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TUCoPS :: Phreaking Technical System Info :: ccittr2.txt

The specification for CCITT R2 MF signaling (aka the rosetta stone of the European Blue Box)





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              The Following was Taken From The Book Called :
                 Signalling in Telecommunications Networks
                        Written by : Samuel Welch
                           ISBN: 0 906048 46 X
                    Text typed by OMEGA / MEGA - Ind.
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                    CEPT interregister signalling (R2)

General
-------
  The combination of this CEPT standardised interregister m.f. signalling
sys-
tem and the associated outband line-signalling system, analogue and digital,
is specified as system R2 (Regional system 2) in the CCITT series. The
analogue R2 interregister signalling is used in the digital application, the
analogue m.f. signals being bit-encoded as for speech. It will be understood
of course that there is no technical reason whatever to limit line
signalling
to outband. CCITT signalling-system specifications specify basic principles
and requirements to be met, and do not embrace design detail. As system R2
is
postulated for a wide field of application, the design realisation of the
sys-
tem may vary greatly, but the basic arrangements apply.
  R2 interregister signalling is 2/6 m.f., continuous compelled, end-to-end,
forward and backward signalling, overlap register operation (including
origi-
nating), with frequencies 120 Hz spaced:

   Forward:     1380, 1500, 1620, 1740, 1860, 1980 Hz
   Backward:    1140, 1020,  900,  780,  660,  540 Hz

  The CCITT is now proposing to specify a semicompelled version for applica-
tion to long-propagation time circuits (e.g. satellite). The specification
detail for this version is not yet complete, but will be based on minimum
change to the basic fully compelled system.
  System R2 is visualised for international, regional and national applica-
tion. The signal repertoire is considerable, embracing all three fields of
application, taking account of repertoire requirements for various national
networks. A main aim of the system is flexibility to cater for the different
conditions of different national networks, particularly in regard to
different
facility requirements and different numbering schemes. This accounts in
large
part for the features of the system.
  In a particular national application, the signal repertoire adopted would
be
the administration's choice from the signals available in the specified sys-
tem, with adequate spare capacity being available for national network
signal(s) requirements not included in the specification, the basic princi-
ples of R2 being the vehicle for transferring the required signals. If
limited
backward signals are required nationally and 2/6 m.f. backward not found
necessary, administrations are free to adopt 2/5 m.f. (or 2/4 m.f.) backward
as appropriate. Equipment would not be provided for the signal channels not
used, but the electrical potential of the system would still be 2/6 m.f.
backward. The backward signal frequencies omitted always start at the
lowest,
i.e. in order 540 Hz, 660 Hz as required. 2/6 m.f. forward signalling would
always apply. In national application, if desired, administrations could
clearly adopt line signalling as dictated by the transmission media,
providing
the R2 line signals are given.

R2 interregister signal code
----------------------------
  Both the forward and backward frequency combinations have primary
meanings,
which, by the use of certain backward signals, may be changed to secondary
meanings. Each signal is acknowledged in the fully compelled sequence, the
backward acknowledgment signals carrying additional information requesting
transmission of the next forward signal or the condition of the called sub-
scriber's line. On seizure of the outgoing circuit, an originating register
automatically sends the first forward signal without requiring a backward
request, and on recognition of this an incoming register returns an acknow-
ledgment which gives instruction as to the next forward signal required.
  The last acknowledgment signal from a transit register also carries an in-
struction as to the first forward signal required by the next register, thus
requesting transmission of this signal from the originating register. Tables
1-5 detail the R2 interregister signal code:

(a) Group I (Table 2) are the primary meanings of the forward signals.
(b) Group II (Table 3) are the secondary meanings, the change from primary
to
    secondary being commanded by backward signals A-3 or A-5. Secondary
    meanings can change back to primary meanings only when the original
change
    from primary to secondary was in response to signal A-5.
(c) Group A (Table 4) are the primary meaningss of the backward signals.
(d) Group B (Table 5) are the secondary meanings, the change from primary to
    secondary being indicated by the backward signal A-3. There is no change
    back to primary once the change to secondary has been made.

