Full-duplex means that data can be sent and received at the same time, and the two are synchronized, much like the feeling of making a phone call, where you can listen while you talk, in both directions.

Half-duplex means that during communication, information can be transmitted in both directions, but only one direction of transmission exists, not both directions at the same time.

Simplex means that data is transmitted in one direction, with one side fixed as the sender and the other side fixed as the receiver.
 

Full-duplex is a way of communication where both sides can send and receive data at the same time. You don’t need to wait for the other person to finish talking. A phone call is a good example — both people can talk and listen at once. Such an arrangement makes the conversation fast and smooth, unlike a walkie-talkie, where you take turns to speak.

In wired communications such as Ethernet, full duplex is usually achieved by physically separating the transmit and receive channels. For example, a twisted pair cable contains separate pairs of wires for transmitting (TX) and receiving (RX) signals, which are physically isolated to avoid signal crosstalk. Similarly, fiber optic communications may utilize optical signals of different wavelengths (WDM technology) or separate fiber cores to differentiate bi-directional data streams. This separation of physical layers allows data to flow in both directions simultaneously with little need for complex conflict detection mechanisms; e.g., the full-duplex mode of Gigabit Ethernet turns off conflict detection (CSMA/CD) by default and relies directly on efficient forwarding by the switch.

Typical application scenarios for full-duplex technology include telephone systems, Ethernet switches, and some wireless communication protocols. For example, traditional telephony separates the upstream and downstream voice channels via analog circuits, while modern VoIP (Voice over Internet Protocol) Network Paging Console relies on digital full-duplex transmission for real-time conversations.

SPON's Network Paging Console is connected via POE, and the device internally handles sending and receiving data separately without interfering with each other. Supporting full-duplex visual intercom, users can speak and listen to each other's voices at the same time without having to wait to switch between sending and receiving states. The video is encoded in H.264, which compresses the video stream to reduce bandwidth usage and maintains stable full-duplex video calls even during network fluctuations.

Half-duplex is a communication mode that allows data to be transmitted in both directions but not simultaneously. Its core feature is that the two communicating parties share the same channel and must alternate between transmit and receive states. For example, the intercom user needs to press the button to speak and release the button to listen to the other party's reply; this “one question, one answer” mode is the typical application of half-duplex.

The realization of half-duplex relies on information transmission channel sharing and direction switching control. In the physical layer, both parties use the same transmission medium (such as a coaxial cable or a radio frequency) through the protocol or hardware signals to coordinate the sending and receiving state. For example, early Ethernet used the CSMA/CD protocol: devices listen to the channel before sending data to see if it is free, and if a conflict is detected (more than one device sends at the same time), the transmission is stopped immediately and retried after a random wait. This mechanism effectively avoids data collision, but frequent conflict detection and retransmission will reduce the overall efficiency.

The advantages of half-duplex are mainly reflected in the lower cost and low complexity. Since only a single channel is required, the hardware design is simple and suitable for scenarios with low real-time requirements. For example, in industrial applications, multiple sensors take turns reporting data through half-duplex mode; workers talk through walkie-talkies. However, its shortcomings come along with it: low channel utilization, switching process generates idle time. Higher latency, the need to wait for channel release, and the risk of conflicts caused by multi-device contention are limitations that make it difficult to meet high throughput or real-time interaction demands.

Half-duplex strikes a balance between efficiency and complexity compared to full-duplex and simplex. Half-duplex is a compromise for bidirectional but alternating communications. It still occupies a large market in intercom systems, some industrial control networks, and low-power IoT. Its low-cost advantage remains key.

Simplex communication is a mode of data transmission in which data can only be transmitted from one direction to another in a fixed manner, and the roles of the two communicating parties are strictly categorized as transmitter and receiver, and the roles are not interchangeable. For example, a radio station transmits signals continuously, and a radio can only receive but not transmit data in the reverse direction. Its core features are the fixation of the communication path and the irreversibility of the signal flow, and it is suitable for scenarios that do not require bidirectional interaction.

The principle of simplex implementation is extremely simple: the hardware at the transmitter and receiver needs to support signal processing in only one direction. The transmitter continuously or on demand transmits signals, and the receiver passively receives and decodes them without coordinating the sending and receiving states or designing complex conflict detection mechanisms. This “just send, do not ask for feedback” feature is so that the simplex system hardware cost is very low and the operation is stable.

The core advantage of simplex communication is its high reliability and low cost. Since there is no need for two-way interaction, the system does not need to deal with channel competition, conflict retransmission, and other issues, so it performs well in scenarios that require high signal stability. For example, in a forest fire monitoring system, temperature sensors only need to send data to the control center periodically in one direction, without receiving commands, which reduces both the complexity of the equipment and energy consumption. However, the limitations are equally obvious: the lack of interaction capability prevents data validation, retransmission, or dynamic control. If data is lost at the receiving end, the sending end cannot sense it and has to rely on repeated transmissions to make up for it.

Data can be transmitted in both directions at the same time in full duplex. With half duplex data can be transmitted in both directions but not at the same time. Simplex means data is transmitted in one direction. Full-duplex transmission mode has the best performance of the three because it maximizes the available bandwidth. All three modes of communication have their own best scenarios for application.

Reference:

https://www.techtarget.com/searchnetworking/definition/full-duplex
https://teachcomputerscience.com/simplex-half-duplex-full-duplex
https://www.comms-express.com/infozone/article/half-full-duplex