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What is a Duplex Communication System?

Beth Turley
Beth Turley

A duplex communication system is a method of transmitting signals, allowing for two people, or pieces of equipment, to communicate with each other in opposite directions—meaning at the same time. This system has become an important standard in the area of telecommunications, especially in telephony and computer networking. Although the definition of duplex means to transmit in opposite directions at the same time, this is not the case in every duplex communication system. Two primary areas exist: full-duplex and half-duplex. In the half-duplex system, both parties can transmit data, so it technically works in opposite directions, but not at the same time.

To understand a full-duplex communication system, think of two groups of people standing at opposite ends of a field in separate lanes. A trigger is released and both sides walk or run toward each other at the same time, with no signals informing the groups to stop. The traffic of people continues in an orderly manner, with no collisions, until another trigger is released informing them to stop. This is the method in which full-duplex operates in a duplex communication system. The telephone, both land-line and mobile varieties, are the most well-known examples of full-duplex systems. Phones allow people to speak and hear concurrently.

Walkie talkies qualify as a half-duplex communication system.
Walkie talkies qualify as a half-duplex communication system.

Computer networks also are examples of full-duplex communications. When connecting to the Internet, a computer on one end sends data while a computer on the other ends sends data at the same time—a simultaneous exchange of information. Both receive and send data without stopping until triggered to do so. Dial-up telephone connections and high-speed broadband Internet are capable of communicating in full-duplex, depending on the networks used. Use of full-duplex allows for faster connections because more information can be sent and received.

Half-duplex communications allows communicating in two directions, but only in one direction at a time. In the example of the two groups standing at opposite ends of a field, only one group can take off when the trigger is released. The opposite group can't take off until the first group is stopped and a second trigger is released. Walkie-Talkie radios are a good example of a half-duplex system—each person has to wait until the other has stopped speaking in order to transmit. One person speaks, and the person on the opposite end receives the message. When the message is completed, the person often states, "over," which informs the other person they may relay their message.

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Discussion Comments


I love walk-talkies. To my kids they’re just toys, but I’ve used them for short term communications to keep tabs on my kids where we’re out in the park and they’re just off on their own, in the woods or something like that.

Admittedly, they find it sometimes annoying, but I tell them to treat it like a toy and act as if I’m playing along. The walkie talkies I use have a 10-mile range so they would have to get pretty far out of range before I lose communication.

They’re half duplex, as I think most walkie talkies are, but I don’t mind that at all. I’m not trying to have an ongoing dialogue; I'm just trying to get a report from time to time of where they’re at and what they’re doing.


@SkyWhisperer - Yes, duplex communication has beat out half-duplex, just as digital communication beat out analog communication signals – as we all learned from what has happened to the television industry. I still feel a little nostalgic at times for the days of HAM radio and analog communications.

One of the things that analog has going for it is accessibility and cost effectiveness. That’s because it just sends continuous signals and modulates the amplitude, and this is far cheaper than digitizing a signal and sending it, because it happens in one step instead of two.

I know that some of the small television stations near where we live were slow to jump on the conversion to digital, even though they knew it was required by law, because it simply cost too much money.

Some made the transition okay, while others, especially the low watt towers, eventually just folded up.


I remember back in the early days of Voice Over Internet Protocol. When the VOIP providers were just getting started, they had to deal with a host of issues.

One of them was latency. I dreaded talking over the Internet and experiencing a delay of two or three seconds before I could hear, or be heard.

But the other issue was half-duplex communication channels. In the early days, you had to take turns when using VOIP technology. I would speak, then wait, and then the other person would speak. It was like using a walkie-talkie and it was annoying because you never knew when the other party was done talking.

Things have gotten better now of course. I called overseas the other day using a VOIP connection and it sounded just like a regular phone call, with no delay and no “signal collision” (I guess that’s the term for it).

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    • Walkie talkies qualify as a half-duplex communication system.
      By: Jamie Wilson
      Walkie talkies qualify as a half-duplex communication system.