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RFC 4734

Definition of Events for Modem, Fax, and Text Telephony Signals

Pages: 47
Proposed Standard
Obsoletes:  2833
Updates:  4733
Part 1 of 2 – Pages 1 to 29
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Network Working Group                                     H. Schulzrinne
Request for Comments: 4734                                   Columbia U.
Obsoletes: 2833                                                T. Taylor
Updates: 4733                                                     Nortel
Category: Standards Track                                  December 2006


    Definition of Events for Modem, Fax, and Text Telephony Signals

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2006).

Abstract

This memo updates RFC 4733 to add event codes for modem, fax, and text telephony signals when carried in the telephony event RTP payload. It supersedes the assignment of event codes for this purpose in RFC 2833, and therefore obsoletes that part of RFC 2833.
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Table of Contents

1. Introduction ....................................................3 1.1. Terminology ................................................3 1.2. Overview ...................................................3 2. Definitions of Events for Control of Data, Fax, and Text Telephony Sessions ..............................................5 2.1. V.8 bis Events .............................................5 2.1.1. Handling of Congestion ..............................9 2.2. V.21 Events ...............................................10 2.2.1. Handling of Congestion .............................11 2.3. V.8 Events ................................................12 2.3.1. Handling of Congestion .............................15 2.4. V.25 Events ...............................................15 2.4.1. Handling of Congestion .............................17 2.5. V.32/V.32bis Events .......................................18 2.5.1. Handling of Congestion .............................19 2.6. T.30 Events ...............................................19 2.6.1. Handling of Congestion .............................23 2.7. Events for Text Telephony .................................23 2.7.1. Signal Format Indicators for Text Telephony ........23 2.7.2. Use of Events with V.18 Modems .....................27 2.8. A Generic Indicator .......................................28 3. Strategies for Handling Fax and Modem Signals ..................29 4. Example of V.8 Negotiation .....................................30 4.1. Simultaneous Transmission of Events and Retransmitted Events Using RFC 2198 Redundancy ............35 4.2. Simultaneous Transmission of Events and Voice-Band Data Using RFC 2198 Redundancy ............................37 5. Security Considerations ........................................39 6. IANA Considerations ............................................40 7. Acknowledgements ...............................................42 8. References .....................................................43 8.1. Normative References ......................................43 8.2. Informative References ....................................44
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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]. In addition to those defined for specific events, this document uses the following abbreviations: Fax facsimile HDLC High-level Data Link Control PSTN Public Switched (circuit) Telephone Network

1.2. Overview

This document extends the set of telephony events defined within the framework of RFC 4733 [5] to include the control events and tones that can appear on a subscriber line serving a fax machine, a modem, or a text telephony device. The events are organized into several groups, corresponding to the ITU-T Recommendation in which they are defined. Their purpose is to support negotiation, start-up and takedown of fax, modem, or text telephony sessions and transitions between operating modes. The actual fax, modem, and text payload is typically carried by other payload types (e.g., V.150.1 [32] modem relay, voice-band data as formalized in ITU-T Rec. V.152 [33], Clearmode [17] for digital data, T.38 [21] for fax, or RFC 4103 [18] for character-mode text). NOTE: implementers SHOULD NOT rely on the descriptions of the various modem protocols described below without consulting the original references (generally ITU-T Recommendations). The descriptions are provided in this document to give a context for the use of the events defined here. They frequently omit important details needed for implementation. The typical application of these events is to allow the Internet to serve as a bridge between terminals operating on the PSTN. This application is characterized as follows: o each gateway will act both as sender and as receiver; o time constraints apply to the exchange of signals, making the early identification and reporting of events desirable so that receiver playout can proceed in a timely fashion;
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   o  the receiver must play out events in their proper order;

   o  transfer of the events must be reliable.  Applications will vary
      in their ability to recover from missing events.

   In some cases, an implementation may simply ignore certain events,
   such as fax tones, that do not make sense in a particular
   environment.  Section 2.4.1 of RFC 4733 [5] specifies how an
   implementation can use the Session Description Protocol (SDP) "fmtp"
   parameter within an SDP description [4] to indicate which events it
   is prepared to handle.

   Regardless of which events they support, implementations MUST be
   prepared to send and receive data signals using payload types other
   than telephone-event, simultaneously with the use of the latter.
   This is discussed further in Section 3.

   In many cases, continuity of playout is critical.  In principle, this
   is achieved through buffering at the receiving end.  It is generally
   desirable to minimize such buffering to reduce round-trip response
   times.  Maintenance of a constant packetization interval at the
   sending end while reporting events is helpful for this purpose.

   A further word on time constraints is in order.  Time constraints
   governing the duration of tones do not pose a problem when using the
   telephone-event payload type: the payload specifies the duration and
   the receiving gateway can play out the tones accordingly.  Problems
   occur when time constraints are specified for the duration of silence
   between tones.  A silent period of "at least x ms" is not a problem
   -- event notifications can be received late, but they can still be
   played out at their specified durations.

   The problem occurs if silence must last for a specific duration or at
   most some specific period.  The most general constraint of the latter
   type has to do with the operation of echo suppressors (ITU-T
   Rec. G.164 [6]) and echo cancellers (ITU-T Rec. G.165 [7]).  These
   devices may re-activate after as little as 100 ms of no signal on the
   line.  As a result, in any situation where echo suppressors or
   cancellers must be disabled for signalling to work, tone events must
   be reported quickly enough to ensure that these devices do not become
   re-enabled.
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2. Definitions of Events for Control of Data, Fax, and Text Telephony Sessions

