sent previously;

   o To start measurement of the call duration for international accounting
     purposes.


1.2.3 Clear-back signal
~~~~~
   A signal sent to the outgoing international exchange to indicate that the
called party has cleared.  In semi-automatic working, this signal has a
supervisory function.  In automatic working, arrangements must be made in
accordance with Recommendation Q.118, and the Notes of Recommendation Q.120, S
1.8 also apply.


1.2.4 Release-guard signal
~~~~~
   A signal sent to the outgoing exchange in response to a clear-forward
signal to indicate that the latter has been fully effective in returning the
switching units at the incoming end of the circuit to idle condition.  An
international circuit is protected against subsequent seizure as long as the
release operations initiated by the clear-forward signal have not been
completed at the incoming end.


1.2.5 Blocking signal
~~~~~
   A signal sent on an idle circuit to the outgoing exchange to cause engaged
conditions (blocking) to be applied to this circuit, guarding it against
subsequent seizure.




 * 1.3 :  FORWARD REGISTER SIGNALS

1.3.1 Address signal
~~~~~
   A signal containing one element of information (digit 1, 2, .  | | , 9 or
0, code 11, code 12 or code 13) about the called or calling party's number or
the end of pulsing indication (code 15).

   For each call a series of address signals is sent (see Recommendations
Q.101 and Q.107).


1.3.2 Country-code and echo-suppressor indicators
~~~~~
   Signals indicating:

   o Whether or not the country-code is included in the address information
     (international transit or terminal call);

   o Whether or not an outgoing half-echo suppressor should be inserted in the
     first international exchange reached;

   o Whether or not an incoming half-echo suppressor should be inserted (an
     outgoing half-echo suppressor having already been inserted in the
     connection).


1.3.3  Language or discriminating digit
~~~~~
   A numerical signal occupying a predetermined position in the sequence of
address signals indicating:

   o In semi-automatic working, the service language to be used in the
     incoming international exchange by the incoming, delay and assistance
     operators when they come in the circuit (language digit);

   o The automatic working or any other special characteristic of the call
     (discriminating digit).


1.3.4 Test call indicator
~~~~~
   A signal occupying the position of the language digit when the call is
originating from test equipment.


1.3.5 Nature of circuit indicators
~~~~~
   Signals only sent on request by certain backward signals and using a second
meaning of some signals, to indicate whether a satellite link is already
included in the connection or not.


1.3.6 End-of-pulsing signal
~~~~~
   An address signal sent indicating (in semi-automatic service) that no other
address signal will follow or (in automatic service) that the transmission of
the code identifying the origin of the call is completed.


1.3.7 Calling party's category signals
~~~~~
   A special group of signals providing, in addition to the information
contained in the language or discrimination digit, supplementary information
concerning the nature of the call (i.e.  whether national or international)
and its origin.

   Typical categories are:

   o Operator capable of sending the forward-transfer signal;

   o Ordinary subscriber or operator with no forward-transfer facility;

   o Subscriber with priority;

   o Data transmission call;

   o Maintenance call.


1.3.8 Signals for use on the national network
~~~~~
   Some of the Group II forward signals (see Recommendation Q.441, S 4.2.3.2)
have been allocated for national use.  When the outgoing international R2
register receives them, it must react as specified in Recommendation Q.480.



 * 1.4 :  BACKWARD REGISTER SIGNALS

1.4.1 Signals requesting transmission of address signals
~~~~~
   Five backward signals without particular names are provided; four of them
are interpreted with reference to the latest address signal sent:

   o Signal requesting the transmission of the address signal following the
     latest address signal sent;

   o Signal requesting repetition of the address signal preceding the latest
     address signal sent (last but one);

   o Signal requesting the repetition of the last but two address signals
     sent;

   o Signal requesting the repetition of the last but three address signals
     sent;

   o Signal requesting the transmission or repetition of the language or
     discrimination digit.


1.4.2 Signal requesting information about the circuit
~~~~~
   A backward signal is provided to request the nature of the circuit.


1.4.3 Signals requesting information about the call or calling party
~~~~~
   Three backward signals without particular names are provided for this
purpose:

    o Signal inquiring the calling party's category;

    o Signal requesting the repetition of the country-code indicator;

    o Signal inquiring whether or not incoming half-echo suppressor should be
      inserted.


