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Versions: 00 01 02 03 04 05 RFC 5244

Audio/Video Transport (avt)                               H. Schulzrinne
Internet-Draft                                               Columbia U.
Expires: June 2, 2006                                          T. Taylor
                                                                  Nortel
                                                       November 29, 2005


     Definition of Events For Channel-Oriented Telephony Signalling
                    draft-ietf-avt-rfc2833biscas-01

Status of this Memo

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   This Internet-Draft will expire on June 2, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This memo updates RFC xxxx (currently draft-ietf-avt-rfc2833bis-12)
   to add event codes for telephony signals used for channel-associated
   signalling when carried in the telephony event RTP payload.







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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Event Definitions  . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Signalling System No. 5  . . . . . . . . . . . . . . . . .  6
       2.1.1.  Signalling System No. 5 Line Signals . . . . . . . . .  6
       2.1.2.  Signalling System No. 5 Register Signals . . . . . . .  7
     2.2.  Signalling System R1 and North American MF . . . . . . . .  8
       2.2.1.  Signalling System R1 Line Signals  . . . . . . . . . .  8
       2.2.2.  Signalling System R1 Register Signals  . . . . . . . .  8
     2.3.  Signalling System R2 . . . . . . . . . . . . . . . . . . . 10
       2.3.1.  Signalling System R2 Line Signals  . . . . . . . . . . 10
       2.3.2.  Signalling System R2 Register Signals  . . . . . . . . 10
     2.4.  ABCD Transitional signalling For Digital Trunks  . . . . . 12
     2.5.  Continuity Tones . . . . . . . . . . . . . . . . . . . . . 13
     2.6.  Trunk Unavailable Event  . . . . . . . . . . . . . . . . . 13
     2.7.  Metering Pulse Event . . . . . . . . . . . . . . . . . . . 14
   3.  Congestion Considerations  . . . . . . . . . . . . . . . . . . 15
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 21
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 21
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
   Intellectual Property and Copyright Statements . . . . . . . . . . 24























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1.  Introduction

1.1.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [1] and
   indicate requirement levels for compliant implementations.

   In addition to those defined for specific events, this document uses
   the following abbreviations:

   MF Multi-frequency

   PSTN  Public Switched (circuit) Telephone Network

   RTP Real-time Transport Protocol [2]

1.2.  Overview

   This document extends the set of telephony events defined within the
   framework of RFC xxxx [3] to include signalling events that can
   appear on a circuit in the telephone network.  Most of these events
   correspond to signals within one of several channel-associated
   signalling systems still in use in the PSTN.

   Trunks (or circuits) in the PSTN are the media paths between
   telephone switches.  A succession of protocols have been developed
   using tones and electrical conditions on individual trunks to set up
   telephone calls using them.  The events defined in this document
   support an application where such PSTN signalling is carried between
   two gateways without being interworked to signalling in the IP
   network: the "RTP trunk" application.

   In the "RTP trunk" application, RTP is used to replace a normal
   circuit-switched trunk between two nodes.  This is particularly of
   interest in a telephone network that is still mostly circuit-
   switched.  In this case, each end of the RTP trunk encodes audio
   channels into the appropriate encoding, such as G.723.1 [10] or G.729
   [11].  However, this encoding process destroys in-band signalling
   information which is carried using the least-significant bit ("robbed
   bit signalling") and may also interfere with in-band signalling
   tones, such as the MF (multi-frequency) digit tones.

   In a typical application, the gateways may exchange roles from one
   call to the next: they must be capable of either sending or receiving
   each signal in the table.




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   This document defines events related to four different signalling
   systems.  Three of these are based on the exchange of multi-frequency
   tones.  The fourth operates on digital trunks only, and makes use of
   low-order bits stolen from the encoded media.  In addition, this
   document defines tone events for supporting tasks such as continuity
   testing of the media path.

      Implementors are warned that the descriptions of signalling
      systems given below are incomplete.  They are provided to give
      context to the related event definitions, but omit many details
      important to implementation.








































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2.  Event Definitions

   Table 1 lists all of the events defined in this document.  These
   events incorporate a considerable number of changes compared with the
   corresponding set of events in RFC 2833 [9], but it would appear that
   the changes have not affected any implementations.

