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Article by Steve Nutt

If you have a monitored intruder alarm system over a British Telecom line then you should know the consequences of sending signals over the new 21CN network. 21CN is not analogue like your existing BT landline, it is digital and uses an Internet Protocol (IP) as its base protocol. On top of that, it uses the Session Initiation Protocol (SIP) which is the same protocol used by most Voice over IP (VoIP) network providers. It therefore stands to reason that the 21CN network will suffer many of the same issues as alarm systems have suffered for many years trying to communicate over networks of 3rd party VoIP providers.

Sadly, many Alarm Companies are holding back to find out just how much their installed systems will be affected by the 21CN upgrade. As BT explains in their 21CN Trials document, the problem is not only with their new network. Carrier Pre-Select and local loop unbundling means that an increasing number of core networks from other Telco’s that are used for sending alarm signals will use VoIP – often without the knowledge of the Alarm Company or the Customer.

Time has proven that alarm panels and digital communicators are not able to transmit their signals reliably over a VoIP network. 21CN may allow consistent results in the early stages, but will start to suffer more as time goes on and as an increasing number of users put the network under load. As a large number of alarm system users already have an Internet connection available to them, it makes sense to tackle the issue now rather than wait for expensive failures in communication.

One problem is that BT will not readily acknowledge the fact that there are likely to be many issues in the area of alarm monitoring, however, other BT documents seem to contradict that view and it has led to a lot of confusion within the security alarm industry.

In 2007 BT told us there would be no problems with alarm panels signalling over the 21CN network. In 2008 we were told “The systems will be ‘groomed out’ of the 21CN Cardiff trial as a result of work undertaken by the British Security Industry Association.”

In April 2009, the BSIA published a BT 21CN test plan. The test plan has been put together after nearly 2 years of development by the BSIA and its members – including extensive testing of alarm systems on the 21st Century Network at BT’s facilities in Adastral Park and Swansea. The primary causes for concern were:

Echo cancellation is always used unless a high-speed fax or data modems are detected

Audio path discontinuities will occur when jitter buffer lengths change, either during a voice call or when modems or faxes are detected

Potential increase in round trip delay

The test plan goes on to say “Please note: Any results obtained from testing will not guarantee that customer premises equipment will work on BT’s 21CN network”.

So, with information like that from British Telecom and your Alarm Company with their head in the sand, you would be well advised to do your own research and search for Alarm Companies and Monitoring Centres that take the 21CN issue a little more seriously.

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