Chapter 24 · ACN 251 C ONTROL OF D EVICES As detailed above, at ACN's functional core are the virtual properties of each device, and each property represents some controllable or monitorable aspect of a device. A property could be only remote-controllable over the ACN network, or, conversely, could represent a value main- tained locally by the device (such as the current value of a local control slider that might be moved by an operator at any time). Properties can also be permanently fixed (i.e., a unit serial number) or held constant by a device, and they can be set to be read only, write only, or set for read/write access. A single, physically controlled unit can be made up of one or more virtual devices; for instance, a dimmer rack might have 96 ACN devices for level control of each dim- mer, and also contain a 97th device related to the whole unit, which reports information such as the monitored temperature, fan speed, incoming voltage, current draw, and so on. Alterna- tively, something like an LED fixture might be represented as a single device, with three color properties (RGB). The Device Management Protocol (DMP) is the mechanism to control, configure, or monitor specific properties in a connected device, and DMP messages are sent over virtual device con- nections (created and handled, typically, by SDT). DMP only manages properties and their values, it doesn't really care about or understand the meaning of any particular property (meaning is handled in the DDL). To accomplish all this sophisticated control, DMP uses only 17 messages, the most common of which are Get Property and Set Property; Sub- scribe and Unsubscribe; Get Property Reply, Get Property Fail, Set Property Fail, Subscribe Accept, Subscribe Reject, and Event.