Power-Supply Infrastructure | VoIP Telephony and Infrastructure

Often overlooked as part of the infrastructure required for secure VoIP is how power issues will be addressed. PBX and PSTN phones run on a common battery system that provides availability for free in the face of a power outage, but VoIP phones and the infrastructure that powers them must be carefully designed to meet equivalent requirements.

Power-over-Ethernet (IEEE 802.3af)

Like the name implies, Power-over-Ethernet (POE) eliminates the need to run a separate power supply to common networking appliances. POE works by injecting power using a switch or special power injector that pushes Direct Current (DC) voltage into the CAT5 cable. POE can be used directly with devices specifically designed for POE or with other DC-powered devices with a converter installed. This converter, called a picker or a tap, diverts the extra voltage from the CAT5 cable and redirects it to a regular power jack.

The major advantage of POE is that it allows greater flexibility in installing networking equipment. Access points can be set up in remote locations that normally would be limited to its proximity to a power outlet. It’s often easier to route cat5 cable outdoors (on an antenna or in a tree, for instance) when only network cable is required. POE is also very popular with supplementary low-power devices, such as IP telephones and webcams, even computers!

POE is regulated by the IEEE 802.3af standard. This standard dictates the device must provide 48 volts of direct current, split over two pairs of a four-pair cable. The maximum current is limited at 350 mA and a maximum load of 16.8 watts. Several vendors have created proprietary (prestandard) implementations of POE, however in most cases newer equipment from these vendors is now available that is compliant with the IEEE standard (although at least one of these vendors now advertises an ability for the client to request a lower or higher amount of current through a proprietary process of negotiation above and beyond specifications within the standard).

To properly address VoIP phone availability concerns using POE, be sure that the power injector, network equipment, and voice servers (and gateways) can all operate on battery power for a sufficient length of time, and consider use of a generator when appropriate.

POE in action is pretty simple. The power source checks to see if the device on the other end of the wire is capable of receiving POE. If it is, the source then checks to see on which pairs of wires the device will accept power. If the device is capable, it will operate in one of two modes, A or B. In mode A, power is sent one way over pins 1 and 2, and is received over pins 3 and 6. In mode B, power is sent over pins 4 and 5 and is received over pins 7 and 8. Although only one mode will be used at a time, a device must be able to use both A or B to be IEEE 802.3af compliant.

UPS

No availability strategy can be considered complete without appropriate use of Uninterruptible Power Supply (UPS) technology. Mission critical equipment such as PBX systems and servers need to be protected from unscheduled power outages and other electrical maladies. Because of the sensitive nature of electronic equipment, safeguards need to be put in place to ensure the safety of this equipment. A UPS protects against several availability threats:

• Power surges When the power on the line is greater than it should be, the UPS acts as a buffer, ensuring that no more power reaches the machine than is supposed to. If a power surge were to occur without a UPS inline, sensitive electronics literally could be zapped out of life.
• Partial loss of power A brownout occurs when the power on the line is less than is required to run an appliance. In many cases a brown out is considered to be more dangerous than a total power failure, as electrical circuitry is very sensitive to power requirements.
• Complete loss of power A blackout occurs when power is completely lost to an area. This is very common during natural disasters, where severe weather may topple the electrical infrastructure of an area. Gas or battery powered UPS systems allow for equipment to continue functioning for a set period of time after the lights have gone out. This is ideal for finicky gear that needs to be completely shut down before going dark, lest system integrity be compromised.

In a call-center environment, downtime to the phone system can be fatal to business. With a properly implemented disaster recovery plan including a network of UPS devices, the phones can continue to work when standard computer systems might not be able to. This may mean the difference between success and doom for some companies.

Energy and Heat Budget Considerations

Given the heat and energy crisis being faced in many data centers due to the rapid increase in equipment densities (without a corresponding decrease in energy efficiency), planning for VoIP availability must include consideration for heat and power capacities in the room where VoIP servers and gateways will be housed. Don’t omit this step only to discover after you’ve deployed that you have no power or cooling headroom for the additional equipment!