Abstract: A T1 digital carrier system having a transmitter, receiver and a plurality of intermediate repeater stages. Each remote repeater has a select switch for automatically selecting that the remote repeater be placed in loop-back mode should a combination of framing bits occur in the serial bit stream that matches that said remote repeaters address therein, thereby allowing remote testing of the communications link and repeaters. In addition, no additional test lines are required to verify a T1 communications link.

Abstract: An integrated data voice multiplexer (IDVM) capable of simultaneously supporting loop-back and communication handshake protocols with no performance degradation. This allows the IDVM to be used in both point-to-point and packet switch networks. The IDVM is of the type using frequency shift keyed FSK modulation of two or more carriers to send a data signal. The presence or absence of carrier signals indicates loop-back state. Narrowband modulation is selectively added to at least one carrier to support handshake protocol. The preferred narrowband modulation is biphase at a rate lower than the FSK modulation rate. Encoder, decoder, data hold and detect circuits for supporting standard RS232 protocol are disclosed.

Abstract: A T1 digital carrier system having a master terminal and four serially coupled remote terminals. Each remote terminal has a by-pass switch for automatically by-passing that remote terminal should a service threatening condition exist therein, thereby maintaining service to all remaining remote terminals. In addition, major and minor alarm information (indicating service requiring conditions) is transmitted by each remote terminal during signaling frames in the T1 serial bit stream.