Table 1 R2 m.f. signal allocation

    Signal Number        Forward signals of          Backward signals of
                     Groups I and II frequency      Groups A and B frequency
                      combination (compounded)       combination
(compounded)
    ------------------------------------------------------------------------
--
          1              1380 + 1500  Hz                1140 + 1020  Hz
          2              1380 + 1620  Hz                1140 + 900   Hz
          3              1500 + 1620  Hz                1020 + 900   Hz
          4              1380 + 1740  Hz                1140 + 780   Hz
          5              1500 + 1740  Hz                1020 + 780   Hz
          6              1620 + 1740  Hz                 900 + 780   Hz
          7              1380 + 1860  Hz                1140 + 660   Hz
          8              1500 + 1860  Hz                1020 + 660   Hz
          9              1620 + 1860  Hz                 900 + 660   Hz
          10             1740 + 1860  Hz                 780 + 660   Hz
          11             1380 + 1980  Hz                1140 + 540   Hz
          12             1500 + 1980  Hz                1020 + 540   Hz
          13             1620 + 1980  Hz                 900 + 540   Hz
          14             1740 + 1980  Hz                 780 + 540   Hz
          15             1860 + 1980  Hz                 660 + 540   Hz

  In international application, the sending of the R2 signals is in a
defined
sequence, the first forward signal giving routing information as follows:

(i)  The country code indicator, followed by the country code itself, for
     transit. In addition to transit indication, the indicator gives
informa-
     tion as to whether an echo-suppressor is required or not (col. a table
2)
(ii) The language (semiautomatic working) or the discriminating (automatic
     working) digit, either digit indicating terminal. Neither the country
     code indicator nor the country code are sent to terminal registers.

  In national application, administrations adopt a signal repertoire
required
for individual national networks. In principle, all frequency combinatios,
primary and secondary, have unique signal meanings whether used internation-
ally or nationally. Fig. 6 shows a typical signalling sequence in a national
application of R2 interregister signalling, a sequence that may be different
for different networks.

Table 2 R2 forward signals Group I

   Signal       When first signal on an      When other than the first
 Designation   international circuit (a)    signal on an international
                                                     circuit (b)
 ---------------------------------------------------------------------------
-
    I-1        Language digit : French        Digit 1 (See note 1)
    I-2                         English             2     "
    I-3                         German              3     "
    I-4                         Russian             4     "
    I-5                         Spanish             5     "
    I-6        Spare (language digit)               6     "
    I-7        Spare (language digit)               7     "
    I-8        Spare (language digit)               8     "
    I-9        Spare (discriminating digit)         9     "
    I-10       Discriminating digit                 0     "
    I-11       Country code indicator,        Operator Code 11 (see note 2)
               outgoing half-echo
               suppressor required
    I-12       Country code indicator,        Operator Code 12 or request
not
               no echo suppressor             accepted. (see note 2, 3, 4)
               required.
    I-13       Test call indicator (call      Code 13 (call to automatic
test
               by automatic test equipment)   equipment)
    I-14       Country code indicator,        Incoming half echo suppressor
               incoming half echo             required (see note 2, 4, 5)
               suppressor required
    I-15       This signal is not used        End of pulsing (code 15)
                                              (see note 6)

 Table 2 Notes

(1) On terminal calls, col. (a) signals I-1/I-10 are the first signals
trans-
    mitted, the country code indicator and the country code (both sent to
    transit exchanges) not being sent to the terminal international
exchange.
(2) Col. (a) signals: It may be decided by bilateral agreement that signal
    I-11, when sent as the first signal, shall serve as a country code indi-
    cator instead of signal I-14 to indicate that the first international
    transit exchange must insert an outgoing half-echo suppressor. If the
    connection passes through two or more international transit exchanges,
    signal I-11 is not sent beyond the first transit exchange. Signal I-12
is
    used solely when no echo suppressor has to be inserted on the interna-
    tional connection. Signal I-14 sent as the first signal, serves as the
    country code indicator and shows that the connection requires echo sup-
    pressors and that the outgoing half-echo suppressor has already been in-
    serted. In response to a signal A-14, the only meaning of signal I-14 is
    that an incoming half-echo suppressor is necessary.
(3) Col. (b) signals: An outgoing international R2 register which receives
    signal A-9 or A-10, the use of which is national, or which receives by
    signal A-13 a request for identification to which it is unable to reply,
    indicates that it cannot answer the request by transmitting I-12.
(4) Col. (a) signals: The sending of signals I-12 or I-14 may be repeated as
    often as necessary, on request by signal A-11.
(5) Col. (b) signals: I-14 is sent in response to signal A-14.
(6) Col. (b) signals: I-15 is also used to indicate (in response to A-13)
    that transmission of the code indentifying the location of the outgoing
    international R2 register is terminated.

