This International Standard specifies the procedures for back-up operation applicable to data transmission on a leased line when implemented through a single interface, controlling both the leased and the back-up facilities, using ITU-T Recommendation V.24 interchange circuits in conjunction with either the 25-pole connector (ISO 2110) or the 26-pole connector (ISO/IEC 11569).
The method used to switch the transmission of data automatically from the leased line to or from the back-up facility is not specified in this International Standard.
This International Standard is not applicable where back-up (calling or answering) is achieved by manual means.
1) The procedures specified in this International Standard may be used at the answering end only, the calling end only, or both ends of a connection.
2) An ordinary DCE (i.e. one not implementing back-up control) may be provided at either end of the back-up circuit and only for back-up purposes. For example, a calling station could provide one or more separate auto-dial GSTN lines to provide back-up on demand to a greater number of leased lines.