1.7 MODES OF DATA TRANSMISSION

 There are 3 modes of data communication:

1) Simplex mode

2) Half duplex mode

3) Full duplex mode

1.7.1 Simplex Mode

In simplex mode of data communication, data flow is unidirectional.

This means that data travels only in one direction i.e., from a sender to a receiver. The receiver can’t respond back to the sender. A simplex system is one i.e., designed for sending messages in one direction only. This is illustrated in Figure 9. This is of limited interest in an industrial communications system as feedback from the instrument is essential to confirm the action requested has indeed occurred. The primary examples of simplex mode are monitor & keyboard.

Monitor can only receive output from the CPU, it can’t give output back to the CPU.


Advantages of simplex mode:

1) It is inexpensive to install.

2) It is very easy to use.

Disadvantages of simplex mode:

The most important disadvantage is that the receiving device can’t confirm to the sending device that weather or not it has received data accurately.

1.7.2 Half Duplex Mode

Half duplex communications occurs when data flows in both directions; although in only one direction at a time as shown in Figure 10. In this mode of transmission, data is transmitted in both the directions but when the one device is sending data to another then the other device has to wait. Other device can only start sending the data when one device has stopped sending. In this mode, the entire bandwidth is available for sending data in either of the direction.

An example of a halfduplex system is a twoparty system such as a "walkietalkie" style twoway radio, wherein one must use "Over" or another previously designated command to indicate the end of transmission, and ensure that only one party transmits at a time, because both parties transmit on the same frequency.

A good analogy for a halfduplex system would be a one lane road with traffic controllers at each end. Traffic can flow in both directions, but only in one direction at a time being regulated by the traffic controllers.


Advantages of half duplex mode:

1) Enables twoway Communications

2) Less costly than Full duplex mode of communication

Disadvantages of half duplex mode:

1) Only one device can transmit at a time.

2) Costs more than simplexFull DuplexFull Duplex refers to the transmission of data in two directions simultaneously.

1.7.3 Full Duplex Mode

In full duplex mode of data transmission, data is transmitted in both the direction simultaneously as shown in Figure 11. This means that both the devices in a network can send receive the data at the same time. It is just like a 2 lanes road with traffic moving in both the direction at the same time. In this mode, signals going in either direction share the half capacity of – one for sending data in one direction other for sending data in another direction.


The primary example of full duplex mode is telephone conversation.

Advantages of full duplex mode:

  • · There is fast transmission of data.
  • · It is more efficient

Disadvantage of full duplex:

  • · It is difficult as well as expensive to install.
  • · High cost is involved
  • · Huge maintenance cost

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