Table 3 R2 forward signals Group II (calling party's category)

    Signal
  designation   Signal
  ------------------------------------------------------------------------
    II-1        Signals assigned for national use (see note 1)
    II-2           "        "     "      "     "    "   "   "
    II-3           "        "     "      "     "    "   "   "
    II-4           "        "     "      "     "    "   "   "
    II-5           "        "     "      "     "    "   "   "
    II-6           "        "     "      "     "    "   "   "
    II-7        Subscriber (or operator without forward transfer facility)
    II-8        Data transmission call
    II-9        Subscriber with priority
    II-10       Operator with forward transfer facility
    II-11       Spare signals for national use (see note 1)
    II-12         "      "     "      "     "    "   "   "
    II-13         "      "     "      "     "    "   "   "
    II-14         "      "     "      "     "    "   "   "
    II-15         "      "     "      "     "    "   "   "

 Table 3 notes

(1) The outgoing international R2 register which receives one of these
signals
    converts it into a signal II-7, II-8, II-9 or II-10.

Table 4 R2 backward signals group A

    Signal
  designation   Signal
  --------------------------------------------------------------------------
--
    A-1         Send next digit (n+1) (see note 1, 2)
    A-2         Send last but one digit (n-1) (see note 1, 2)
    A-3         Address complete, changeover to reception of B signals
                (see note 3, 4, 9)
    A-4         Congestion in national network (see note 4)
    A-5         Send calling party's category (see note 5)
    A-6         Address complete, charge, set up speech conditions
                (see note 4, 9)
    A-7         Send last but two digits (n-2) (see note 1, 2)
    A-8         Send last but three digits (n-3) (see note 1, 2)
    A-9         Spare for national use (see note 6)
    A-10          "    "     "      "
    A-11        Send country code indicator
    A-12        Send language or discriminating digit (see note 2, 7)
    A-13        Send location of outgoing R2 internation register (see note
2)
    A-14        Request for information on use of echo suppressor (is an in-
                coming half echo suppressor required?) (see note 8)
    A-15        International exchange congestion (see note 4)

 Table 4 notes

(1) Last received digit = n.
(2) Reply expected in the form of a Group I forward signal.
(3) Reply expected in the form of a Group II forward signal.
(4) This signal may be sent:
    (i) either as an acknowlegment of any forward signal, or
    (ii) automatically in pulse form when there is no forward signal.
(5) This signal, used to acknowledge a Group I forward signal, requests
trans-
    mission of a Group II signal. It may be followed by any other A signal,
    but the latter will be linked to the sequence of Group I forward signals
    already received and will automatically cause the forward signals to
    revert to their Group I primary meanings.
(6) Reply by signal I-12 (request not accepted) (see note 3 table 2)
(7) The outgoing international R2 register is informed by the first signal
    A-12 that an international circuit connecting to a terminal
international
    exchange has been made.
(8) This signal is used at an international exchange (incoming) where it is
    possible to insert an incoming half-echo suppressor. It is sent to ack-
    nowledge the discriminating digit or the language digit and the reply
    is received:
    (a) signal I-14 when an incoming half-echo suppressor is required, or
    (b) next digit of the address information when no incoming half-echo
        suppressor is required.
(9) When the terminal exchange is unable to send detailed information on the
    condition of the called subscriber's line, signal A-3 followed by a
Group
    B signal do not apply, and signal A-6 is used.

