telemetry Telemetry is the in situ data collection, collection of measurements or other data at remote points and their automatic data transmission, transmission to receiving equipment (telecommunication) for monitoring. The word is derived from the Gr ...

, commutation is a process whereby multiple data streams ("measurands"), possibly with differing data rates, are combined into a single frame-based stream for

transmission Transmission may refer to: Medicine, science and technology * Power transmission ** Electric power transmission ** Propulsion transmission, technology allowing controlled application of power *** Automatic transmission *** Manual transmission *** ...

, before being separated again (decommutated) upon reception; it is a form of

time-division multiplexing Time-division multiplexing (TDM) is a method of transmitting and receiving independent signals over a common signal path by means of synchronized switches at each end of the transmission line so that each signal appears on the line only a fracti ...

Frame synchronization In telecommunication, frame synchronization or framing is the process by which, while receiving a stream of framed data, incoming frame alignment signals (i.e., a distinctive bit sequences or syncwords) are identified (that is, distinguished from ...

must be achieved before a data stream can be decommutated.

Etymology

Commutation is named by analogy with electric commutators, which engage multiple electrical contacts in sequence as they rotate; similarly, telemetry commutation involves sampling a sequence of data points in turn, before returning to the first data point. Hardware or software which performs commutation is referred to as a commutator; its opposite at the receiving end is a decommutator. Dedicated hardware generally supports faster commutation and decommutation than software on a general purpose architecture.

Mechanism

A set of

data word In computing, a word is the natural unit of data used by a particular processor design. A word is a fixed-sized datum handled as a unit by the instruction set or the hardware of the processor. The number of bits or digits in a word (the ''word si ...

s, together with synchronization and ID or counter words, constitute a minor frame; a set number of minor frames are combined to form a major frame. Measurands occupy fixed positions within each major frame, with these positions defined in a

database In computing, a database is an organized collection of data stored and accessed electronically. Small databases can be stored on a file system, while large databases are hosted on computer clusters or cloud storage. The design of databases sp ...

, allowing them to be extracted. Measurands may be sampled multiple times within each minor frame (supercommutation), or they may only be sampled once in several frames (subcommutation), depending on the required data rate for each measurand. Commutated frames may also contain

asynchronous Asynchrony is the state of not being in synchronization. Asynchrony or asynchronous may refer to: Electronics and computing * Asynchrony (computer programming), the occurrence of events independent of the main program flow, and ways to deal with ...

data, which require further processing to extract. Decommutators may be equipped to handle multiple frame formats, containing different sets of measurands in different positions, for use in different operating modes; a switch in format may be signaled with a specific measurand. Translators exist to allow support for equipment-specific database formats. Decommutated measurand values may be stored in a "current value table" or other architecture.

References

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Etymology

Mechanism

See also

References

Further reading