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Successful VOIP Over DSL, Part 5: Power Considerations

One of the great things about the traditional PSTN is that it keeps working when the power goes out. I’ve repeatedly read others recommending that people sustain traditional POTS service at least in part because of this fact. Their theory being that VOIP service isn’t sustained during a power outage. But this need not be the case given just a little forethought.

Prior to migrating to Asterisk we had been using a Panasonic KX-TG4000 KSU (seen left). This phone system has four FXO interfaces for analog lines.

It also featured a built-in battery backup so our phones stayed up through power outages. In migrating to VOIP within our home and office I felt it necessary to strive for this kind of reliability. It has certainly made my wife happier.

There are a number of factors involved in my consideration of power for the phone system as a whole.

1. Asterisk Servers

I have long been a believer in embedded systems and my Asterisk servers reflect this fact. During my initial experimentation with Asterisk I ran it on traditional PC hardware, but eventually I migrated to a mini-itx system, and then later to embedded systems like the Soekris Net 4801 and HP T5700 thin clients.

Rear view of a H-P T5700 Thin Client

The embedded systems offer a number of advantages but two of the biggest are low noise and low power consumption. Both of the embedded platforms mentioned draw less than twelve watts. That means that they can be kept running a long time from a relatively low cost UPS.

Along with low power consumption comes the added benefit of low heat output. This can be important if you lose power and your air conditioner stops running. Living in South Texas when the AC unit stops the whole place can heat up quickly.

2. UPS Power For Network Components

Various key network devices also need to be on UPS power. In my case this includes:

  • DSL modem
  • Router
  • Netgear 24 port gigabit switch
  • Power over Ethernet insertion devices
  • Wifi access point
  • Charging cradle for Aastra cordless handset

I recommend that you keep your phones and network components on their own UPS. All of the devices listed have very low power requirements. This means that an inexpensive UPS (1500 KVA, approx $120) can keep the entire network running for a good long while.

Home vs Home Office

My office is actually in what some people would call the “Garage Apartment.” I prefer to think of it as the “Carriage House” or “Executive Suite.” There are a couple of underground cat 5 runs from the office to the house so it’s all one network.

There is a small networking cabinet in the house that contains a 16 port switch and a Linksys ATA for the home phones. This gear needs to stay powered up 24/7 /365 so I also had to provide a second, smaller UPS (700 KVA) in the house.

3. Power Over Ethernet

In my opinion this is one of the most overlooked conveniences in SOHO networking. Providing power-over-Ethernet (POE hereafter) is tremendously useful. It lets me keep my Polycom and Aastra phones powered by the same UPS as the rest of network closet. If ever I need to replace my ATAs I will definitely seek new units that are POE capable.

A small Netgear switch capable of POE

I especially feel that POE is useful for wifi access points. It lets you position the AP in a location that is selected for ideal wireless propagation (even outside in a weatherproof housing) without concern for providing an outlet. It also makes it easy to provide physical security for your WLAN from the wiring closet. That is, when I’m out of office for a week the AP is powered down by simply unplugging the cat 5 jumper running to the AP.

A small Linksys switch capable of POE

POE can be provided by careful selection of your network switch. Some low cost 8 and 16 port switches provide POE on a limited number of ports, ideal for SOHO use.

Mid-Span POE Insertion

It’s also possible to add POE via “mid-span insertion.” This involves placing a small power insertion device on the network line between the switch and the device to be powered. This is how I started using POE as my Aastra 480i phones came with POE insertion devices. I was so happy with them that I purchased a couple more for my Polycom phones.

Midspan POE Inserters

Mid-span POE insertion devices come in single and multi-port models. The single port models look like “line bump” power supplies but with two RJ-45 jacks. Multi-port POE insertors look a lot like network switches.


POE capable switches are definitely more expensive than non-POE switches. If you shop wisely you may find a POE capable switch that meets your needs, while superficially more expensive, is actually cheaper than a non-POE switch and a mid-span insertion device.

If you only need a few POE ports then using mid-span insertion is typically less expensive.


Current Capability

In examining POE insertors or POE capable switches it’s worth noting how much current each port can provide. The Linksys 8 port switch pictured above, for instance, provides 15 watts per port when 4 ports are powered or only 7 watts per port when all 8 ports are powered. You need to be aware of the power requirements of each of your upstream devices and be certain that the POE power source can handle all of them. Phones are not generally a large power draw. Wifi access points and security cameras draw a little more.

Beware Older Phones

The standard for POE is referred to as 802.3af and specifies not only the wiring standard, but also a protocol for POE power sources to detect if the upstream device is also POE capable.

Prior to this standard becoming widespread several manufacturers made equipment based upon their own standards. This is especially true for older Polycom and Cisco IP phones. These may require special network adapter cables to be powered by standard POE power sources.

Alternatively, some larger midspan POE inserters (ex Belden Power Sense) can switch between standard and device specific POE on a per-port basis. That can be very handy if you need to power a variety of devices.


While 100% VOIP we are still able to keep our phones, our entire network for that matter, running when the power fails. The combination of a decent UPS and POE makes this possible.

Perhaps one day I’ll pull the plug on the UPS and see how long everything runs. Its never been needed for more than 10-15 minutes at a time.

It’s a truly amazing and wonderful thing to be sitting at my desk when the power goes out suddenly. Then, in the silence created by the total lack of PC noise, I find myself basking in the faint glow of the backlight from an Aastra 480i. The silence is shattered by the ringing of my phone. It’s my wife calling from the house telling me that the power is out.

It’s even more amazing when the entire network stays up throughout a power outage and I’m able to easily transition to working on a laptop complete with internet access over wifi.


Upon reading this last passage my wife insisted that I also add her wish that we go one step further and install a generator.  It’s a prudent, if costly idea, given our location in hurricane alley. Perhaps one day.

She also adds that geeks do get the girl…more often than one might think.

This Post Has 8 Comments
  1. Looks like a nice piece of equipment you got there.

    Will the Panasonic KX-TG4000B KSU work with most voip providers like Onesuite? I still have my POTS though I seldom use it for outgoing calls. I am still wary of power failures or internet downtime to fully dropped the land line altogether.

  2. The KXTG4500 has enough built-in battery power to ride out maybe an hour without power. Beyond that you need a real beefy external UPS. If you want to run without power for long periods then a standby generator is the only answer.

    The KXTG4500 is not at all VoIP capable itself. It has 4 FXO interfaces, so accepts four analog POTS lines. It could easily handle any ITSP if you add a suitable 4 port FXS adapter. There are many to choose from. Perhaps a pair of Linksys-SPA-2002?

    The result would be a phone system capable of four simultaneous calls over IP.

  3. I thought it was a pretty ingenious idea to have not just the UPS, but a generator installed as well to avoid system crashes and/or desktops. The added security that your phone line won’t go down if a blackout occurs. Knowing this and knowing the consumer prices for service, this is more than a way to eliminate having a solid land line altogether. I have only experienced one power outage, but luckily I had a UPS and a backup UPS in the event that the 1st one gets drained. I never tried the Aastra series of products, but I can see why having one would simplify how one stores messages, limited bandwidth being all one client needs for a maintain a good connection while talking.on an important call.

  4. Pity I didn’t get it installed before Hurricane Ike hit. We were without power for 10 days 12 hours.

    But we have the unit picked and just need to assemble the funding before we can proceed. A 17 kw natural gas fired generator will run our entire house and office indefinitely should we lose power again.

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