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WiFi connections are running at half-duplex while the wired part of the LAN are on full-duplex. So it seems that by connecting through WiFi, something had to give. Were we shortchanged? Do you like losing half of anything? Worse, will we not be able to perform certain stuff with our computers and peripheral devices if they were connected through WiFi?

Duplex versus Simplex

In networking, the term ‘duplex’ signifies the ability for two points or devices to communicate with each other, as opposed to ‘simplex’ which refers to unidirectional communication. In a duplex communication system, both points (devices) can transmit and receive information. Examples of duplex systems include telephones and walkie-talkies.

On the other hand, simplex systems only permit one device to transmit information, while other receives. The common infrared remote control a prime example of a simplex system, where the IR remote controller transmits signals but never receives any in return.

Full and Half-Duplex

Full-duplex communication between two components means that both can transmit and receive information between each other simultaneously. Telephones are full-duplex systems so both parties on the phone can talk and listen at the same time.

In half-duplex systems, the transmission and reception of information must happen alternately. While one point is transmitting, the other must only receive. Walkie-talkie radio communication is a half-duplex system, this is characterised by saying “over” at the end of a transmission to signify that the party is ready to receive information.

Image credit: Wikipedia
A simple illustration of a half-duplex communication system. Image credit: Wikipedia

How Duplexing Affects WiFi Routers

WiFi routers are devices that modulate and schedule the flow of information to and from any WiFi-capable electronic device (like a laptop or smartphone) to the Internet, using a specific standard or protocol called IEEE 802.11 which works at half-duplex. WiFi is just the trademark brand for this specific IEEE standard (understand the common WiFi standards Understanding the Most Common Wi-Fi Standards Understanding the Most Common Wi-Fi Standards Wi-Fi can be a bit confusing because there are a handful of different standards being used. Here's what you need to know. Read More ).

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WiFi devices wirelessly connect to the router using radio waves at 2.4GHz or at 5GHz. The router schedules and makes sure the correct information flows between each connected device and the Internet; without collision and loss; by a process call Time Division Duplexing (TDD) to behave like full-duplexing.

TDD emulates full-duplexing by setting up or dividing time periods that alternate between transmission and reception. Data packets flow both ways as dictated by the time divisions. By chopping these time periods finely, devices connected this way seem to be transmitting and receiving simultaneously.

Why Can’t Current Routers Run At Full-Duplex?

The biggest problem to achieving full-duplex capability over radio is self-interference. This interference or noise is more intense than the actual signal itself. Simply put, interference in a full-duplex system occurs when a single point is transmitting and receiving simultaneously, and it will receive its own transmission too, hence self-interference is produced.

Diagram illustrating self-intereference. Credit: Kumu Network
Diagram illustrating self-intereference. Credit: Kumu Network

Practical full-duplex wireless is possible in the realms of research and academia. This is largely achieved by cancelling the self-interference on two levels. The first is by signal inversion of the noise signal itself and then the process of noise-cancellation is further enhanced digitally. A few Stanford University students have built working full-duplex radio prototypes in 2010 and 2011 (read the white paper). Some of these students have gone on to form a commercial startup called KUMU Networks, committed to revolutionizing wireless networking.

Other works such as IBFD (In-Band Full-Duplex) by Cornell University and STAR (Simultaneous Transmit and Receive) by Photonic Systems Inc. can also be found.

What About Wired LAN?

The wired portion of the LAN communicates at full-duplex with two pairs of twisted wires forming the ethernet cable connection. Each pair is dedicated to transmit and receive information packets simultaneously, hence no collision of data and no interference.

Here’s everything you need to know about Ethernet cables Everything You Need To Know About Ethernet Cables Everything You Need To Know About Ethernet Cables Read More .

