📄 rfc2833.txt
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Table 6 summarizes the events and tones that can appear on a trunk. Note that trunk can also carry line events (Section 3.12), as MF signaling does not include backward signals [15]. ABCD transitional: 4-bit signaling used by digital trunks. For N- state signaling, the first N values are used.Schulzrinne & Petrack Standards Track [Page 17] RFC 2833 Tones May 2000 Event encoding (decimal) ___________________________________________________ Acceptance tone 96 Confirmation tone 97 Dial tone, recall 98 End of three party service tone 99 Facilities tone 100 Line lockout tone 101 Number unobtainable tone 102 Offering tone 103 Permanent signal tone 104 Preemption tone 105 Queue tone 106 Refusal tone 107 Route tone 108 Valid tone 109 Waiting tone 110 Warning tone (end of period) 111 Warning Tone (PIP tone) 112 Table 5: Country-specific Line events The T1 ESF (extended super frame format) allows 2, 4, and 16 state signaling bit options. These signaling bits are named A, B, C, and D. Signaling information is sent as robbed bits in frames 6, 12, 18, and 24 when using ESF T1 framing. A D4 superframe only transmits 4-state signaling with A and B bits. On the CEPT E1 frame, all signaling is carried in timeslot 16, and two channels of 16-state (ABCD) signaling are sent per frame. Since this 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 [16], 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. Wink: A brief transition, typically 120-290 ms, from on-hook (unseized) to off-hook (seized) and back to onhook, used by the incoming exchange to signal that the call address signaling can proceed. Incoming seizure: Incoming indication of call attempt (off-hook).Schulzrinne & Petrack Standards Track [Page 18] RFC 2833 Tones May 2000 Event encoding (decimal) __________________________________________________ MF 0... 9 128...137 MF K0 or KP (start-of-pulsing) 138 MF K1 139 MF K2 140 MF S0 to ST (end-of-pulsing) 141 MF S1... S3 142...143 ABCD signaling (see below) 144...159 Wink 160 Wink off 161 Incoming seizure 162 Seizure 163 Unseize circuit 164 Continuity test 165 Default continuity tone 166 Continuity tone (single tone) 167 Continuity test send 168 Continuity verified 170 Loopback 171 Old milliwatt tone (1000 Hz) 172 New milliwatt tone (1004 Hz) 173 Table 6: Trunk events Seizure: Seizure by answering exchange, in response to outgoing seizure. Unseize circuit: Transition of circuit from off-hook to on-hook at the end of a call. Wink off: A brief transition, typically 100-350 ms, from off-hook (seized) to on-hook (unseized) and back to off-hook (seized). Used in operator services trunks. Continuity tone send: A tone of 2010 Hz. Continuity tone detect: A tone of 2010 Hz. Continuity test send: A tone of 1780 Hz is sent by the calling exchange. If received by the called exchange, it returns a "continuity verified" tone. Continuity verified: A tone of 2010 Hz. This is a response tone, used in dual-tone procedures.Schulzrinne & Petrack Standards Track [Page 19] RFC 2833 Tones May 20004 RTP Payload Format for Telephony Tones4.1 Introduction As an alternative to describing tones and events by name, as described in Section 3, it is sometimes preferable to describe them by their waveform properties. In particular, recognition is faster than for naming signals since it does not depend on recognizing durations or pauses. There is no single international standard for telephone tones such as dial tone, ringing (ringback), busy, congestion ("fast-busy"), special announcement tones or some of the other special tones, such as payphone recognition, call waiting or record tone. However, across all countries, these tones share a number of characteristics [17]: o Telephony tones consist of either a single tone, the addition of two or three tones or the modulation of two tones. (Almost all tones use two frequencies; only the Hungarian "special dial tone" has three.) Tones that are mixed have the same amplitude and do not decay. o Tones for telephony events are in the range of 25 (ringing tone in Angola) to 1800 Hz. CED is the highest used tone at 2100 Hz. The telephone frequency range is limited to 3,400 Hz. (The piano has a range from 27.5 to 4186 Hz.) o Modulation frequencies range between 15 (ANSam tone) to 480 Hz (Jamaica). Non-integer frequencies are used only for frequencies of 16 2/3 and 33 1/3 Hz. (These fractional frequencies appear to be derived from older AC power grid