Link Protocol

• Using the data link or link protocol consists of using a set of data communication rules that define the parameters surrounding the transmission and receipt of information. These protocols also define the communication channels used, as well as the process of retrieving lost information. The data link layer is the second layer in the Open Systems Interconnection Reference Model, a hierarchical structure consisting of seven layers of communication. The seven layers include the physical, data link, network, transport, session, presentation and application layers.
  
  

Simplex Communication

• Simplex is the most common form of data link communication, where all information is sent in only one direction, as opposed to bi-directional information packets. The signals in a simplex circuit are therefore called unidirectional. Your keyboard is an excellent example of a simplex data link. You type on the keyboard, and the letters appear on your monitor. Another example is a TV program; the program is broadcast from the point of origin to your television set.
  
  

Half-Duplex

• Half-duplex data links consists of signals going in two directions, but not at the same time. The signal first goes in one direction, and then a second signal is returned. The concept is similar to a single frequency walkie-talkie. First you talk, and then your friend talks, but he has to wait until you finish and let go of the transmit button on the walkie-talkie before he can speak. In the half-duplex system of communication the first signal must be received and the transmission stopped before a second signal can be picked up and transmitted back.
  
  

Duplex Communication

• Duplex data links enable signals to go back and forth at the same time. Multiplexing is a term used to describe multiple duplex data links. An example of a duplex data link is a telephone system. You and your friend can both speak at the same time and the signals will be sent simultaneously.