2.1. V.8 bis Events

Recommendation V.8 bis [10] is a general procedure for two endpoints to establish each other's capabilities and to transition between different operating modes, both at call startup and after the call has been established. It supports many of the same terminals as V.8 [9] (Section 2.3 below), but allows more detailed parameter negotiation. It lacks support for some of the older V-series modems defined in V.8, but adds capabilities for simultaneous or alternating voice and data, H.324 [20] multilink, and T.120 [23] conferencing. Following V.8 bis capability negotiations, if the terminals have negotiated a modem-based operating mode, they initiate the actual modem session using either V.8, a truncated version of V.8 (preferred), or V.25 start-up. V.25 is described in Section 2.4. V.8 bis distinguishes between "signals" and "messages". The V.8 bis signals -- ESi/ESr, MRe/MRd, and CRe/CRd -- consist of tones, as described in the next few paragraphs. The V.8 bis messages -- MS, CL, CLR, ACK(1), ACK(2), NAK(1), NAK(2), NACK(3), and NACK(4) -- consist of sequences of bits transported over V.21 [12] modulation. Signals are intended to be comprehensible at the receiver even in the presence of voice content. They consist of two tone segments. The first segment consists of a dual-frequency tone held for 400 ms, and has the function of preparing the receiver and any in-line echo suppressor or canceller for what follows. The specific frequencies depend only on whether the signal is from the initiator or the responder in a transaction. When using the telephone-event payload, the V8bISeg and V8bRSeg events in Table 1 represent the first segment of any V.8 bis signal in the initiating and responding case, respectively. The complete V.8 bis strategy for dealing with echo suppressors or cancellers is described in Rec. V.8 bis Appendix III. The only silent period constraints imposed are of the "at least" type, posing no difficulties for the use of the telephone-event payload. The second segment follows immediately after the first, and is a single tone held for 100 ms. The frequency used indicates the specific signal of the six signals defined. When using the telephone-event payload, the second segment of a V.8 bis signal is represented by the applicable event: CRdSeg, CReSeg, MRdSeg, MReSeg,
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   ESiSeg, or ESrSeg, as defined in Table 1.  ESiSeg and ESrSeg use the
   same frequencies as V.21 low and high channel '1' bits, respectively
   (see Table 2), and are therefore assigned the same event codes.

   V.8 bis messages use V.21 [12] frequency-shift signalling to transfer
   message content.  V.21 is described in the next section.  V.8 bis
   uses V.21 in half-duplex mode at 300 bits/s, with the lower channel
   assigned to the initiator and the upper channel to the responder.

   Each V.8 bis message is preceded by a 100-ms preamble of continuous
   V.21 marking frequency except if it was immediately preceded by an
   ESi or ESr signal (the second segment of which is that same V.21
   marking frequency).  The sender SHALL NOT report this preamble tone
   using the ESiSeg or ESrSeg events; these are to be used only for the
   V.8 bis signals to which they pertain.

      Spelling this out, continuous V.21 marking tone immediately
      following V8bISeg and V8bRSeg is reported as ESiSeg or ESrSeg,
      respectively.  Continuous V.21 marking tone occurring in any other
      context, and particularly after CRdSeg, CReSeg, MRdSeg, or MReSeg,
      is reported by other means such as a different payload type or
      using the V.21 '1' bit events defined in Section 2.2.

   No events are defined for V.8 bis messages, but a brief description
   follows.

   o  the V.8 bis CL message describes the sending terminal's
      capabilities;

   o  the CLR message also describes capabilities, but indicates that
      the sender wants to receive a CL in return;

   o  the MS establishes a particular operating mode;

   o  the ACK and NAK messages are used to terminate the message
      transactions.

   The V.8 bis messages are organized as a sequence of octets.  The
   first two to five octets are HDLC flags (0x7E).  Then comes a message
   type identifier (four bits), a V.8 bis version identifier (four
   bits), zero to two more octets of identifying information, followed
   by zero or more information field parameters in the form of bit maps.
   An individual bit map is one to five octets in length.  Up to 64
   octets of non-standard information may also be present.  The
   information fields are followed by a checksum and one to three HDLC
   flags.  Because of limits on the size of any one information field,
   V.8 bis defines segmentation procedures.  Excess data is sent in an
   additional message, but only after prompting from the receiving end.
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   Applications supporting V.8 bis signalling using the telephone-event
   payload MAY transfer V.8 bis messages in the form of sequences of
   bits, using the V.21 bit events defined in the next section.  If they
   do so, the transmitted information MUST include the complete contents
   of the message: the initial HDLC flags, the information field, the
   checksum, and the terminating HDLC flags.

   Transmission MUST also include the extra '0' bits added according to
   the procedures of Rec. V.8 bis, clause 7.2.8, to prevent false
   recognition of HDLC flags at the receiver.  Implementers should note
   that these extra '0' bits mean that in general V.8 bis messages as
   transmitted on the wire will not come out to an even multiple of
   octets.  Sending implementations MAY choose to vary the packetization
   interval to include exactly one octet of information plus any extra
   '0' bits inserted into that octet; the resulting variation will be
   insignificant compared with the amount of buffering required to guard
   against network delays in delivery of packets to the receiver (see
   below).

      One reason for reporting the V.21 bits exactly as presented on the
      wire is to match the corresponding content if it is also carried
      by other means, such as voice-band data.

   The power levels of the V.8 bis and V.21 signals are subject to
   national regulation.  Thus, it seems suitable to model V.8 bis events
   as tones for which the volumes SHOULD be specified by the sender.  If
   the receiver is rendering the V.8 bis tones as audio content for
   onward transmission, the receiver MAY use the volumes contained in
   the event reports, or MAY modify the volumes to match downstream
   national requirements.

   Table 1 summarizes the event codes defined for V.8 bis signalling in
   this document.  The individual events are described following the
   table.  Each event begins when the beginning of the tone segment is
   detected and ends when the tone is no longer detected.
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    +---------+-------------+-----------+------------+------+---------+
    | Event   | Freq.  (Hz) | Dur. (ms) | Event Code | Type | Volume? |
    +---------+-------------+-----------+------------+------+---------+
    | ESiSeg  |      980    |    100    |     38     | tone |   yes   |
    |         |             |           |            |      |         |
    | ESrSeg  |     1650    |    100    |     40     | tone |   yes   |
    |         |             |           |            |      |         |
    | CRdSeg  |     1900    |    100    |     23     | tone |   yes   |
    |         |             |           |            |      |         |
    | CReSeg  |      400    |    100    |     24     | tone |   yes   |
    |         |             |           |            |      |         |
    | MRdSeg  |     1150    |    100    |     25     | tone |   yes   |
    |         |             |           |            |      |         |
    | MReSeg  |      650    |    100    |     26     | tone |   yes   |
    |         |             |           |            |      |         |
    | V8bISeg | 1375 + 2002 |    400    |     28     | tone |   yes   |
    |         |             |           |            |      |         |
    | V8bRSeg | 1529 + 2225 |    400    |     29     | tone |   yes   |
    +---------+-------------+-----------+------------+------+---------+

                    Table 1: Events for V.8 bis Signals

   ESiSeg:

      The second segment of a V.8 bis initiating Escape Signal (ESi).
      The complete ESi signal is represented by events V8bISeg followed
      by ESiSeg.  ESi will be followed by an MS, CL, or CLR message from
      the same terminal.  A 1.5-s silent interval may come between the
      ESi signal and the transmission of the MS, CL, or CLR message to
      accommodate network echo suppressors.

