Figure 1-11.—UHF/HF full-duplex FSK single-channel teleprinter relay circuit

stations are required to keep transmitters on the air at all times and to send a phasing signal at the request of the distant end. Figure 1-11 shows a diagram of a UHF/HF full-duplex FSK (frequency-shift keying) single- channel teleprinter relay circuit. There are two types of duplex operation: full duplex and half duplex. Full duplex (FDX) refers to a communications system or equipment capable of transmitting simultaneously in two directions. Half duplex (HDX) pertains to a transmission over a circuit capable of transmitting in either direction, but only one direction at a time. Small ships traveling in company normally use duplex in a task group common net in which they terminate with a larger ship that is serving as net control. The net control ship provides the ship-shore relay services. Ships traveling independently can use this system for anon-call ship-shore termination to transmit their outgoing messages. Simplex Simplex is a method of operation that provides a single channel or frequency on which information can be exchanged (figure 1-12). Simplex communications operation is normally reserved for UHF and those ships that do not have sufficient equipment for duplex operation. In some cases, a simplex circuit can be established when equipment casualties occur. Where no HF simplex frequency is indicated or guarded, ships requiring a simplex ship-shore circuit must call on a duplex ship send frequency. The ship must state “SIMPLEX” in the call-up, indicating that the ship cannot transmit and receive simultaneously. Figure 1-11.—UHF/HF full-duplex FSK single-channel teleprinter relay circuit. Figure 1-12.—UHF/HF netted simplex FSK teleprinter relay circuit. 1-11