Special information tone
Encyclopedia
In telephony
, a special information tone (SIT) is an international standard
three beep signal indicating a call did not go through and usually precedes a recorded announcement explaining the problem. As the SIT is well known in many countries, callers who do not speak the language of the announcement will still understand that the call has failed.
Additionally, there are eight variations of the SIT signal to indicate why the call failed (e.g. disconnected number, busy circuits, dialing error, etc). All eight SIT signals are defined below with recorded examples.
The SIT was developed because automated dialing equipment, modems, and network call detector/classifiers simply cannot decipher between a live answer or a recording, or what is said in a recorded announcement.
- Telecommunications Standardization Sector (ITU-T
), consists of a sequence of three precise tone segments with frequencies of 950 ±50 Hz
, 1400 ±50 Hz
, and 1800 ±50 Hz
, sent in that order.
Each segment is allowed a duration of 330 ±70 ms
with a silent interval of up to 30 ms
between segments. The nominal tone level is -24 dBm0 (decibels relative to 1 mW measured at the 0 dB TLP) with limits of ±1.5 dB measured with a continuous tone.
The difference in level between any two segments is required to be less than 3 dB.
The above requirements apply at the point at which tones are applied to the network.
further refined the above ITU standard to include the following variants to convey condition encountered. This enables telephone equipment to determine and log the reason a call failed.
In the Bellcore specification the first and second tone segments vary based on the condition encountered: having either a short or long duration, and either a lower or higher part of the frequency band allowed by the ITU-T
. The third tone segment may be of long or short duration but is limited to the lower frequency state. Currently, the third tone segment has been assigned both a fixed long duration and a fixed lower frequency. This fixed assignment of the third tone provides a reference or calibration point for detection devices.
The interval between the segments of SITs is between 0 and 4 ms
. To minimize the number of callers who may abandon the call without listening to the announcement, the nominal time gap between the third tone segment and the beginning of the announcement is set as close to zero as possible, with an allowed maximum of 100 ms
.
s (used in telemarketing
) respond to SITs, consumer devices such as the Telezapper
play an Intercept SIT to trick the telemarketer's equipment into flagging a called number as disconnected.
Additionally, the above recordings of SITs could be used on a voicemail
or answering machine
to achieve a similar effect.
Some telephone companies advise against playing false SITs on active lines and/or message equipment as it could cause caller confusion - especially in an emergency.
Telephony
In telecommunications, telephony encompasses the general use of equipment to provide communication over distances, specifically by connecting telephones to each other....
, a special information tone (SIT) is an international standard
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
three beep signal indicating a call did not go through and usually precedes a recorded announcement explaining the problem. As the SIT is well known in many countries, callers who do not speak the language of the announcement will still understand that the call has failed.
Additionally, there are eight variations of the SIT signal to indicate why the call failed (e.g. disconnected number, busy circuits, dialing error, etc). All eight SIT signals are defined below with recorded examples.
The SIT was developed because automated dialing equipment, modems, and network call detector/classifiers simply cannot decipher between a live answer or a recording, or what is said in a recorded announcement.
International Telecommunication Union definition
A SIT, as defined by the ITUInternational Telecommunication Union
The International Telecommunication Union is the specialized agency of the United Nations which is responsible for information and communication technologies...
- Telecommunications Standardization Sector (ITU-T
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
), consists of a sequence of three precise tone segments with frequencies of 950 ±50 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
, 1400 ±50 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
, and 1800 ±50 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
, sent in that order.
Each segment is allowed a duration of 330 ±70 ms
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
with a silent interval of up to 30 ms
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
between segments. The nominal tone level is -24 dBm0 (decibels relative to 1 mW measured at the 0 dB TLP) with limits of ±1.5 dB measured with a continuous tone.
The difference in level between any two segments is required to be less than 3 dB.
The above requirements apply at the point at which tones are applied to the network.
AT&T/Bellcore Standard SIT composition
AT&TAT&T
AT&T Inc. is an American multinational telecommunications corporation headquartered in Whitacre Tower, Dallas, Texas, United States. It is the largest provider of mobile telephony and fixed telephony in the United States, and is also a provider of broadband and subscription television services...
further refined the above ITU standard to include the following variants to convey condition encountered. This enables telephone equipment to determine and log the reason a call failed.
In the Bellcore specification the first and second tone segments vary based on the condition encountered: having either a short or long duration, and either a lower or higher part of the frequency band allowed by the ITU-T
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
. The third tone segment may be of long or short duration but is limited to the lower frequency state. Currently, the third tone segment has been assigned both a fixed long duration and a fixed lower frequency. This fixed assignment of the third tone provides a reference or calibration point for detection devices.