   ESrSeg:

      The second segment of a V.8 bis responding Escape Signal (ESr).
      The complete ESr signal is represented by events V8bRSeg followed
      by ESrSeg.  ESr is always sent by the calling terminal in response
      to an MRe or CRe from an automatic answering station.  It will be
      followed by an MS, CL, or CLR message.  The ESr signal turns off
      any announcement being generated by the automatic answering
      station.

   CRdSeg:

      The second segment of a V.8 bis Capabilities Request signal (CRd).
      The first segment of the CRd signal is represented either by
      V8bISeg or V8bRSeg, depending on context.  The other end will
      return a capabilities list (CL or CLR message).
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   CReSeg:

      The second segment of a V.8 bis Capabilities Request signal (CRe)
      initiated by an automatic answering terminal.  The complete CRe
      signal is represented by events V8bISeg followed by CReSeg.  The
      calling terminal will respond with a CRd signal or a CL or CLR
      message.

   MRdSeg:

      The second segment of a V.8 bis Mode Request signal (MRd).  The
      first segment of the MRd signal is represented either by V8bISeg
      or V8bRSeg, depending on context.  The other end will return a CRd
      signal or an MS message.

   MReSeg:

      The second segment of a V.8 bis Mode Request signal (MRe)
      initiated by an automatic answering terminal.  The complete MRe
      signal is represented by events V8bISeg followed by MReSeg.  The
      calling terminal will respond with an MRd or CRd signal or an MS
      message.

   V8bISeg:

      The first segment of an initiating V.8 bis signal, which may be
      one of ESi, CRd, CRe, MRd, or MRe.

   V8bRSeg:

      The first segment of a responding V.8 bis signal, which may be one
      of ESr, CRd, or MRd.

2.1.1. Handling of Congestion

V.8 bis implementations are unlikely to tolerate gaps or extensions in playout times due to congestion-caused packet delay. At a minimum, the current transaction is liable to be reset when these defects in playout occur. As a result, careful management of the playout buffer is required at the receiver to increase robustness in the face of possible lost or delayed packets. The playout algorithm should also be such as not to cause event playout to exceed the nominal duration of the event. V.8 bis does not appear to offer opportunities for dynamic adaptation to congestion through manipulation of the packetization interval.
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2.2. V.21 Events

V.21 [12] is a modem protocol offering data transmission at a maximum rate of 300 bits/s. Two channels are defined, supporting full duplex data transmission if required. The low channel uses frequencies 980 Hz for '1' (mark) and 1180 Hz for '0' (space); the high channel uses frequencies 1650 Hz for '1' and 1850 Hz for '0'. The modem can operate synchronously or asynchronously. V.21 is used by other protocols (e.g., V.8 bis, V.18, T.30) for transmission of control data, and is also used in its own right between text terminals. The V.21 events are summarized in Table 2. Sending implementations SHOULD report a completed event for every bit transmitted (i.e., rather than at transitions between '0' and '1'). Bit events are assumed to begin and end with the clock interval for the event, neglecting the rise and fall times between bit transitions. Thus, it is important for a gateway to determine the actual bit rate in use before beginning to report V.21 events. Sometimes determination of the bit rate is not immediately possible, as in the case of the 100-ms training signal at V.21 mark frequency used before V.8 bis messages. Transmission of a single longer-duration V.21 event is reasonable under these circumstances and should not cause any difficulties at the receiving end. Implementations SHOULD pack multiple events into one packet, using the procedures of Section 2.5.1.5 of RFC 4733 [5]. Eight to ten bits is a reasonable packetization interval. Reliable transmission of V.21 events is important, to prevent data corruption. Reporting an event per bit rather than per transition increases reporting redundancy and thus reporting reliability, since each event completion is transmitted three times as described in Section 2.5.1.4 of RFC 4733 [5]. To reduce the number of packets required for reporting, implementations SHOULD carry the retransmitted events using RFC 2198 [2] redundancy encoding. This is illustrated in the example in Section 4.1. The time to transmit one V.21 bit at the nominal rate of 300 bits/s is 3.33 ms, or 26.67 timestamp units at the default 8000-Hz sampling rate for the telephone-event payload type. Because this duration is not an integral number of timestamp units, accurate reporting of the beginning of the event and the event duration is impossible. Sending gateways SHOULD round V.21 event starting times to the nearest whole timestamp unit.
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   When sending multiple consecutive V.21 events in a succession of
   packets, the sending gateway MUST ensure that individual event
   durations reported do not cause the last event of one packet to
   overlap with the first event of the next, taking into account the
   respective initial event timestamps.  To accomplish this, the sending
   gateway MUST derive the individual event durations as the succession
   of differences between the event starting times (so that, at 8000 Hz,
   every third event has reported duration 26 units, the remainder 27
   units).

   Where a receiving gateway recognizes that a packet reports a
   consecutive series of V.21 bit events, it SHOULD play them out at a
   uniform rate despite the possible one-timestamp-unit discrepancies in
   their reported spacing and duration.

   +--------------------+----------------+------------+------+---------+
   | Event              | Frequency (Hz) | Event Code | Type | Volume? |
   +--------------------+----------------+------------+------+---------+
   | V.21 channel 1,    |           1180 |         37 | tone |     yes |
   | '0' bit            |                |            |      |         |
   |                    |                |            |      |         |
   | V.21 channel 1,    |            980 |         38 | tone |     yes |
   | '1' bit            |                |            |      |         |
   |                    |                |            |      |         |
   | V.21 channel 2,    |           1850 |         39 | tone |     yes |
   | '0' bit            |                |            |      |         |
   |                    |                |            |      |         |
   | V.21 channel 2,    |           1650 |         40 | tone |     yes |
   | '1' bit            |                |            |      |         |
   +--------------------+----------------+------------+------+---------+

                     Table 2: Events for V.21 Signals

   Implementations that choose to transmit V.21 content using a
   different payload type may wish to use one of the indicator events
   defined in Table 7 to alert the receiver to the nature of the
   content.  It is not expected that an implementation will send both
   one of these indicator events and the V.21 bit events defined above
   for the same content.