   +---------------------+------------+-------------+--------+---------+
   | Event               |  Frequency |  Event Code | Event  | Volume? |
   |                     |    (Hz)    |             | Type   |         |
   +---------------------+------------+-------------+--------+---------+
   | MF 0...9            |  (Table 2) |  128...137  | tone   | yes     |
   |                     |            |             |        |         |
   | MF Code 11 (SS No.  |  700+1700  |     138     | tone   | yes     |
   | 5) or KP3P/ST3P     |            |             |        |         |
   | (R1)                |            |             |        |         |
   |                     |            |             |        |         |
   | MF KP (SS No. 5) or |  1100+1700 |     139     | tone   | yes     |
   | KP1 (R1)            |            |             |        |         |
   |                     |            |             |        |         |
   | MF KP2 (SS No. 5)   |  1300+1700 |     140     | tone   | yes     |
   | or KP2P/ST2P (R1)   |            |             |        |         |
   |                     |            |             |        |         |
   | MF ST (SS No. 5 and |  1500+1700 |     141     | tone   | yes     |
   | R1)                 |            |             |        |         |
   |                     |            |             |        |         |
   | MF Code 12 (SS No.  |  900+1700  |     142     | tone   | yes     |
   | 5) or KP'/STP (R1)  |            |             |        |         |
   |                     |            |             |        |         |
   | ABCD signalling     |     N/A    |  144...159  | state  | no      |
   |                     |            |             |        |         |
   | Continuity          |    2000    |     167     | tone   | yes     |
   | check-tone          |            |             |        |         |
   |                     |            |             |        |         |
   | Continuity          |    1780    |     168     | tone   | yes     |
   | verify-tone         |            |             |        |         |
   |                     |            |             |        |         |
   | Metering pulse      |     N/A    |     174     | other  | no      |
   |                     |            |             |        |         |
   | Trunk unavailable   |     N/A    |     175     | other  | no      |
   |                     |            |             |        |         |
   | MFC Forward 1...15  |  (Table 4) |  176...190  | tone   | yes     |
   |                     |            |             |        |         |
   | MFC Backward 1...15 |  (Table 5) |  191...205  | tone   | yes     |
   +---------------------+------------+-------------+--------+---------+

                     Table 1: Trunk signalling events




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2.1.  Signalling System No. 5

   Signalling System No. 5 (SS No. 5) is defined in ITU-T
   Recommendations Q.140 through Q.180 [4].  It has two systems of
   signals: "line" signalling, to acquire and release the trunk, and
   "register" signalling, to pass digits forward from one switch to the
   next.

2.1.1.  Signalling System No. 5 Line Signals

   No. 5 line signalling uses tones at two frequencies: 2400 and 2600
   Hz.  The tones are used singly for most signals, but together for the
   Clear-forward and Release-guard.  (This reduces the chance of an
   accidental call release due to carried media content duplicating one
   of the frequencies.)  The specific signal indicated by a tone depends
   on the stage of call set-up at which it is applied.

   No events are defined in support of No. 5 line signalling.  However,
   implementations MAY use the ABCD events described in Section 2.4 and
   shown in Table 1 to propagate SS No. 5 line signals.  If they do so,
   they MUST use the following mappings.  These mappings are based on an
   underlying mapping equating A=0 to presence of 2400 Hz signal and B=0
   to presence of 2600 Hz signal in the indicated direction.

   o  both 2400 and 2600 Hz present: event code 144;

   o  2400 Hz present: event code 149;

   o  2600 Hz present: event code 154;

   o  neither signal present: event code 159.

   The initial event report for each signal SHOULD be generated as soon
   as the signal is recognized, and in any case no later than the time
   of recognition as indicated in ITU-T Recommendation Q.141, Table 1
   (i.e. 40 ms for "seizing" and "proceed-to-send", 125 ms for all other
   signals).  The packetization interval following the initial report
   SHOULD be chosen with considerations of reliable transmission given
   first priority.  Note that the receiver must supply its own volume
   values for converting these events back to tones.  Moreover, the
   receiver MAY extend the playout of "seizing" until it has received
   the first report of a KP event (see below), so that it has better
   control of the interval between ending of the seizing signal and
   start of KP playout.

      The KP has to be sent beginning 80 +/- 20 ms after the SS No. 5
      "seizing" signal has stopped.




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2.1.2.  Signalling System No. 5 Register Signals

   No. 5 register signalling uses pairs of tones to convey digits and
   signals framing them.  The tone combinations and corresponding
   signals are shown in the Table 2.  All signals except KP1 and KP2 are
   sent for a duration of 55 ms.  KP1 and KP2 are sent for a duration of
   100 ms.  Inter-signal pauses are always 55 ms.