Table 5 R2 backward signals group B (condition of called subscriber's line)

    Signal
  designation   Signal
  --------------------------------------------------------------------------
--
    B-1         Spare for national use (see note 2)
    B-2         Subscriber transferred (see note 3)
    B-3         Subscriber line busy (see note 4)
    B-4         Congestion (encountered after change from Group A to Group B
                signals) (see note 5)
    B-5         Vacant national number (see note 6)
    B-6         Subscriber line free - charge (see note 4, 7)
    B-7         Subscriber line free - no charge (see note 7)
    B-8         Subscriber line out of order (see note 3)
    B-9         Spare for national use (see note 8)
    B-10          "    "     "      "
    B-11        Spare for international use
    B-12          "    "       "         "
    B-13          "    "       "         "
    B-14          "    "       "         "
    B-15          "    "       "         "

 Table 5 notes

(1) Any Group B backward signal acknowledges a Group II forward signal and
is
    always preceded by an A-3 signal which indicates that the incoming
regis-
    ter has received all the Group I forward signals it requires from an in-
    ternational R2 register.
(2) Signal B-1 is always interpreted by the outgoing international R2
register
    as signal B-6.
(3) After recognising B-2 or B-8, the outgoing international R2 register
    clears forward and causes the transmission of a recorded announcement or
    an appropriate tone. If the destination national network cannot
recognise
    transferred subscriber, or subscriber's line out of order, A-3 may be
    followed by B-5 instead of by B-2 or B-8 to ensure that an appropriate
    tone is sent to the caller.
(4) If the destination national network can only distinguish called
line-free
    or busy, A-3 is followed by B-3 when line-busy and by B-6 when free (or
    A-6 only shall be sent without being followed by a Group B signal so
that
    the caller may receive tone or recorded annoucement sent by the incoming
    equipment).
(5) When the congestion condition is encountered following the change-over
    from Group A to Group B signals, B-4 is sent on the conditions for A-4.
(6) After recognising B-5, the outgoing international R2 register clears the
    forward connection and causes transmission of an appropriate tone to the
    caller.
(7) After recognising B-6 or B-7, the outgoing international R2 register
sets
    up speech conditions to enable the caller to receive ring tone.
(8) Signals B-9 and B-10 are always interpreted by the outgoing
international
    R2 register as signal B-5.

Basic philosophy of the R2 interregister signal code
----------------------------------------------------

(a) The R2 interregister signal code is framed to take account of various
con-
    ditions arising with R2 in the international network and R2 variants in
    national networks.
(b) All the information necessary for setting up the connection is
transmitted
    by Group I signals. An outgoing R2 register sends the first signal
spont-
    aneously immediatly after outgoing circuit seizure, this being possible
as
    the signal is continuous. All other signals are sent in reply to appro-
    priate Group A backward signals.
(c) All incoming R2 registers (transit and terminal) receive the first
signal
    without request from the register receiving this signal, which indicates
    transit or terminal. Transit is indicated by the country code indicator
    I-11, I-12 or I-14 and terminal by the language digit I-1 to I-5 or the
    discriminating digit I-10. This obviates the need for two different
line-
    seizure signals, transit and terminal. The R2 register is required to
de-
    termine transit or terminal in the forward routing, and the last acknow-
    ledgment signal from a transit register request the transmission of the
    first signal to be received by the next register.
(d) The country code transit indicator serves to condition the receiving re-
    gister to analyse for transit routing, normally on the country code, and
    also conveys relevant echo-suppressor information:

    I-12, which may be requested as many times as required by A-11, is used
         when no echo suppressor has to be inserted.
    I-14 is used when a half-echo suppressor is required as an outgoing
half-
         echo suppressor has already been inserted. When sent in reply to
         A-14, I-14 has its sole meaning 'incoming half-echo suppressor re-
         quired'.
    I-11 indicates to the first transit exchange that an outgoing half-echo
         suppressor should be inserted and that no half-echo suppressor has
         been inserted at the outgoing exchange. I-11 is not sent beyond the
         first transit exchange and the outgoing exchange, having sent I-11
         once, sends I-14 if it is again asked by A-11 for the transit indi-
         cation.
(e) Signal I-15, end-of-pulsing, is used to indicate the end of the indenti-
    fication procedure (see (l)), and in the international semi-automatic
    service that there are no more digit signals to follow.
(f) Information concerning the nature of the call can be transmitted on the
    network by the Group II forward signals, and sent in reply to A-3 or
A-5.
    The signals cater for both international and national calls.
    II-1 to II-6 are assigned for national use as follow:

        II-1 Subscriber-initiated call (national)
        II-2 Priority call (national)
        II-3 maintenance-equipment call (national)
        II-4 Spare (national)
        II-5 Operator-initiated call (national)
        II-6 Data-transmission call (national)

    When an international outgoing R2 register receives a Group II signal
with
    national meaning from the national network, the signal is converted into
    an internationally accepted signal as follows : II-1 to II-4 and the
    national spares II-11 to II-15 into II-7, II-5 into II-7 or II-10, and
    II-6 into II-8.
(g) The potential for error-detection and correction by retransmission
exists.
    If the time allowed for a signal to settle down to 2-and-2 only is rela-
    tively long, it would not normally be necessary to request a repeat of a
    digit n signal as the signal would remain online until acknowledged. A
    short waiting time would strengthen the need for error-correction and if
    adopted is achieved by returning A-2 requesting digit n-1 and
acknowledg-
    ing this by A-1 requesting digit n+1, to receive digit n. This procedure
    is not applicable when the digit n is the first in the store of the out-
    going register.
(h) The concept of requesting send last but so many digits by various
backward
    A signals facilitates the successive transmission of the country code
in-
    dicator and the country code to successive transit registers in a
flexible
    manner. The same flexibility applies in national application, the
transit
    registers receiving the trunk area code.
(j) The condition of the incoming-exchange switching equipment or of the
    called line can be transmitted back on the network by Group B signals,
the
    outgoing register then taking appropriate action. These signals are sent
    to acknowledge receipt of Group II forward signals after changeover to B
    signals has been requested by A-3, which signal must always precede the
B
    signals.
(k) In some circumstances the signal A-3 changeover to B signals may be sent
    before the connection is made with the called line. Congestion in selec-
    tion stages can therefore still occur after changeover from A to B
    signals. A similar situation may arise when interworking system R2 with
    other signalling systems. B signals may then be sent from a centre of a
    higher category in the network hierarchy than the exchange where the
    called line is connected, so that congestion in a group of circuits can
be
    encountered after changeover to B signals. Since A-4 is no longer
applica-
    ble in these cases, B-4 is sent.
(l) Certain identification procedures can be transmitted over the network.
An
    international transit register, or a register in the destination
country,
    can request, by A-13, the location of an international R2 register as
soon
    as at least one forward signal has been received from that register. The
    international outgoing register replies with the first digit of its own
    country code. A further digit of the country code is sent in reply to
each
    subsequent request by A-13. Further requests by A-13 may elicit
successive
    digits of the trunk code of the exchange where the international R2
    register is situated. When all the digits required to indicate the loca-
    tion have been sent, the next A-13 signal is acknowledged by the end of
    pulsing signal I-15.

  The calling subscriber's number may be transmitted over the connection,
for
example by repeating A-5, or by the use of A-9 or A-10. This procedure is
limited to national networks, and international registers prevent its opera-
tion on international circuits by replying with I-12 (request not accepted).