2.2.1. Handling of Congestion

The duration of V.21 bits cannot be extended from its nominal value (which depends on the transmission rate). The playout algorithm at the receiver should take this constraint into account when compensating for the delay or loss of packets due to congestion.
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   Other congestion-related considerations depend on the specific
   application for which the V.21 bit events are being used.

2.3. V.8 Events

V.8 [9] is an older general negotiation and control protocol, supporting startup for the following terminals: H.324 [20] multimedia, V.18 [11] text, T.101 [22] videotext, T.30 [8] send or receive fax, and a long list of V-series modems including V.34 [28], V.90 [29], V.91 [30], and V.92 [31]. In contrast to V.8 bis [10], in V.8 only the calling terminal can determine the operating mode. V.8 does not use the same terminology as V.8 bis. Rather, it defines four signals that consist of bits transferred by V.21 [12] at 300 bits/s: the call indicator signal (CI), the call menu signal (CM), the CM terminator (CJ), and the joint menu signal (JM). In addition, it uses tones defined in V.25 [13] and T.30 [8] (described below), and one tone (ANSam) defined in V.8 itself. The calling terminal sends using the V.21 low channel; the answering terminal uses the high channel. The basic protocol sequence is subject to a number of variations to accommodate different terminal types. A pure V.8 sequence is as follows: 1. After an initial period of silence, the calling terminal transmits the V.8 CI signal. It repeats CI at least three times, continuing with occasional pauses until it detects ANSam tone. The CI indicates whether the calling terminal wants to function as H.324, V.18, T.30 send, T.30 receive, or a V-series modem. 2. The answering terminal transmits ANSam after detecting CI. ANSam will disable any G.164 [6] echo suppressors on the circuit after 400 ms and any G.165 [7] echo cancellers after one second of ANSam playout. 3. On detecting ANSam, the calling terminal pauses at least half a second, then begins transmitting CM to indicate detailed capabilities within the chosen mode. 4. After detecting at least two identical sequences of CM, the answering terminal begins to transmit JM, indicating its own capabilities (or offering an alternative terminal type if it cannot support the one requested).
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   5.  After detecting at least two identical sequences of JM, the
       calling terminal completes the current octet of CM, then
       transmits CJ to acknowledge the JM signal.  It pauses exactly 75
       ms, then starts operating in the selected mode.

   6.  The answering terminal transmits JM until it has detected CJ.  At
       that point, it stops transmitting JM immediately, pauses exactly
       75 ms, then starts operating in the selected mode.

   The CI, CM, and JM signals all consist of a fixed sequence of ten '1'
   bits followed by a signal-dependent pattern of ten synchronization
   bits, followed by one or more octets of variable information.  Each
   octet is preceded by a '0' start bit and followed by a '1' stop bit.
   The combination of the synchronization pattern and V.21 channel
   uniquely identifies the message type.  The CJ signal consists of
   three successive octets of all zeros with stop and start bits but
   without the preceding '1's and synchronizing pattern of the other
   signals.

   Applications MAY report each instance of a CM, JM, and CJ signal,
   respectively, as a series of V.21 bit events (Section 2.2), or may
   use another payload type to carry this information.  Applications
   supporting V.8 signalling using the telephone-event payload MAY
   report the synchronization part of the CI signal (ten '1's followed
   by '00000 00001') both as a series of V.21 bit events and, when it
   has been recognized, as a single CI event.

      Note that the CI event covers only the synchronization part of the
      CI signal.  The remaining call function octet and its start and
      stop bits need to be transmitted also, either as a series of V.21
      bit events or in some other payload format.  Presumably, the
      calling end gateway will use the same format for the CM and CJ
      signals.

   The overlapping nature of V.8 signalling means that there is no risk
   of silence exceeding 100 ms once ANSam has disabled any echo control
   circuitry.  However, the 75-ms pause before entering operation in the
   selected data mode will require both the calling and the answering
   gateways to recognize the completion of CJ, so they can change from
   playout of telephone-event to playout of the data-bearing payload
   after the 75-ms period.
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      +--------+----------------------+------------+------+---------+
      | Event  |       Frequency (Hz) | Event Code | Type | Volume? |
      +--------+----------------------+------------+------+---------+
      | ANSam  |            2100 x 15 |         34 | tone |     yes |
      |        |                      |            |      |         |
      | /ANSam | 2100 x 15 phase rev. |         35 | tone |     yes |
      |        |                      |            |      |         |
      | CI     |          (V.21 bits) |         53 | tone |     yes |
      +--------+----------------------+------------+------+---------+

                      Table 3: Events for V.8 Signals

   ANSam:

      The modified answer tone ANSam consists of a sinewave signal at
      2100 Hz, amplitude-modulated by a sine wave at 15 Hz.  The
      beginning of the event is at the beginning of the tone.  The end
      of the event is at the sooner of the ending of the tone or the
      occurrence of a phase reversal (marking the beginning of a /ANSam
      event).  Phase reversals are used to disable echo cancellation; if
      they are being applied, they occur at 450-ms intervals.

      An ANSam event packet SHOULD NOT be sent until it is possible to
      discriminate between an ANSam event and an ANS event (see V.25
      events, below).

      The modulated envelope for the ANSam tone ranges in amplitude
      between 0.8 and 1.2 times its average amplitude.  The average
      transmitted power is governed by national regulations.  Thus, it
      makes sense to indicate the volume of the signal.

   /ANSam:

      /ANSam reports the same physical signal as ANSam, but is reported
      following the first phase reversal in that signal.  It begins with
      the phase reversal and ends at the end of the tone.  The receiver
      of /ANSam MUST reverse the phase of the tone at the beginning of
      playout of /ANSam and every 450 ms thereafter until the end of the
      tone is reached.

   CI:

      CI reports the occurrence of the V.21 bit pattern '11111 11111
      00000 00001' indicating the beginning of a V.8 CI signal.  The
      event begins at the beginning of the first bit and ends at the end
      of the last one.  This event MUST NOT be reported except in a
      context where a V.8 CI signal might be expected (i.e., at the
      calling end during call setup).  Note that if the calling modem
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      sends the CI signal at all, it will typically repeat the signal
      several times.

      It is expected that the CI event will be most useful when the
      modem content is being transmitted primarily using another payload
      type.  The event acts as a commentary on that content, allowing
      the receiver to recognize that V.8 signalling is in progress.