frequencies.) o Tones that are not continuous have durations of less than four seconds. o ITU Recommendation E.180 [18] notes that different telephone companies require a tone accuracy of between 0.5 and 1.5%. The Recommendation suggests a frequency tolerance of 1%.4.2 Examples of Common Telephone Tone Signals As an aid to the implementor, Table 7 summarizes some common tones. The rows labeled "ITU ..." refer to the general recommendation of Recommendation E.180 [18]. Note that there are no specific guidelines for these tones. In the table, the symbol "+" indicates addition ofSchulzrinne & Petrack Standards Track [Page 20] RFC 2833 Tones May 2000 the tones, without modulation, while "*" indicates amplitude modulation. The meaning of some of the tones is described in Section 3.12 or Section 3.11 (for V.21). Tone name frequency on period off period ______________________________________________________ CNG 1100 0.5 3.0 V.25 CT 1300 0.5 2.0 CED 2100 3.3 -- ANS 2100 3.3 -- ANSam 2100*15 3.3 -- V.21 "0" bit, ch. 1 1180 0.00333 V.21 "1" bit, ch. 1 980 0.00333 V.21 "0" bit, ch. 2 1850 0.00333 V.21 "1" bit, ch. 2 1650 0.00333 ITU dial tone 425 -- -- U.S. dial tone 350+440 -- -- ______________________________________________________ ITU ringing tone 425 0.67--1.5 3--5 U.S. ringing tone 440+480 2.0 4.0 ITU busy tone 425 U.S. busy tone 480+620 0.5 0.5 ______________________________________________________ ITU congestion tone 425 U.S. congestion tone 480+620 0.25 0.25 Table 7: Examples of telephony tones4.3 Use of RTP Header Fields Timestamp: The RTP timestamp reflects the measurement point for the current packet. The event duration described in Section 3.5 extends forwards from that time.4.4 Payload Format Based on the characteristics described above, this document defines an RTP payload format called "tone" that can represent tones consisting of one or more frequencies. (The corresponding MIME type is "audio/tone".) The default timestamp rate is 8,000 Hz, but other rates may be defined. Note that the timestamp rate does not affect the interpretation of the frequency, just the durations. In accordance with current practice, this payload format does not have a static payload type number, but uses a RTP payload type number established dynamically and out-of-band. It is shown in Fig. 3.Schulzrinne & Petrack Standards Track [Page 21] RFC 2833 Tones May 2000 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | modulation |T| volume | duration | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |R R R R| frequency |R R R R| frequency | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |R R R R| frequency |R R R R| frequency | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |R R R R| frequency |R R R R| frequency | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: Payload format for tones The payload contains the following fields: modulation: The modulation frequency, in Hz. The field is a 9-bit unsigned integer, allowing modulation frequencies up to 511 Hz. If there is no modulation, this field has a value of zero. T: If the "T" bit is set (one), the modulation frequency is to be divided by three. Otherwise, the modulation frequency is taken as is. This bit allows frequencies accurate to 1/3 Hz, since modulation frequencies such as 16 2/3 Hz are in practical use. volume: The power level of the tone, expressed in dBm0 after dropping the sign, with range from 0 to -63 dBm0. (Note: A preferred level range for digital tone generators is -8 dBm0 to -3 dBm0.) duration: The duration of the tone, measured in timestamp units. The tone begins at the instant identified by the RTP timestamp and lasts for the duration value. The definition of duration corresponds to that for sample- based codecs, where the timestamp represents the sampling point for the first sample. frequency: The frequencies of the tones to be added, measured in Hz and represented as a 12-bit unsigned integer. The field size is sufficient to represent frequencies up to 4095 Hz,Schulzrinne & Petrack Standards Track [Page 22] RFC 2833 Tones May 2000 which exceeds the range of telephone systems. A value of zero indicates silence. A single tone can contain any number of frequencies. R: This field is reserved for future use. The sender MUST set it to zero, the receiver MUST ignore it.4.5 Reliability This payload format uses the reliability mechanism described in Section 3.7.5 Combining Tones and Named Events⌨️ 快捷键说明
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