The limited availability of frequencies keeps these systems out of metro areas. Utilities in outlying areas are big users of licensed systems. The radio signal is transmitted through repeaters by the same service providers used by taxicabs. Cellular telephones also can be used to transmit data in such situations. As noted above, any data acquisition system might mix and match components and transmit data over multiple interfaces.
Protocol converters help a great deal in the interfacing of various hardware.
Provides a transparent interface between two dissimilar devices. For example, it allows an IEEE 488 instrument to communicate with an RS-232 printer or plotter. RS-422 supports up to 32 devices. RS-485 supports up to 64 devices. An auto-dialer monitors conditions at a remote site and, if an alarm condition develops, alerts people or other equipment.