1.4.4 Congestion signals
~~~~~
   Two congestion signals are provided:

   o a signal indicating international congestion, i.e.  that the call set-up
     attempt has failed owing to congestion of the group of international
     circuits, or congestion in the international switching equipment, or to
     time-out or abnormal release of an incoming R2 register in an
     international transit exchange;

   o a signal indicating national congestion, i.e.  that the call set-up
     attempt has failed owing to congestion in the national network (excluding
     a busy called subscriber's line) or to time-out or abnormal release of an
     incoming R2 register in a terminal international exchange or a national
     exchange.


1.4.5 Address-complete signals
~~~~~
   Signals indicating that it is no longer necessary to send another address
signal, and

   o Either cause immediate passage to the speech position to enable the
     calling subscriber to hear a tone or a recorded announcement of the
     national incoming network;

   o Or announce the transmission of a signal indicating the condition of the
     called subscriber's line.


1.4.6 Signals indicating the condition of the called subscriber's line
~~~~~
   Six signals sent in the backward direction are provided to give information
about the called subscriber's line and to indicate the end of interregister
signalling.  These signals are:

   o Send special information tone a signal sent in the backward direction
     indicating that the special information tone should be returned to the
     calling party.  This tone indicates that the called number cannot be
     reached for reasons not covered by other specific signals and that the
     unavailability is of a long term nature.  (See also Recommendation Q.35);


   o Subscriber line busy :  a signal indicating that the line or lines
     ~~~~~~~~~~~~~~~~~~~~    connecting the called subscriber to the exchange
     are busy;


   o Unallocated number :  A signal indicating that the number received is
     ~~~~~~~~~~~~~~~~~~    not in use (e.g.  an unused country code or an
     unused trunk code or subscriber number that has not been allocated);


   o Subscriber line free, charge :  A signal indicating that the called
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~   subscriber's line is free and that the
     call is to be charged on answer;


   o Subscriber line free, no charge :  A signal indicating that the called
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~    subscriber's line is free and that the
     call is not to be charged on answer.  This signal is used only for calls
     to special destinations;


   o subscriber line out of order :  a signal indicating that the subscriber's
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~    line is out-of-service or faulty.



1.4.7 Signals for use in the national network
~~~~~
   Some of the backward signals have been allocated for national use.  Since
not all incoming registers can know the origin of the connection and since
end-to-end signalling is used, it may happen that the above-mentioned signals
are sent to the outgoing international R2 register.  When this register
receives them it must react as indicated in Recommendations Q.474 and Q.480.




                                    SECTION 2

                       LINE SIGNALLING, ANALOGUE VERSION
                             ____________________
                             Recommendation Q.411
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 2.1 :  LINE SIGNALLING CODE

2.1.1 General
~~~~~
   The System R2 line signalling, analogue version, is intended for use on
carrier circuits.  The line signals are transmitted link-by-link.  The code
for the transmission of line signals is based on the tone-on-idle signalling
method employed are equipped in each direction of transmission with a
signalling channel outside the speech frequency band.  When the circuit is in
the idle state, a low-level signalling tone is sent continuously in both
directions over the signalling channels.  The tone is removed in the forward
direction at the moment of seizure and in the backward direction when the
called subscriber answers.

   The connection is released when the signalling tone is restored in the
forward direction; release causes the tone to be restored in the backward
direction.  If the called party is the first to clear, the signalling tone is
restored in the backward direction first.  It is then restored in the forward
direction either when the caller clears or when a certain interval has elapsed
after recognition of the signalling tone in the backward direction.  This
signalling method, requiring only simple equipment, provides rapid signal
recognition and retransmission.  The signal transfer speed provided by
continuous type signalling compensates for the need of signal repetition
inherent in link-by-link transmission.

   The signalling system is specified for one-way operation of 4-wire carrier
circuits.


2.1.2 Line conditions
~~~~~
   Tone-on or tone-off denotes a certain line signalling condition.  The line
thus has two possible conditions in each direction, i.e.  a total of four line
signalling conditions.  Taking into account the time sequence, the circuit may
resume one of the six characteristic states shown in Table 1/Q.411.

                                                          TABLE [1/Q.411] p.

   The transition from one signalling condition to another corresponds to the
transfer of a line signal according to the definitions in S 1.  To change from
the release state to the idle state additional criteria (timing) are necessary
to ensure a defined sequence corresponding to the transfer of the
release-guard signal (see S 2.2.2.6 below).



