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The tech term “always on” or “always-on” refers to systems that are continuously available, plugged in, or connected to power sources and networks. Always on may also refer to systems that are continually operational — that do not take breaks, but continue to hum along through all hours of the day and night.
As computing became more sophisticated, always-on systems have started to replace older on-demand systems that functioned on different types of resources. For example, some of the best examples of modern always-on systems are cable modems and DSL connections. That is a contrast to yesterday's dial-up connections that were only "on" when they were connected through a particular method using the public switched telephone network.
Other systems can also be described as "always on." For instance, there are the Always On Available Groups featured in SQL Server 2012, involving high-availability disaster recovery resources. In general, always on means that the system is always operational and responding to demands from users.