2.3.1. Handling of Congestion

The tolerances built into V.8 suggest that it may be mostly robust in the face of packet losses or delays. Playout of ANSam and /ANSam can be extended for multiple packetization periods without harm, provided that phase reversals occur on schedule at 450-ms intervals during playout of the latter. To increase robustness of transmission of the V.21-based signals, sending applications using the V.21 events SHOULD include an integral number of octets, including start and stop bits, in each packet. The presence of start and stop bits provides some hope that receiving implementations can withstand unavoidable gaps in playout between octets. When a message is being repeated (as is possible for CI, CM, and JM), an even stronger robustness measure would be for the receiver to retain a copy of the message when it is first received, and when a packet is delayed or lost to continue playing out the current message instance and commence a new repetition as if packets had continued to arrive on schedule.

2.4. V.25 Events

V.25 [13] is a start-up protocol predating V.8 [9] and V.8 bis [10]. It specifies the exchange of two tone signals: CT and ANS. CT (calling tone) consists of a series of interrupted bursts of 1300-Hz tone, on for a duration of not less than 0.5 s and not more than 0.7 s and off for a duration of not less than 1.5 s and not more than 2.0 s. [13]. Modems not starting with the V.8 CI signal often use this tone. ANS (Answer tone) is a 2100-Hz tone used to disable echo suppression for data transmission [13], [8]. For fax machines, Recommendation T.30 [8] refers to this tone as called terminal identification (CED) answer tone. ANS differs from V.8 ANSam in that, unlike the latter, it has constant amplitude. V.25 specifically includes procedures for disabling echo suppressors as defined by ITU-T Rec. G.164 [6]. However, G.164 echo suppressors have now for the most part been replaced by G.165 [7] echo
Top   ToC   RFC4734 - Page 16
   cancellers, which require phase reversals in the disabling tone (see
   ANSam above).  As a result, Recommendation V.25 was modified in July
   2001 to say that phase reversal in the ANS tone is required if echo
   cancellers are to be disabled.

   One possible V.25 sequence is as follows:

   1.  The calling terminal starts generating CT as soon as the call is
       connected.

   2.  The called terminal waits in silence for 1.8 to 2.5 s after
       answer, then begins to transmit ANS continuously.  If echo
       cancellers are on the line, the phase of the ANS signal is
       reversed every 450 ms.  ANS will not reach the calling terminal
       until the echo control equipment has been disabled.  Since this
       takes about a second, it can only happen in the gap between one
       burst of CT and the next.

   3.  Following detection of ANS, the calling terminal may stop
       generating CT immediately or wait until the end of the current
       burst to stop.  In any event, it must wait at least 400 ms (at
       least 1 s if phase reversal of ANS is being used to disable echo
       cancellers) after stopping CT before it can generate the calling
       station response tone.  This tone is modem-specific, not
       specified in V.25.

   4.  The called terminal plays out ANS for 2.6 to 4.0 seconds or until
       it has detected calling station response for 100 ms.  It waits
       55-95 ms (nominal 75 ms) in silence.  (Note that the upper limit
       of 95 ms is rather close to the point at which echo control may
       reestablish itself.)  If the reason for ANS termination was
       timeout rather than detection of calling station response, the
       called terminal begins to play out ANS again to maintain
       disabling of echo control until the calling station responds.

   The events defined for V.25 signalling are shown in Table 4.

   +-------------------+----------------+------------+------+---------+
   | Event             | Frequency (Hz) | Event Code | Type | Volume? |
   +-------------------+----------------+------------+------+---------+
   | Answer tone (ANS) | 2100           |         32 | tone |     yes |
   |                   |                |            |      |         |
   | /ANS              | 2100 ph. rev.  |         33 | tone |     yes |
   |                   |                |            |      |         |
   | CT                | 1300           |         49 | tone |     yes |
   +-------------------+----------------+------------+------+---------+

                     Table 4: Events for V.25 Signals
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   ANS:

      The beginning of the event is at the beginning of the 2100-Hz
      tone.  The end of the event is at the sooner of the ending of the
      tone or the occurrence of a phase reversal (marking the beginning
      of a /ANS event).

      An initial ANS event packet SHOULD NOT be sent until it is
      possible to discriminate between an ANS event and an ANSam event
      (see V.8 events, above).

   /ANS:

      /ANS reports the same physical signal as ANS, but is reported
      following the first phase reversal in that signal.  It begins with
      the phase reversal and ends at the end of the tone.  The receiver
      of /ANS MUST reverse the phase of the tone at the beginning of
      playout of /ANS and every 450 ms thereafter until the end of the
      tone is reached.

   CT:

      The beginning of the CT event is at the beginning of an individual
      burst of the 1300-Hz tone.  The end of the event is at the end of
      that tone burst.  The gateway at the calling end SHOULD use a
      packetization interval smaller than the nominal duration of a CT
      burst, to ensure that CT playout at the called end precedes the
      sending of ANS from that end.

2.4.1. Handling of Congestion

The V.25 sequence appears to be robust in the face of lost or delayed packets, provided that the receiver continues to play out any tone it is in the process of playing until more packets are received. The receiver must play out the phase transitions for /ANS on schedule, at 450-ms intervals, even if updates of the /ANS event have been delayed. It also appears to be possible for the sender to temporarily increase the packetization interval to reduce packet volumes when congestion is encountered. The one risk is that extended playout proceeds past the actual end of the tone (as determined retroactively), and the receiver is forced to continue imposing an additional playout buffering lag in order to meet the constraint on maximum duration of the nominal 75-ms silent period following tone playout.
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2.5. V.32/V.32bis Events

ITU-T Recommendation V.32 [14] is a modem using phase-shift keying with quadrature amplitude modification. It operates on a carrier at 1800 Hz, modulated at 2400 symbols/s. The basic data rates for V.32 are 4800 and 9600 bits/s. V.32bis [15] extends the data rates up to 14,400 bits/s. Most or all existing deployments are V.32bis, typically in support of point-of-sale terminals and the like. One reason V.32bis is still used is because of its relatively rapid start-up sequence, particularly on leased lines. Operating over the public telephone network, the start-up begins as follows: a. the answering end begins with the V.25 answering procedure (1.8 to 2.5 s of silence followed by continuous ANS tone to a maximum of 3.3 s, with possible phase reversals to disable echo cancelling equipment); b. the calling end waits in silence until it has detected ANS for 1 s; c. the calling end begins to transmit a V.32/V.32bis pattern designated AA, i.e., a series of '0000' bit sequences transmitted at 4800 bits/s; d. upon detecting the AA pattern for at least 100 ms, the called modem is silent for 75 +/- 20 ms, then responds with an AC pattern, which is a series of '0011' bit sequences transmitted at 4800 bits/s. The difference in leased line operation is that the calling modem starts the session by sending AA. After that, the called modem responds with AC, and the rest of the sequence is unchanged. In support of V.32/V.32bis operation, Table 5 defines two events, V32AA and V32AC. +----------------+------------------+------------+------+---------+ | Event | Bit Pattern | Event Code | Type | Volume? | +----------------+------------------+------------+------+---------+ | V32AA | b'0000' repeated | 63 | tone | yes | | | | | | | | V32AC | b'0011' repeated | 27 | tone | yes | +----------------+------------------+------------+------+---------+ Table 5: Events for V.32/V.32bis Signals
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   V32AA:

      Indicates that the AA calling pattern of a V.32/V.32bis terminal
      has been detected.

