US 4,910,763 AGranted

Switching device for connecting a data transmission system to the telephone switched network

AssigneeInternational Business Machines Corporation

InventorsFrancois Caron, Jean Cholat-Namy, Emile Morlec

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Abstract

A data transmission system is described which includes a switching circuit that enables the connection of a tributary modem to a control modem via the public switched network. Switching occurs when it is no longer possible to transmit data on the leased network because the line connected to the tributary modem has failed. The switching circuit can connect either the permanent leased line or the switched network to the receiver of the control modem. It comprises a narrow pass-band filter and detector which is permanently connected between the reception side of the switched network and the control modem receiver. When the tributary modem answers a call from the control modem, it sends a tone ahead of the data message, the frequency of which tone corresponds to the frequency of the pass-band filter and is out of the data band. When such a tone is detected by the detector and pass-band filter, the control modem toggles a two position switch which isolates the leased line and connects the switched network to the receiver of the control modem.

Claims5

Dependent claims

In a data transmission system connectable to the telephone switched network, comprising a control device (11) including a transmitter (12) and a receiver (16), a plurality of tributary stations also including each a transmitter and a receiver, and a permanent transmission network for exchanging data between said control device and each of said plurality of tributary stations, said permanent transmission network including a principal link connected to said control device and tributary links each one being connected on the one hand to one of said tributary stations and on the other hand to said principal link according to a multipoint configuration, a switching device (20) enabling said control device to be connected to the telephone switched network so as to continue exchanging data with one tributary station (41) the tributary link of which has failed said switching device comprising:

connections (17, 18) for permanently connecting the transmitter of said control device to the transmitting side of said telephone switched network, as soon as said control device has been warned that said tributary link has failed;
a detecting circuit (31) connected between the receiving side of said telephone switched network and the receiver of said control device, adapted to detect a heading signal transmitted by the tributary station associated with said failed tributary link;
and
a two-position switch (30) controlled by said control device for disconnecting the receiver of said control device from the receiving side of said principal link and connecting it to the receiving side of said telephone switched network in response to the reception of said heading signal by said control device.

1 more element

Dependencies are inferred from the claim text.