Voice Over Internet (VoIP) and Burglar Alarms

Voice over internet systems (VoIP) are designed to carry voice signals, and are typically not optimised for carrying fax, dial-up modem, and alarm reporting signals.  It's not usually recommended that VoIP phone systems be used for alarm monitoring due to problems with reliability.  If you've got Naked DSL and don't have a normal POTS analogue phone line but still want your alarm monitored, don't disspare, there are ways around the problem.

The best solution of it use an alarm capable of reporting events directly over the internet (IP reporting).  If you have an older alarm that doesn't have this feature, you can use adaptors that convert the phone signals to internet signals.  Or, if you already have a VoIP analogue phone adaptor (ATA) with a spare line, you can connect it directly to Monitoring Plus.

VoIP Basics

A traditional analogue phone systems sends unprocessed voice signals from a telephone to the exchange.  An old style exchange (which don't really exist any more) would directly connect the phone line of the caller to the phone line of the person receiving the call, allowing the unprocessed analogue signals to flow from one end to the other.  This system has a number of drawbacks, and modern phone exchanges convert the analogue signals into a digital format as they enter the exchange.  The use of digital technology allows for much more efficient switching of calls, especially over long distances, as the call can now be routed over a digital network.  Of course, before being passed to the receiving phone line, the digital signals must be converted back to analogue.

VoIP takes this digital approach one step further.  Instead of doing the analogue to digital conversion at the exchange, it is done at the phone.  This way, the call can be routed digitally right from one phone to another, over the internet.  Purely digital VoIP phones do all this in one package, but normal analogue phones can also be used with the addition of an Analogue Telephone Adaptor (ATA).

VoIP and burglar alarms

A typical VoIP set-up does not directly transmit the digitised signals, but instead will use data compression to reduce the bandwidth and data speed, to avoid taxing the internet connection.  This compression is design to work specifically with human speech, but does not work very well with other types of signals, such as DTMF tones (the tones used for dialling phone numbers), faxes, and dial-up modems.  When these non-voice signals are compressed they get badly distorted and are are not transmitted correctly.  Because the DTMF dialling tones are use quite often during voice calls, on automated menus for example, the problem is overcome by detecting these tones in the ATA and sending them separately as special events.

Burglar alarm systems typically report events to central monitoring stations by placing phone calls using your normal phone line.  (Note than some modern systems have options for internet TCP/IP based reporting.)  To communicate with the monitoring station, the alarm uses a series of beeps and DTMF tones.  The DTMF tones are normally handled by the ATA without problems, but other non-DTMF tones are also used for handshaking (controlling the flow of data) are not recognised by the ATA and are sent as though they are speech.  Typically the alarm is not able to communicate with the monitoring station because the handshake tones are distorted too much by the compression.

Fortunately, it is often possible to choose what the compression is used, indeed it can be disables altogether.  The compression is done by a piece of software called a Codec (short for Coder-Decoder).  Popular compression codecs have names like G.711, G.723, G.729, GSM, ILBC.  The interesting one here is G.711, also called A-law and µ-law (pronounced mu-law), which actually do not compress the signal.  They have the disadvantage of having a high bit-rate needing a fast connection, but the advantage of very high quality (if the network can keep up).  For non-voice uses over VoIP, it's important to use the G.711 codec.

However, choosing a non-compressing codec between your ATA and your VoIP provider doesn't guarantee that this codec will be used throughout the system.  Your provided may choose to re-encode (called transcode) your data using a different codec, compromising the quality of the connection for non-voice signals.

What's worse, even with the right settings, everything doesn't always go according to plan.  When the alarm is communicating with the monitoring station, it's expecting all the right tones at the right time.  The internet however, is not renowned for delivering data in a timely fashion.  If you are surfing the web and you have to wait a tenth of a second longer for your web page to arrive, you'll probably not too concerned.  But, a tenth of a second gap in a handshake tone can make all the difference to an alarm.  This means you need to make sure you use a high quality VoIP provider and a good internet connection.

Also, some alarms are simply not very compatible with some ATAs.  Because alarm systems want to send their messages as quickly as possible, they use very short tone burst for their DTMF signalling.  These tones are just on the border line of what is considered an acceptable length as far as normal dialling tones are concerned.  Therefore, equipment that is designed to detect tones from a person dialling a phone does not always work correctly with the fast tones sent by alarms.  It's often better to turn off the ATA's DTMF detection completely (set it to in-band mode), and let the monitoring station take care of it.

Monitoring Plus VoIP alarm reporting

Not many monitoring companies support VoIP as a method of alarm monitoring.  This is because calls from your alarm to the monitoring company must be routed through your VoIP provided like any other phone call you make, leading to poor reliability (as discussed above).

Monitoring Plus takes a different approach, and uses a specialised VoIP server to communicate with your alarm.  By connecting your alarm to the spare line of your ATA (or a separate ATA), and configuring it to connect directly to the Monitoring Plus server, the problems of call routing and transcoding are avoided.  Also, by using a non-compressing codec and in-band DTMF signalling, high call quality is maintained.  At our monitoring station, we use specialised DTMF detection algorithms that are specifically designed for communicating with alarm systems, avoiding the problems of missed digits.

On our burglar alarm phone wiring page we discuss the best way to connect ADSL broadband and burglar alarms.  When connecting you alarm to Monitoring Plus VoIP monitoring, the set-up is a little different.  With VoIP, the ATA output takes the place of the phone line entering your house, and phones are be connected to the first line.  The second line output of the ATA (or a separate ATA) is configured to directly connect to the Monitoring Plus servers (and cannot be used for normal phone calls), and is connected to the alarm system only.


Figure 1 - Wiring for burglar alarm using Monitoring Plus VoIP monitoring.