Simplex, Half-Duplex, and Full Duplex

A communications network can provide three basic types of service:

• Simplex: The most basic type of service is known as simplex. This service provides one-way communication and is also known as uni-directional. Examples of this type of service are point-to-multipoint DMX control distribution, and the point-to-point transmission of GPS coordinates using NMEA GPS Sentences.
• Half Duplex: Some networks connect more than two devices and operate in half duplex. RDM and CAN bus are examples of this type of service. This is sometimes known as two-way alternate, denoting that nodes take it in turn to transmit. Nodes that are not transmitting generally act as receivers.
• Full Duplex: Many data networks transfer data in two directions and are known as duplex communications links. In full duplex, both local and remote nodes may simultaneously transmit. (A telephone line is capable of full duplex transmission, although most human users choose to adopt a half duplex procedure to listen to what the remote person has to say.)

Addresses and Identifiers

Labeling data by adding an address or indetifier (ID) allows receivers to select which data they wish to process and use. There are two distinct ways that data can be labled:

• Address: This is the simplest approach, in which the sender decides which node should process the data it sends. Every receiver checks the addressing information when it receives each frame (or packet). If a network node sees its own address, then the corresponding packet or frame is processed, otherwise frames/packets with all other IDs are silently discarded. This requires the sender to know, or be able to find out, the address associated with a node to whom it wishes to send.
• ID: This is a different approach, in which the sender labels/describes the data rather than the node that will receive it. Each receiver is configured with a set of IDs and a network node only processs a packet or frame that has an ID in the list, otherwise it is silently discarded. A multicast group ID is a similar concept defining an address of channel over which a specific stream of information flows.The use of IDs to label data is common in control networks. It requires the receiver to know what sets of IDs it wishes to receive.

A CAN bus node is configured with an explicit IDs at the start of each CAN frame. A DMX nodes is configured with a profile that sets the base address of a receiver and atatches meaning to a set of following slots. An RDM node uses a pre-configured ID to select the set of messages to which it will respond.

Unicast, Broadcast, and Multicast

At the link layer there are also three methods of communication:

• Unicast: This is the simplest method of communication, in which data is sent in simplex mode from a sender to a specific receiver. Pairs of transmitters and receivers can also send in unicast simultaneously using full duplex, or by taking it in turns using half-dupex. In an environment where the physical layer supports broadcast, this requires a link layer address to identify the intended receiver (e.g., Ethernet MAC address or the UID in RDM).
• Broadcast: Some networks have been designed to transmit a message that is received by all the nodes connected to the network. This is known as broadcast or point-to-multipoint transmission and uses simplex mode. It is common in control networks. In DMX, data is broadcast to fixtures connected to the bus, and the position within the frame is used to identify specific information of interest.
• Multicast: Some networks allow simultaneous transmission that allow a group of receivers to receive only the messages that are important (i.e., identified by a specific group ID number). This is known as multicast transmission. Multicast utilises the same physical layer as used for broadcast, but uses addresses or IDs to separate the transmissions into separate groups as in unicast. An example of multicast transmission is the distribution of messages using CAN bus (where receivers can select which message IDs are of interest to a particular receiver).

Prof. Gorry Fairhurst, School of Engineering, University of Aberdeen, Scotland (2025).