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VoIP and Home Security Systems Don't Get Along
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."
Depend for emergency communication on a shared bandwidth communications link whose functioning depends on utility power availability coupled with some ISP's service plan, and maybe when the bad guys break in you won't get the call? Huh? You think? Or, to put it another way, there's no guarantee that The Phone Company's own landline will work perfectly either, but if I had to bet my home on it, I'd go with TPC over VoIP. In fact, personally, I've stuck with TPC landline because of E911, because my landline has always worked during NYC blackouts even when my cellular phone didn't, and because I have yet to see a VoIP service provider that would guarantee that if some guy in Afghanistan (or Milwaukee, for that matter) somehow manages to clone my SIP identity and proceeds to make N-billion dollars (well, amounts are relative to my savings account balance) worth of international phone calls, that they won't hold my feet to the fire if I refuse to pay the bill. But of course, you may see things differently.
It smells because there are easy solutions to the problems. First of all, you can supply backup power to your ATA and not have to worry.
Secondly you should wire your setup as RJ31X so the alarm system can cut in and take control.
Thirdly - you can set your bandwidth so that fax and modem signals will work. Better yet, how come no alarm company has an IP based monitoring setup? Be pretty simple to do with VPN's, etc.
Finally the E-911 issue was resolved a long time ago. I have full E-911 service through Vonage.
All this leads me to believe that ILEC's are behind these stories. They're losing business left and right to less expensive VoIP carriers. And Verizon for one is in a particularly bad spot, their little fiber build out isn't generating the returns they expected.
Is anyone (here) surprised by this? It seems painfully obvious to me, that most such services obviously wouldn't work. That this guy wasn't notified BY THE SECURITY SERVICE that his alarm system wasn't functioning for over a year, speaks volumes about how useless that service really is.
It's only too easy to cut a POTS line, or tie it up by dialing-in to it, which is exactly what any competent burglar will do... Maybe with a (pre-paid?) cell-based service, your alarm will have a fighting chance, but not a lot even then.
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works great, doesn't require any phone line, and has gone down in price recently.
POTS lines are no longer needed.
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There is a lot of signal degradation converting from analog to digital, lossy compression of the digital signal, and converting it back to analog. Not to mention the analog to digital conversion has to happen twice (once over the VoIP carrier, and again when it's received).
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
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