                                 Upper Frequency (Hz)

   +-----------------+---------+---------+---------+---------+---------+
   | Lower Frequency |     900 |    1100 |    1300 |    1500 |    1700 |
   |            (Hz) |         |         |         |         |         |
   +-----------------+---------+---------+---------+---------+---------+
   |             700 | Digit 1 | Digit 2 | Digit 4 | Digit 7 | Code 11 |
   |                 |         |         |         |         |         |
   |             900 |         | Digit 3 | Digit 5 | Digit 8 | Code 12 |
   |                 |         |         |         |         |         |
   |            1100 |         |         | Digit 6 | Digit 9 |     KP1 |
   |                 |         |         |         |         |         |
   |            1300 |         |         |         | Digit 0 |     KP2 |
   |                 |         |         |         |         |         |
   |            1500 |         |         |         |         |      ST |
   +-----------------+---------+---------+---------+---------+---------+

                    Table 2: SS No. 5 Register Signals

   The KP signals are used to indicate start of digit signalling.  KP1
   indicates a call expected to terminate in a national network served
   by the switch to which the signalling is being sent.  KP2 indicates a
   call that is expected to transit through the switch to which the
   signalling is being sent, to another international exchange.  The end
   of digit signalling is indicated by the ST signal.  Code 11 or Code
   12 following a country code (and possibly another digit) indicates a
   call to be directed to an operator position in the destination
   country.  A Code 12 may be followed by other digits indicating a
   particular operator to whom the call is to be directed.

   Implementations using the telephone-events payload to carry SS No. 5
   register signalling MUST use the following events from Table 1 to
   convey the register signals shown in Table 2:

   o  event code 128 to convey Digit 0

   o  event codes 129-137 to convey Digits 1 through 9 respectively

   o  event code 138 to convey Code 11




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   o  event code 139 to convey KP1

   o  event code 140 to convey KP2

   o  event code 141 to convey ST

   o  event code 142 to convey Code 12.

   The sending implementation SHOULD send an initial event report for
   the KP signals as soon as they are recognized, and MUST send an event
   report for all of these signals as soon as they have completed.

2.2.  Signalling System R1 and North American MF

   Signalling System R1 is mainly used in North America, as is the more
   common variant designated simply "MF".  R1 is defined in ITU-T
   Recommendations Q.310-Q.332 [5], while MF is defined in [8].

   Like SS No. 5, R1/MF has both line and register signals.  The line
   signals (not counting Busy and Reorder) are implemented on analogue
   trunks through the application of a 2600 Hz tone, and on digital
   trunks by using ABCD signalling.  Interpretation of the line signals
   is state-dependent (as with SS No. 5).

2.2.1.  Signalling System R1 Line Signals

   In accordance with Table 1/Q.311, implementations MAY use the ABCD
   events described in Section 2.4 and shown in Table 1 to propagate R1
   line signals.  If they do so, they MUST use the following mappings.
   These mappings are based on an underlying mapping equating A=0 to
   presence of 2600 Hz signal in the indicated direction and A=1 to
   absence of that signal.

   o  2600 Hz present: event code 144;

   o  no signal present: event code 159.

2.2.2.  Signalling System R1 Register Signals

   R1 has a signal capacity of 15 codes for forward inter-register
   signals but no backward inter-register signals.  Each code or digit
   is transmitted by a tone pair from a set of 6 frequencies.  The R1
   register signals consist of KP, ST, and the digits "0" through "9".
   The frequencies allotted to the signals are shown in Table 3.  Note
   that these frequencies are the same as those allotted to the
   similarly-named SS No. 5 register signals, except that KP uses the
   frequency combination corresponding to KP1 in SS No. 5.  Table 3 also
   shows additional signals used in North American practice: KP', KP2P,



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   KP3P, STP or ST', ST2P, and ST3P [8].

