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DSPs (PVDM3-x) requirements for SRST operation


  First off  let me explain the scenario...We have some remote sites that are connected to our VOIP network using ASA VPNs over the Internet. I have QOS all set (took me quite a while to get it all set properly and tested) and we have been having no problems at the sites for more than a year. I have each site configured with an SRST with a few POTs lines (911 backup and a few outside lines) just in case the VOIP is down - although it never is.)


   I am now being asked to convert even smaller remote sites with 4 to 10 users from Centrix to this same type of setup. My telephone counterpart (I am the network admin) tells me that we must have an SRST at the site since it is a requirement by law for 911 backup purposes????? Seems illogical to me. (During normal operation the switches at the remote site utilize the CER at our main site and have an ELIN for the remote address.) This SRST addition adds a significant expense to each remote site especially with the monthly POTs line charges from the ISP that are never used. Is this really a legal requirement for a business in NY (we are a county government)?


   Assuming worse case that I need an SRST at these smaller sites, when do the DSP calculations really come into play. From what I have read it seems that the DSP requirements are calculated based on the usage of active POTs lines attached to the SRST in my case. So as an example if I have an SRST with 25 licenses and I have 15 phones at the site and only 2 POTS lines connected to the SRST, it seems that even a PVDM3-16 would easily handle the load if the main connection was lost and SRST mode kicked in. True?  Can I also assume that these 15 phones at the site are not using any DSP mem if they are making local calls to each other within that building? Would the DSP also not be used if these local phones had conferences taking place between local phones within this site (while the main VOIP connection is still down)? It seems I could get away with a 2911 or 2901 SRST with a PVDM3-16 card as my cheapest solution.


   If I legally do need a backup, any other ideas for a cheaper solution. I am thinking of possibly using a cellular connection as a backup to the main site for ISP outages but if the main site is down then there is no local 911 available again (if it is REALLY a legal requirement).