[asterisk-users] life safety system and VOIP

[Wilton Helm]

>The dial tone for the phone line still comes from the CO. The phone companies >loop there copper cable in and out of the remote cabinets. 


Remote terminals are served by T1 or higher density carrier circuits, which can be either copper or fiber, often employing statistical multiplexing.  While the DT may originate in the CO, it does so only in a data sense, not an analog POTS sense.  The remote terminal actually generates the POTS analog signal, and is dependent on the life of the batteries in the box.  They are good for several hours, maybe even a day, but definitely not weeks.

Some RTs also have a DSLAM associated with them for DSL, but that is a separate topic and involves more batteries.

>This is true, that is why most fire panels have to have 2 phone lines.

Which only catches about half of the problems, assuming both come through the same cable from the same CO or RT (and, in the latter case, the same carrier circuit).  If a card fails or the I & R guy opens or shorts the loop, the other line can take over.  If the CO or RT crashes, or batteries die or cable gets dug through by a backhoe, guess what goes down!  For serious mission critical circuits the engineer specifies two different operating companies and requires each to provide complete circuit details so he can insure that one isn't leasing lines from the other, or other scenarios that would be vulnerable to a single incident.

>Time was a copper pair was supervised with a DC current from end to end,

Another variation on this theme used by central alarm monitoring companies of years ago was to have the telco provide a copper loop that included a number of customer sites.  Basically each site was in series.  At the monitoring station was the DC power and a relay.  If all was well the loop was complete and the relay operated.  Each site had a mechanical interrupter--a spring wound gear mechanism that pulsed out digits by breaking the loop momentarily.  When an alarm condition occurred (such as water movement in a sprinkler riser) the spring would wind down, turning the gears and pulsing opens on the loop.  In some cases, this caused ink mark square waves that could be counted on paper.  The pulses were similar to rotary dial pulses in groups for digits, but slower speed.  They represented the ID number of the sender reporting, which identified the customer and location.

Of course, if anything in the loop, any sender, any telco drop, failed, the whole set of customers was unmonitored until it was fixed--which could be a day or two in extreme cases.  I was called out once to service a site that had these.  The one good thing about them was the only electrical requirement was at the monitoring station.

Wilton
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.digium.com/pipermail/asterisk-users/attachments/20090217/19973a94/attachment.htm