Transmission Modes

2.2 Transmission Modes

Data transmission may take place in one direction only or it may be bi-directional. There are 3 groups into which the channel can be classified:

  • Simplex
  • Half Duplex
  • Full Duplex

Simplex

Data is transmitted from the sender to receiver only, eg: from a central computer to a dumb terminal. The communication can only take place in one direction and it is not possible for the receiver to send data back. An example of simplex transmission would be data being sent to an electronic notice board such as those found in train stations and airports.

Half-Duplex

Data can travel in both directions but not at the same time. Each end of the communications link acts as sender and receiver, eg: two-way communication between computers and other computers that may be connected to a hub. Controls will exist to ensure that the devices do not send at the same time. One human example of this type of communication is the use of walkie-talkies, where each person communicating must indicate when they have finished speaking.

Full Duplex

Data can travel in both directions simultaneously, eg: two or more computers connected to a network device such as a switch that provides full duplex activity.

Note: as well as the data that is sent between systems there is also control information, eg: ready to send and ready to receive. Therefore, although the data may be only one way in a passive simplex system there may be the need for control information to be collected and used by the sender.

Next: Baud and Data Rates