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Technical bulletin: Telecommunications networks & DTMF protocols

Posted: 8/15/2012 7:02:21 AM

We have received reports since 31st July 2012 of telecare service providers experiencing problems in relation to the transmission of social alarm protocols. This was reinforced by recent TSA statements which cite difficulties that relate to telecommunications networks issues. Our technical team has investigated these issues in some detail across a range of products and communication protocols, by examining the data and recordings associated with alarm transmissions. Given the nature of the problem reports we have also been in contact with telecommunications network providers, the NICC (Network Interoperability Consultative Committee) and other technology providers.

Our summary understanding of the issue is that network components responsible for the digital coding and decoding of voice band signals have been the subject of changes. Such changes or upgrades may arise for example where telecommunications providers seek to optimise the performance and handle increasing data traffic on their networks. The relevant aspect of the most recent changes is that they have the effect of intercepting DTMF signalling tones previously carried wholly in the audible ‘voice band’, and regenerating these tones ‘out of band’. The normal behaviour of a typical social alarm transmission is shown in figure 1. Here, audible DTMF tones are switched on and off, usually with 80millisecond time periods, to enable exchange of information between the alarm unit and the alarm receiving centre (ARC). These are the audible waveforms that can be recorded at either end of the alarm communication link and analysed.

Traditional ‘circuit-switched’ telephone networks (analogue and digital) have provided high reliability for such signalling methods for many years. However, networks now incorporate “next-generation” components which employ ‘packet-switching’ (IP packets), and can exhibit different behaviour. The most recent network changes have resulted in a more intrusive characteristic. Here, alarm units have continued to create the DTMF sequence as shown in figure 1, but what is received at the ARC shows the markedly changed pattern of figure 2. The interpretation of this waveform is that the network components are detecting the DTMF tones being sent after approximately 50msec. Then, rather than allowing continued transmission in their traditional form, they are interrupted and re-generated by the network. The net result is that the ARC receives a lengthened and somewhat distorted set of DTMF tones. It is apparent that the overall cycle of a tone and its associated ‘gap’ is retained at 160msec. For the ARC receiving these DTMF transmissions this has the effect of reducing the gap between tones to 20-30msecs. The ARC equipment is evidently unable to process this distorted signalling pattern reliably and can result in failed receipt of alarms.

In terms of equipment affected, we have observed this behaviour across multiple suppliers and types of ARC and end-user equipment that use standard DTMF signalling formats, and across multiple protocol types, including BS8521 and TT92.

This effect has not been observed at all on STMF protocols. This latter point is not surprising, since STMF was originally designed to avoid similar interception and corruption of DTMF signalling that can be caused by analogue-to-digital (Voice over Internet Protocol) telephone adapters in the home.

We are continuing to work with network providers, and we have already seen benefits where network changes have been ‘rolled-back’ to alleviate the problems encountered in recent days. However, the interconnections between networks from different providers, along with the reported variabilities by geography are proving to be complex, and technology and service providers therefore need to remain vigilant. We are continuing to support service providers on a case by case basis, initially to assist with resolutions to network issues, and where necessary to provide workarounds within the alarm systems. It is therefore recommended that service providers follow best practice through: regular test calls; raising any issues immediately with their telecommunications providers; reporting the issues to the relevant alarm system providers. We will continue to work with NICC and network providers to provide a more sustainable environment for DTMF signalling.

Meanwhile we are recommending that wherever possible our alarm systems are configured to communicate using STMF-signalling, to provide added resilience in the presence of the failure mode described above.

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Tunstall Healthcare (UK) Ltd Head Office
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