   V32AC:

      Indicates that the AC answering pattern of a V.32/V.32bis terminal
      has been detected.

   Each of these two events begins at the beginning of its pattern, and
   ends nominally when the pattern stops being received.  Following the
   sending of either of these events the session may continue using
   V.150.1 modem relay [32] or Clearmode [17] as negotiated or
   configured in advance.  To help make the transition as quickly as
   possible, the V32AA or V32AC event SHOULD be reported as soon as the
   corresponding pattern is detected.  It seems likely that the
   implementation will be transmitting the event reports simultaneously
   with the same data in an alternate form, typically using RFC 2198 [2]
   redundancy.

2.5.1. Handling of Congestion

The primary issue raised by congestion is the loss or undue delay of the initial report. Once the receiver is aware that an AA or AC pattern has been detected, further reports are of no interest. The actual duration of the AC pattern may be as short as 27 ms. On this basis, the appropriate sender behavior may be to send at least three packets reporting the event using normal event updates and end of event retransmission behavior and a fairly short packetization interval (20-30 ms).

2.6. T.30 Events

ITU-T Recommendation T.30 [8] defines the procedures used by Group III fax terminals. The pre-message procedures for which the events of this section are defined are used to identify terminal capabilities at each end and negotiate operating mode. Post-message procedures are also included, to handle cases such as multiple document transmission. Fax terminals support a wide variety of protocol stacks, so T.30 has a number of options for control protocols and sequences. T.30 defines two tone signals used at the beginning of a call. The CNG signal is sent by the calling terminal. It is a pure 1100-Hz tone played in bursts: 0.5 s on, 3 s off. It continues until timeout
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   or until the calling terminal detects a response.  Its primary
   purpose is to let human operators at the called end know that a fax
   terminal has been activated at the calling end.

   The called terminal waits in silence for at least 200 ms.  It then
   may return CED tone (which is physically identical to V.25 ANS), or
   else V.8 ANSam if it has V.8 capability.  If called and calling
   terminals both support V.8, the called terminal will detect CI or
   more likely CM in response to its ANSam and will continue with V.8
   negotiation.  Otherwise, the called terminal stops transmitting CED
   after 2.6 to 4 seconds, waits 75 +/- 20 ms in silence, then enters
   the T.30 negotiation phase.

   In the T.30 negotiation phase the terminals exchange binary messages
   using V.21 signals, high channel frequencies only, at 300 bits/s.
   Each message is preceded by a one-second (nominal) preamble
   consisting entirely of HDLC flag octets (0x7E).  This flag has the
   function of preparing echo control equipment for the message that
   follows.

   The pre-transfer messages exchanged using the V.21 coding are:

   Digital Identification Signal (DIS):

      Characterizes the standard ITU-T capabilities of the called
      terminal.  This is always the first message sent.

   Digital Transmit Command (DTC):

      A possible response to the DIS signal by the calling terminal.  It
      requests the called terminal to be the transmitter of the fax
      content.

   Digital Command Signal (DCS):

      A command message sent by the transmitting terminal to indicate
      the options to be used in the transmission and request that the
      other end prepare to receive fax content.  This is sent by the
      calling end if it will transmit, or by the called end in response
      to a DTC from the calling end.  It is followed by a training
      signal, also sent by the transmitting terminal.

   Confirmation To Receive (CFR):

      A digital response confirming that the entire pre-message
      procedure including training has been completed and the message
      transmissions may commence.
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   Each message may consist of multiple frames bounded by HDLC flags.
   The messages are organized as a series of octets, but like V.8 bis,
   T.30 calls for the insertion of extra '0' bits to prevent spurious
   recognition of HDLC flags.

   T.30 also provides for the transmission of control messages after
   document transmission has completed (e.g., to support transmission of
   multiple documents).  The transition to and from the modem used for
   document transmission (V.17 [24], V.27ter [26], V.29 [27], V.34 [28])
   is preceded by 75 ms (nominal) of silence).

   Applications supporting T.30 signalling using the telephone-event
   payload MAY report the preamble preceding each message both as a
   series of V.21 bit events and, when it has been recognized, as a
   single V.21 preamble event.  The T.30 control message following the
   preamble MAY be reported in the form of a sequence of V.21 bit events
   or using some other payload type.  If transmitted as bit events, the
   transmitted information MUST include the complete contents of the
   message: the initial HDLC flags, the information field, the checksum,
   the terminating HDLC flags, and the extra '0' bits added to prevent
   false recognition of HDLC flags at the receiver.  Implementers should
   note that these extra '0' bits mean that in general T.30 messages as
   transmitted on the wire will not come out to an even multiple of
   octets.

   The training signal sent by the transmitting terminal after DCS
   consists of a steady string of V.21 high channel zeros (1850-Hz tone)
   for 1.5 s.  Since the bit rate (nominally 300 bits/s) should have
   been clearly established when processing the preceding signalling, it
   is natural that if the telephony-event payload type is being used,
   this training signal will also be sent as a series of V.21 bit events
   at that bit rate.  However, if the sending gateway is capable of
   recognizing the transition from the end of the DCS to the start of
   training, it MAY report the training signal as a single extended V.21
   (high channel) '0' event.