FTP cable3 by Baran Ivo - Own work. Licensed under Public domain via Wikimedia Commons
FTP cable3 by Baran Ivo – Own work. Licensed under Public domain via Wikimedia Commons

Progress In WiFi Connectivity

Within the IEEE 802.11 protocol, changes were made to achieve either better range or better data throughput, or both. From its formative days in 1997 to 2013, WiFi standards have been amended from 802.11 to 802.11b/a, 802.11g, 802.11n, and finally, 802.11ac (should you buy a wireless-AC router Should You Buy A Wireless 802.11ac Router? Should You Buy A Wireless 802.11ac Router? 802.11ac promises blistering speeds, but many consumers are just now getting around to upgrading to 802.11n, leaving many to wonder if the new version is worthwhile. Read More ?). No matter how advanced they’ve become, they still belong to the 802.11 family, which will always run at half-duplex. Although improvements have been made, most notably with the inclusion of MIMO (what’s MIMO What You Need To Know When Buying A WiFi Router For Your Home What You Need To Know When Buying A WiFi Router For Your Home Many people find that they don’t know much about routers when it comes time to buy one. There’s no shame in that. Consumers usually don’t bother to learn much about them because we don’t often... Read More ?), running at half-duplex reduces the overall spectral efficiency by half.

Interestingly, MIMO-supported routers (multiple-input multiple-output) advertise much faster data rates. These routers utilize multiple antennas to transmit and receive multiple data streams simultaneously, which can boost the overall transfer rates. This is commonly found in 802.11n and newer routers, which advertise speeds from 600 megabits per second and higher. However, since they operate at half-duplex, 50 percent (300 megabits per second) of the bandwidth is reserved for transmitting while the other 50 percent is used for receiving.

Full-Duplex WiFi In The Future

There is increasing commercial interest in full-duplex wireless connectivity. The main reason being that advances in half-duplex FDD and TDD are saturating. Software enhancements, modulation advances, and MIMO improvements are getting harder and harder. As more devices connect wirelessly, the need for increased spectral efficiency will eventually be paramount. Full-duplex wireless connection have successfully demonstrated the instantaneous doubling of this spectral efficiency.

In areas where there is minimum impact on hardware, software reconfiguration, regulatory changes and monetary investments, this change from half-duplex to full-duplex will start to figure more and more prominently. Driven initially by the need for more capacity, we may find full-duplex WiFi sometime in the near future, initially side by side with the latest half-duplex components.

  1. Murthy sagi
    August 4, 2016 at 9:46 am

    Very Nice :-)

  2. Deepak
    January 31, 2016 at 4:58 pm

    Why cant we use cellphone type 2 channels on wifi one for receiving and other for receving?

    • Eddie
      November 17, 2016 at 3:55 pm

      Hi, I have the same question as Deepak. Can someone tell us why 802.11 can't use 2 different channels per client, one for sending and the other one for receiving? (as is cellphone)

  3. Saw Chaw
    November 19, 2015 at 10:51 pm

    Explained very well in plain lucid language. We hear a lot about 802.11 being half-duplex, but rarely question why? This article helps explain it. Thanks.

  4. Saketh Balaji
    April 26, 2015 at 4:49 pm

    Great article. Just one doubt though.If routers can't have full duplex connections over radios,how do cell phones do it over radio?

    • Luke Chan
      October 28, 2015 at 3:54 am

      You may wanna take a look at this:

      http://electronics.howstuffworks.com/cell-phone1.htm

    • Saw Chaw
      November 30, 2015 at 10:25 pm

      Because Cellphones operate on 2 radios/2 channel frequencies simultaneously. Radio 1/Channel 1 (up-link) from cellphone to base station and Radio 2/Channel 2 (down-link) from base-station to cellphone.

      • Eddie
        November 17, 2016 at 3:57 pm

        And why 801.11 doesn't use 2 channels frequencies simultaneasouly ? this could be easily implemented I guess

  5. Jeba
    February 19, 2015 at 11:13 pm

    Simple, and Easily Understandable Explanation... Thanks to make to me understand what is simplex vs Duplex... :)

  6. Guy M
    September 24, 2014 at 4:51 pm

    Phoon. That was AWESOME.

  7. Shu
    September 24, 2014 at 1:26 pm

    Agreed with the comments above, easy to read a highly technical concept with layman language. Thanks!

  8. Eric S
    September 23, 2014 at 3:35 pm

    Good article. Explained in a way everyone can understand!

  9. Ethan
    September 22, 2014 at 7:17 pm

    Fantastic article. Hope to see more from Phoon.

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