As it becomes an increasingly vital service for carriers, providing continuous availability with 24x7 power protection and environmental control will be essential.
September 1, 2002
Wireless data is experiencing tremendous growth as carriers roll out an array of capabilities to differentiate their services. This growth represents economic benefits and brand loyalty for carriers and greater convenience and productivity for users.
It is estimated that user demand for wireless data will escalate from less than one megabyte (MB) per month per user in 2001 to more than 200 MBs per month per user by 2006.
In addition to greater service capabilities, a wireless carrier must also offer service-level agreements (SLAs) that deliver availability equal to the current public switched telephone network’s (PSTN) uptime measurement of 99.999 per cent.
Total service assurance is a strategy that proactively combines improvements in network reliability, service quality and operational efficiency. This article will address these areas as they relate to the infrastructure supports for power protection and environmental control. Topics covered include transient voltage surge suppression (TVSS), hybrid (AC and DC) power configurations, uninterruptible power supply (UPS) and generator back-up, power for cooling requirements, redundancy architectures, and monitoring capabilities.
SUPPRESSING THE SURGE
The growing availability requirements for communications networks at carrier and enterprise organizations have resulted in increased demand for surge suppression that protects critical voice and data networks. Transient Voltage Surge Suppression (TVSS) provides an initial layer of security for anything from single servers to entire facilities. TVSS technology is particularly useful for protecting remote telecom sites, which have sensitive electronic equipment that can be damaged by even the slightest power disruptions.
TVSS provides noise filters that remove small surges for continuous distortion limitation at every point of the sign wave, whether it’s a surge or spike. The filtration provided by TVSS is one of the few methods of limiting ring waves, which are produced by immediate reductions or increases in the power supply.
For critical communications networks, TVSS units may be applied to electrical distribution panels to protect both linear and non-linear loads from damaging transients and electric line noise. These units are designed to limit power spikes on voice and data lines while lessening smaller power aberrations as well. Through circuit sensors, these units report of potential failure through status contacts with equipment load.
TVSS can be a cost-effective solution for reducing UPS usage, thus preserving battery life and lowering maintenance costs. Compared to a UPS, surge suppression equipment is less costly to maintain and replace, making this initial layer of protection a significant cost savings for many organizations.
The effectiveness of even the most advanced surge suppression device depends as much on how it is grounded and installed as the technology itself. Before installing any power protection device, a thorough site audit needs to be conducted that include some less obvious factors that contribute to interrupted service. Disruptive problems include ground impedance factors such as the type of soil, the humidity, and the methods of grounding used.
DISTRIBUTED REDUNDANT HYBRID POWER
Communication networks were previously made up of analogue equipment for voice systems, which typically required a DC power source. In contrast, data systems generally need an AC power source. The development of combined data and voice networks has created a situation where both AC and DC are needed for the same systems operation.
There are a variety of power protection strategies for data and voice systems including a DC power supply with inverters for AC loads, a DC power supply and an AC UPS for AC loads, and an AC UPS with rectifiers for DC loads. The use of generators will also be covered as a practical substitute in lieu of substantial battery back up. There are advantages and disadvantages to each of these methods, and the proper arrangement is typically a matter of budgets and application criteria.
For networks that must remain up and running, a hybrid distributed redundant power system may be the best strategy (see figure 1). Systems of this type utilize distributed, redundant DC rectifier systems supplied from large, dual redundant AC UPS systems. Small, self-contained DC rectifier systems along with AC Power Distribution Units (PDUs) can be located throughout a facility to supply either AC or DC power to the load equipment. The high availability power protection solutions of data centres are merged with telecom DC power systems to optimize reliability. Redundant, standby generators can also be utilized for dependable power in the event of a sustained utility power failure.
A hybrid distributed redundant power configuration provides substantial reliability for wireless systems by using dual redundant AC UPSs with redundant power distribution paths. A high degree of fault tolerance is obtained without any single points of failure in the AC or DC power system. Any component within the AC or DC power system can fail or be maintained without disrupting equipment operation.
UNINTERRUPTIBLE POWER SUPPLIES
A UPS provides conditioned and dependable AC power to keep sensitive computing systems up and running. For situations where a cost-effective solution is needed, a line-interactive topology offers power conditioning to protect against sags, spikes, surges, and brownouts, as well as providing battery back up.
For sensitive wireless equipment, a true online double conversion UPS is the optimal solution for true isolation from problems originating from utility or generator power. In this configuration, these units provide “bullet-proof” protection for mission critical systems. Double conversion technology runs input AC power through the battery via the rectifier at the same time the inverter converts this power back to conditioned AC power.
