Abstract: A calling telephone terminal and a called telephone terminal optimize the cost reduction of real time communication via a first communication network operating in real time. The calling terminal recognizes the status of a called terminal as “real time communication” in the case of a called terminal requiring real time communication or “off-line communication” in the case of a called terminal tolerating off-line communications and constructs and temporarily stores a message off-line and then sends it to the called terminal via a second communication network operating off-line if the status of the called terminal is “off-line communication”.

Abstract: To supervise the operation of an arithmetic unit of a digital transmultiplexer intended to convert a TDM-signal into a FDM-signal and vice versa, a pilot signal is utilized as the test signal which pilot signal is also used to control the level of the received FDM-signals and is processed in the transmultiplexer. The pilot signal selection filter used for controlling the level is used, also in time-division multiplex, for selecting a component having a frequency equal to the pilot signal frequency and occurring in each of the two input and output signals of the arithmetic unit.

Abstract: A circuit for converting into an FDM format spatially separated signalling signals and pilot signals, comprises a digital filter bank for converting the signals into the FDM format. A receiver for separating the signalling signals and pilot signals from the FDM signal comprises another digital filter bank for producing baseband signals. These filter banks have a transfer characteristic formed by a number of passband curves which are derived from the characteristic of a selected low-pass filter.