Figure 1 Typical National Network R2 interregister signalling sequence.
         (assuming two-digit trunk-area code)

 originating         transit 1             transit 2                terminal
  |                      |                     |                        |
  |first digit trunk     |                     |                        |
  |--------------------->|                     |                        |
  |area code (signal     |                     |                        |
  |group I)              |                     |                        |
  |                      |                     |                        |
  |acknow. and request   |                     |                        |
  |<---------------------|                     |                        |
  |for next digit        |                     |                        |
  |(signal A-1)          |                     |                        |
  |                      |                     |                        |
  |second digit trunk    |                     |                        |
  |--------------------->|                     |                        |
  |area code (signal     |                     |                        |
  |group I)              |                     |                        |
  |                      |                     |                        |
  |acknow. and request   |                     |                        |
  |<---------------------|                     |                        |
  |for first digit trunk |                     |                        |
  |area code (signal A-2)|                     |                        |
  |                      |                     |                        |
  |first digit trunk area code (signal group I)|                        |
  |------------------------------------------->|                        |
  |acknow. and request for next digit (signal  |                        |
  |<-------------------------------------------|                        |
  |A-1)                                        |                        |
  |                                            |                        |
  |second digit trunk area code (signal        |                        |
  |------------------------------------------->|                        |
  |group I)                                    |                        |
  |                                            |                        |
  |acknow. and request for next digit-the      |                        |
  |<-------------------------------------------|                        |
  |first digit of local number (signal A-1)    |                        |
  |                                            |                        |
  |first digit local number (signal group I)                            |
  |-------------------------------------------------------------------->|
  |acknow. and request for next digit (signal A-1)                      |
  |<--------------------------------------------------------------------|
  |                               |\Subsequent digits                   |
  |                               |/of local number                     |
  |last digit local number (signal group I)                             |
  |-------------------------------------------------------------------->|
  |acknow. change over to B signals (signal A-3)                        |
  |<--------------------------------------------------------------------|
  |calling subs category (signal group II)                              |
  |-------------------------------------------------------------------->|
  |acknow. called subs line free (signal B-6 register dismissal)        |
  |<--------------------------------------------------------------------|
  |if called sub busy:                                                  |
  |                   last digit local number (signal group I)          |
  |-------------------------------------------------------------------->|
  |acknow. change over to B signals (signal A-3)                        |
  |<--------------------------------------------------------------------|
  |calling subs category (signal group II)                              |
  |-------------------------------------------------------------------->|
  |acknow. called subs line busy (signal B-3 register dismissal)        |
  |<--------------------------------------------------------------------|


Operational Features
--------------------
  Various features of system R2 are made flexible so as to cater for
different
conditions arising in different applications, typically:

* Determination of number complete : Any one of the following criteria may
be
  used to determine whether or not the address information received by an
in-
  coming register is complete, depending on the following conditions:

    (i) Analysis of the number received. This is applicable when the connec-
        tion set-up extends to the exchange at which the called line
        terminates. If the incoming register is equipped to determine the
con-
        dition of the called line, signal A-3 is returned on receipt of the
        last digit. A Group B signal is then sent indicating the condition
of
        the called line. If the incoming register is not equipped to
determine
        the condition of the called line, signal A-6 is returned and no
Group
        B signal follows.
   (ii) Criteria given by the switching equipment subsequent to the m.f.
reg-
        ister. To avoid delay in sending the answer signal, no B signal is
        sent when the called line is free. A-6 is sent to set up speech con-
        ditions.
  (iii) When given, receipt of I-15, the end of pulsing signal. I-15 is
always
        sent in certain procedures such as identification (See note(l)
above).
        It is not normally sent in the transfer of the called party's
address
        information in the automatic service. I-15 is acknowledged by A-1,
        A-3, A-4, A-6 or A-15.
   (iv) The assumption, after a time delay, typically 5 +/- 1 s, that no
        further addres digits will be received. A-6 is returned as a pulse
        signal when this condition applies.

* Termination of m.f. signalling when a connection cannot be completed :
  A register ceases m.f. signalling immediately any condition preventing
call
  set-up is recognised, signal A-4, A-15 or an appropriate B signal being
  returned.

* Transmission of pulse signals : Particular conditions can arise when the
  fully-compelled sequence cannot be used, it then being necessary to have
  the option in the system of sending a backward pulse signal without prior
  receipt of a forward signal, e.g.

  (a) When an incoming register, after acknowledging a forward signal, is
un-
      able to complete the call (e.g. congestion) and the next forward
signal
      does not appear.
  (b) When the address complete signal A-6 is sent, the last forward signal
      having already been acknowledged.

