1

Scope

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.

NOTES 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.