Forward compatibility

Forward compatibility or upward compatibility is a design characteristic that allows a system to accept input intended for a later version of itself. The concept can be applied to entire systems, electrical interfaces, telecommunication signals, data communication protocols, file formats, and computer programming languages. A standard supports forward compatibility if a product that complies with earlier versions can 'gracefully' process input designed for later versions of the standard, ignoring new parts which it does not understand. Forward compatibility or upward compatibility is a design characteristic that allows a system to accept input intended for a later version of itself. The concept can be applied to entire systems, electrical interfaces, telecommunication signals, data communication protocols, file formats, and computer programming languages. A standard supports forward compatibility if a product that complies with earlier versions can 'gracefully' process input designed for later versions of the standard, ignoring new parts which it does not understand. The objective for forward compatible technology is for old devices to recognise when data has been generated for new devices. Forward compatibility for the older system usually means backward compatibility for the new system, i.e. the ability to process data from the old system; the new system usually has full compatibility with the older one, by being able to both process and generate data in the format of the older system. Forward compatibility is not the same as extensibility. A forward compatible design can process at least some of the data from a future version of itself. An extensible design makes upgrading easy. An example of both design ideas can be found in web browsers. At any point in time, a current browser is forward compatible if it gracefully accepts a newer version of HTML. Whereas how easily the browser code can be upgraded to process the newer HTML determines how extensible it is. The introduction of FM stereo transmission, or color television, allowed forward compatibility, since monophonic FM radio receivers and black-and-white TV sets still could receive a signal from a new transmitter. It also allowed backward compatibility since new receivers could receive monophonic or black-and-white signals generated by old transmitters. HTML is designed to treat all tags in the same way (as inert, unstyled inline elements) unless their appearance or behavior is overridden; either by the browser's default settings, or by scripts or styles included in the page. This makes most new features degrade gracefully in older browsers. One case where this did not work as intended was script and style blocks, whose content is meant to be interpreted by the browser instead of being part of the page. Such cases were dealt with by enclosing the content within comment blocks. Some products are not designed to be forward compatible, which has been referred to as NUC (not upwardly compatible). In some cases this might be intentional by the designers as a form of vendor lock-in or software regression.