                                 Upper Frequency (Hz)

   +------------+---------+---------+---------+---------+--------------+
   |      Lower |     900 |    1100 |    1300 |    1500 |         1700 |
   |  Frequency |         |         |         |         |              |
   |       (Hz) |         |         |         |         |              |
   +------------+---------+---------+---------+---------+--------------+
   |        700 | Digit 1 | Digit 2 | Digit 4 | Digit 7 | KP3P or ST3P |
   |            |         |         |         |         |              |
   |        900 |         | Digit 3 | Digit 5 | Digit 8 |   KP' or STP |
   |            |         |         |         |         |              |
   |       1100 |         |         | Digit 6 | Digit 9 |           KP |
   |            |         |         |         |         |              |
   |       1300 |         |         |         | Digit 0 | KP2P or ST2P |
   |            |         |         |         |         |              |
   |       1500 |         |         |         |         |           ST |
   +------------+---------+---------+---------+---------+--------------+

                      Table 3: R1/MF Register Signals

   Implementations using the telephone-events payload to carry North
   American R1 register signalling MUST use the following events from
   Table 1 to convey the register signals shown in Table 3:

   o  event code 128 to convey Digit 0;

   o  event codes 129-137 to convey Digits 1 through 9 respectively;

   o  event code 138 to convey KP3P or ST3P.

   o  event code 139 to convey KP;

   o  event code 140 to convey KP2P or ST2P;

   o  event code 141 to convey ST;

   o  event code 142 to convey KP' or STP;

      As with the original telephony signals, the receiver interprets
      codes 138, 140, and 142 as KPx or STx signals based on their
      position in the signalling sequence.

   Unlike SS No. 5, R1 allows a large tolerance for the time of onset of
   register signalling following the recognition of start-dialling line
   signal.  This means that sending implementations MAY wait to send a
   KP event report until the KP has completed.



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2.3.  Signalling System R2

   International Signalling System R2 is described in ITU-T
   Recommendations Q.400-Q.490 [6], but there are many national
   variants.  R2 line signals are continuous, out-of-band, link by link,
   and channel associated.  R2 (inter)register signals are
   multifrequency, compelled, in-band, end to end, and also channel
   associated.

2.3.1.  Signalling System R2 Line Signals

   R2 line signals may be analog, one-bit digital using the A bit in the
   16th channel, or digital using both A and B bits.  Implementations
   MAY use the ABCD events described in Section 2.4 and shown in Table 1
   to propagate these signals.  If they do so, they MUST use the
   following mappings.

   1.  For the analog R2 line signals shown in Table 1 of ITU-T
       Recommendation Q.411, implementations MUST map as follows.  This
       mapping is based on an underlying mapping of A bit = 0 when tone
       is present.

       *  event code 144 (Table 1) is used to indicate the Q.411
          "tone-on" condition

       *  event code 159 (Table 1), is used to indicate the Q.411 "tone-
          off" condition.

   2.  The digital R2 line signals as described by ITU-T Recommendation
       Q.421 are carried in two bits, A and B. The mapping between A and
       B bit values and event codes SHALL be the same in both directions
       and SHALL follow the principles for A and B bit mapping specified
       in Section 2.4.  Note that the mapping from line states to event
       codes thus generated differs from the mappings generated for
       analog signalling.

2.3.2.  Signalling System R2 Register Signals

   In R2 signalling, the signalling sequence is initiated from the
   outgoing exchange by sending a line "seizing" signal.  After line
   "seizing" signal (and "seizing acknowledgment" signal in R2D), the
   signalling sequence continues using MF register signals.  ITU-T
   Recommendation Q.441 classifies the forward MF register signals into
   Groups I and II, the backward MF register signals into Groups A and
   B. These groups are significant with respect both to what sort of
   information they convey and where they can occur in the signalling
   sequence.




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   R2 is a compelled tone signalling protocol, meaning that one tone is
   played until an "acknowledgment or directive for the next tone" is
   received which indicates that the original tone should cease.  In R2
   signalling, the signalling sequence is initiated from the outgoing
   exchange by sending a forward Group I signal.  The first forward
   signal is typically the first digit of the called number.  The
   incoming exchange typically replies with a backward Group A-1
   indicating to the outgoing exchange to send the next digit of the
   called number.

   The tones have meaning; however, the meaning varies depending on
   where the tone occurs in the signalling.  The meaning may also depend
   on the country.  Thus, to avoid an unmanageable number of events,
   this document simply provides means to indicate the 15 forward and 15
   backward MF R2 tones (i.e., using event codes 176-190 and 191-205
   respectively as shown in Table 1).  The frequency pairs for these
   tones are shown in Table 4 and Table 5.