   The events defined for T.30 signalling are shown in Table 6.  The CED
   and /CED events represent exactly the same tone signals as V.25 ANS
   and /ANS, and are given the same codepoints; they are reproduced here
   only for convenience.
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   +--------------------+----------------+------------+------+---------+
   | Event              | Frequency (Hz) | Event Code | Type | Volume? |
   +--------------------+----------------+------------+------+---------+
   | CED (Called tone)  | 2100           |         32 | tone |     yes |
   |                    |                |            |      |         |
   | /CED               | 2100 ph. rev.  |         33 | tone |     yes |
   |                    |                |            |      |         |
   | CNG (Calling tone) | 1100           |         36 | tone |     yes |
   |                    |                |            |      |         |
   | V.21 preamble flag | (V.21 bits)    |         54 | tone |     yes |
   +--------------------+----------------+------------+------+---------+

                     Table 6: Events for T.30 Signals

   CED:

      The beginning of the event is at the beginning of the 2100-Hz
      tone.  The end of the event is at the sooner of the ending of the
      tone or the occurrence of a phase reversal (marking the beginning
      of a /CED event).

      An initial CED event packet SHOULD NOT be sent until it is
      possible to discriminate between a CED event and an ANSam event
      (see V.8 events, above).

   /CED:

      /CED reports the same physical signal as CED, but is reported
      following the first phase reversal in that signal.  It begins with
      the phase reversal and ends at the end of the tone.  The receiver
      of /CED MUST reverse the phase of the tone at the beginning of
      playout of /CED and every 450 ms thereafter until the end of the
      tone is reached.

   CNG:

      The beginning of the CNG event is at the beginning of an
      individual burst of the 1100-Hz tone.  The end of the event is at
      the end of that tone burst.

   V.21 preamble flag:

      This event begins with the first V.21 bits transmitted after a
      period of silence.  It ends when a pattern of V.21 bits other than
      an HDLC flag is observed.  This means that the V.21 preamble event
      absorbs the initial HDLC flags of the following message.
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      It is expected that the V.21 preamble flag event will be most
      useful when the modem content is being transmitted primarily using
      another payload type.  The event acts as a commentary on that
      content, allowing the receiver to prepare itself to transition to
      fax mode.

2.6.1. Handling of Congestion

T.30 appears to be an intermediate case in terms of its vulnerability to congestion. Tone playout in the face of packet delay or loss is subject to the same considerations as for V.25 (see Section 2.4.1). Similarly, the receiver may extend playout of the preamble event while waiting for further reports. However, gaps or extended playout of the V.21 sequences are not feasible. This means, as with V.8 bis, that the receiver must manage its playout buffer appropriately to increase robustness in the face of congestion.

2.7. Events for Text Telephony

2.7.1. Signal Format Indicators for Text Telephony

Legacy text telephony uses a wide variety of terminals, with different standards favored in different parts of the world. Going forward, the vision is that new terminals will work directly into the packet network and be based on RFC 4103 [18] packetization of character data. In anticipation of this migration, it is RECOMMENDED that text carried in the PSTN by legacy modem protocols be converted to RFC 4103 packets at the sending gateway. During a transitional period, however, gateways of a lesser capability may be able to recognize the nature of incoming content, but may only be able to encode it as voice-band data on the packet side. In such circumstances, it will help to optimize processing of the signal at the receiving end if that end receives an indication of the nature of the voice-encoded data signals. The events defined in this section provide such indications, and MAY be used in conjunction with ITU-T Recommendation V.152 [33], as one example, to carry the content as voice-band data. Implementers should take note of an additional class of text terminals not considered in the events below. These terminals use dual tone multi-frequency (DTMF) tones to encode and exchange signals. This application is described in RFC 4733 [5], Section 3.1, in conjunction with the registration of DTMF events.
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   The events shown in Table 7 correspond to signals coming from the
   following modem types:

   o  Baudot [34], a five bit character encoding nominally operating at
      45.45 or 50 bits/s with frequencies 1800 Hz = '0', 1400 Hz = '1';

   o  EDT, which is V.21 [12] operating at 110 bits/s in half-duplex
      mode (lower channel only); characters are 7-bit IA5 plus initial
      start bit, trailing parity bit, and two stop bits;

   o  Bell 103 mode (documented in Recommendation V.18 Annex D), which
      is structurally similar to V.21, but uses different frequencies:
      lower channel, 1070 Hz = '0', 1270 Hz = '1'; upper channel, 2025
      Hz = '0', 2225 Hz = '1'; characters are US ASCII framed by one
      start bit, one trailing parity bit, and one stop bit;

   o  V.23 [25] based videotex, in Minitel and Prestel versions.  V.23
      offers a forward channel operating at 1200 bits/s if possible
      (2100 Hz = '0', 1300 Hz = '1') or otherwise at 600 bits/s (1700 Hz
      = '0', 1300 Hz = '1'), and a 75 bits/s backward channel, which is
      transmitting 390 Hz (continuous '1's) except when '0' is to be
      transmitted (450 Hz);

   o  a non-V.18 text terminal using V.21 [12] at 300 bits/s.
      Characters are 7-bit national (e.g., US ASCII) with a start bit,
      parity, and one stop bit.
Top   ToC   RFC4734 - Page 25
   +----------+-----------+----------------+---------+-------+---------+
   | Event    | Bit Rate  | Frequency (Hz) |   Event |  Type | Volume? |
   |          | bits/s    |                |    Code |       |         |
   +----------+-----------+----------------+---------+-------+---------+
   | ANS2225  | N/A       | 2225           |      52 |  tone |     yes |
   |          |           |                |         |       |         |
   | V21L110  | 110       | 980/1180       |      55 | other |      no |
   |          |           |                |         |       |         |
   | V21L300  | 300       | 980/1180       |      30 | other |      no |
   |          |           |                |         |       |         |
   | V21H300  | 300       | 1650/1850      |      31 | other |      no |
   |          |           |                |         |       |         |
   | B103L300 | 300       | 1070/1270      |      56 | other |      no |
   |          |           |                |         |       |         |
   | V23Main  | 600/1200  | 1700-2100/1300 |      57 | other |      no |
   |          |           |                |         |       |         |
   | V23Back  | 75        | 450/390        |      58 | other |      no |
   |          |           |                |         |       |         |
   | Baud4545 | 45.45     | 1800/1400      |      59 | other |      no |
   |          |           |                |         |       |         |
   | Baud50   | 50        | 1800/1400      |      60 | other |      no |
   |          |           |                |         |       |         |
   | XCIMark  | 1200      | 2100/1300      |      62 |  tone |     yes |
   +----------+-----------+----------------+---------+-------+---------+

                  Table 7: Indicators for Text Telephony

   ANS2225:

      indicates that a 2225-Hz answer tone has been detected.  This is a
      pure tone with no amplitude modulation and no semantics attached
      to phase reversals, if there are any.  The sender SHOULD report
      the beginning of the event when the tone is detected.  The sender
      MAY send updates as the tone continues, and MUST report the end of
      the event when the tone ceases.  The tone concerned is generated
      by a Bell 103-type modem in answer mode.  This event MUST NOT be
      reported outside of the startup context (i.e., on the answering
      side at the beginning of a call).

