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

, 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 or transmit may refer to: Science and technology * Power transmission ** Electric power transmission ** Transmission (mechanical device), technology that allows controlled application of power *** Automatic transmission *** Manual tra ...

, before being separated again (decommutated) upon reception; it is a form of time-division multiplexing.

Frame synchronization In telecommunications, frame synchronization or framing is the process by which, while receiving a stream of fixed-length frames, the receiver identifies the frame boundaries, permitting the data bits within the frame to be extracted for decodin ...

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 sources in turn, before returning to the first data source. 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 words, 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 or a type of data store based on the use of a database management system (DBMS), the software that interacts with end users, applications, and the database itself to capture and a ...

, 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 any dynamic far from synchronization. If and as parts of an asynchronous system become more synchronized, those parts or even the whole system can be said to be in sync. Asynchrony or asynchronous may refer to: Electronics and com ...

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