Comparing Full Duplex & Half Duplex Connections

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Are you planning to take the Network+ exam? If so, make sure you understand the difference between half duplex and full duplex connections. These are the most common modes used for computers and network devices communication. Additionally, you should understand how to configure a switch using proper features in different scenarios.

For example, can you answer this question?

Q. What is created by separate switch ports?

A. Collisison domains

B. Broadcast domains



More, do you know why the correct answer is correct and the incorrect answers are incorrect? The answer and explanation is available at the end of this post.

Duplex Connections and Collisions

Computers and network devices can typically communicate using different modes. The most common modes are half duplex and full duplex, but simplex mode is used in some specialized connections.

  • Simplex. Data is sent over the connection in one direction only. For example, a computer can send data over a simplex connection but not receive data on the same connection.
  • Half duplex. The same connection supports both transmitting and receiving but only one at a time. For example, while a computer is sending data over the connection, it cannot receive data on the same connection. This is similar to how push-to-talk cellular phones or old-fashioned walkie-talkies operate.
  • Full duplex. The same connection supports both transmitting and receiving simultaneously. The connection has separate wires used for both transmitting and receiving. This eliminates the possibility of data colliding with each other on the same cable. Most twisted pair cables and network interface cards (NICs) support full duplex mode, but some legacy hardware does not support it.

Learn more about common network  components mentioned in the Network+ objectives.

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The following figure shows a switch with two computers connected. Computer A has a full duplex connection, so it can use the same connection to send and receive data at the same time. Computer B is connected with a half duplex connection. It can send data to the switch using this connection, or receive data back from the switch on this connection, but it cannot send and receive data at the same time with this connection.

Full duplex

Full duplex and half duplex connections

Most interfaces support auto-negotiation for the correct duplex mode and speed. If both devices can operate using full duplex mode, they automatically configure themselves using full duplex mode. If one of the devices can only operate at half duplex mode, the other device will configure itself to use the slower half duplex mode.

If you run across a connection running in half duplex mode, check to see if one of the interfaces is manually configured to use half duplex. If it is, you can often change it to full duplex to increase the speed of the connections.

The connection will not work if both devices are manually configured with different modes. For example, the connection fails if one device is manually configured with half duplex and the other device is manually configured with full duplex. The link light on the devices will show that they are connected, but they won’t be transferring data back and forth between each other.

Remember This

Auto-negotiation automatically configures interfaces with the fastest duplex mode and speed of the other device. When using manual settings, both devices must be manually configured with the same duplex mode and speed or the connection fails.

There is a subtle point worth mentioning here. If you look at the full duplex connection in the figure, you can see collisions are impossible. The only way a collision can occur is if the same line is used to send and receive data, but you can see that the computer sends data on one line and receives data on the other line.

However, even though collisions are impossible if both devices are using full duplex mode, you should still think of each port on a switch creating separate collision domains, especially when taking the Network+ exam.


Q. What is created by separate switch ports?

A. Collision domains

B. Broadcast domains



Answer is A is correct. A switch creates separate collision domains between each port and the device connected to the switch’s port.

All devices connected to a switch are in the same collision domain, so separate switch ports do not create different collision domains.

You can create separate virtual local area networks (VLANs) with a switch, but not all switches support VLANs so this isn’t the best answer

An access control list (ACL) is a group of rules used on a router or firewall to define network access.

You might like to check out these posts too:

Network Segments and Domains
Broadcast Domains and Collision Domains

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