  The problem concerns signals A-3, A-4, A-6 and A-15, it being required
that
  these be pulse (150+/-40 ms) in those circumstances not allowing the
fully-
  compelled sequence. No forward signal is sent by the outgoing register on
  receipt of A-4, A-6 or A-15, the pulse signals releasing the register. On
  receipt of A-3 pulse, the outgoing register sends a Group II signal
forward,
  the incoming register acknowledging this in the normal compelled manner by
  sending a B signal.

* Setting up of speech conditions : The last backward signal releases the
out-
  going and incoming registers to set up speech conditions by switching the
  speech path through. This register-dismissal signal will vary depending
upon
  the conditions of a particular network and is normally A-6 or B signal.

* Release of transit registers : For international R2 transit exchanges, it
is
  specified that the last forward signal received by a transit register be
  acknowledged by a backward signal inviting a specific signal which is the
  first forward signal to the succeeding register. The following backward
  signals are used:

   (i) A-11 if the next exchange is international transit, which causes I-12
       or I-14 (country code indicators) to be sent by the outgoing
register,
       this being the first signal to the next transit register.
  (ii) A-12 if the next exchange is internationall terminal, which causes
the
       language digit (semiautomatic) or the discriminating digit
(automatic)
       to be sent by the outgoing register, this being the first signal to
the
       international terminal register.

  The basic R2 concept permits the termination of m.f. signalling and the
  setting up of speech conditions at transit exchanges in any desired way to
  suit the requirements of particular networks, but the adoption of the
inter-
  national arrangements for national networks would give uniformity and ease
  of interworking with international R2.

* Abnormal release of registers : National application-register time-outs
  would be a matter for individual administrations, but the recommended
inter-
  national time-outs are specified as follows :
    An outgoing international R2 register times-out:

    (a) 15+/-3 s during the sending of forward m.f. signals, which time-out
        delay is a function of the time required in the extreme for the
        switching procedures in a transit exchange.
    (b) Not less than 24 s during intervals when no forward signalling is
        sent, which time delay is a function of the maximum interval between
        the dialing of two successive digits and the time-out delay of in-
        coming registers.

  On time-out, appropriate tone is returned to the caller and the register
  released. An incoming international R2 register, transit or terminal,
times
  out in 8-24 s, this time delay being a function of:

    (i) The maximum intervals between the dialing of two successive digits.
   (ii) The maximum time required to set up a connection.
  (iii) The incoming register being required to be released before the
expiry
        of the outgoing register time-out.
   (iv) The interval between register seizure and the receipt of the first
        forward signal.
    (v) The interval between two successive signals in the forward
direction.

  On time-out:

  (a) The congestion signal is sent in pulse form, which prompts release of
      the international connection.
  (b) The incoming register and other equipment in the incoming exchange is
      released.
  (c) The incoming circuit is blocked until the clear-forward (release)
signal
      is received.

* Interworking between international system R2 and national systems derived
  from it : National systems derived from system R2 may, or may not, have
the
  full 2/6 m.f. capacity in either direction, but more usually would in the
  forward direction. International R2, having 2/6 m.f. in each direction,
  readily interworks with national R2 systems having the same capacity. The
  system can also be adapted for interworking with national R2 with less
than
  2/6 m.f. in the backward (and in the forward if this ever arose)
direction.
  On a routing outgoing national network - international network - incoming
  national network, it is logical for signal transmission and other reasons
  that the overall multilink route would be divided into sections, each
being
  end-to-end signalling in its own right. In this event:

   (i) The outgoing register in a given end-to-end section must be able to
re-
       cognise at least all the backward signals in that section. Every in-
       coming register in that section must be able to recognised at least
all
       the forward signals used on that section and directed to that
register.
  (ii) When the number of signals provided is not the same on all parts of
the
       route, the division is logically made at a connection point between
       links having different numbers of signals, and thus at the interface
       international gateway exchanges in the originating and destination
       countries.

