1,056,113. Automatic exchange systems. SIEMENS & HALSKE A.G. April 29, 1964 [April 29, 1963], No. 17765/64. Heading H4K. In a time division multiplex exchange system in which each sub-station is connectible to two multiplex highways both of which must be used for each call, the highways are connected in series by an amplifier. In the system shown in Fig. 2, a calling station A1 is connected by switch Sal to an outgoing (calling) highway switch Sb2 to an incoming (called) highway Mab and a called station A2 is connected via a switch Sb2 to an incoming (called) highway Man during one time slot whereby the stations exchange signals via both highways and a bidirectional amplifier Vz. In Fig. 3 the highways Ms and Me act respectively as Go and Return highways and are interconnected by a uniirectional amplifier Ve so that in this case two time slots P1 and P2, one for each direction of transmission, are required. The amplifier may comprise valves or transistors or it may be an inductive parametric amplifier (Fig. 4, not shown) of the type described in Specification 999, 231.