                                 Upper Frequency (Hz)

    +----------------------+-------+-------+-------+--------+--------+
    | Lower Frequency (Hz) | 1500  | 1620  | 1740  | 1860   | 1980   |
    +----------------------+-------+-------+-------+--------+--------+
    | 1380                 | Fwd 1 | Fwd 2 | Fwd 4 | Fwd 7  | Fwd 11 |
    |                      |       |       |       |        |        |
    | 1500                 |       | Fwd 3 | Fwd 5 | Fwd 8  | Fwd 12 |
    |                      |       |       |       |        |        |
    | 1620                 |       |       | Fwd 6 | Fwd 9  | Fwd 13 |
    |                      |       |       |       |        |        |
    | 1740                 |       |       |       | Fwd 10 | Fwd 14 |
    |                      |       |       |       |        |        |
    | 1860                 |       |       |       |        | Fwd 15 |
    +----------------------+-------+-------+-------+--------+--------+

                   Table 4: R2 Forward Register Signals
















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                                 Upper Frequency (Hz)

   +-----------------+---------+---------+---------+---------+---------+
   | Lower Frequency | 1140    | 1020    | 900     | 780     | 660     |
   | (Hz)            |         |         |         |         |         |
   +-----------------+---------+---------+---------+---------+---------+
   | 1020            | Bkwd 1  |         |         |         |         |
   |                 |         |         |         |         |         |
   | 900             | Bkwd 2  | Fwd 3   |         |         |         |
   |                 |         |         |         |         |         |
   | 780             | Bkwd 4  | Bkwd 5  | Bkwd 6  |         |         |
   |                 |         |         |         |         |         |
   | 660             | Bkwd 7  | Bkwd 8  | Bkwd 9  | Bkwd 10 |         |
   |                 |         |         |         |         |         |
   | 540             | Bkwd 11 | Bkwd 12 | Bkwd 13 | Bkwd 14 | Bkwd 15 |
   +-----------------+---------+---------+---------+---------+---------+

                   Table 5: R2 Backward Register Signals

2.4.  ABCD Transitional signalling For Digital Trunks

   ABCD is a 4-bit signalling system used by digital trunks, where A, B,
   C, and D are the designations of the individual bits.  For N-state
   (N<=16) signalling, the first N values are used.  ABCD signalling
   events are all mutually exclusive states.  The most recent state
   transition determines the current state.

   When using Extended Super Frame (ESF) T1 framing, signalling
   information is sent as robbed bits in frames 6, 12, 18, and 24.  A D4
   superframe only transmits 4-state signalling with A and B bits.  On
   the CEPT E1 frame, all signalling is carried in timeslot 16, and two
   channels of 16-state (ABCD) signalling are sent per frame.

   The meaning of ABCD signals varies with the application.  One example
   of a specification of ABCD signalling codes is T1.403.02 [13], which
   reflects North American practice for "loop" signalling as opposed to
   the trunk signalling discussed in previous sections.

   Since ABCD information is a state rather than a changing signal,
   implementations SHOULD use the following triple-redundancy mechanism,
   similar to the one specified in ITU-T Rec. I.366.2 [12], Annex L.  At
   the time of a transition, the same ABCD information is sent 3 times
   at an interval of 5 ms.  If another transition occurs during this
   time, then this continues.  After a period of no change, the ABCD
   information is sent every 5 seconds.

   As shown in Table 1, the 16 possible states are represented by event
   codes 144 to 159 respectively.  Implementations using these event



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   codes MUST map them to and from the ABCD information based on the
   following principles:

   1.  State numbers are derived from the used subset of ABCD bits by
       treating them as a single binary number, where the A bit is the
       high-order bit.  Unused bits are populated by propagating values
       from the used bits.  Specifically, if only the A bit is used, its
       value is propagated to the other three bits.  If the A and B bits
       are used, the value of the A bit is propagated to the C bit and
       the value of the B bit is propasgated to the D bit.  As examples:
       if only A and B bits are used, A=0, B=1, then C and D are 0 and 1
       respectively and the state number is 5 (binary 0101); if all four
       bits are used and A=1, B=1, C=0, D=1, then the state number is 13
       (binary 1101).

   2.  State numbers map to event codes by order of increasing value
       (i.e., state number 0 maps to event code 144, ..., state number
       15 maps to event code 159).