Segment durations
- Short duration = 276 msMillisecondA millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
- Long duration = 380 msMillisecondA millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
Frequencies for use in SITs
| First segment | Second segment | Third segment |
|---|---|---|
| (high) 985.2 Hz Hertz The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications.... |
(high) 1428.5 Hz Hertz The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications.... |
(low) 1776.7 Hz Hertz The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications.... |
| (low) 913.8 Hz Hertz The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications.... |
(low) 1370.6 Hz Hertz The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications.... |
The interval between the segments of SITs is between 0 and 4 ms
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
. To minimize the number of callers who may abandon the call without listening to the announcement, the nominal time gap between the third tone segment and the beginning of the announcement is set as close to zero as possible, with an allowed maximum of 100 ms
Millisecond
A millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
.
SIT example recordings and encoding scheme
| Name | Code | Duration | Frequency | Description | Example |
|---|---|---|---|---|---|
| Reorder – intraLATA Local access and transport area Local access and transport area is a term used in U.S. telecommunications regulation. It represents a geographical area of the United States under the terms of the that precipitated the breakup of the original AT&T into the "Baby Bells" or created since that time for wireline... |
RO' | short, long, long | low, high, low | Incomplete digits, internal office or feature failure – local office | |
| Vacant Code | VC | long, short, long | high, low, low | Unassigned N11 code N11 code An N11 code or N11 number is a special abbreviated dialing telephone number within the North American Numbering Plan, which allows access to special services. Each of these eight numbers prevents 10,000 telephone numbers from being used... , CLASS code Vertical service code A vertical service code or VSC is a special code dialed prior to a telephone number that engages some type of special telephone service... or prefix Telephone prefix A telephone prefix is the first set of digits of a telephone number; in the North American Numbering Plan countries , it is the first three digits out of a seven-digit phone number... |
|
| No Circuit – intraLATA Local access and transport area Local access and transport area is a term used in U.S. telecommunications regulation. It represents a geographical area of the United States under the terms of the that precipitated the breakup of the original AT&T into the "Baby Bells" or created since that time for wireline... |
NC' | long, long, long | high, high, low | All circuits busy – local office | |
| Intercept | IC | short, short, long | low, low, low | Number changed or disconnected | |
| Reorder – interLATA Local access and transport area Local access and transport area is a term used in U.S. telecommunications regulation. It represents a geographical area of the United States under the terms of the that precipitated the breakup of the original AT&T into the "Baby Bells" or created since that time for wireline... |
RO' ' | short, long, long | high, low, low | Call failure, no wink or partial digits received – distant office | |
| No Circuit – interLATA Local access and transport area Local access and transport area is a term used in U.S. telecommunications regulation. It represents a geographical area of the United States under the terms of the that precipitated the breakup of the original AT&T into the "Baby Bells" or created since that time for wireline... |
NC' ' | long, long, long | low, low, low | All circuits busy – distant office | |
| Ineffective/Other | IO | long, short, long | low, high, low | General misdialing, coin deposit required or other failure | |
| Future Use | – | short, short, long | high, high, low | Reserved for future use. |
Other uses
Because many predictive dialerPredictive dialer
A predictive dialer dials a list of telephone numbers and connects answered dials to people making calls, often referred to as agents. Predictive dialers use statistical algorithms to minimize the time that agents spend waiting between conversations, while minimizing the occurrence of someone...
s (used in telemarketing
Telemarketing
Telemarketing is a method of direct marketing in which a salesperson solicits prospective customers to buy products or services, either over the phone or through a subsequent face to face or Web conferencing appointment scheduled during the call.Telemarketing can also include recorded sales pitches...
) respond to SITs, consumer devices such as the Telezapper
Telezapper
The TeleZapper is a device designed to reduce the number of telemarketing-related phone calls a household receives by imitating the tone signal normally played by a phone company to indicate a line has been disconnected. The Telezapper was created by Privacy Technologies, Inc, a wholly owned...
play an Intercept SIT to trick the telemarketer's equipment into flagging a called number as disconnected.
Additionally, the above recordings of SITs could be used on a voicemail
Voicemail
Voicemail is a computer based system that allows users and subscribers to exchange personal voice messages; to select and deliver voice information; and to process transactions relating to individuals, organizations, products and services, using an ordinary telephone...
or answering machine
Answering machine
The answering machine or message machine, also known as the telephone answering machine in the UK and some Commonwealth countries) and previously known as an ansaphone, ansafone, or telephone answering device is a device for answering telephones and recording callers' messages.Unlike voicemail,...
to achieve a similar effect.
Some telephone companies advise against playing false SITs on active lines and/or message equipment as it could cause caller confusion - especially in an emergency.
External links
- World PSTN Tone Database - Special information tones
- Call Progress Tones (Cisco SystemsCisco SystemsCisco Systems, Inc. is an American multinational corporation headquartered in San Jose, California, United States, that designs and sells consumer electronics, networking, voice, and communications technology and services. Cisco has more than 70,000 employees and annual revenue of US$...
) - artofhacking.com's database of SIT tones for the US and other countries