📄 rfc2833.txt
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Schulzrinne & Petrack Standards Track [Page 11]
RFC 2833 Tones May 2000
ANSam: The modified answer tone (ANSam) [3] is a sinewave signal
at 2100 +/- 1 Hz without phase reversals, amplitude-modulated
by a sinewave at 15 +/- 0.1 Hz. This tone is sent by modems
if network echo canceller disabling is not required.
/ANSam: The modified answer tone with phase reversals (ANSam) [3]
is a sinewave signal at 2100 +/- 1 Hz with phase reversals at
intervals of 450 +/- 25 ms, amplitude-modulated by a sinewave
at 15 +/- 0.1 Hz. This tone [10,8] is sent by modems [11] and
faxes to disable echo suppressors.
CNG: After dialing the called fax machine's telephone number (and
before it answers), the calling Group III fax machine
(optionally) begins sending a CalliNG tone (CNG) consisting
of an interrupted tone of 1100 Hz. [9]
CRdi: Capabilities Request (CRd), initiating side, [12] is a
dual-tone signal with tones at 1375 Hz and 2002 Hz for 400
ms, followed by a single tone at 1900 Hz for 100 ms. "This
signal requests the remote station transition from telephony
mode to an information transfer mode and requests the
transmission of a capabilities list message by the remote
station. In particular, CRdi is sent by the initiating
station during the course of a call, or by the calling
station at call establishment in response to a CRe or MRe."
CRdr: CRdr is the response tone to CRdi (see above). It consists
of a dual-tone signal with tones at 1529 Hz and 2225 Hz for
400 ms, followed by a single tone at 1900 Hz for 100 ms.
CRe: Capabilities Request (CRe) [12] is a dual-tone signal with
tones at tones at 1375 Hz and 2002 Hz for 400 ms, followed by
a single tone at 400 Hz for 100 ms. "This signal requests the
remote station transition from telephony mode to an
information transfer mode and requests the transmission of a
capabilities list message by the remote station. In
particular, CRe is sent by an automatic answering station at
call establishment."
CT: "The calling tone [8] consists of a series of interrupted
bursts of binary 1 signal or 1300 Hz, 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."
Modems not starting with the V.8 call initiation tone often
use this tone.
Schulzrinne & Petrack Standards Track [Page 12]
RFC 2833 Tones May 2000
ESi: Escape Signal (ESi) [12] is a dual-tone signal with tones at
1375 Hz and 2002 Hz for 400 ms, followed by a single tone at
980 Hz for 100 ms. "This signal requests the remote station
transition from telephony mode to an information transfer
mode. signal ESi is sent by the initiating station."
ESr: Escape Signal (ESr) [12] is a dual-tone signal with tones at
1529 Hz and 2225 Hz for 400 ms, followed by a single tone at
1650 Hz for 100 ms. Same as ESi, but sent by the responding
station.
MRdi: Mode Request (MRd), initiating side, [12] is a dual-tone
signal with tones at 1375 Hz and 2002 Hz for 400 ms followed
by a single tone at 1150 Hz for 100 ms. "This signal requests
the remote station transition from telephony mode to an
information transfer mode and requests the transmission of a
mode select message by the remote station. In particular,
signal MRd is sent by the initiating station during the
course of a call, or by the calling station at call
establishment in response to an MRe." [12]
MRdr: MRdr is the response tone to MRdi (see above). It consists
of a dual-tone signal with tones at 1529 Hz and 2225 Hz for
400 ms, followed by a single tone at 1150 Hz for 100 ms.
MRe: Mode Request (MRe) [12] is a dual-tone signal with tones at
1375 Hz and 2002 Hz for 400 ms, followed by a single tone at
650 Hz for 100 ms. "This signal requests the remote station
transition from telephony mode to an information transfer
mode and requests the transmission of a mode select message
by the remote station. In particular, signal MRe is sent by
an automatic answering station at call establishment." [12]
V.21: V.21 describes a 300 b/s full-duplex modem that employs
frequency shift keying (FSK). It is used by Group 3 fax
machines to exchange T.30 information. The calling transmits
on channel 1 and receives on channel 2; the answering modem
transmits on channel 2 and receives on channel 1. Each bit
value has a distinct tone, so that V.21 signaling comprises a
total of four distinct tones.
Schulzrinne & Petrack Standards Track [Page 13]
RFC 2833 Tones May 2000
In summary, procedures in Table 2 are used.
