SHIP-SHORE CIRCUITS

As we mentioned earlier, the fleet broadcast is the primary means for delivering messages to afloat commands. This section discusses a few of the other types of circuits by which a ship can transmit its message traffic ashore or to other ships for delivery or relay.

SHIP-SHORE CIRCUIT MODES OF OPERATION

There are three methods of operating communications circuits: duplex, simplex, and semiduplex. The mode of operation at any given time depends upon equipment and frequency availability.

Duplex

Duplex describes a communications circuit designed to transmit and receive simultaneously. In such operations, each station transmits on a different frequency and both stations transmit concurrently. Both

Figure 1-10.-Full-duplex time-diversity modem.

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

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

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 1-12.-UHF/HF netted simplex FSK teleprinter relay circuit.

When a ship requests simplex operation on duplex circuits, the shore station may be required to shift transmitters prior to acknowledging call-up. If no reply is received within 45 seconds, the ship should repeat the call-up procedures. If a third attempt is required, the ship should check equipment to ensure proper operation.

Semiduplex

Semiduplex communications circuits, used primarily on task force/task group/ORESTES, are a combination of the simplex and duplex modes. All stations except the net control station (NECOS) transmit and receive on the same frequency. The NECOS transmits and is received on a second frequency. The NECOS may transmit continuously, whereas all other stations must transmit in accordance with simplex procedures.