Time to Go On Line?
A Comparison of Powering Strategies for the ONT
by: Kevin Borders
During the first few years of Fiber-to-the-Home (FTTH) deployments, the most prevalent method of powering Optical Network Terminals (ONT) was to provide Uninterruptible Power Supplies (UPS) or battery backup systems at the customer premises. Some providers placed the unit inside the subscriber’s home and turned over ownership and maintenance to the customer. Others, preferring to maintain control of their network, placed the backup system on the side of the house. Both solutions were viable, but were also vulnerable to extended power outages. As FTTH service matured and became more widespread, some service providers began to seek ways to avoid placing batteries at the edge of network, where maintenance costs were highest.
The search for new and different techniques for powering ONTs coincided with a shift in network deployment trends. The initial targeted applications were greenfield, or new plant builds, such as new subdivisions. These applications were the low-hanging fruit of FTTH deployments since the cost of placing the fiber in an open trench was so much less than in an existing neighborhood. The telco would install the Optical Line Terminal (OLT) at the Central Office (CO), build a Passive Optical Network (PON) in the outside plant (OSP), and place ONTs at each home with some form of power and battery backup to run the ONT. Providing power at the home for the ONT became known as Local Power.
As the greenfield deployments saturated and FTTH installation techniques matured, telcos began to look at upgrading existing neighborhoods where fiber would replace an existing copper network. It was then that service providers and vendors began to explore the use of copper cable to deliver current from the existing CO power system to energize the ONT. This method, called Line Power, has caught on in many areas as service providers opt for the high reliability of the CO power plant to supply power to the ONTs.
Both Local Power and Line Power techniques have roles in today’s FTTH networks. The technology choice is dependent on many factors including:
- Copper availability
- Cost of maintaining both the copper and fiber networks
- Frequency of extended outages in an area
- Installed initial cost
- Overall life cycle cost analysis
The purpose of this article is to identify the issues and concerns in powering ONTs, and then to compare the pros and cons of each method. Our intent is for the service provider to understand the best scenarios for deploying each powering technique.
There are 2 primary methods of Local Power, both involving AC supplied at the home and some form of Uninterruptible Power Supply (UPS) for battery backup. The AC provided at the home or business is converted by a UPS to 12Vdc to run the ONT, and is then backed up by a lead acid battery in case of a commercial power failure.
Customer UPS Method
One local power technique is called the Customer UPS Method. This approach involves placing the power supply/battery back-up unit inside the residence or perhaps in a garage. This approach is depicted in Figure 1.
Gaining access inside the home can be a logistical issue, but it is usually a “one-and-done” event. The telco incurs an upfront cost of deploying the ONT and battery backup system, and then turns the power asset as well as the maintenance responsibility over to the customer. While this technique lowers the total cost of the installation, it does have some drawbacks.
The Customer UPS Method is prevalent with a couple of very large carriers, but is less acceptable to many Independent Operating Companies (IOCs) due to concerns about customer satisfaction. Many feel that yielding control of the battery and power system to the consumer might result in the perception of poor service, especially if the customer is not technically savvy enough to maintain or even replace the battery. Even if the customer fails to take the proper action, these IOCs believe the lack of service will still reflect poorly on the telco. In smaller communities, this can become a real issue since the subscribers often know who works for the phone company and may opt to lodge their complaint personally. In larger towns, this may not be as much of a concern due to the relative anonymity of the telco personnel.
Though batteries typically last longer indoors than outdoors, they do eventually fail. When they do, the customer not only has the responsibility of procuring and replacing the batteries, but also recycling the failed battery. Some telcos have programs for customers that simplify the recycling process, but it’s safe to assume that more than a few batteries have ended up in landfills. One other concern with local batteries is that they are vulnerable to extended AC power outages. So while the CO may be up and running, consumers may not be able to access the network because the battery is out of juice.
The Customer UPS Method can offer the lowest costs since the subscriber bears the responsibility for ongoing battery maintenance as well as the small, but recurring, cost of supplying AC power for the device. The Customer UPS Method played an important role in the early adoption of FTTH, but it does have some negative attributes that may offset the low-cost offering.
Telco UPS Method
To overcome the perception of poor customer service, many service providers place the UPS and/or the ONT outdoors on the side of the home. This not only gives them control of their entire network, but also avoids the hassle of coordinating with the customer to gain access to the home. The technique, known as the Telco UPS Method, is shown in Figure 2.
As with the Customer UPS Method, the AC power for the ONT and for the battery backup equipment for powering subscriber equipment is provided at the house. Sometimes, the subscriber supplies the power in the form of an existing AC outlet. In most cases, however, the telco is responsible for adding an AC connection, either as a new receptacle or as a connection to the meter. When a new AC connection is required, the telco must incur the expense of an electrician and often the recurring monthly cost of electricity (albeit a very low recurring amount).
