|  | 8.11 | Network Power Supply Issues: Surge Suppressors and Uninterruptible Power Supply (UPS) Functions
| 8.11.7 | UPS: description and operation
As a rule, UPS devices that offer fewer features, and cost less money, are used as standby power systems only. This means that they monitor power lines. If, and when, a problem occurs, the UPS switches over to the inverter, which is powered by its batteries. The time needed for this switch to occur is called the transfer time. Usually, the transfer time lasts for only a short time. This does not usually present a problem for most modern computers, which are designed to coast on their own power supplies for at least a hundred milliseconds. UPS devices that offer more features, and cost more money, typically operate online. This means that they constantly supply power from inverters, which are powered by their batteries. While they do this, their batteries continue charging from the power line. Because their inverters supply freshly generated AC, such UPS devices have the added benefit of ensuring that no spikes from the power line reach the networking devices that they serve. If, and when, the AC power line goes down, however, the UPS batteries will switch, smoothly, from recharging to providing power to the inverter. Consequently, this type of UPS effectively reduces the transfer time needed to zero. Other UPS products fall into a hybrid category. While they appear to be online systems, they do not run their inverters all the time. Because of these differences, be sure to investigate the features of any UPS you plan to incorporate as part of a LAN installation. In any event, a good UPS should be designed to communicate with the file server. This is important so that the file server can be warned to shut down files when the UPS battery power nears its end. Additionally, a good UPS reports instances when the server starts to run on battery power, and supplies this information to any work stations running on the network, after the power outage has occurred.