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Half Duplex vs Full Duplex

Half duplex and full duplex define whether data can travel in both directions simultaneously across a network connection. Hubs enforce half duplex communication, while switches support full duplex, allowing multiple devices to send and receive at the same time.

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About this video

Half duplex and full duplex describe how data flows between two points on a network connection. In a half duplex setup, data can only move in one direction at a time, and transmission must stop completely before information can flow back the other way. A classic telegraph line illustrates this well — a single wire carries a signal in one direction, and a separate physical line would be required to support simultaneous communication in both directions. Full duplex eliminates this limitation by allowing sending and receiving to occur at the same time over the same connection. These concepts have direct implications for network hardware. A hub is a simple multi-port repeater that broadcasts every incoming signal out through all of its ports. When two devices attempt to transmit simultaneously, the hub cannot handle the conflicting signals, so the entire network is forced to operate in half duplex mode where only one device can communicate at a time. This creates inefficiency and limits throughput as the number of connected devices grows. A switch addresses this by intelligently directing frames to specific destination ports rather than broadcasting to all of them. This targeted approach allows multiple devices to communicate simultaneously without interference, enabling full duplex operation across the network. The internal mechanisms switches use to manage frame forwarding and avoid collisions are what make full duplex communication reliable at scale, and they represent a foundational reason why switches have largely replaced hubs in modern network infrastructure.

What you'll learn

What's covered

Half Duplex vs Full Duplex

Aligned to

CompTIA Network+
1.2 Compare and contrast networking appliances, applications, and functions.
Cisco CCNA
1.13 Describe switching concepts
Cisco CCST Networking
4.5 Explain basic switching concepts
CompTIA A+ Core 1
2.2 Compare and contrast common networking hardware.

Key terms

Half Duplex
A communication mode that allows data to be transmitted in both directions but only one direction at a time.
Full Duplex
A communication mode that allows data to be transmitted in both directions simultaneously.
Hub
A basic Layer 1 network device that receives a signal on one port and repeats it out every other port simultaneously, creating a single shared collision domain among all connected devices. Hubs have been replaced by switches in modern networks because they waste bandwidth and cause collisions.
Switch
A network device that connects devices within a LAN and forwards traffic based on MAC addresses.
Collision Domain
A network segment where data packets can collide with each other when transmitted simultaneously.

Transcript

Let's get out of the protocols for a second and talk about half duplex and full duplex, because with these different technologies, some of them can communicate in half duplex and others can communicate in full duplex. So let's talk about what the difference is between those.

The Telegraph Example

Here, I've just got our telegraph example once again. When that signal is being sent across the line, this device right here is either sending electrons over or not sending electrons over. We only have two states really that this can be in: it's either on or off. So it's using up this line between the two, point A and point B.

What happens with a telegraph is when you're on one side and you're communicating using that one side, and information is being sent over to the other side, no other communication can happen. So this telegrapher cannot send information back to this location right here. You would have to run a whole separate line and have separate equipment in order to be able to transfer data in both directions at the same time.

So this is the difference between half duplex and full duplex. Half duplex just means that data can flow in only one direction at a time, and that data has to stop before information can flow back, versus full duplex, where both sending and receiving can happen at the same time.

Hubs and Half Duplex

So now let's talk about half duplex and full duplex in a network example. Right here, let's say we have a hub that's sitting in between, and this machine right here needs to talk. So what it does is it sends out a communication, and that communication gets broadcasted out all of its lines, all of its ports. That is how a hub will work. We know that a hub is just a multi-port repeater, so whatever comes into it on one of the ports, it gets sent out every other port.

Now, let's say there's a machine that wants to talk at the same time. So this machine right here communicates at the same time. Now what this hub is doing is it's trying to communicate the same signal, or two different signals, out every single port. It can't do that. It can't have two signals that are going out every single port. So what a hub does is it creates a half duplex network, a network where only one device can talk at a time.

Switches and Full Duplex

That is not ideal. Obviously, we want communication to flow much, much better than that. We want a full duplex system where multiple devices can talk at the same time. So to do that, what we would do is replace this hub with a switch.

In a switch scenario, a computer tries to talk. It's sending out to a single machine. So let's say it's trying to talk to the server right here, and it sends that frame out to this server right here. If another device is trying to talk at the same time, a switch is able to send that to another device, and so communication can actually happen in a two-way format on this switch. That's because a switch is much more intelligent on how it can handle those frames and where those frames are being sent to. It does a lot of mechanisms to help facilitate this.

We're going to be talking about those different mechanisms that switches use in order to facilitate full duplex. But a switch will operate in full duplex, versus a hub.

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