Double conversion technology produces only pure sinewave output, the most superior power quality for maximum systems availability.
A UPS system cannot keep systems running forever, but they do allow enough time for a graceful shutdown. For applications where downtime is unacceptable, other standby power sources may be needed, such as additional batteries or back-up generators.
GENERATORS AND UPS TECHNOLOGY
Generators provide back-up power that can withstand a variety of disruptions, supplying power for hours or even days. As a practical alternative, a power system that includes generators may reduce the number of large, heavy batteries that are required for critical systems. Specific to wireless organizations that have hundreds of sites in a geographic area, a cost effective alternative may benefit from the use of portable generators that can be send out as needed. These portable units can provide a significant cost savings and eliminate the hassles of obtaining permits for permanently sited generators.
A common problem that occurs during a power outage involves the starting of loads on a generator, which causes output frequency to vary, thereby triggering a line-interactive UPS to cycle to battery operation. The problem is especially pronounced with natural gas generators. This repetitive battery cycling can completely discharge the battery, significantly shortening its life. Another possible concern is the generator instability that occurs when the UPS load is transitioned to the generator. The UPS load transfer causes the generator voltage to sag, which triggers the UPS to go back to battery operation.
Once the UPS senses steady generator output, it switches the load back to the generator. If generator output drops again, the load will shift back to battery operation.
For a power protection strategy that includes generators, a UPS that has double-conversion technology is ideal. A double-conversion UPS can handle erratic changes in power supply frequency while maintaining regulated, consistent output power frequency, without the use of a battery. Incoming AC power is rectified to DC power to supply the internal DC bus of the
The output inverter takes the DC power and creates regulated AC power to support the critical load. Batteries attached to the DC bus are float charged during normal operation. When the input power is outside the normal operating limits, the batteries supply power to sustain the inverter and critical load. During this transfer in power from the utility to the generator, the UPS is the control bridge supporting transfer between utility and generators until these generators reach full capacity.
COOLING SYSTEMS AND POWER
For critical wireless networks, power protection must be just as reliable as cooling systems. During a blackout, a UPS kicks in to provide power to critical computers and components. However, most UPS systems do not have the power available to run cooling systems.
At this point, the temperature in a data centre may escalate due to built-in delays on HVAC restarts after emergency power generators have started. During this delay, the temperature can rapidly climb to the point where reliability of vital systems may be compromised.
As this situation demonstrates, an infrastructure that maintains continuous availability must include design and implementation of an integrated system for both power and cooling solutions. This mandates a comprehensive approach to infrastructure supports that take into consideration the individual needs of every communications network and operating environment.
Most power protection systems can be paralleled to provide N+1 redundancy both in systems and on the component level for enhanced protection. A redundancy configuration offers substantial reliability and can be economical and easy to deploy. Continuous availability requires dual bus architectures, with redundancy throughout, to eliminate single points of failure and maintainability. For power protection, a dual bus configuration provides a mirrored system, which may be electronically tied together. In the event that one of the buses goes down, an identical system is ready to handle the load for a flawless transition.
Access and control of infrastructure support systems is essential to understanding the operation and availability of communications networks, particularly for remote and unmanned wireless shelters.
The most recent solutions include out-of-band monitoring using a modem or a Internet-based system, and in-band monitoring through an internal network connection. For continuous communications capabilities, an in-band connection can be used in conjunction with out-of-band access in the event that the network goes down or is not available. Most network monitoring systems allow for management and control of infrastructure supports using either SNMP (Simple Network Management Protocol) or http access.
Most monitoring solutions include trending and analysis reports to precisely track down events. In many cases, it is possible to map out the underlying cause of a problem by looking at historical logs and documentation. This is supplied by system messaging, event and status logs and time stamps, which can point to an exact reason and the time of incidence for a specific problem.
Internet-based monitoring may be particularly valuable for some applications like remote sites and shelters. As a complete part of a communications system, a single site has the capacity to take down an entire network. Therefore, the ability to monitor and control power from any location that has Web access is an exceptional benefit for individuals responsible for network uptime.
As wireless data continues to grow as increasingly vital services for carriers, being ready with networks that offer complete reliability will be essential. This includes an infrastructure that offers power protection and environmental control using technologies and configurations that maximize availability. For many carriers, this will be a crucial element in gearing up for the increasingly competitive wireless market of the future.
James Hall is telecom market manager for Liebert Corporation in Columbus, OH. He can be reached via email at firstname.lastname@example.org.