   The R2 system has considerable end-to-end signalling capability, allowing
   the theoretical possibility of extending the R2 end-to-end signalling
into
   the destination national network to an extent depending upon the national
   network transmission characteristic. Assuming international circuits of
   nominal loss 0-5 dB and deviation 1 dB, and a fourlink international
   routing, the nominal transmission loss between the incoming international
   gateway exchange and any R2 register in the destination county should not
   exceed 11-4 dB for a country using three 4-wire switched links at most,
and
   11 dB for four. For the general case, however, it is preferred that the
   end-to-end signalling be divided at the incoming, as well as at the out-
   going, gateway exchange.


R2 Interregister signalling on satellite circuits
-------------------------------------------------
  With the fully-compelled mode, the speed of address-information transfer
is
slow on long-propagation circuits such as satellite, which increases the
post-
dialing delay and the CCITT is at present considering the adoption of a
semi-
compelled mode as an option to be applied on such circuits. In the approach,
the forward signals are continuous and the backward acknowledgment pulse
(75 ms or 150 ms, yet to be decided) instead of continous, approximately two
propagation and two signal recognition times being involved per signal se-
quence. This approach does not involve major changes to the basic fully-
compelled system and is at present being considered for this reason. Any
weakness the backward pulse signals may have compared with continous is
judged to be acceptable in the interest of improving the speed of
information
transfer. The detail is not yet finalised, but it is clearly desirable that
register-holding on the satellite links should be such that networks, in-
cluding cases where the R2 fully-compelled sequence is used, after a
satellite
link be buffered against the satellite link connection set-up delays.
  Long propagation times will also have reaction on the R2 line signalling
system. The T1 and T2 times of the clear forward/release guard sequence, and
the double seizure detection time on bothway working, which times are propa-
gation-time-dependent, would need to take account of the longer propagation
time of satellite circuits. The increase in these times is yet to be
decided.


Relevant data
-------------
* Transmit:

  - Absolute power level of each signal frequency -11.5 dBm +/- 1dB.
  - Signal frequency tolerance +/- 4 Hz.
  - Time interval between start of sending of each of the two frequencies
not
    to exceed 1 ms.
  - Interval between cessation of each frequency not to exceed 1 ms.

* Receive:

  - Receiver response range -5 dBm to -35 dBm.
  - Difference in level between the two frequencies of a signal not greater
    than 5 dB for adjacent frequencies and 7 dB for nonadjacent. In applica-
    tion this allows a 4 dB attenuation distortion of the end-to-end chain
for
    two adjacent frequencies and a 6 dB distortion for two nonadjacent,
provi-
    ded the level of the weaker signal frequency is not lower than -35 dBm
at
    the receiver input.
  - Receiver not to recognise a signal of two frequencies of level -5 dBm
and
    a duration less than 7 ms.
  - Receiver not to recognise a signal of two frequencies having a
difference
    in level of 20 dB or more.
  - Received signal frequency variation +/- 10 Hz.
  - The sum of the operate and release times to a two frequency signal not
to
    exceed 80 ms (the operate and release times are not specified
seperately)
  - Receiver not to release to interruptions to signal 7 ms or less.
  - System malfunction to signal interruptions greater than 7 ms (typically
    20 ms at administration's choice) is prevented by further logic
elements.


General comment on R2 interregister signalling
----------------------------------------------
  The R2 interregister signalling system is postulated as a flexible
general-
purpose system for a wide field of application, catering for the different
conditions likely to arise in different networks. Individual administrations
would adopt features to meet the requirements of their own networks, but
with-
in the basic principles of the R2 system. This could well mean that applica-
tion of R2 in a particular network may not give optimum interregister
signal-
ling for the conditions of that network, e.g. the numbering scheme may be
such
that group acknowledgment as distinct from per-signal acknowledgment, would
meet the requirements. This, however, must be balanced against the potential
of system R2 for rationalised interregister signalling, which is clearly
very
desirable, and in this regard system R2 has significant merit.


-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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                   This was taken From The Book Called:
                 Signalling in Telecommunications Networks
                        Written by : Samuel Welch
                           ISBN: 0 906048 46 X
                    Text typed by OMEGA / MEGA - Ind.
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