Making Line Power Work for You
Has powering today’s communications network come full circle?
By John S. Pendergrass
Has powering today’s communications network come full circle? Since the 1930’s, the telephone network has been powered from the central office. As the need for greater bandwidth became apparent, the power began moving away from the central office and closer to the customer. Now, in many applications it is the customers’ responsibility to provide the power, including energy reserve.
Line power solutions move the responsibility back upstream.
Then and Now
Plain Old Telephone Service (POTS) is provided over copper wires. The copper originates in the central office, which also houses the energy reserve such as batteries and generators, and is terminated at the customer site. Communications signals and power travel over the copper. Your phone operates during a power outage, assuming the line has not been damaged, even when your electricity is off. The telephone uses power equivalent to a 9V battery and increases when the phone is ringing. Because of the energy reserve, the power is virtually uninterruptable.
Today, our expectations have gone from the POTS to triple play, and we require greater bandwidth in the network. Triple-play services and IP-based television is one of the biggest changes in the telecom network in our generation. These services require power in the range of 5W to 50W.
Service providers are committed to providing high data rate services to customers. To do so, fiber optic cable must be deployed. Equipment providing triple play services is located close to or on the customer premise. Unlike copper, fiber is a poor conductor. Should the utility power fail, power for telephony services such as 911, security monitoring and alarm, and inbound and outbound calls must come from a different source.
Many service providers use a rechargeable battery back-up unit. For example, Verizon provides a unit with each service installation. Older units allow for four hours of power to the optical network terminal during a power outage, while newer units installed in 2008 allow for eight hours.
In the case of many fiber deployments the responsibility for maintaining the battery is ambiguous. Some service providers offer replacement plans while others require subscribers to replace their own battery.
Another solution is to remotely power the new electronics using existing and available telephone copper pairs to deliver power from larger, upstream sites with batteries. Remote power is known by many other names: Line power, Express power, Span power and others.
By any name, the power distribution network can be represented as shown in Figure 1. The upstream converter, shown at the top, is located near both the power source and the batteries. It provides a step-up voltage conversion to minimize power loss in the network copper.
The power available from a single converter is strictly limited by safety concerns. In order to transmit this power as far as possible, multiple twisted pairs of copper can be used in parallel on one converter output. Two pairs are shown. PU is the output Power from the Upstream converter. R is the resistance of the copper and PD is the input Power to the Downstream converter, which steps the remaining voltage down for use by the Phone, Video and Data Equipment.
Line power deployments face several obstacles such as safety, stability, and network planning. Network providers raised concerns over the safety of placing high voltages in the network. Would high voltages provide the same reliability we’ve come to expect? Are the tools available to determine the viability of utilizing line power in the network? Let’s address these three issues independently.
Safety: Established Limits
Underwriters Laboratory released UL 60950-21 in 2003. The benchmark standard for the effects of electricity on people is IEC479-1: Effects of Current on Human Beings and Livestock.Figure 15 of that document indicates the two parameters that must be controlled are: Body Current and Duration of Body Current. Realize that normal operational currents for +/-190V systems operate with currents less than 250 mA. Particular emphasis is placed on recognizing a fault condition and shutting down the circuit as quickly as possible. The red annotation shows how the circuit acts quickly to recognize line-to-line faults and make the conductors safe in a +/-190V environment. The DC-2 range represents the area where no lasting physiological damage occurs.
Stability: All Operating Voltages are Not Equal
As customers grow their networks, they often ask fundamental questions about the powering consequences of different power architecture decisions. While customers can choose from -130V, -190V, or +/-190V backgrounds, we have engineered our solutions around the +/-190V. While the -130V and -190V are options, we feel the +/-190V provides the most network-deployment flexibility.
Compared to the -190V system shown in Figure 3 the +/-190V network modeled in Figure 4 presents a completely different scenario, and a completely different future for the network going forward. The entire operational envelope for the +/-190V service provider is stable and well behaved even at powers up to 60 Watts per pair.
Looking closely, one can see that the 100VA safety limit of 250 mA further protects the system from instabilities. With this architecture the service provider can focus on providing services rather than worrying about network stability.
Plan Your Network: Reliable Tools Do Exist
Whether a network is brand new or an upgrade for existing customers, the power needs to be planned. There are several network planning tools available. The following spreadsheet-based tool, with data validation controls on the input cells, allows customers to conduct “what if” scenarios to thoroughly consider all alternatives before deciding how to provision the network. Of particular interest to some customers is the provisioning of multiple pairs per 100VA RFT-V circuit. This approach provides the benefits of effectively bringing your remote customer closer to you. Round trip loop resistance is halved the first time one combines pairs.
One concern customers have with this approach is recognizing latent failures when one of the multiple conductors is severed in a construction mishap, but not properly repaired afterward. This condition will manifest as additional power loss in the partially repaired circuit. Since all circuits feeding a particular remote cabinet will share, group sharing thresholds can be set to alarm deviant behavior. This fault detection circuitry requires at least two 100VA RFT-V circuits feeding a common load to operate. A screen shot of the most commonly used planning tool is shown here.
In conclusion, powering the fiber-based network over existing copper pairs is a viable solution. There are established and accepted safety standards. +/-190V networks are stable for the range of triple-play relevant downstream loads. Network planning tools are now available to coordinate the use of your existing copper network to power your triple-play deployments. Maximize your return on investment in the copper network. Don’t waste it.
About the Author:
John Pendergrass is the outside plant product line manager for Lineage Power’s Energy Systems Division. He has more than 25 years experience in the telecommunications market. John has served Lineage Power in sales, marketing, product and project management, and merger, acquisition and alliance roles for 12 years.