                             ____________________
                             Recommendation Q.412
                             ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 * 2.2 :  CLAUSES FOR EXCHANGE LINE SIGNALLING EQUIPMENT

2.2.1 Recognition time for transition of signalling condition
~~~~~
   The recognition time trfor a changed condition (transition from tone-on to
tone-off or vice versa) is 40 _ 10 ms | duration that the presence or absence
of a direct current signal must have

                  [ Although the signalling condition (tone-on or tone-off)
                  [ physically only appears in transmission equipment, it is
                  [ used in this section as a reference criterion to specify
                  [ functions of exchange equipment.  Originally this value
                  [ was (20 _ 7) ms.  Since there will be no problem in the
                  [ interworking between equipment having the original
                  [ recognition time tr = (20 _ 7) ms, and equipment having
                  [ the new recognition time tr = (40 _ 10) ms, existing
                  [ equipment need not necessarily be changed to the value
                  [ tr = (40 _ 10) ms.

   at the output of the signal receiver in order to be recognized as a valid
signalling condition by the exchange equipment.  Thus the specified value does
not include the response time tr\dsof signalling receivers (see Recommendation
Q.415).  However, it is determined on the assumption that there is
interruption control (see Recommendation Q.416).


2.2.2 States and procedures under normal conditions
~~~~~
      2.2.2.1 Seizure

      The outgoing end removes the tone in the forward direction.  If seizure
      is immediately followed by release, removal of the tone must be
      maintained for at least 100 ms to make sure that it is recognized at the
      incoming end.


      2.2.2.2 Answering

      The incoming end removes the tone in the backward direction.  When
      another link of the connection using tone-on-idle continuous signalling
      precedes the outgoing exchange, the tone-off condition must be
      established on this link immediately after it is recognized in this
      exchange.  When another signalling system is used on the preceding link,
      the rules for interworking are applicable.


      2.2.2.3 Clear-back

      The incoming end restores the tone in the backward direction.  When
      another link of the connection using tone-on-idle continuous signalling
      precedes the outgoing exchange the tone-on condition must be established
      on this link immediately after it is recognized in this exchange.  When
      another signalling system is used on the preceding link, the rules for
      interworking are applicable.  The provisions set forth in S 2.2.2.6
      below must also be taken into consideration.


      2.2.2.4 Clear-forward procedure

      The outgoing end restores the tone in the forward direction (see S
      2.2.2.1 above).  The forward connection is released and the
      release-guard sequence begins as soon as the changed signalling
      condition is recognized at the incoming end.  In the outgoing exchange
      the circuit remains blocked until the release-guard sequence is
      terminated (see S 2.2.2.6 below).


      2.2.2.5 Blocking and Unblocking procedure

      At the outgoing exchange the circuit stays blocked so long as the tone
      remains off in the backward direction.

      Restoration of the tone in the backward direction - accompanied by the
      presence of the tone in the forward direction - restores the circuit to
      the idle state.  The circuit may then be seized for a new call.


      2.2.2.6 Release and release-guard sequence

      Release-guard must be ensured whatever the state of the circuit at the
      moment the clear-forward signal is sent - seized prior to answer,
      answered or cleared by the called party.  It may also happen that
      answering or clearing by the called party occurs when release has
      already begun at the outgoing exchange.  The cases are described below
      and shown in the Figures 2/Q.412 to 4/Q.412.  The exact timing is shown
      in Figure 5/Q.412.

      a) Release prior to answered state

         The clear-forward signal is sent from the outgoing end by restoring
         the tone in the forward direction (see Figure 2/Q.412).  Recognition
         of this tone has the following consequences at the incoming end:

         i) The tone in the backward direction is removed;

        ii) The release of the switching units is initiated;

       iii) The release-guard sequence starts.

         When release operations at the incoming end are complete, but not
         before an interval T2has elapsed after the removal, the tone is again
         restored at the incoming end in the backward direction.

                                                         FIGURE 2/Q.412 p.

         When T1has elapsed the outgoing end must recognize that the tone-off
         condition in the backward direction is established.  After this
         recognition the restoring of the tone in the backward direction
         returns the circuit to the idle state and completes the release-guard
         sequence.

