Failover or failsafe?

I recently had a conversation about the need for PSTN failover. The basic proposition being that VoIP, IP telephony, SIP, Skype, etc. are OK, but if you have an IP network - be it broadband/DSL or SIP trunking, or an Ethernet WAN/LAN - you must have the option of being able to failover to the PSTN. Why? Just in case; for safety's sake; as a backup - because IP is perceived to be somehow less reliable. Surely this can't be true.

It's either a myth propagated by those with something to gain/lose i.e., the big fixed line telcos, carriers and service providers, or it's a kind of 'security blanket' that's needed, because we all just can't let go.

Here's the typical scenario:
A corporate enterprise buys a shiny new IP-PBX, which runs SIP and provides HD Voice capable feature phones to the desktop of every executive in the building. It procures a SIP trunk(s) - depending on where you are in the world, you need either one trunk per multiple calls or one trunk per call - from an ITSP and installs everything in a green field site on the new technology business park that's just been built with European Commission funding. There's not a PSTN line in sight. But, wait, what's that advice it's been getting? It's got to have a PSTN line(s) as backup in case its SIP trunks or IP networks fail, or power is lost and its VoIP phones can't make emergency calls. Who gives this advice? I don't know, but it's being followed.

My difficulty isn't with the problem - backup is needed - it's with the solution. Redundancy, resilience, failover and call recovery - all these things are needed in order to achieve high availability and preserve continuity of service. Such things are taken for granted in certain scenarios. Ask anyone responsible for disaster recovery in any major corporate enterprise. And the principles are applied to any data centre operation, particularly in the financial sector with banks and share trading.

Backup is needed. In the 'old days', a corporate telecommunications manager would ensure that regardless of how many ISDN lines he needed, they were brought into the building via separate ducts. That was to protect against the so-called 'JCB effect', which is oblivious to the type of network you're running, by the way. It was also common practice to source lines from more than one service provider - for resilience as well as to gain the advantages of least cost routing. Couple that with the 'n+1' redundant hardware architecture inherent in the PABX, uninterruptible power supplies (UPS) and an emergency generator and you've got your own enclave that's impervious to most man made disasters and a few of Mother Nature's.

Backup is needed. But if you quite naturally backed up an ISDN line with another ISDN line, why not back up a SIP trunk with another SIP trunk? Is the copper wire carrying the TDM signals really inherently more reliable than that routing SIP and RTP media? I don't think it is.

Surely, the real solution is to have a backup SIP trunk or a backup broadband or ADSL/SDSL line. Then you can employ all those good disaster recovery methods I mentioned earlier on your IP network. If you're smart, you've already implemented redundant SIP signalling on separate SIP trunks, your IP-PBX has redundant hardware, you've got a UPS and, perhaps, a generator.

As for emergency calls, now that you are at least as adequately protected as in the past, there's less need to worry. However, it will make sense to ensure that your ITSP is responsible enough to have a mechanism in place to be able to route 112/911/999 calls to a Public Service Answering Point (PSAP) at all times. If the PSAP isn't IP-based, it will have to have a gateway interconnect in any case.
And, there's always that personalised, hands free, voice activated, unified communications device i.e., your cell phone, as a last resort.
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