Now that I’ve setup Polycoms SoundPoint IP500 phone system to talk to a local Asterisk system and everything is working as expected, the next step is to think of fault-tolerance. In my mind, a VoIP system has two distinct responsibilities regarding reliability: to provide business critical and life critical services. Obviously, the latter is more important (If you have your priorities straight!), and is the topic of this article.
There are a few possibilities when looking to provide access to emergency services:
- local switch connected to the local PSTN,
- termination provider that supports E911, including location information,
- dedicated POTS phone connected to the local PSTN,
- dedicated local SIP gateway such as a Sipura 3000.
The first solution, which would involve having a single port FXO card in your local PBX assumes that you actually have a local PBX which is not always the case. It also adds complexity to your infrastructure, which then must provide backup power to another server. Remember, when talking about life critical services, 2 or 3 hours of runtime is probably not enough.
The second solution appears very promising since it removes the burden of local systems however, since you must still provide power to the VoIP terminals themselves, you are not much farther ahead. In addition, the reliability of your connection to your provider must now be reexamined, something that is often very costly. Finally, since the majority of VoIP termination providers do not yet offer 911 access, this is a mout point.
Thirdly is what I term the “Red Handset” approach. A very simple and reliable solution: go out and buy the the most brightly colored POTS phone you can find, mount it on a prominent location, and connect it directly to the local PSTN. Label it, in your country’s official languages, “Emergency Use Only”, and you’re done. The only downside, as I see it, is that a distressed person may not have the presence of mind to use this phone when they are frantically pounding at the uncaring keys of your fancy IP desk phones.
Last, a very plausible solution comes in the form of an ATA device setup for the purposes of emergency access. For example, the Sipura 3000 has both an FXS and an FXO port; this allows you to use it simultaneously as an adapter for analog devices, such as that rather expensive conference room unit you bought during The Boom, and as a gateway to the local PSTN. You could, and probably should, implement solution three by connecting a handset to the ATAs FXS port since the Sipura 3000 has a relay that connects the FXS device to the FXO line in the event of power failure. Program the Sipura for open, unauthenticated access and then configure your IP phones to use it for emergency access.
Configuring Polycom SoundPoint IP Phones for Emergency Services Access
In section 4.6.2.1.4.2.2 of the “Administrator Guide for SoundPoint IP SIP”, version 1.3.0, the configuration of emergency services access is described:
create a server for your dedicated ATA (4.6.2.1.2)
<server voIpProt.server.2.address="1.1.1.1"
voIpProt.server.2.transport="UDPonly"
voIpProt.server.2.register="0" />
make sure your local dialplan ensures your locations emergency services access number is immediately dialed (4.6.2.1.4.1)
<digitmap dialplan.digitmap="911|[2-9]xxxxx|1[2-9]xx[2-9]xxxxxx" />
add an emergency entry for the same number (4.6.2.1.4.2.2)
<dialplan><digitmap dialplan.digitmap="911|[2-9]xxxxx|1[2-9]xx[2-9]xxxxxx" /><routing><emergency dialplan.routing.emergency.1.value="911"
dialplan.routing.emergency.1.server.1="2" /></routing></dialplan>
General Fault-Tolerance
You need a few more things to make this a truly fault-tolerant system:
- have your IP phones on a dedicated switch and make sure you are using Power over Ethernet (PoE),
- put the above switch on a dedicated UPS,
- put the ATA on the same dedicated UPS,
- ensure that this UPS will provide a full-day of runtime with this load,
One more, not so obvious, issue: check with your local PSTN access provider to verify that your lines will function during a power outage. Many business lines are actually digital and have local equipment that requires power. Usually, battery backup is installed but often is not sufficient. This would be an issue whether or not you are using VoIP.
Contributions to this article from Bill Street.
Well, I intend to get myself fully setup this way. To heck with all the other ways of getting voice service. Also, 
The sound quality of the