153:
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. 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.
1123:
875:
795:
729:
663:
594:
525:
459:
390:
313:
80:
automated equipment; indicating not only a failed call, but also the specific reason for the failure (e.g., disconnected number, busy circuits, dialing error, etc.). The equipment can then make an intelligent choice about what to do next. If the circuits were busy, then calling again later makes sense; if the number was disconnected, then calling again is futile.
168:. 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.
152:
SIT follows the above ITU standard but uses more tightly controlled signals within the ITU limits to signal the condition encountered. 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
79:
signal intended both to be heard by the caller, and to be detected by automated dialing equipment to determine a call has failed. In North
America, the AT&T/Bellcore SIT standard allows the frequency and duration of the tones to vary slightly - making eight distinct messages specifically for
248:. 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
59:
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
681:
612:
813:
477:
331:
679:
610:
747:
543:
408:
475:
329:
811:
745:
541:
406:
680:
611:
812:
476:
330:
746:
542:
407:
987:
134:. 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.
976:, E: Overall Network Operation, Telephone Service, Service Operation and Human Factors / Q: Switching and Signalling, ITU-T Rec. E.180/Q.35
17:
923:
100:
1089:
1019:
914:
639:
570:
435:
289:
164:: having either a short or long duration, and either a lower or higher part of the frequency band allowed by the
1028:
906:
1146:
1094:
672:
603:
1109:
1104:
88:
1099:
1012:
375:
8:
804:
468:
322:
91:
allows call progress advisories to be played/displayed to callers in their own language.
931:
850:
49:
1127:
879:
107:), consists of a sequence of three precise tone segments with frequencies of 950 ±50
53:
52:
consisting of three rising tones indicating a call has failed. It usually precedes a
950:
BOC Notes on the LEC Networks - 1994 Signalling - SR-TSV-002275 Issue 2, April, 1994
1005:
927:
379:
137:
The difference in level between any two segments is required to be less than 3 dB.
1064:
997:
738:
534:
399:
970:
1140:
1074:
991:
1059:
1049:
131:
1054:
971:
249:
245:
184:
178:
140:
These requirements apply at the point the tones are sent on the network.
127:
123:
72:
1084:
866:
832:
762:
696:
627:
558:
492:
423:
346:
1039:
969:
959:
860:
855:
68:
61:
31:
304:
Incomplete digits, internal office or feature failure – local office
1079:
1044:
371:
157:
94:
585:
Call failure, no wink or partial digits received – distant office
149:
84:
76:
43:
27:
Three-beep signal indicating a call did not go through correctly
244:
The interval between the segments of SITs is between 0 and 4
236:
230:
222:
216:
210:
165:
116:
112:
108:
104:
46:
973:
Technical characteristics of tones for the telephone service
720:
General misdialing, coin deposit required or other failure
255:
143:
130:
between segments. There is a silence period of 1000 ±250
262:
The recordings below play each SIT sequence two times.
1027:
160:specification the first and second tone segments
1138:
95:International Telecommunication Union definition
907:"Special-Information-Tone Frequency Detection"
122:Each segment is allowed a duration of 330 ±70
1013:
863:, the voice on most U.S. intercept recordings
103:- Telecommunications Standardization Sector (
960:International Telecommunication Union (1998)
1020:
1006:
946:
944:
942:
940:
924:American Telephone and Telegraph Company
937:
162:vary based on the condition encountered
14:
1139:
900:
898:
896:
894:
256:Example recordings and encoding scheme
144:AT&T/Bellcore standard composition
1001:
904:
171:
891:
126:with a silent interval of up to 30
24:
932:10.1002/j.1538-7305.1981.tb00267.x
793:
727:
661:
654:All circuits busy – distant office
592:
523:
457:
388:
311:
25:
1158:
981:
915:The Bell System Technical Journal
64:or some other intended function.
1121:
873:
831:Problems playing this file? See
809:
761:Problems playing this file? See
743:
695:Problems playing this file? See
677:
626:Problems playing this file? See
608:
557:Problems playing this file? See
539:
491:Problems playing this file? See
473:
450:All circuits busy – local office
422:Problems playing this file? See
404:
345:Problems playing this file? See
327:
1029:Signaling (telecommunications)
953:
516:Number changed or disconnected
190:
13:
1:
884:
7:
844:
687:No Circuit – interLATA (2x)
483:No Circuit – intraLATA (2x)
10:
1163:
1118:
1095:Multi-frequency signaling
1090:Dual-tone multi-frequency
1035:
220:
99:A SIT, as defined by the
83:As an alternative to the
18:Special information tones
1070:Special information tone
786:Reserved for future use.