2.5.  Continuity Tones

   Continuity tones are used for testing circuit continuity during call
   setup.  Two basic procedures are used.  In international practice,
   clause 7 of ITU- T Recommendation Q.724 [7] describes a procedure
   applicable to four-wire trunk circuits, where a single 2000 +/- 20 Hz
   check-tone is transmitted from the initiating telephone switch.  The
   remote switch sets up a loopback, and continuity check passes if the
   sending switch can detect the tone on the return path.  Q.724 clause
   8 describes the procedure for two-wire trunk circuits.  The two-wire
   procedure involves two tones: a 2000 Hz tone sent in the forward
   direction, and a 1780 +/- 20 Hz tone sent in response.

   If implementations use the telephone-events payload type to propagate
   continuity check-tones, they MUST map these tones to event codes as
   follows:

   o  For four-wire continuity testing, the 2000 Hz check-tone is mapped
      to event code 167.

   o  For two-wire continuity testing, the initial 2000 Hz check-tone Hz
      tone is mapped to event code 167.  The 1780 Hz continuity verify
      tone is mapped to event code 168.

2.6.  Trunk Unavailable Event

   This event indicates that the trunk is unavailable for service.  The
   length of the downtime is indicated in the duration field.  The
   duration field is set to a value that allows adequate granularity in



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   describing downtime.  A value of 1 second is RECOMMENDED.  When the
   trunk becomes unavailable, this event is sent with the same timestamp
   three times at an interval of 20 ms.  If the trunk persists in the
   unavailable state at the end of the indicated duration, then the
   event is retransmitted, preferably with the same redundancy scheme.

   Unavailability of the trunk might result from a failure or an
   administrative action.  This event is used in a stateless manner to
   synchronize trunk unavailability between equipment connected through
   provisioned RTP trunks.  It avoids the unnecessary consumption of
   bandwidth in sending a continuous stream of RTP packets with a fixed
   payload for the duration of the downtime, as would be required in
   certain E1-based applications.  In T1-based applications, trunk
   conditioning via the ABCD transitional events can be used instead.

2.7.  Metering Pulse Event

   The metering pulse event may be used to transmit meter pulsing for
   billing purposes.  Since the metering pulse is a discrete event, each
   metering pulse event report MUST have both the 'M' and 'E' bits set.
   Meter pulsing is normally transmitted by out-of-band means while
   conversation is in progress.  Senders MUST therefore be prepared to
   transmit both the telephone-event and audio payload types
   simultaneously.  Metering pulse events MUST be retransmitted as
   recommended in section 2.5.1.4 of RFC xxxx [3].  It is RECOMMENDED
   that the retransmission interval be the lesser of 50 ms and the
   pulsing rate, but no less than audio packetization rate.
























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3.  Congestion Considerations

   The ability to adapt to congestion varies with the signalling system
   being used and also differs between line and register signals.

   With the specific exception of register signalling for S.S. No. 5 and
   R1/MF, the signals desribed in this document are fairly tolerant of
   lengthened durations should these be necessary.  Thus in congested
   conditions, the sender may adapt by lengthening the reporting
   interval for the tones concerned.  At the receiving end, if a tone is
   being played out and an under-run occurs due to delayed or lost
   packets, it is best to continue playing the tone until the next
   packet arrives.  Interrupting a tone prematurely, with or without
   resumption, can cause the call setup attempt to fail, whereas
   extended playout just increases the call setup time.

   Register signalling for S.S. No. 5 and R1/MF is subject to time
   constraints.  Both the tone signals and the silent periods between
   them have specified durations and tolerances of the order of 5 to 10
   ms.  The durations of the individual tones are of the order of two to
   three packetization intervals (55/68 ms, with the initial KP lasting
   100 ms).  The critical requirement for transmission of the telephony-
   event payload is that the receiver knows which signal to play out at
   a given moment.  It is less important that the receiver receive
   timely notification of the end of each tone.  Rather, it should play
   out the sequence with the durations specified by the signalling
   standard rather than the actual durations reported.

   These considerations suggest that as soon as a register signal has
   been reliably identified, the sender should emit a report of that
   tone.  It should then provide an update within 5 ms for reliability,
   and no more updates until reporting the end of the tone.

   Increasing the playout buffer at the receiver during register
   signalling will increase reliability.  This has to be weighed against
   the implied increase in call setup time.















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4.  Security Considerations

   Because the events for which event codes are provided in this
   document relate directly to the setup, billing, and takedown of
   telephone calls, they may be used to commit toll fraud in the PSTN.
   Thus PSTN gateways MUST authenticate the source of trunk signalling
   event reports and ensure that the authenticated entity is authorized
   to originate them.

