Simplex, Half Duplex, Full Duplex

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There are three modes of transmission, namely: simplex, half duplex, and full duplex.  The transmission mode defines the direction of signal flow between two connected devices.

The primary difference between three modes of transmission is that in a simplex mode of transmission the communication is unidirectional, or one-way; whereas in the half duplex mode of transmission the communication is two-directional, but the channel is interchangeably used by both of the connected devices.  On the other hand, in the full duplex mode of transmission, the communication is bi-directional or two-way, and the channel is used by both of the connected devices simultaneously.

Comparison Chart

Basis for Comparison Simplex Half Duplex Full Duplex
Direction of Communication Unidirectional Two-directional, one at a time Two-directional, simultaneously
Send / Receive Sender can only send data Sender can send and receive data, but one a time Sender can send and receive data simultaneously
Performance Worst performing mode of transmission Better than Simplex Best performing mode of transmission
Example Keyboard and monitor Walkie-talkie Telephone

Simplex

In simplex transmission mode, the communication between sender and receiver occurs in only one direction.  The sender can only send the data, and the receiver can only receive the data.  The receiver cannot reply to the sender.

Simplex transmission can be thought of as a one-way road in which the traffic travels only in one direction—no vehicle coming from the opposite direction is allowed to drive through.

To take a keyboard / monitor relationship as an example, the keyboard can only send the input to the monitor, and the monitor can only receive the input and display it on the screen.  The monitor cannot reply, or send any feedback, to the keyboard.

Half Duplex

The communication between sender and receiver occurs in both directions in half duplex transmission, but only one at a time.  The sender and receiver can both send and receive the information, but only one is allowed to send at any given time.  Half duplex is still considered a one-way road, in which a vehicle traveling in the opposite direction of the traffic has to wait till the road is empty before it can pass through.

For example, in walkie-talkies, the speakers at both ends can speak, but they have to speak one by one.  They cannot speak simultaneously.

Full Duplex

In full duplex transmission mode, the communication between sender and receiver can occur simultaneously.  The sender and receiver can both transmit and receive at the same time. Full duplex transmission mode is like a two-way road, in which traffic can flow in both directions at the same time.

For example, in a telephone conversation, two people communicate, and both are free to speak and listen at the same time.

Key Differences of the Three Transmission Modes

  • In simplex mode, the signal is sent in one direction. In half duplex mode, the signal is sent in both directions, but one at a time.  In full duplex mode, the signal is sent in both directions at the same time.
  • In simplex mode, only one device can transmit the signal. In half duplex mode, both devices can transmit the signal, but one at a time.  In full duplex mode, both devices can transmit the signal at the same time.
  • Full duplex performs better than half duplex, and half duplex in turn performs better than simplex.
  • Simplex: The keyboard sends the command to the monitor. The monitor cannot reply to the keyboard.
  • Half duplex: Using a walkie-talkie, both speakers can communicate, but they have to take turns.
  • Full duplex: Using a telephone, both speakers can communicate at the same time.
  • The full duplex transmission mode offers the best performance among the three, on account of the fact that it maximises the amount of bandwidth available.