618:Reorder – interLATA (2x)
337:Reorder – intraLATA (2x)
87:SIT tone and recording,
56:describing the problem.
36:special information tone
1110:Signalling System No. 7
1105:Signalling System No. 6
1100:Signaling System No. 5
905:Feuer, A. (Sep 1981),
798:
753:Ineffective/Other (2x)
732:
666:
597:
528:
462:
393:
316:
119:, sent in that order.
47:international standard
1128:Telephones portal
880:Telephones portal
797:
731:
665:
596:
527:
461:
392:
315:
177:Short duration = 276
54:recorded announcement
183:Long duration = 380
988:Call Progress Tones
779:short, short, long
509:short, short, long
851:Call-progress tone
799:
733:
713:long, short, long
707:Ineffective/Other
667:
638:No Circuit – inter
598:
578:short, long, long
529:
463:
434:No Circuit – intra
394:
363:long, short, long
317:
297:short, long, long
50:call progress tone
1147:Telephony signals
1134:
1133:
842:
841:
814:
748:
682:
647:long, long, long
613:
544:
478:
443:long, long, long
409:
332:
242:
241:
172:Segment durations
16:(Redirected from
1154:
1126:
1125:
1124:
1022:
1015:
1008:
999:
998:
977:
962:
957:
951:
948:
935:
934:
911:
902:
878:
877:
876:
816:
815:
796:
782:high, high, low
750:
749:
730:
684:
683:
664:
615:
614:
595:
546:
545:
526:
480:
479:
460:
446:high, high, low
414:Vacant Code (2x)
411:
410:
391:
334:
333:
314:
265:
264:
195:
194:
75:, the SIT is an
21:
1162:
1161:
1157:
1156:
1155:
1153:
1152:
1151:
1137:
1136:
1135:
1130:
1122:
1120:
1114:
1065:Disconnect tone
1031:
1026:
984:
966:
965:
958:
954:
949:
938:
909:
903:
892:
887:
874:
872:
847:
838:
837:
829:
827:
826:
825:
824:
817:
810:
807:
800:
794:
768:
767:
759:
757:
756:
755:
754:
751:
744:
741:
734:
728:
716:low, high, low
702:
701:
693:
691:
690:
689:
688:
685:
678:
675:
668:
662:
633:
632:
624:
622:
621:
620:
619:
616:
609:
606:
599:
593:
581:high, low, low
569:Reorder – inter
564:
563:
555:
553:
552:
551:
550:
547:
540:
537:
530:
524:
498:
497:
489:
487:
486:
485:
484:
481:
474:
471:
464:
458:
429:
428:
420:
418:
417:
416:
415:
412:
405:
402:
395:
389:
366:high, low, low
352:
351:
343:
341:
340:
339:
338:
335:
328:
325:
318:
312:
300:low, high, low
288:Reorder – intra
258:
201:Second segment
193:
174:
146:
115:, and 1800 ±50
97:
28:
23:
22:
15:
12:
11:
5:
1160:
1150:
1149:
1132:
1131:
1119:
1116:
1115:
1113:
1112:
1107:
1102:
1097:
1092:
1087:
1082:
1077:
1072:
1067:
1062:
1057:
1052:
1047:
1042:
1036:
1033:
1032:
1025:
1024:
1017:
1010:
1002:
996:
995:
983:
982:External links
980:
979:
978:
964:
963:
952:
936:
889:
888:
886:
883:
870:
869:
864:
858:
853:
846:
843:
840:
839:
828:
818:
808:
803:
802:
801:
792:
791:
790:
788:
783:
780:
777:
774:
770:
769:
758:
752:
742:
737:
736:
735:
726:
725:
724:
722:
717:
714:
711:
708:
704:
703:
692:
686:
676:
671:
670:
669:
660:
659:
658:
656:
651:
650:low, low, low
648:
645:
642:
635:
634:
623:
617:
607:
602:
601:
600:
591:
590:
589:
587:
582:
579:
576:
573:
566:
565:
554:
549:Intercept (2x)
548:
538:
533:
532:
531:
522:
521:
520:
518:
513:
512:low, low, low
510:
507:
504:
500:
499:
488:
482:
472:
467:
466:
465:
456:
455:
454:
452:
447:
444:
441:
438:
431:
430:
419:
413:
403:
398:
397:
396:
387:
386:
385:
383:
367:
364:
361:
358:
354:
353:
342:
336:
326:
321:
320:
319:
310:
309:
308:
306:
301:
298:
295:
292:
285:
284:
281:
278:
275:
272:
269:
257:
254:
240:
239:
233:
226:
225:
219:
215:(high) 1428.5
213:
206:
205:
204:Third segment
202:
199:
198:First segment
192:
189:
188:
187:
181:
173:
170:
145:
142:
96:
93:
26:
9:
6:
4:
3:
2:
1159:
1148:
1145:
1144:
1142:
1129:
1117:
1111:
1108:
1106:
1103:
1101:
1098:
1096:
1093:
1091:
1088:
1086:
1083:
1081:
1078:
1076:
1075:Off-hook tone
1073:
1071:
1068:
1066:
1063:
1061:
1058:
1056:
1053:
1051:
1048:
1046:
1043:
1041:
1038:
1037:
1034:
1030:
1023:
1018:
1016:
1011:
1009:
1004:
1003:
1000:
993:
992:Cisco Systems
989:
986:
985:
975:
974:
968:
967:
961:
956:
947:
945:
943:
941:
933:
929:
926:: 1289–1312,
925:
921:
917:
916:
908:
901:
899:
897:
895:
890:
882:
881:
868:
865:
862:
859:
857:
854:
852:
849:
848:
836:
834:
822:
806:
789:
787:
784:
781:
778:
775:
772:
771:
766:
764:
740:
723:
721:
718:
715:
712:
709:
706:
705:
700:
698:
674:
657:
655:
652:
649:
646:
643:
641:
637:
636:
631:
629:
605:
588:
586:
583:
580:
577:
574:
572:
568:
567:
562:
560:
536:
519:
517:
514:
511:
508:
505:
502:
501:
496:
494:
470:
453:
451:
448:
445:
442:
439:
437:
433:
432:
427:
425:
401:
384:
382:
381:
377:
373:
368:
365:
362:
359:
356:
355:
350:
348:
324:
307:
305:
302:
299:
296:
293:
291:
287:
286:
282:
279:
276:
273:
270:
267:
266:
263:
260:
253:
251:
247:
238:
235:(low) 1370.6
234:
232:
228:
227:
224:
221:(low) 1776.7
218:
214:
212:
209:(high) 985.2
208:
207:
203:
200:
197:
196:
186:
182:
180:
176:
175:
169:
167:
163:
159:
154:
151:
141:
138:
135:
133:
129:
125:
120:
118:
114:
110:
106:
102:
92:
90:
86:
81:
78:
74:
70:
65:
63:
57:
55:
51:
48:
45:
41:
37:
33:
19:
1069:
1060:Reorder tone
1050:Ringing tone
972:
955:
919:
913:
871:
830:
820:
819:Future Use (
785:
760:
719:
694:
653:
625:
584:
556:
515:
490:
449:
421:
369:
357:Vacant Code
344:
303:
280:Description
261:
259:
243:
229:(low) 913.8
161:
155:
147:
139:
136:
121:
98:
82:
66:
58:
39:
35:
29:
1055:Busy signal
773:Future Use
370:Unassigned
191:Frequencies
111:, 1400 ±50
73:busy signal
885:References
867:TeleZapper
833:media help
805:Future SIT
763:media help
697:media help
628:media help
559:media help
503:Intercept
493:media help
424:media help
376:CLASS code
347:media help
277:Frequency
1040:Dial tone
861:Pat Fleet
856:Telephone
673:NC' ' SIT
604:RO' ' SIT
274:Duration
69:dial tone
62:voicemail
32:telephony
1141:Category
1080:Zip tone
1045:Ringtone
845:See also
821:Not Used
372:N11 code
283:Example
158:Bellcore
150:AT&T
42:) is an
1085:2600 Hz
469:NC' SIT
323:RO' SIT
156:In the
85:in-band
77:in-band
67:Like a
44:in-band
823:) (2x)
739:IO SIT
644:NC' '
575:RO' '
535:IC SIT
400:VC SIT
380:prefix
922:(7),
910:(PDF)
271:Code
268:Name
166:ITU-T
105:ITU-T
640:LATA
571:LATA
440:NC'
436:LATA
294:RO'
290:LATA
148:The
34:, a
928:doi
710:IO
506:IC
378:or
360:VC
101:ITU
89:SS7
71:or
40:SIT
30:In
1143::
939:^
920:60
918:,
912:,
893:^
776:–
374:,
252:.
250:ms
246:ms
237:Hz
231:Hz
223:Hz
217:Hz
211:Hz
185:ms
179:ms
132:ms
128:ms
124:ms
117:Hz
113:Hz
109:Hz
1021:e
1014:t
1007:v
994:)
990:(
930::
835:.
765:.
699:.
630:.
561:.
495:.
426:.
349:.
38:(
20:)
Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.