   Additional security considerations are described in RFC xxxx [3].









































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5.  IANA Considerations

   This document defines the event codes shown in Table 6.  These events
   are additions to the telephone-event registry established by RFC xxxx
   [3].  The reference for all of them is the present document.

   +---------+---------------------------------------------------------+
   |   Event | Event Name                                              |
   |    Code |                                                         |
   +---------+---------------------------------------------------------+
   |     128 | SS No. 5 or R1 digit "0"                                |
   |         |                                                         |
   |     129 | SS No. 5 or R1 digit "1"                                |
   |         |                                                         |
   |     130 | SS No. 5 or R1 digit "2"                                |
   |         |                                                         |
   |     131 | SS No. 5 or R1 digit "3"                                |
   |         |                                                         |
   |     132 | SS No. 5 or R1 digit "4"                                |
   |         |                                                         |
   |     133 | SS No. 5 or R1 digit "5"                                |
   |         |                                                         |
   |     134 | SS No. 5 or R1 digit "6"                                |
   |         |                                                         |
   |     135 | SS No. 5 or R1 digit "7"                                |
   |         |                                                         |
   |     136 | SS No. 5 or R1 digit "8"                                |
   |         |                                                         |
   |     137 | SS No. 5 or R1 digit "9"                                |
   |         |                                                         |
   |     138 | MF Code 11 (SS No. 5) or KP3P/ST3P (R1)                 |
   |         |                                                         |
   |     139 | MF KP (SS No. 5) or KP1 (R1)                            |
   |         |                                                         |
   |     140 | MF KP2 (SS No. 5) or KP2P/ST2P (R1)                     |
   |         |                                                         |
   |     141 | MF ST (SS No. 5 and R1)                                 |
   |         |                                                         |
   |     142 | MF Code 12 (SS No. 5) or KP'/STP (R1)                   |
   |         |                                                         |
   |     144 | A bit signalling state '0' or AB state '00' or ABCD     |
   |         | state '0000'                                            |
   |         |                                                         |
   |     145 | ABCD signalling state '0001'                            |
   |         |                                                         |
   |     146 | ABCD signalling state '0010'                            |
   |         |                                                         |
   |     147 | ABCD signalling state '0011'                            |



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   |     148 | ABCD signalling state '0100'                            |
   |         |                                                         |
   |     149 | AB signalling state '01' or ABCD state '0101'           |
   |         |                                                         |
   |     150 | ABCD signalling state '0110'                            |
   |         |                                                         |
   |     151 | ABCD signalling state '0111'                            |
   |         |                                                         |
   |     152 | ABCD signalling state '1000'                            |
   |         |                                                         |
   |     153 | ABCD signalling state '1001'                            |
   |         |                                                         |
   |     154 | AB signalling state '10' or ABCD state '1010'           |
   |         |                                                         |
   |     155 | ABCD signalling state '1011'                            |
   |         |                                                         |
   |     156 | ABCD signalling state '1100'                            |
   |         |                                                         |
   |     157 | ABCD signalling state '1101'                            |
   |         |                                                         |
   |     158 | ABCD signalling state '1110'                            |
   |         |                                                         |
   |     159 | A bit signalling state '1' or AB state '11' or ABCD     |
   |         | signalling state '1111'                                 |
   |         |                                                         |
   |     167 | Continuity check-tone                                   |
   |         |                                                         |
   |     168 | Continuity verify-tone                                  |
   |         |                                                         |
   |     174 | Metering pulse                                          |
   |         |                                                         |
   |     175 | Trunk unavailable                                       |
   |         |                                                         |
   |     176 | MFC forward signal 1                                    |
   |         |                                                         |
   |     177 | MFC forward signal 2                                    |
   |         |                                                         |
   |     178 | MFC forward signal 3                                    |
   |         |                                                         |
   |     179 | MFC forward signal 4                                    |
   |         |                                                         |
   |     180 | MFC forward signal 5                                    |
   |         |                                                         |
   |     181 | MFC forward signal 6                                    |
   |         |                                                         |
   |     182 | MFC forward signal 7                                    |
   |         |                                                         |
   |     183 | MFC forward signal 8                                    |



