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.)

Unicast, Broadcast, and Multicast

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

• Unicast: This is often seen as the simplest method of communication, in which data is sent from a sender to a specific receiver. This is the basic mode supported by RS-232, since there are only two devices connected to the link, e.g. to connect a computer to a GPS device. 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 are able to transmit a message that can be received by all nodes connected to the network (this is common in LANs and control networks). This is known as broadcast or point-to-multipoint transmission. DMX uses broadcast to distribute data to fixtures connected to the bus. IPv4 also supports broadcast.
• Multicast: Some networks allow simultaneous transmission that allow a group of receivers to receive only the messages that are important. 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. 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) and the use of the IP network protocol to support packet video allowing remote participation in conferences across the Internet (e.g. this maps an IP multicast address to an Ethernet multicast MAC address). DMX Universe data can also be sent over IP networks using multicast.

Prof. Gorry Fairhurst, School of Engineering, University of Aberdeen, Scotland. (2016)