Special information tones

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In telephony, special information tones are a set of eight, similar sounding ITU-T audible signals and are made up of three audio tones. Each SIT tone indicates that a telephone call cannot be completed and is typically followed by a recorded announcement explaining the reason for the call failure.

In addition to indicating a general call failure to a caller, because automated dialing equipment, modems, and network call detector/classifiers cannot determine the content of a recording, each of the eight SIT's permit a way for such equipment to automatically classify why a call failed (e.g. number disconnected, circuits busy, mis-dialing,etc). Recorded examples of all SIT signals are below.

Contents

[edit] International Telecommunication Union definition. [1]

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.

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 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.

[edit] SIT composition[1]

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.

[edit] Segment durations

  • Short Duration = 276 ms
  • Long Duration = 380 ms

[edit] Frequencies for use in SITs

First Segment Second Segment Third Segment
(low) 913.8 Hz (low) 1370.6 Hz (low) 1776.7 Hz
(high) 985.2 Hz (high) 1428.5 Hz

The interval between the segments of SITs is between 0 and 4 ms. To minimize the 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.

[edit] SIT example recordings and encoding scheme[1]

Name Code Duration Frequency Description Example
Reorder - intraLATA RO' Short, Long, Long low, high, low Incomplete digits, internal office or feature failure - local office

RO' SIT

Reorder - intraLATA (2x)
Problems listening to the file? See media help.
Vacant Code VC Long, Short, Long high, low, low Unassigned N11 code, CLASS code or prefix

VC SIT

Vacant Code (2x)
Problems listening to the file? See media help.
No Circuit - intraLATA NC' Long, Long, Long high, high, low All circuits busy - local office

NC' SIT

No Circuit - intraLATA (2x)
Problems listening to the file? See media help.
Intercept IC Short, Short, Long low, low, low Number changed or disconnected

IC SIT

Intercept (2x)
Problems listening to the file? See media help.
Reorder - interLATA RO' ' Short, Long, Long high, low, low Call failure, no wink or partial digits received - distant office

RO' ' SIT

Reorder - interLATA (2x)
Problems listening to the file? See media help.
No Circuit - interLATA NC' ' Long, Long, Long low, low, low All circuits busy - distant office

NC' ' SIT

No Circuit - interLATA (2x)
Problems listening to the file? See media help.
Ineffective/Other IO Long, Short, Long low, high, low General misdialing, coin deposit required or other failure

IO SIT

Ineffective/Other (2x)
Problems listening to the file? See media help.
Future Use -- Short, Short, Long high, high, low Reserved for future use.

Future SIT

Future Use (Not Used) (2x)
Problems listening to the file? See media help.

[edit] Other uses

Because many predictive dialers (used in telemarketing) respond to SITs, consumer devices such as the Telezapper have been developed which play an Intercept SIT in order to trick the telemarketer's equipment into flagging a called number as disconnected. (See Telezapper for more information.)

Additionally, the above recordings of SITS could be used on a voicemail or answering machine to achieve a similar effect.

Note that 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.

[edit] References

  1. ^ a b c BOC Notes on the LEC Networks - 1994 Signalling - SR-TSV-002275 Issue 2, April, 1994

[edit] External links

[edit] See also

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Telephony signals

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