Posted on 24 Jun 2020
Introduced in 1963 ‘Dual Tone Multi Frequency’ (DTMF) signalling has been a mainstay of telecommunications networks all over the world. Often referred to by the original Bell System commercial name ‘touch tone’, every time you “Push 1 for Sales…” on a telephone keypad you are using DTMF signalling.
Most lift emergency telephones installed today, use DTMF signalling to transfer operator signals (playing location messages, muting in car microphones etc.) and, in the case of telephones conforming to EN81-28, transfer test information as part of the 3-day test call procedure.
As telecoms networks evolve to meet the latest demands of consumers and businesses the level of support for DTMF signalling (in fact all analogue signalling) is changing and sadly not for the better.
DTMF signally is an analogue signalling system which consists of 16 audible tones generated by the 0-9, ‘#’, and ‘*’ keys on a standard telephone keypad and A, B, C, D keys typically only found on telephone engineer testing equipment.
DTMF tones are transmitted, over the voice channel of the telephone network, between two devices to transfer information. Using the example above, pressing the ‘1’ key generates a specific tone telling the automated switchboard that you want to be connected to the sales department.
In addition to the consumer telecoms application, DTMF is used in machine to machine (M2M) communication. A device generates a stream of DTMF tones to transfer information over the telephone network to a receiver which then decodes the tones into readable data. It’s this M2M application that includes the elevator industries operator instructions and 3-day test call information.
Any disruption to the sequence of the tones, or distortion to the tones themselves, will result in the information being unintelligible at the receiving end. Because DTMF tones are an analogue audio signal, any interference on the voice channel will distort the tones.
As new technologies have emerged, analogue signalling has become less and less popular as a means of transferring data. In turn telecoms networks have begun to provide less support for these legacy signalling types, including DTMF.
At a national level, Public Switch Telephone Networks (PSTN, aka “landlines”) across the globe have begun to set dates, typically 2025 onwards, for the end of DTMF support on their networks. This is in conjunction with the rollout of fibre optic telephone lines more suited to carrying data. At that point emergency telephones trying to place three-day background calls over the PSTN, will be unable to communicate using DTMF and therefore will not be able to confirm they are in working order (resulting in “failed” test calls).
When mobile networks were introduced in the 1990s support for DTMF was carried over from the PSTN. However Mobile Network Operators (MNO) use compression technology to transfer tones through the voice channel. This compression can then result in the tones being distorted either by affecting the timing of the tones or “clipping” the upper or lower frequencies. Each generation of mobile technology from 2G onwards has provided less and less support for the high quality DTMF needed for M2M communication.
The latest generation of emergency telephones, such as Avire’s Digital Communications Platform (DCP), use the data channel of the mobile network to transfer test information and therefore aren’t reliant on DTMF support.
However, work will need to be done in order to support existing units installed in the field.
When PSTNs remove support for DTMF an ‘analogue telephone adaptor’ (ATA) will need to be installed to translate the DTMF tones from the telephone into data packets that can be transferred over the new fibre telephone lines. ATAs need to be provided by the network operator, can be costly and often require significant support for correct configuration. This is because each PSTN will implement their data handling slightly differently.
A solution for legacy telephones connected to either the PSTN or mobile networks (GSM) is to switch to a GSM gateway which can translate the DTMF tones from the telephone into data packets and then transfer these over the mobile network. This is much easier as MNOs (?) all handle their data channel in a similar way so the GSM equipment can use standard data protocols. An additional benefit to this approach is the fact that SIM card packages are significantly cheaper than landline rental charges from PSTNs.
Avire’s DCP provides both a cellular gateway (GSM/GPRS or 3G options and soon 4G) and a digital emergency telephone, this system uses data packets as opposed to DTMF to transfer information. When used with Avire’s Digital Audio Units (DAU) the DCP forms “the brain” of a fully digital emergency communications system.
Alternatively the DCP can be paired with an older generation of telephone such as Memcom, C100, or other analogue emergency telephones (contact our sales department for specific models supported), to convert DTMF into data packages, thus prolonging the life of these units past the point at which DTMF support ends or DTMF becomes too unreliable.
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