Don't Get Caught Without Backup

You can control a lot of things about your point-to-multipoint network, but the commercial ac power source isn't one of them. Although power is integral to network reliability, it is one of the most overlooked aspects of site design. In fact, many carriers don't even employ a power engineer.

You must consider power requirements when you first prepare your business plan. By doing so, you can ensure the survivability of your service, regardless of the state of external power in an area. Adequate power is a quality issue and a systems-reliability issue, but most of all, it's a business issue.

For the most part, point-to-multipoint radio equipment uses dc battery power rather than 110Vac power. Therefore, you have the option of providing back-up power in case of emergency. With dc power systems, a rectifier receives the power from the 110Vac main and converts it to 48Vdc. This, in turn, float-charges a battery plant connected to the radio equipment. When you lose ac power, the battery plant will continue to provide power for a predetermined length of time.

Not all carriers provision back-up battery facilities at all sites. Some assume that because utilities are located underground in core business districts, power outages are not a threat. In reality, power-grid failures and building-maintenance-related outages are common, but you can take steps to ensure your network is immune to ac-related power failures.

Radio-Hub Backup Options

The first line of defense is to install a battery backup plant, which will provide power after an outage. The battery plant's size depends on the amount of power the equipment draws and how many hours of backup power you want (normally four to eight).

Unlike cellular technology, which requires large amounts of power, point-to-multipoint technology has relatively low requirements (hundreds of watts vs. thousands of watts). These low requirements, and the fact that modern gel-cell batteries are smaller, safer and easier to handle than lead-cell batteries, minimize space considerations. Providing battery backup in a typical radio hub, for example, only requires a space large enough to hold the equivalent of four to eight automobile batteries.

To accommodate the time between the end of battery backup and when you re-establish commercial power, consider installing a generator port and manual-transfer switch. This solution provides an easy way to hook up trailer-mounted generators via an external power port, which usually is located at ground level of the building's exterior. Although this method is an inexpensive way to safeguard a radio hub, disaster-prone areas might not have a generator available to rent. You may want to purchase some generators to share across all of your facilities. Assuming all sites don't fail simultaneously, it is more cost-effective than placing a generator at each location.

To ensure there is never a service interruption, regardless of how long the power grid is off-line, you can permanently installan ac generator with an automatic-transfer switch. However, this approach causes a few environmental concerns. For example, the type of fuel you use often is restricted for in-building generator applications, and local fire codes may prohibit generators altogether. Because generators cost approximately 20,000 dollars to 35,000 dollars, you must weigh the expense against the investment's value.

End-Site Battery Backup

You've protected the main switch, and the radio hubs are backed up, but what if power to a customer's building fails, and all customers lose service? To make sure this doesn't happen, you need to address the end site (radio interface) at the customer's premise.

Because the power requirements of the radio interface are even smaller than those of the radio hub, the cost to provide the necessary backup protection is small. Costs add up, however. Even a small battery plant, which costs about 800 dollars to 1,000 dollars per building, represents a significant portion of the overall cost of radio equipment deployed at the end site. In addition, you might deploy your system in thousands of buildings, and the cost to provide battery backup for the radio-interface portion of a network can be significant.

When justifying the cost of this added protection, remember that you are in the business to provide customers with an end-to-end network. The power considerations at the end building are just as important as the power considerations elsewhere. As point-to-multipoint networks evolve, you must ensure reliability to earn new business.

Protecting the Network End-to-End

If a commercial ac main supplies power to the radio hub and interface, ultimate network reliability is out of your hands. Everything from utility or in-building maintenance to natural disasters can lead to outages. Anytime the ac main is touched, the potential for disaster increases.

When planning a network, engineers normally provision appropriate dc power backup for main switching centers. They view these areas as the central activity points in the network -- if they die, everything dies. The same can be said for the radio hub sites and customer equipment.

Buying backup power is like buying insurance. How much you need depends on the amount of risk vs. the cost you want to assume, given the site's characteristics. Normally, eight hours of battery backup is recommended at the radio hubs and main switching centers when there is no on-site generator.

You also should equip these sites with a manual-transfer switch and an external power port to accommodate a generator. You will recoup the added expense during one power outage. Give customer equipment at least four to eight hours of coverage if you want to assure end-to-end network survivability.

You do not want to be at the mercy of an external environment, so controlling the events that affect service is essential. Making power requirements part of your initial planning process is one way to ensure consistent, high-quality service. That way, you leave nothing to chance -- especially your customers' business.