         At the incoming end the sending of an answer signal can only be
         prevented after the clear-forward signal has been recognized.  To
         avoid any false operation should answer coincide with release,
         transition from tone-on to tone-off in the backward direction must
         not be interpreted, at the outgoing end, as part of the release-guard
         sequence during an interval T1.  The interval T1starts with the
         sending of the tone in the forward direction.  It is long enough to
         make quite sure that the clear-forward signal is recognized and the
         tone-off condition established at the incoming end.

         For the calculation of the intervals T1and T2see S 2.2.2.7 below.


      b) Release in answered state

         In this case, the release operations differ only from a) above in so
         far as i) does not apply.  At the incoming end, however, sending of a
         clear-back signal can only be prevented after the clear-forward
         signal has been recognized.  Should there be a clear-back signal the
         specified interval T1enables any resultant difficulties to be
         obviated (see Figure 3/Q.412).

                                                            Figure 3/Q.412 p.

      c) Release in clear-back state

         The release operations are identical to those described in a) above.
         Should there be a second answer signal, the specified interval
         T1 enables any resultant difficulties to be obviated (see Figure
         4/Q.412).


      2.2.2.7 Calculation of the intervals T1and T2specified for release and
              release-guard operations

      Figure 5/Q.412 shows the factors involved in calculating the intervals
      T1 and T2.

      At the outgoing end when the interval T1 has elapsed (Figure 5/Q.412,
      point D) the tone-off condition in the backward direction can be
      expected and recognized with certainty in all cases mentioned in S
      2.2.2.6 above.

      Similarly, the interval T2preceding the restoring of the tone in the
      backward direction may be applied in all cases.  To avoid false
      operation in the event of coinciding forward and backward signals or an
      irregular sequence of signals, the time interval T2must also be observed
      with the release operation [see S 2.2.2.6 b) above].

                                                      Figure 4/Q.412, p.11

                                                      Figure 5/Q.412, p.12

      The minimum value of T1is the sum of the maximum values of the times
      required for the various operations which take place between application
      of the tone in the forward direction and recognition of the absence of
      tone in the backward direction.

      T2 delays release of the circuit.  It must therefore be as short as
      possible.  However, it must be long enough to ensure recognition of the
      tone-off condition in the backward direction at the outgoing end when T1
      is at its maximum, even when this condition begins as early as possible.

      a) Exclusive terrestrial circuits

         The calculation is based on a maximum one-way transmission delay of
         30 ms for a circuit.  Thus, for circuits on high-velocity
         transmission systems via terrestrial lines (including submarine
         cables) the maximum operating range of the specified line signalling
         system is 4800 km

         Values used for the calculation of T1and T2:

               0 < tp < 30 ms

               0 < to < 30 ms (see S 2.3.2.4)

               30 ms < tr < 50 ms

               0 < ti < 20 ms


         Calculation of intervals T1and T2:

               T 1 > AC         T 1 > 2 (t p + t o ) max. + t r max. + t i
       max.
 
       T 1 > (2 x 60 + 50 + 20) ms

       T 1 > 190 ms



         Allowing a safety margin of 10 ms and a tolerance of _ | 0%, the
         specified value of T1is (250 _ 50) ms.

               T 2 > BF         T 2 > T 1 max. + t r max. + t i max.  -  2
       (t p + t o ) min. -  t r min. - t i min.

       T 2 > (300 + 50 + 20 - 0 - 30 - 0) ms

       T 2 > 340 ms

                  [ See Recommendation G.114, Fascicle III.1.



         Allowing a safety margin of 20 ms and a tolerance of _ | 0%, the
         specified value of T2is (450 _ 90) ms.

      b) Circuits including a satellite link

         The calculation is based on the assumption that the complete
         connection includes two terrestrial sections with a maximum one-way
         transmission delay of 15 ms each and a satellite section with a
         one-way transmission delay of (270 _ 20) ms.

         Values used for the calculation of T1and T2:

               250 < tp < 320 ms

         0 < to <  30 ms (see S 2.3.2.4)

        30 < tr <  50 ms

         0 < ti <  20 ms


            Calculation of intervals T1and T2:

               T 1 > AC         T 1 > 2 (t p + t o ) max. + t r max. + t i
       max.

       T 1 > (2 x 350 + 50 + 20) ms

       T 1 > 770 ms



            Allowing a safety margin of 30 ms and a tolerance of _  |  0%,
       the specified value of T1is (1000 _ 200) ms.




               T 2 > BF         T 2 > T 1 max. +  t r max. + t i max. -  2
       (t p + t o ) min. -  t r min. - t i min.

