Multi-frequency signaling
Intelephony,multi-frequency signaling(MF) is a type ofsignalingthat was introduced by theBell SystemafterWorld War II.It uses a combination of audible tones for address (telephone number) transport and supervision signaling ontrunk linesbetween central offices. The signaling is sentin-bandover the same channel as the bearer channel used for voice traffic.
Multi-frequency signaling defines electronic signals that consist of a combination of two audible frequencies, usually selected from a set of six frequencies. Over several decades, various types of MF signaling were developed, including national and international varieties. TheCCITTstandardization process specified the American Bell System version asRegional Standard No. 1,orSignalling System R1,[1]and a corresponding European standard asSignalling System R2.Both were largely replaced by digital systems, such asSignalling System 7,which operate out-of-band on a separate data network.
Because of the in-band transmission characteristic of MF signaling, the systems proved vulnerable to misuse and fraud byphone phreakingwith devices such as ablue box.
Multifrequency signaling is a technological precursor ofdual-tone multi-frequency signaling(DTMF,Touch-Tone), which uses the same fundamental principle, but was used primarily for signaling address information and control signals from a user's telephone to the wire-center's Class-5 switch. DTMF uses a total of eight frequencies.
Operation
[edit]
Digits are represented by two simultaneous tones selected from a set of five (MF 2/5), six (MF 2/6), or eight (MF 2/8) frequencies. The frequency combinations are played, one at a time for each digit, to the remotemulti-frequency receiverin a distanttelephone exchange.MF is used for signaling intrunkingapplications.
Using MF signaling, the originating telephone switch sends a start signal to seize the line, taking the circuit off-hook. The terminating office acknowledges the seizure with a ready state by responding with a wink start signal, which is a momentary off-hook condition. The originating office then sends address information to the terminating switch. In R1 MF signaling this address information normally is a KP tone, the numeric digits of the destination number, and an ST tone to indicate the end of the address. Other information may also be added, such as the caller's number, using KP2 as a delimiter.
MF is a type ofin-band signaling.Depending on the type and configuration of switching equipment, it may or may not be audible to the telephone user, but the technology was vulnerable to abuse with a method calledphreakingwith ablue boxwhich generates the tones required to control remote telephone switches.
Multi-frequency signals
[edit]| Code | 700 Hz | 900 Hz | 1100 Hz | 1300 Hz | 1500 Hz | 1700 Hz |
|---|---|---|---|---|---|---|
| 1 | X | X | ||||
| 2 | X | X | ||||
| 3 | X | X | ||||
| 4 | X | X | ||||
| 5 | X | X | ||||
| 6 | X | X | ||||
| 7 | X | X | ||||
| 8 | X | X | ||||
| 9 | X | X | ||||
| 0/10 | X | X | ||||
| 11/ST3 | X | X | ||||
| 12/ST2 | X | X | ||||
| KP | X | X | ||||
| KP2 | X | X | ||||
| ST | X | X |
TheBell Systempublished the following standards for MF tone timing:[citation needed]
- KP (110 ms)
- Digits 0-9, ST: 55 ms
The interval between digits is the same as the digit duration (55 ms)
These standards are, for the most part, still in place where MF signaling is in use in legacy exchanges. MF signaling is still used in North America for inter-office signaling, although it is increasingly rare.[citation needed]
Demise
[edit]In-band signaling fell into disfavor in thepublic switched telephone network(PSTN) as electronic switching systems displaced electro-mechanical switching systems, but legacy offices may still exist in some countries that are still using some electromechanical and other legacy switching equipment.[2]
Out-of-bandCommon Channel Signaling(CCS) became nearly universal by the end of the 20th century in theUnited States.Benefits include higher connection establishment rate and better fraud security.
Most 911Public Safety Answering Points(PSAPs) use the MF format to identify the calling party to the PSAP when processing calls fromMobile Telephone Switching Offices(MTSOs) and landline telephone exchanges.[3]This is based on an earlier system which used MF to identify the calling party to afeature group'D' (101xxxx) alternate long-distance provider.
See also
[edit]References
[edit]- ^Pearce, J. Gordon (2013).Telecommunications Switching.Springer. p. 243.
- ^"In-Band Signaling in the former Soviet Union".Binary Revolution Forums.Archived from the original on 2021-05-16.Retrieved2019-08-08.
{{cite web}}:CS1 maint: bot: original URL status unknown (link) - ^"NENA 03-002 v2: Enhanced MF Signaling, E9-1-1 Tandem to PSAP".
External links
[edit]- "Speeding Speech",a 1950sBell Systemfilm, depicts a 2-1-1 long-distance operator manually entering a number on an MF keypad just prior to the introduction ofdirect distance dialing.The keypad, visible at 0:01:41 and 0:05:20, has two columns of five digits plus KP (key pulse) and ST (start).
- [1]Details of MF signaling from Bell Laboratories Record. Pages 221-225
