This International Standard describes the implementation of a back-up operation for data transmission on a leased line by means of the Rec. V.24 Interchange circuits when using the 25-pin connector ISO 2110.It answers to an increasing need for the ability to provide a back-up facility for the case where data transmission over a leased line is no longer possible.There are several possible back-up facilities:Back-up of a 2-wire leased line by one GSTN line; Back-up of a 4-wire leased line by one GSTN line; Back-up of a 4-wire leased line by two GSTN lines;Back-up of a 2-wire/4-wire leased line by one/two lines of a private switched network.NOTE: Back-up of a 2-wire/4-wire leased line by one/two alternate leased private lines is for further study. This list may not be exhaustive.This variety of facilities should not be visible in the protocol used at the DTE/DCE interface to exchange the interface control and status information. It is understood that only one back-up facility is provided as alternate to data transmission on a leased line. The case of manual back-up calling and answering is excluded from the protocol. Further, this International Standard applies only where it is required to have, in one interface, the control of both leased and back-up facilities.This International Standard does not preclude any method to be used to switch automatically data transmission from leased line(s) to the back-up facility.In automatic back-up switching, the following cases have to be distinguished:Switching to the back-up facility is done at the DTE initiative. It is called "switching in direct mode" in this International Standard.Switching to the back-up facility is done at the DCE initiative when the DCE is authorized by the DTE to do so. It is called "Switching in authorized mode" in this International Standard. In the event that the DTE cannot control this facility over the interface, selection of this mode within the DCE during installation is permitted.This International Standard also distinguishes between:Answering StationIn this station, the DCE switches to the back-up facility upon a back-up request received (e.g. an incoming call).Calling StationIn this station, the DCE has to send to the called station a back-up request (e.g. a call for the other end to switch to the back-up facility.It should be noted that these procedures may be used at the Answering end only, the Calling end only, or both ends of a connection. It is possible for a simple DCE (i.e. one not implementing back-up control) to be provided at either end of the back-up circuit and for back-purposes only, for instance the 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.
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