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.

Abstract: A DTE can control the modems attached thereto by transmitting command messages over the data path and analyzing report messages supplied by the modems in return. Each command message is identified by a specific header. A command message intended for a remote modem (34) is intercepted by the local modem (30) and reformatted so as to obtain a supervisory message that is then transmitted at the rate of one bit per baud. A command message intended for a remote multichannel modem (48) is transmitted either by a supervisory message or in the data mode of operation.

Abstract: In a variable configuration multiplex transmission method, correct sending of data to their respective addressees (terminals 22, 24, 26 and/or 28) supposes that reception station (34) can determine which is the channel configuration from amongst configurations (A, B, C and/or D) that is active. This invention is adapted to proceed to such a channel determination by multiplexing a predetermined particular character from the active channels, directly transmitting over transmission channel (32) the multiplexed predetermined character burst, receiving (34) the multiplexed character burst, and using the received character burst in order to read a pre-registered table which directly gives out that channel configuration which has active channels.

Abstract: A method of testing a communication network that comprises a first and a second multiport modem communicating with each other through a transmission channel, each of said modems being provided with a number of input/output interfaces and each of the interfaces on either modem being associated with a corresponding interface on the other modem. In accordance with the method of the present invention, a test request is applied to one of the interfaces on the first multiport modem, which responds thereto by generating a test configuration code identifying that interface. This modem is then caused to assume a so-called test configuration, in which only that interface to which the test request was applied is active, and a test command comprising an indication of the test to be performed and a link level identifier is applied to that interface. If the identifier designates the link level to which the first modem belongs, this modem will control the test specified in the test command.