FULL is specified. If FULL is specified, full-duplex mode is forced. PU 2s and PU 1s operate in half-duplex data mode. This is where most of the technological advancement has been achieved with respect to half and full duplex. The modem can be synchronous or asynchronous and can be analog or digital.
Older modems are capable of transmitting or receiving only at a given time.
The modem does not activate the CTS signal, and the DTE does not transmit, because DCD is in the active state. One method uses multiple carrier signals at different frequencies, so that the local modem's transmit and receive signals, as well as the remote modem's transmit and receive signals, each have their own carrier frequency. DTE equipment in an SDLC environment have configuration options that specify which mode of operation is supported by DCE equipment. The default parameters for most PU 2 devices are set for half duplex, although they can also support full-duplex operation. If the facility is capable of full duplex, RTS can be asserted at all times.