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work, period. Fortunately, uninterruptible power supplies (UPSes) are designed especially for networking to preserve, at least for a limited time, the power flowing through network components in the event of power loss.

Today's UPSes not only protect network components from power disturbances, but also serve as actual network components. UPSes communicate both with users and with other components on the network. UPS communication possibilities include notifying admini strators of power problems (before more serious ones occur), providing detailed statistics on power usage and bringing down servers gracefully.

Problem Solving--UPS Style Aside from the disasters a total loss of power can cause, other less-noticeable power anomalies occur, including powers sags, surges and spikes. Surges (over voltage) happen when too much power is supplied by the utility company. Because power is supplied in a grid over a geographical location, regions close to utilities will react to major changes in the grid when segments lose power and when they regain it. Transformers and other equipment that drive and monitor the utilities' power supply frequently can't respond in the time that sensitive equipment requires. Sags (under voltage) occur when not enough power is supplied by the utility.

Conditions such as intense weather can generate problems as well. Spikes often are caused by lightning or high winds that short out power lines. If you've ever been within a mile of a transfor mer when it blows up, you can understand the magnitude of that force--it's deafening.

Interruptions to the normal sine wave of AC power can freeze the operation of hardware, requiring the power cycle of a machine to get it back on track. Worse, severe spikes and lengthy power swells can damage electrical components, including circuits, power supplies and motors that handle moving parts such as hard drives. This can be expensive, both for the cost of the hardware and the cost of downtime while the equipment is replaced.

Even within a building's power structure, servers located on the same circuit as an elevator in a building may behave erratically because of the starting and stopping of large motors. Remember, when a server freezes, data cached in memory that hasn't been written to disk yet is lost forever. The worst part is you don't know exactly what was lost until a user looks for that data. Another adverse effect of power disturbances is the corruption of data. This is less likely to be noticed bec ause it can happen even if the victimized machines do not freeze or show any other type of strange behavior.

For instance, a slight power sag might not affect a server enough to freeze it, but could impede the transfer of data to a power-sensitive external RAID array or tape autoloader sensitive to minor power changes. Another situation finds the server and its components unchanged, but somewhere a hub loses its cool and fails when hit by a spike, rendering the server on that segment unreachable.

Choosing a UPS for Your Network There are three types of UPSes: offline, line interactive and online. Offline UPSes are the cheapest on the market. An offline UPS basically consists of inverters and it does not condition or treat the power until the most unfavorable conditions occur. Aside from offering simple spike and surge suppression, an offline UPS simply waits for something to happen. It requires minimal equipment and serves as a passive system that constantly monitors the flow of electricity. W hen a power surge or sag reaches a certain threshold, or if power is lost completely, the inverters take over and move the power supply from the wall to the UPS' batteries. When the power returns to normal, the UPS turns off the inverters and returns the normal utility power to the components being served.