Practical Power Guidelines for VoIP and Internet Telephony Applications

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Practical Power Guidelines for VoIP and
Internet Telephony Applications
by David Slotten
WHITEPAPER95-2616Integrate Backup Power into VoIP Networks
Voice over IP (VoIP) is exploding in popularity as an application for businessdata networks. VoIP promises to consolidate a company's data andtelecommunications infrastructure as well as its support resources. As aresult, a company can lower its hardware and service costs while raisingproductivity through the use of more elaborate and customizable telephonyapplications.
Unfortunately, there are serious limitations inherent to the data networksthat are increasingly called upon to support VoIP. The primary limitation ispower availability. Before moving voice traffic from traditional circuit-switched public phone systems to private data network connections, onemust consider a public phone system's unique attribute-battery support. Inorder to deliver extremely high availability for such vital services asemergency 911 support in the event of extended power outages, publicphone systems are connected to massive battery arrays.
While most data networks have some type of backup support during poweroutages (provided by UPS Systems and/or generators), the backup runtimeis generally much less than the 4 to 8 hours of backup that is typicallyprovided for public phone systems. Because of this shortcoming, VoIPapplications generally require an increase in the UPS System-supportedpower capacity (e.g. more or larger UPS Systems). Increased UPS Systemcapacity provides power for network-dependent phones and increasesoverall backup runtime to ensure that normal telephone operation (including911 service) remains available in the event of an extended power outage.
Reflecting on important lessons learned during its own transition to IPtelephony, Cisco provides several best-practice recommendations. One ofthe most important recommendations is installing a UPS System toguarantee availability:
"Plan Your Power: When an IP network carries voice,reliability is essential. In case of an emergency,people need to summon assistance by dialing 911.When using inline power to switches and routers, makesure they are connected to an uninterruptible powersupply [UPS System] to guarantee dial tone if thepower should go out."
Source: Cisco Systems white paper "The Transition to IP Telephony at Cisco Systems".http://www.cisco.com/en/US/tech/tk652/tk701/technologies_white_paper09186a00800cb7fd.shtml
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Before selecting a UPS System to ensure 100% availability of IP telephonysystems, it's important to consider the unique requirements of VoIP networkequipment. Network designs hosting VoIP applications will vary widely frombusiness to business due to a number of variables, including the scale ofthe network and the variety of legacy equipment involved. However, threedevices are common to all networks:
Client Devices (phones, PC-based soft phones, etc.)During the transition to IP telephony, these devices will either (a) derivetheir power from the network cable via a Power over Ethernet (PoE)connection scheme, or (b) plug into a local AC source.
If they plug into a local AC source, they must be protected by a UPSTypical VoIP System. Often a desktop UPS not only safeguards phone service, but alsoNetwork Design guarantees file integrity for associated PC users.
Networking Devices (switches, routers, etc.)During the transition to IP telephony, port capacity on the network and inwiring closets will increase to accommodate additional devices (phones)connected to the network. Increased port capacity will increase the powerrequirements placed on your UPS System, either reducing runtime oroverloading the UPS. Note that if a networking device also supplies Powerover Ethernet, the aggregate load of all client devices will also be borne bythe networking device's UPS System.
Generally, an existing UPS will be inadequate to (a) power the increasedload [watts] and (b) power the load for an acceptable length of time. Five toSource: Cisco Systems white paper"Power and Cooling for VoIP and IP fifteen minutes of runtime provided to gracefully... [download for more]