Procedure indications
___________________________________________________
V.25 and V.8 ANS
V.25, echo canceller disabled ANS, /ANS, ANS, /ANS
V.8 ANSam
V.8, echo canceller disabled /ANSam
Table 2: Use of ANS, ANSam and /ANSam in V.x recommendations
Event encoding (decimal)
___________________________________________________
Answer tone (ANS) 32
/ANS 33
ANSam 34
/ANSam 35
Calling tone (CNG) 36
V.21 channel 1, "0" bit 37
V.21 channel 1, "1" bit 38
V.21 channel 2, "0" bit 39
V.21 channel 2, "1" bit 40
CRdi 41
CRdr 42
CRe 43
ESi 44
ESr 45
MRdi 46
MRdr 47
MRe 48
CT 49
Table 3: Data and fax named events
3.12 Line Events
Table 4 summarizes the events and tones that can appear on a
subscriber line.
ITU Recommendation E.182 [13] defines when certain tones should be
used. It defines the following standard tones that are heard by the
caller:
Dial tone: The exchange is ready to receive address information.
Schulzrinne & Petrack Standards Track [Page 14]
RFC 2833 Tones May 2000
PABX internal dial tone: The PABX is ready to receive address
information.
Special dial tone: Same as dial tone, but the caller's line is
subject to a specific condition, such as call diversion or a
voice mail is available (e.g., "stutter dial tone").
Second dial tone: The network has accepted the address
information, but additional information is required.
Ring: This named signal event causes the recipient to generate an
alerting signal ("ring"). The actual tone or other indication
used to render this named event is left up to the receiver.
(This differs from the ringing tone, below, heard by the
caller
Ringing tone: The call has been placed to the callee and a calling
signal (ringing) is being transmitted to the callee. This
tone is also called "ringback".
Special ringing tone: A special service, such as call forwarding
or call waiting, is active at the called number.
Busy tone: The called telephone number is busy.
Congestion tone: Facilities necessary for the call are temporarily
unavailable.
Calling card service tone: The calling card service tone consists
of 60 ms of the sum of 941 Hz and 1477 Hz tones (DTMF '#'),
followed by 940 ms of 350 Hz and 440 Hz (U.S. dial tone),
decaying exponentially with a time constant of 200 ms.
Special information tone: The callee cannot be reached, but the
reason is neither "busy" nor "congestion". This tone should
be used before all call failure announcements, for the
benefit of automatic equipment.
Comfort tone: The call is being processed. This tone may be used
during long post-dial delays, e.g., in international
connections.
Hold tone: The caller has been placed on hold.
Record tone: The caller has been connected to an automatic
answering device and is requested to begin speaking.
Schulzrinne & Petrack Standards Track [Page 15]
RFC 2833 Tones May 2000
Caller waiting tone: The called station is busy, but has call
waiting service.
Pay tone: The caller, at a payphone, is reminded to deposit
additional coins.
Positive indication tone: The supplementary service has been
activated.
Negative indication tone: The supplementary service could not be
activated.
Off-hook warning tone: The caller has left the instrument off-hook
for an extended period of time.
The following tones can be heard by either calling or called party
during a conversation:
Call waiting tone: Another party wants to reach the subscriber.
Warning tone: The call is being recorded. This tone is not
required in all jurisdictions.
Intrusion tone: The call is being monitored, e.g., by an operator.
CPE alerting signal: A tone used to alert a device to an arriving
in-band FSK data transmission. A CPE alerting signal is a
combined 2130 and 2750 Hz tone, both with tolerances of 0.5%
and a duration of 80 to. 80 ms. The CPE alerting signal is
used with ADSI services and Call Waiting ID services [14].
The following tones are heard by operators:
Payphone recognition tone: The person making the call or being
called is using a payphone (and thus it is ill-advised to
allow collect calls to such a person).
Schulzrinne & Petrack Standards Track [Page 16]
RFC 2833 Tones May 2000
Event encoding (decimal)
_____________________________________________
Off Hook 64
On Hook 65
Dial tone 66
PABX internal dial tone 67
Special dial tone 68
Second dial tone 69
Ringing tone 70
Special ringing tone 71
Busy tone 72
Congestion tone 73
Special information tone 74
Comfort tone 75
Hold tone 76
Record tone 77
Caller waiting tone 78
Call waiting tone 79
Pay tone 80
Positive indication tone 81
Negative indication tone 82
Warning tone 83
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