Accessing AC at the meter provides a very reliable source. The meter must be retrofitted with a power ring that taps AC power from the utility meter base and converts it to a nominal 12Vdc. That 12Vdc source connects to a battery backup system to ensure service in the event of an outage. While this technique is ideal for ensuring a simple, professional, and reliable installation, some power utilities are reluctant to coordinate with the telephone companies. A typical Power Ring Converter is shown in Figure 3.
More commonly, an AC outlet is added by an electrician and fed from the subscriber’s breaker panel. Either approach eliminates the need for coordinating access inside the home. As with other OSP applications, the life cycle of lead acid batteries in outdoor environments is shortened due to being subjected to temperatures that fluctuate above and below the recommended operating temperature of nominal 25°C (77°F). Consequently, it is not uncommon to have to replace batteries every 2-3 years. Ongoing battery maintenance is a big issue for the Telco UPS Method since it drives up the total life cycle cost due to the cost of truck rolls to replace and/or maintain the batteries.
On the plus side, the replaced batteries will be included in the telephone company’s normal recycling process, ensuring that these batteries do not end up in landfills. But because the power is derived locally, this method is vulnerable to extended outages. In other words, once the battery reserve is exhausted, the consumer is offline until utility AC service is restored.
The alternative to locally supplied power is power delivered over copper cables from a centralized location. This approach, called the Line Power Method, uses the highly reliable centralized power systems to power ONTs. In using abandoned copper cable to deliver the current, the Line Power Method makes use of existing telco assets. And by using the central power system as the source, the telco can entirely avoid placement of batteries at the edge of the network. A diagram of the Line Power Method is provided in Figure 4.
Typically, a reliable -48Vdc power plant and battery backup system serves as the ultimate source for the line power. The line power system converts the -48V to ±190Vdc through the deployment of dc-dc converters, called Up Converters. The ±190Vdc is carried across the copper pairs until it reaches its destination, where the elevated voltage is converted by a Down Converter back to 12Vdc to power the single family ONT. The process is depicted in Figure 5.
Unlike the Local Power Methods, the Line Power approach requires equipment at the central site as well as at the home. The CO equipment consists of a shelf of Up Converter modules. The 48V power plant connects to the input side of the shelf; the output of the shelf connects to the Main Distribution Frame (MDF) for connection to OSP cables. The CO power may need to be augmented (add rectifiers and/or batteries), depending on the spare capacity available in the CO.
At the customer’s premises, the Down Converter equipment mounts to the side of the house. The OSP drop cable connects to the input of the Down Converter while the output connects to the ONT. A typical CO installation and a home installation are shown in Figure 6.
Since the power in the Line Power Method comes from the CO, this technique is the most robust, reliable solution. Of course, a cable cut would interrupt power to the ONT, but would likely sever the communications path as well. In all other cases, this design enables the circuit to continue operating even during periods of extended outages; in essence, as long as the CO is operating, the line-powered circuits are as well.
One drawback for Line Power is the need for copper. This limits Line Power to overbuild and rehab applications since service providers do not normally deploy new copper for the purpose of powering the ONTs. There have been some cases where a service provider elected to deploy composite fiber/copper cable, but those instances have been fairly rare. For cost comparisons with local power, cable is presumed to be available and fully amortized for any application considered suitable for Line Power.
Since there is a CO component, additional power and batteries may be required at the CO. Though many carriers have excess CO power capacity from their switch modernization programs, a true comparison of power methods will include a cost component for CO power augments.
There are no batteries at the customers’ premises, so the Line Power Method minimizes operating expenses. Truck rolls are virtually eliminated since there are no remote batteries to maintain or replace.
This approach is environmentally friendly because it:
- Avoids the proliferation of lead acid batteries.
- Reduces fuel consumption and carbon emissions by minimizing truck rolls.
- Reuses deployed copper cable.
- Places the responsibility for managing the ONT power in the hands of professionals rather than untrained subscribers.
The Bottom Line
Both Local and Line Powering techniques have roles in today’s FTTH networks. The Customer UPS Method has traction with service providers that emphasize cost containment and are willing to endure occasional push back from customers.
For service providers that prefer to maintain and control their networks, both the Telco UPS Method and the Line Power Method have roles. The Telco UPS Method is the best solution for greenfield applications. But for overbuilds and rehab applications, the Line Power Method offers compelling benefits. By using copper that has already been deployed, it avoids stranding costly assets. It offers superior reliability to locally powered ONTs, plus lowers the total cost of ownership by eliminating truck rolls to maintain batteries at the customer’s premises.
By avoiding the use of small, distributed batteries, Line Power is a more environmentally acceptable solution as well. Perhaps most importantly, the trained professional service provider supplies the battery backup, resulting in improved reliability and increased customer satisfaction.
Unlike the early days of FTTH, the network designer has several viable options for powering ONTs. Line Power should now be considered one of the leading alternatives.
Kevin Borders is a Director of Product Marketing at Alpha Technologies. He has more than 30 years of experience in the Telecommunications and Power sectors, having worked extensively in Marketing, Sales, and Product Management roles at PECO II and Marconi Communications, Inc. For more information, visit www.alpha.ca.