Data transmission system connectable to the telephone switched network.

摘要

Data transmission system including a switching circuit (20) which enables the connection of a tributary modem to a control modem by means of the public switched network when it is no longer possible to transmit data on the leased network because the line associated to the tributary modem has failed. The switching circuit connects both the permanent leased line and the switched network to the receiver (16) of the control modem. It comprises a narrow pass-band filter (31) which is permanently connected between the reception side of the switched network and the control modem receiver (16). When the tributary modem answers after a call from the control modem, it sends a tone ahead the data message the frequency of which corresponds to the frequency of the pass-band filter (31) and is out of the data band. Such a tone being detected by the pass-band filter (31), the control modem toggles a two position switch (30) which isolates the leased line and connects the switched network to the receiver (16) of the control modem.