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   |     184 | MFC forward signal 9                                    |
   |         |                                                         |
   |     185 | MFC forward signal 10                                   |
   |         |                                                         |
   |     186 | MFC forward signal 11                                   |
   |         |                                                         |
   |     187 | MFC forward signal 12                                   |
   |         |                                                         |
   |     188 | MFC forward signal 13                                   |
   |         |                                                         |
   |     189 | MFC forward signal 14                                   |
   |         |                                                         |
   |     190 | MFC forward signal 15                                   |
   |         |                                                         |
   |     191 | MFC backward signal 1                                   |
   |         |                                                         |
   |     192 | MFC backward signal 2                                   |
   |         |                                                         |
   |     193 | MFC backward signal 3                                   |
   |         |                                                         |
   |     194 | MFC backward signal 4                                   |
   |         |                                                         |
   |     195 | MFC backward signal 5                                   |
   |         |                                                         |
   |     196 | MFC backward signal 6                                   |
   |         |                                                         |
   |     197 | MFC backward signal 7                                   |
   |         |                                                         |
   |     198 | MFC backward signal 8                                   |
   |         |                                                         |
   |     199 | MFC backward signal 9                                   |
   |         |                                                         |
   |     200 | MFC backward signal 10                                  |
   |         |                                                         |
   |     201 | MFC backward signal 11                                  |
   |         |                                                         |
   |     202 | MFC backward signal 12                                  |
   |         |                                                         |
   |     203 | MFC backward signal 13                                  |
   |         |                                                         |
   |     204 | MFC backward signal 14                                  |
   |         |                                                         |
   |     205 | MFC backward signal 15                                  |
   +---------+---------------------------------------------------------+

   Table 6: Channel-oriented signalling events to be added to the audio/
                    telephone-event event code registry




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6.  Acknowledgements

   The complete list of acknowledgements for contribution to the
   development and revision of RFC 2833 is contained in RFC xxxx [3].
   The Editor believes that the following people contributed
   specifically to the present document: Flemming Andreasen, Rex
   Coldren, Bill Foster, Rajesh Kumar, Oren Peleg, Moshe Samoha, Adrian
   Soncodi, and Yaakov Stein.











































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7.  References

7.1.  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to indicate requirement
        levels", RFC 2119, March 1997.

   [2]  Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
        "RTP: A Transport Protocol for Real-Time Applications",
        RFC 3550, STD 0064, July 2003.

   [3]  Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF Digits,
        Telephony Tones and Telephony Signals", Work in
        progress: draft-ietf-avt-rfc2833bis-12.txt, November 2005.

   [4]  International Telecommunication Union, "Specifications for
        signalling system no. 5", ITU-T Recommendation Q.140-Q.180,
        November 1988.

   [5]  International Telecommunication Union, "Specifications of
        Signalling System R1", ITU-T Recommendation Q.310-Q.332,
        November 1988.

   [6]  International Telecommunication Union, "Specifications of
        Signalling System R2", ITU-T Recommendation Q.400-Q.490,
        November 1988.

   [7]  International Telecommunication Union, "Telephone user part
        signalling procedures", ITU-T Recommendation Q.724,
        November 1988.

   [8]  Telcordia Technologies, "LSSGR: signalling for Analog
        Interfaces", Generic Requirement GR-506, June 1996.

7.2.  Informative References

   [9]   Schulzrinne, H. and S. Petrack, "RTP Payload for DTMF Digits,
         Telephony Tones and Telephony Signals", RFC 2833, May 2000.

   [10]  International Telecommunication Union, "Speech coders : Dual
         rate speech coder for multimedia communications transmitting at
         5.3 and 6.3 kbit/s", ITU-T Recommendation G.723.1, March 1996.

   [11]  International Telecommunication Union, "Coding of speech at 8
         kbit/s using conjugate-structure algebraic-code-excited linear-
         prediction (CS-ACELP)", ITU-T Recommendation G.729, March 1996.

   [12]  International Telecommunication Union, "AAL type 2 service



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         specific convergence sublayer for trunking", ITU-T
         Recommendation I.366.2, February 1999.

   [13]  ANSI/T1, "Network and Customer Installation Interfaces -- DS1
         Robbed-Bit signalling State Definitions", American National
         Standard for Telecommunications T1.403.02-1999, May 1999.













































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Authors' Addresses

   Henning Schulzrinne
   Columbia U.
   Dept. of Computer Science
   Columbia University
   1214 Amsterdam Avenue
   New York, NY  10027
   US

   Email: schulzrinne@cs.columbia.edu


   Tom Taylor
   Nortel
   1852 Lorraine Ave
   Ottawa, Ontario  K1H 6Z8
   CA

   Email: taylor@nortel.com































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