Before the digital T-carrier system, carrier wave systems such as 12-channel carrier systems worked by frequency division multiplexing; each call was an analog signal. A T1 trunk could transmit 24 telephone calls at a time, because it used a digital carrier signal called Digital Signal 1 (DS-1). DS-1 is a communications protocol for multiplexing the bitstreams of up to 24 telephone calls, along with two special bits: a "framing bit" (for frame synchronization) and a "maintenance-signaling bit". T1's maximum data transmission rate is 1.544 megabits per second.