VoIP and Home Security Systems Don't Get Along 187
coondoggie writes "Here is a story about consumer VoIP services that can cause your home security alarm system to malfunction or not work at all. There have been problems with customer phone systems in Canada who were using Primus but Vonage customers in the U.S have complained too. A number of sites have popped up offering suggestions to help deal with the problem."
Aren't these known issues? (Score:1, Informative)
Simple answer: Basic analog dialtone (Score:3, Informative)
Re:This one smells (Score:3, Informative)
You mean like this? [nextalarm.com]
Re:alternative alarm monitoring: Internet (Score:1, Informative)
It's not just that. Mainly timing. (Score:5, Informative)
It's not just that.
POTS signals are generally converted to digital samples at the first switching center they hit (or at curbside equipment along the way), switched as a digital signal, and converted to analog again similarly near the far end. To avoid clicks and pops (and persistent phase jumps) the sampling rates at the D->A and A->D conversion must match - exactly. The phone companies use very accurate clocks, synchronized across their whole network, to make this happen.
The phone companies originally used digital just to pack multiple phone calls for a hop from one analog switching center to another - and D->A->switch->A->D converted at each switch - with synchronization only needed between the ends of the hop. This saved a lot on cabling and gave better signal than analog transport, but not as good as digital from one "last mile" to the other. Then they added digital switching to eliminate the degradation of the multiple A/D conversions and simplify the switch - and spent a decade or more getting clocking synchronized across the whole network to eliminate the resulting glitches. Even today, in the being-retired POTS network, "timing is a third of the problem".
(These days the clocks are synchronized even between carriers by essentially all of them getting their master clocking from the atomic clocks of the GPS system. Before that they used things like LORAN D - a pre-satellite clocking-based radio navigation system for ships - or generated them in their own committee of atomic clocks and distributed the clocking along with the signals using the carriers of the SONET optical fibers or the T1 and E1 carriers of copper and microwave days, and these methods are still used to synchronize boxes that aren't in installations big enough to rate their own satellite-derived clock.)
The signal is encoded as a "DS0" stream of 8,000 8-bit samples per second, in one of two closely related floating-point-like coding schemes ("A-law" or "u-law" where "u" is "mu"), depending on whether you're using European or American-style standards.
So the signal is only capable of carrying 64,000 bits per second. (In fact the LSB may be "stolen" every few samples for ringing, off-hook, and dialing information, so only 56,000 bps are reliable - and it's actually a bit lower since some code sequences are forbidden by a regulation.)
Modern modems are designed around this and try to use as much as possible of these bits for data. In typical ISP-type modem banks the ISP end is connected to the phone company by a digital link and can directly control the bits, without incuring an A->D penalty, so the downlink can approach 56k, with the modem figuring out the actual sampling boundaries as part of the decoding. The uplink (or both sides in communication between two modems on analog POTS lines) comes pretty close to it - though it has to sacrifice some bandwidth to use a coding scheme that can survive clocking-rate errors between the modem's transmitter and the digitizer.
Of course if your VoIP link uses compression to carry your signal in less than 64k bps of payload, you're totally hosed. (And many of them do. For starters, if you're working over a dialup line you don't HAVE 64k bps to use.) Your modem assumes it's working over the POTS network and tries to use the bandwidth. And its signal gets totally hashed by the compression.
But even if you have the bandwidth (or the modem figures out that it's got a "noisy link" and down-speeds), you're still hosed. Because the clocking used for VoIP A->D and D->A steps is just not stable enough for the modem to take advantage of the bandwidth in the digital link.
One of the big pieces of persistent fallout from the war between
Pure FUD (Score:3, Informative)
Obviously the author of the article (and the submitter) didn't do their homework.
A great place to start looking for how to make your alarm work with Vonage can be found here [vonage-forum.com]
And as for the people posting that using VOIP for an alarm is foolish because all a thief would have to do is cut the power: A thief is more likely to cut the phone line going from the PSTN to your house than he is the power. He isn't going to think, "Hmm, this person might have VOIP. I'd better cut the power, the cable, and the phone line outside the house just in case".
Re:Bah! (Score:2, Informative)
The irony is that the current security protocols would get IP/IT security professionals giggling like school girls and saying things like 'Awww, how quaint'.