       T 2 > (1200 + 50 + 20 - 2 x 250 - 30 - 0) ms

       T 2 > 740 ms


   Allowing a safety margin of 60 ms and a tolerance of _ | 0%, the specified
value of T2is (1000 _ 200) ms.

   c) Terrestrial circuits and circuits including a satellite link

      The situation in which equipment can serve terrestrial circuits as well
      as circuits including a satellite link is not to be preferred because
      for the case of terrestrial circuits the release sequence is unduly
      prolonged.  The calculation is based on the same assumption as in b)
      above, but with a minimum for (tp + to) = 0.

      This does not affect the value of T1, so also in this situation T1 =
      1000 _ 200 ms.

      Values used for the calculation of T2:

                0 < tp < 320 ms

        0 < to <  30 ms (see S 2.3.2.4)

       30 < tr <  50 ms

        0 < ti <  20 ms



            Calculation of interval T2:

               T 2 > BF         T 2 > T 1 max. + t r max. + t i max.  -  2
       (t p + t o ) min. -  t r  | min. - t i min.

       T 2 > (1200 + 50 + 20 - 0 - 30 - 0) ms

       T 2 > 1240 ms


   Allowing a safety margin of 40 ms and a tolerance of _ | 0%, the specified
value of T2is (1600 _ 320) ms.

2.2.3 Abnormal conditions
~~~~~
   The situations described below are those in which interruption control of
signalling channels (see Recommendation Q.416) does not function and which
occur only during interruptions of individual channels or in the event of a
fault in a line-signalling equipment.  In addition, the situations described
in SS 2.2.3.3 and 2.2.3.4 below may also result from operation of interruption
control at the incoming end of the circuit.  In that case the circuit
automatically returns to normal at the command of interruption control.

   2.2.3.1 If an exchange recognizes tone-off condition in the backward
           direction (premature answer) before the outgoing R2 register has
           received a signal A-6 or a Group B-signal, the con- nection must be
           released.  Congestion information is then sent back-wards or a
           repeat attempt is made to set up the call.

   2.2.3.2 In the case of non-reception of the answer signal, of delay in
           clearing by the calling subscriber in automatic working, or of
           non-reception of the clear-forward signal by the incoming exchange
           after the clear-back signal has been sent, the provisions of
           Recommendation Q.118 apply.

   2.2.3.3 If, in the cases given in S 2.2.2.6 a) or c) above, the tone in the
           backward direction is not removed, the circuit will remain blocked,
           since it cannot return to the idle state of its own accord.  The
           action to be taken in such cases is described in S 6.6.

   2.2.3.4 If after sending of the clear-forward signal the signalling tone in
           the backward direction is not restored, the circuit stays blocked,
           as described in section S 2.2.2.5 above.  The same occurs when, in
           the idle state, the signalling tone in the backward direction is
           interrupted by a fault.

   2.2.3.5 When the signalling tone in the forward direction of an idle
           circuit is interrupted owing to a fault, the incoming end
           recognizes seizure and connects multifrequency signalling
           equipment, but no interregister signalling follows.

           a) When the interruption is greater than the incoming R2 register
              time-out (see Recommendation Q.476) the regis- ter will release
              and the circuit must be brought into blocked state by removal of
              the signalling tone in the backward direction.  As soon as the
              fault is cleared and the tone in the forward direction
              consequently restored, the circuit returns to the idle state in
              accordance with S 2.2.2.6 b) above.

            b) When the interruption is shorter than the time-out, restoration
               of the signalling tone in the forward direction will return the
               circuit to the idle state in accordance with S 2.2.2.6 a)
               above.

2.2.4 Alarms for technical staff
~~~~~
   According to Recommendation Q.117, an alarm should in general be given to
technical staff as soon as an abnormal condition is recognized as probably due
to a fault.

   It is recommended that a delayed action alarm should be operated at the
outgoing end for the conditions described in SS 2.2.2.5, 2.2.3.3 and 2.2.3.4
above, i.e.  when the circuit does not revert to the idle state after sending
of the clear-forward signal or receipt of the blocking signal.

   Arrangements for the operation of the alarm will be made by each
Administration.

   At both the outgoing and the incoming end, when interruption control (see
Recommendation Q.416) functions, alarm condition must first be established for
the transmission equipments.  However, in this case a delayed-action alarm may
also be given to the technical staff of the exchange.




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