Backward compatibility

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In the context of telecommunications and computing a device or technology is said to be backward or downward compatible if it can work with input generated by an older device.[1] If products designed for the new standard can receive, read, view or play older standards or formats, then the product is said to be backward-compatible; examples of such a standard include data formats and communication protocols. Jocularly referred to as "hysterical raisins" i.e., a homophone like phrase for "historical reasons".

The reverse is forward compatibility, which implies that old devices allow (or are expected to allow) data formats generated by new (or future) devices, perhaps without supporting all new features. A standard supports forward compatibility if older product versions can receive, read, view or play the new standard.

For example, the introduction of FM stereo transmission allowed backward compatibility since new FM radio receivers could receive monaural signals generated by old transmitters. It also allowed forward compatibility, since old monaural FM radio receivers still could receive a signal from a new transmitter.

In programming languages, backward compatibility refers to the ability of a compiler for version N of the language to accept programs or data that worked under version N - 1.[2] (By this definition, if previous versions (N - 1, N - 2, etc.) were also backward compatible, which is often the case, then, by induction, version N will also accept input that worked under any prior version after, and including, the latest one that was not backward compatible. However, in practice, features are often deprecated and support is dropped in a later release, which is yet thought of as backward compatible.)

In other contexts, a product or a technology is said to be backward compatible when it is able to fully take the place of an older product, by inter-operating with products that were designed for the older product.[3][Need quotation to verify]

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Description

Backward compatibility is a relationship between two components, rather than being an attribute of just one of them. More generally, a new component is said to be backward compatible if it provides all of the functionality of the old component.[citation needed]

Backward compatibility is the special case of compatibility in which the new component has a direct historical ancestral relationship with the old component. If this special relationship does not exist then it not usually spoken of as "backward" compatibility but is instead just "compatible"—a consistent interface allowing interoperability between components and products that were each developed separately.

Data does nothing in the absence of an interpreter, so the notion of compatibility does not apply to document files, it only applies to software. In the case of a program that creates document files, a new version of that program ("v2") is said to be backward compatible with the old version of the program ("v1") when it can both read and write documents that work with v1. Everything that v1 could do must also be possible with v2, including saving documents that can be read by v1 (which is something that v1 could do.)

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