Historians have interfaces toward the most common industrial protocols and fieldbuses, including PROFIBUS, MODBUS, DeviceNet, and OPC, and sometimes also legacy protocols for old plants or DCS. These interfaces implement the most efficient ways to gather data according to the abilities of the specific protocol. To store large amounts of data with minimum disk usage and acceptable approximation errors, Historians rely on a data-filtering and compression engine. The values are stored according to the dynamic of the signal and their filtering and compression mechanisms. Signals coming from the same device, such as the pressure and the flow rate of a pump, could have, and typically do have, their row data stored in different timestamps. Typically, Historians store time-series data following a hierarchical data model.