Half Duplex vs Full Duplex: The Role of Transmission Modes in Networking

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25 Oct, 2021

Half Duplex vs Full Duplex: The Role of Transmission Modes in Networking

For over five decades enterprises and consumers have been using a variety of telecommunication systems for many different purposes, including business communications, personal communications, data transmission, access to internet resources and more.

Full Duplex, Half Duplex or Other Modes of Transmission?

For over five decades enterprises and consumers have been using a variety of telecommunication systems for many different purposes, including business communications, personal communications, data transmission, access to internet resources and more. These systems are based on diverse networking technologies, spanning different physical media, networking protocols and terminal devices [1]. One of the of the distinguishing features of these systems is their transmission mode. Specifically, there are simplex systems that transmit only in one direction i.e., from a data producer to a data consumer. At the same time, there are duplex systems that enable both parties of a networked interaction to send and receive data. Duplex communication systems enable point to point interactions between two different connected parties in both directions. They are used in the scope of different communication networks in support of bi-directional communications, as well as of systems that that provide need a reverse channel to provide feedback about their operation (e.g., industrial control systems). Duplex communication systems can be further classified into half duplex and full duplex systems [2]. 

Full Duplex

Full Duplex systems enable the communicating parties to interact simultaneously. Hence, they enable communications in both directions. From a technical perspective, full-duplex communications are implemented in two different ways:

  • Using a single communication channel that handles both directions at the same time.
  • Using two distinct channels, each one handles one of the directions i.e., a dual-simplex mode.

This technical difference is however fully transparent to the end users, which witness fully duplex communication.

One of the most prominent examples of full duplex communications is land-line telephone networks. As we all know, landline telephones allow both callers and callees to speak and hear simultaneously. To this end, they employ a dual wire circuit in the telephony line. This full duplex mode has been extended to modern telephony networks, such as cellular telephony technologies.

Half Duplex

Half-duplex systems enable participate parties to communicate with each other, yet they do not permit such communication to occur simultaneously. Rather communications take place in a unidirectional fashion i.e., in one direction at a time. To facilitate both parties to communicate half duplex communication network obligates every party that receives a signal to wait until the transmission of the signal is complete. Only when the transmission is completed can this party reply.

A characteristic example of a half-duplex communication system is the legacy walkie-talkie. Walkie-talkie systems are two-way radio systems that come with a push-to-talk button. Prior to starting speaking, every speaker presses this button, which turns on the transmitter and turns off the receiver. In this way, the speaker talks without hearing the other person. When they are done speaking, they can release the button, which allows them to listen to the other party. In traditional systems, a designated keyword like “over” was used to indicate the end of a message transmission. This aims at avoiding collisions i.e., cases where more than one party transmits data at the same time. Collisions lead to lost messages or distorted interactions, which must be avoided.

Nowadays, the most popular communication systems (e.g., smartphones) are full duplex, which makes half duplex deployments quite rare. The rationale behind the deployment of a half-duplex system lies in preserving bandwidth. For instance, in the scope of frequency division multiplexing systems, half duplex systems use a single rather than multiple frequencies for communications. However, half duplex systems can be also implemented based on time division multiplexing. In these cases, each one of the parties is allowed to transmit for a specific time interval. Then, they wait for the other party, and the cycle repeats. 

Simplex

There are many cases where one of the parties needs to transmit information, while other parties are constrained to listening. This is the case with many broadcasting services like radio, television, and other unidirectional broadcasting networks. Moreover, unidirectional interactions take place in various sensing and automation applications such as applications involving wireless microphones, alarms, and cameras. These systems are characterized as Simplex communication systems. Systems that operate in a simplex mode can benefit from using the entire capacity of a communication channel. However, many of the above-listed applications end up operating in a duplex mode, given the need for providing synchronous or asynchronous feedback to the transmitting party.

Comparing the Different Communication Modes

As already outlined, simplex, half duplex and full duplex systems vary in terms of directionality, sender options, performance, and cost. The following table illustrates how each of these transmission modes compares to the others.

AspectSimplexHalf duplexFull duplex
DirectionalityUni-directionalBi-directionalBi-directional 
Sender OptionsCan only send dataCan Send and Receive Data (one at a time)Can Send and Receive Data  (simultaneously)
Performance++++++
Cost++++++

In wireline communication systems the physical media plays an important role in supporting simplex or duplex communications. For instance, in fiber networks, there are simplex and duplex fiber cables. Simplex fiber cables comprise a single strand of plastic fiber glass, while duplex fiber cables comprise two strands of glass. Simplex fiber networks are useful in cases where there is a need for single transmission lines. There are however cases where signals are multiplexed over a single fiber to support bi-directional communications. Likewise, duplex fiber cables are used to support duplex communications requiring both data transmission and data reception.

As another example, modern Ethernet networks (e.g., Fast Ethernet and Gigabit Ethernet) operate in full duplex mode. To this end, Ethernet cables with two pairs of wires are required: One for data transmission and another for simultaneous data reception. Full duplex operation is very important in Ethernet networks since it is the foundation for reducing collisions, increasing available bandwidth, and boosting network performance. In Ethernet networks, the benefits of full-duplex modes over half-duplex communications are evident and important, as they obviate the need for packet retransmissions due to excessive collisions.

Overall, the different communication modes provide a rich set of options for designing and deploying networked systems. Full duplex systems offer distinct advantages in all cases where high-performance bi-directional communications are required. In cases where simultaneous communications are not of uttermost importance, other modes can be considered to economize on bandwidth resources and implementation costs [3], including modes that combine full duplex and half duplex transmission [4]. Network designers and engineers must understand the operation of all the different modes to increase their versatility and efficiency in designing effective networked systems that address diverse business requirements. 


References

  1. James Kurose, Keith Ross, Computer Networking: A Top-Down Approach 7th Edition, ISBN-13: 978-0133594140, ISBN-10: 9780133594140.
  2. Andrew S. Tanenbaum, Nick Feamster, David J. Wetherall, "Computer Networks, 6th Edition", ©2021 Pearson, ISBN-13: 9780137523214.
  3. Z. Zhang, K. Long, A. V. Vasilakos and L. Hanzo, "Full-Duplex Wireless Communications: Challenges, Solutions, and Future Research Directions," in Proceedings of the IEEE, vol. 104, no. 7, pp. 1369-1409, July 2016, doi: 10.1109/JPROC.2015.2497203.
  4. W. Cheng, X. Zhang and H. Zhang, "Full/half duplex based resource allocations for statistical quality of service provisioning in wireless relay networks," 2012 Proceedings IEEE INFOCOM, 2012, pp. 864-872, doi: 10.1109/INFCOM.2012.6195835.

More by John Soldatos

John Soldatos holds a PhD in Electrical & Computer Engineering from the National Technical University of Athens (2000) and is currently Honorary Research Fellow at the University of Glasgow, UK (2014-present). He was Associate Professor and Head of the Internet of Things (IoT) Group at the Athens In...

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