In half-duplex transmissions, data can flow in both directions over the medium, but only in one direction at a time [1]. A classic example of half-duplex communication is two-way radios. When using a two-way radio, only one person can speak at a time. If signals are sent simultaneously, they collide and neither receiver gets the information. Half-duplex transmissions are usually synchronous [2] because in order for the information to be exchanged reliably, the transmissions must be timed and in sync. Half-duplex transmission also use the full bandwidth of the medium [3].
Finally, when transmissions are full-duplex, this means that information can be sent in both directions simultaneously [1]. Telephone communication is the most common example of full-duplex communication. Full-duplex transmissions are usually asynchronous [2]. Since data can flow in both directions there is less need to time and synchronize the transmission, data can be sent and received "on-demand". In addition, with full-duplex transmissions bandwidth is divided so that data can flow in both directions over the same medium [3].
Regarding bandwidth . . . In general, full-duplex transmissions are going to require more than twice the bandwidth of simplex and half-duplex transmission. I say in general because there are cases where transmissions require very little bandwidth in the first place, so full-duplex transmissions experience very little hit in performance in these cases. However, in cases where large amounts of data are being sent in both directions, low bandwidth mediums are going to prove inadequate. I fall back to online gaming as being a good example of this, which is why most popular online games require a broadband connection to play, although they may not require a full-duplex connection. You will understand this more after reading Part 5 on Baseband and Broadband Transmission.
Think of it like this: Let's say we have a four-lane, one-way street in a busy city. All four lanes are consistently full and travelling in one direction. This is how simplex transmission works. The four lanes are your bandwidth and the cars are your data.
Now take that same four-lane street and make a law that says that every other day the traffic can travel in the other direction. This is a physical example of half-duplex transmission. Again, the lanes are your bandwidth, cars are your data, but this time we have a law, which represents a protocol (more on protocols in future posts). Notice what happened though. Even though our cars can still use the full four lanes available to travel in one direction, the amount of traffic that can can travel in one direction has been reduced by the law. Likewise, half-duplex is going to reduce the amount of data that can travel over the medium when compared to simplex communication. So even though the same amount of data is being transferred over all, only half the data is being transferred in either direction.
Now again, let's take that same four-lane street and say, "okay we are going to make this a two-way street." Now cars can travel either way at the same time and we are going to discard the old law and create a new one that says, "cars must stay in the right two lanes relative to the direction they are travelling" to avoid collisions. This is a physical example of full-duplex transmission. Things all remain the same, except the type of transmission and the protocol have changed. Now notice how the traffic or amount of data being exchanged has changed. Essentially the same amount of data is being transferred in either direction, i.e in a two-day period, four lanes in one direction with a one day wait = two lanes in one direction with no wait. However, there are more things to consider. Even though we are transferring at the same rate, look at the congestion we have created by making the street two-way. What was a busy, but efficient one-way-only or one-way-every-other-day street, is now a clogged-up two-way street. And even though we have a new protocol intended to prevent collisions, due to congestion, collisions are highly likely to occur as traffic tries to get to where it is supposed to be going. Therefore, even though half and full-duplex can send at the same rate over a period of time, the rate at any given instant is at least half and maybe even more due to congestion. So in situations where you will be switching to full-duplex transmission, bandwidth is definitely plays a factor.
This concludes the Part 4 of the the Data Transmission and Protocol series. I spoke a bit about bandwidth in this part, so in Part 5 I be discussing baseband and broadband transmissions. Baseband and broadband are two sides of the same coin and often can dictate the way in which you set up a network. After reading Part 5 you will know the difference between the two as well as their capabilities and limitations.
[1] Tomsho, G., Tittel, E., & Johnson, G. (2004). Guide to networking essentials. Thomson Course Technology: Boston, MA. ISBN-13: 978-1-4188-3718-1
[2] InetDaemon Enterprises. (2010). Asynchronous vs. synchronous. Retrieved from http://bit.ly/pOhOl8
[3] Kioskea. (2008). Data transmission - Transmission modes. Retrieved from http://bit.ly/pu9GeN
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