📄 rfc2833.txt
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Intrusion tone 84
Calling card service tone 85
Payphone recognition tone 86
CPE alerting signal (CAS) 87
Off-hook warning tone 88
Ring 89
Table 4: E.182 line events
3.13 Extended Line Events
Table 5 summarizes country-specific events and tones that can appear
on a subscriber line.
3.14 Trunk Events
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.
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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).
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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.
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RFC 2833 Tones May 2000
4 RTP Payload Format for Telephony Tones
4.1 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 of
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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 tones
4.3 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.
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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,
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