[asterisk-users] VOIP with PSTN backup

[Rich Adamson]

Brian Candler wrote:
> I'm looking for a way to set up a VOIP network in branch offices where one
> or more phones have "lifeline" capability, i.e. can place calls if the IP
> network or VOIP service dies, or even if power goes down. (I'm thinking of
> business continuity here, not just emergency services)
> 
> This seems to limit my choice of products somewhat, and I was wondering if
> anyone had recommendations for use in this scenario.
> 
> The approaches I'm thinking of are:
> 
> (1) Use an ATA with PSTN passthrough or FXO port, and connect an old
> analogue telephone to the FXS port.
> 
> In this case, the analogue phone has lifeline. If there's a true FXO port
> then PSTN calls can in principle be routed to/from other VOIP phones in the
> office (but see below)
> 
> There seem to be a few to choose from, although far fewer with a true FXO
> port.
> 
> (2) Find a VOIP phone with integrated PSTN or FXO port
> 
> In this case, the only one I have found so far by searching the web is
> Clipcomm CP101.
> 
> I have also read that many FXO devices tend to be badly implemented; in
> particular, on seeing ringing voltage, they actually pick up and answer the
> call, instead of sending off a SIP INVITE and waiting for the OK before
> connecting. I'd certainly like the device to behave properly in this regard.
> 
> As a second part of this question, it would be extremely desirable if the
> backup PSTN service were available to all the phones in the office. That
> means:
> 
> (a) incoming PSTN calls could ring *all* the VOIP phones in the office, not
> just the one phone or ATA connected to the PSTN line; and
> 
> (b) any VOIP phone could route a call out over the LAN to the local FXO PSTN
> port, e.g. by dialling a prefix to access it.
> 
> This isn't so essential but it's definitely desirable. Any recommendations
> for how to do this too?
> 
> A large number of offices is going to be involved, and I want to keep as
> much switching "intelligence" centralised as possible, both for ease of
> management and to keep the cost down. That is, I don't want to install a
> PC + TMD400P + Asterisk in each location, but just a small media gateway or
> VOIP phone.
> 
> However I can see that the incoming ringing issue will require call forking,
> so I am happy to install an OpenWrt box running Asterisk or siproxd or
> whatever in each site. Being diskless and low power should mean little
> maintenance is required. But such a box isn't going to be able to take an
> FXS/FXO card, so I'll still rely on an ATA or VOIP phone to present a PSTN
> interface. So that's the key part I'm looking for.
> 
> Finally, the devices must be robust (i.e. not need power cycling every 24
> hours) and centrally manageable.
> 
> I think that's about it - many thanks for your ideas and experience!

If you get real serious about this, then do a "risk assessment" for each 
component involved in the end-to-end communications system. The risk 
assessment should include an analysis of each component answering 
questions like:
1. What's a reasonable business "down time" for the communications 
system (and that answer is not zero)
2. How important is the component (high, medium, low)
3. What's the likely restoration time for the component
4. What are some of the potential causes for a component failure
etc, etc.

Once that is done, I think you'll find that you can prioritize which 
assets need to be addressed in what order. For example, a "fiber seeking 
backhoe" will likely disable all forms of communications (eg, analog and 
digital). Therefore, trying to locate a phone (or ATA) with an analog 
fxo port is of no value. Finding an alternative carrier maybe based on 
some form of wireless service, cable broadband, etc, might be a 
reasonable approach.

Some companies will actually bury telecomm communications facilities 
into a building, arriving from two distinct locations, thus reducing the 
exposure to the fiber seeking backhoe.