Abstract: In a ring network system including two transmission lines transmitting a signal in directions opposite to each other, and a plurality of line concentrators connected in the form of a loop by the two transmission lines and each accommodating a plurality of terminal stations connected to one of the two transmission lines, a signal detecting unit and a terminating circuit are disposed in each of the line concentrators on the signal output side of each of the transmission lines so as to be connectable to and disconnectable from the transmission lines, so that, when the signal detecting unit in one of the line concentrators is connected to the associated transmission line, the transmission line is disconnected in the line concentrator, and the terminating circuit is connected to the downstream segment of the disconnected transmission line.

Abstract: A multiplexer for trunking, over at least one high-speed trunk line on a multiplex basis under statistical time-division multiplexing control, transmission data on low-speed lines connected to a plurality of data terminal units, operates normally to transmit multiplexed data over leased high-speed trunk lines and, when a data buffer which temporarily stores data on the low-speed lines has its contents in excess of a certain level, operates by adding a public line as a high-speed trunking line to the leased high-speed trunk line for implementing the multiplexed data trunking.

Abstract: A relay circuit comprise first and second relays having respective terminals at which positive cross-talk of a signal of positive polarity is produced when the other terminals to which the positive polarity signal is applied are in the make and break states, respectively, and third and fourth relays having respective terminals at which negative cross-talk of a signal of negative polarity is produced when the other terminals to which the negative polarity signal is applied are in the make and break states, respectively, wherein with regards to the terminals where the cross-talk occurs, the first and fourth relays are connected to each other and said second and third relays are connected to each other, respectively.