   V21L110:

      indicates that the sender has detected V.21 modulation operating
      in the lower channel at 110 bits/s.  Note that it may take some
      time to distinguish between 300 bits/s and 110 bits/s operation.
      It is expected that implementations will not transmit both this
      event and individual V.21 bit events for the same content.
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   V21L300:

      indicates that the sender has detected V.21 modulation operating
      in the lower channel at 300 bits/s.  Note that it may take some
      time to distinguish between 300 bits/s and 110 bits/s operation.
      It is expected that implementations will not transmit both this
      event and individual V.21 bit events for the same content.

   V21H300:

      indicates that the sender has detected V.21 modulation operating
      in the upper channel at 300 bits/s.  It is expected that
      implementations will not transmit both this event and individual
      V.21 bit events for the same content.

   B103L300:

      indicates that the sending device has detected Bell 103 class
      modulation operating in the low channel at 300 bits/s.

   V23Main:

      indicates that the sending device has detected V.23 modulation
      operating in the high-speed channel.  As described below, this
      indicator may alternate with the XCIMark indication.

   V23Back:

      indicates that the sending device has detected V.23 modulation
      operating in the 75 bit/s back-channel.

   Baud4545:

      indicates that the sending device has detected Baudot modulation
      operating at 45.45 bits/s.

   Baud50:

      indicates that the sending device has detected Baudot modulation
      operating at 50 bits/s.

   XCIMark:

      Indicates that the sending device has detected the specific bit
      pattern (0) 1111 1111(1)(0)1111 1111(1) sent at 1200 bits/s using
      V.23 upper-channel modulation, following a period of V.23 main
      channel "mark" (1300 Hz).
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   It is assumed in all cases that the event reports described here are
   being transmitted in addition to another media encoding, typically
   G.711 [19] voice-band data, reporting the same information.  A
   natural method to do this is to combine the voice-band data with
   event reports in an RFC 2198 [2] redundancy payload.

   The handling of ANS2225 has been indicated above.  Since it is a
   specific tone, it can be handled like any other tone event.

   For all of the other indicators, the sender SHOULD generate an
   initial event report as soon as the nature of the audio content has
   been recognized.  For reliability, the initial event report SHOULD be
   retransmitted twice at short intervals. (20 ms is a suggested value,
   although the packetization period of the associated media may be
   sufficient.)  The sender MAY continue to send additional reports of
   the same indicator event, although these have little value once the
   receiver has adjusted itself to the type of content it is receiving.

   If the nature of the content changes (e.g., because it is coming from
   a V.18 terminal in the probing stage), the sender MUST send an event
   report for the new content type as soon as it is recognized.  If the
   sender has been sending updates for the previous indicator, it SHOULD
   report the end of that previous indicator event along with the
   beginning of the new one.

2.7.1.1. Handling of Congestion
In the face of packet loss or delay, it is appropriate for the receiver to continue to play out the ANS2225 event until further packets are received. For the other events, the issue is loss of the initial event report rather than maintenance of playout continuity. The advice on retransmission of these other events already given above is sufficient to deal with packet loss or delay due to congestion.

2.7.2. Use of Events with V.18 Modems

ITU-T Recommendation V.18 [11] defines a terminal for text conversation, possibly in combination with voice. V.18 is intended to interoperate with a variety of legacy text terminals, so its start-up sequence can consist of a series of stimuli designed to determine what is at the other end. Two V.18 terminals talking to each other will use V.8 to negotiate startup and continue at the physical level with V.21 at 300 bits/s carrying 7-bit characters bounded by start and stop bits.
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   The V.18 terminal is also designed to interoperate with the text
   modems listed in the previous sub-section.  The startup sequences for
   all these different terminal types are naturally quite different.
   The V.18 initial startup sequence specifically addresses itself to
   V.8-capable terminals and V.21 terminals and, by the combination of
   signals, to V.23 videotex terminals.  During the initial startup
   sequence, the V.18 terminal listens for frequency responses
   characterizing the other terminal types.  If it does not make contact
   in the preliminary step, it probes for each type specifically.  By
   the nature of the application, V.18 has been designed to provide an
   extremely robust startup capability.

   The handling of the V.18 XCI signal is a specific case of the
   procedures described in the previous section.  XCI is a signal
   transmitted in high-band V.23 modulation to stimulate V.23 terminals
   to respond and to allow detection of V.18 capabilities in a DCE.  The
   3-second XCI signal uses the V.23 upper channel having periods of
   "mark" (i.e., 1300 Hz) alternating with the XCIMark pattern.  The
   full definition is found in V.18, Section 3.13.  The sender SHOULD
   indicate V23Main during the transmission of the "mark" portion of
   XCI, and change the indication to XCIMark when that pattern is
   detected.

2.8. A Generic Indicator

Numerous proprietary modem protocols exist, as well as standardized protocols not identified above. Table 8 defines a single indicator event that may be used to identify modem content when a more specific event is unavailable. Typically, this would be sent in combination with another payload type, for example, voice-band data as specified by ITU-T Recommendation V.152 [33]. As with the indicators in the previous section, the sender SHOULD generate an initial event report as soon as the nature of the audio content has been recognized. For reliability, the initial event report SHOULD be retransmitted twice at short intervals. (20 ms is a suggested value, although the packetization period of the associated media may be sufficient.) The sender MAY continue to send additional reports of the VBDGen event, although these have little value once the receiver has adjusted itself to the type of content it is receiving.
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   +--------+---------------+------------+-----------+-------+---------+
   | Event  | Bit Rate      | Frequency  |     Event |  Type | Volume? |
   |        | bits/s        | (Hz)       |      Code |       |         |
   +--------+---------------+------------+-----------+-------+---------+
   | VBDGen | Variable      | Variable   |        61 | other |      no |
   +--------+---------------+------------+-----------+-------+---------+

                  Table 8: Generic Modem Signal Indicator

   VBDGen:

      indicates that the sender has detected tone patterns indicating
      the operation of some form of modem.  This indicator SHOULD NOT be
      sent if a more specific event is available.



(page 29 continued on part 2)

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