Special information tone
In
Because the SIT is well known in many countries, callers can understand that their call has failed, even though they do not understand the language of the recorded announcement (e.g., when calling internationally) instead of assuming the recording is voicemail or some other intended function.
Like a
As an alternative to the
International Telecommunication Union definition
A SIT, as defined by the ITU - 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.[2][3]
Each segment is allowed a duration of 330 ±70
The difference in level between any two segments is required to be less than 3 dB.
These requirements apply at the point the tones are sent on the network.
AT&T/Bellcore standard composition
The AT&T SIT follows the above ITU standard but uses more tightly controlled signals within the ITU limits to signal the condition encountered.[1] By using slight variations of tone frequencies and durations, the AT&T SIT conveys to automated equipment why the call failed. The ITU specification requires the frequency to be within ±50 Hz of a nominal value. AT&T deliberately transmits a frequency that varies from the ITU nominal; it is about 35 Hz high to signal one set of conditions or about 35 Hz low to signal another set. Similarly, AT&T also modulates the duration of the tone within the ITU limits of 330±70 ms.[1] Using this scheme, AT&T could encode six bits of information, representing 64 different combinations; only four bits are actually used, encoding eight different conditions, with eight further combinations currently left unused.
In the
Segment durations
Frequencies
First segment | Second segment | Third segment |
---|---|---|
(high) 985.2 Hz | (high) 1428.5 Hz | (low) 1776.7 Hz |
(low) 913.8 Hz | (low) 1370.6 Hz |
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.
Example recordings and encoding scheme
The recordings below play each SIT sequence two times.
Name | Code | Duration | Frequency | Description | Example |
---|---|---|---|---|---|
Reorder – intraLATA | 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, CLASS code or prefix | |
No Circuit – intraLATA | 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 | RO' ' | short, long, long | high, low, low | Call failure, no wink or partial digits received – distant office | |
No Circuit – interLATA | 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. |
See also
- Call-progress tone
- Telephone
- Pat Fleet, the voice on most U.S. intercept recordings
- TeleZapper
References
- International Telecommunication Union (March 1998), "Special information tone", Technical characteristics of tones for the telephone service, E: Overall Network Operation, Telephone Service, Service Operation and Human Factors / Q: Switching and Signalling, ITU-T Rec. E.180/Q.35
External links
- Call Progress Tones (Cisco Systems)