The term duplex, on its own, refers to the capability to send and receive data. Duplex is often used when talking about conversations over a telephone or computer. A full-duplex Ethernet environment can use a pair of twisted cable for packet receiving and a pair of twisted cable for transmission.
Half-Duplex Devices
Half-duplex devices can only transmit in one direction at one time. So although data can move in two directions, it cannot be at the same time. Both devices are capable of transmitting and receiving so when one device is sending, the other is receiving. A pair of walkie-talkies is an example of this because communications are sent both ways but can only be transmitted one at a time.
Full-Duplex Devices
The term full-duplex describes simultaneous data transmission and receptions over one channel. A full-duplex device is capable of bi-directional network data transmissions at the same time. Where communication from both devices is required at all times, this mode is used. For example, telephone networks are made up of full-duplex devices to allow simultaneous transmitting and receiving.
Full/Half-Duplex Devices
In data communications, the full/half-duplex device allows users to choose either full or half-duplex modes. With half-or-full duplex devices, such as modems, a switch will be set to either full or half mode. The mode can be modified to correspond with each specific type of communication program. When set to a half-duplex system, connections alternate use of the communication channel and hardware can determine the time each data link in the system is allotted for transmissions.
Differences Between Full and Half-Duplex Systems
There are distinct differences between full and half-duplex systems. With half-duplex mode, each transmitted character is immediately displayed on a monitor. If a device is operating in full-duplex mode, transmitted data does not appear on-screen until it is received and returned. Full-duplex Ethernet does save time when compared to half-duplex because it alleviates collisions and frame retransmissions. Sending and receiving are separate functions, creating a system where there is full data capacity in each direction. In contrast, half-duplex can be used to conserve bandwidth.
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Bidirectional (one at a time) |
Bidirectional (simultaneously) | Unidirectional |
| Sender can send and receive data separately | Sender can send and receive data at the same time |
Data can only be sent |
| Used to conserve bandwidth when only single communication is needed | Used when communication is required in both directions without any delay |
When maximum bandwidth is required for the transmission and only one direction is required |
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Data transmission refers to the process of transferring data between two or more digital devices. Data is transmitted from one device to another in analog or digital format. Basically, data transmission enables devices or components within devices to speak to each other.
How does data transmission work between digital devices?
Data is transferred in the form of bits between two or more digital devices. There are two methods used to transmit data between digital devices: serial transmission and parallel transmission. Serial data transmission sends data bits one after another over a single channel. Parallel data transmission sends multiple data bits at the same time over multiple channels.
What is serial transmission?
When data is sent or received using serial data transmission, the data bits are organized in a specific order, since they can only be sent one after another. The order of the data bits is important as it dictates how the transmission is organized when it is received. It is viewed as a reliable data transmission method because a data bit is only sent if the previous data bit has already been received.
Example of Serial Data Transmission
Serial transmission has two classifications: asynchronous and synchronous.
Asynchronous Serial Transmission
Data bits can be sent at any point in time. Stop bits and start bits are used between data bytes to synchronize the transmitter and receiver and to ensure that the data is transmitted correctly. The time between sending and receiving data bits is not constant, so gaps are used to provide time between transmissions.
The advantage of using the asynchronous method is that no synchronization is required between the transmitter and receiver devices. It is also a more cost effective method. A disadvantage is that data transmission can be slower, but this is not always the case.
Synchronous Serial Transmission
Data bits are transmitted as a continuous stream in time with a master clock. The data transmitter and receiver both operate using a synchronized clock frequency; therefore, start bits, stop bits, and gaps are not used. This means that data moves faster and timing errors are less frequent because the transmitter and receiver time is synced. However, data accuracy is highly dependent on timing being synced correctly between devices. In comparison with asynchronous serial transmission, this method is usually more expensive.
When is serial transmission used to send data?
Serial transmission is normally used for long-distance data transfer. It is also used in cases where the amount of data being sent is relatively small. It ensures that data integrity is maintained as it transmits the data bits in a specific order, one after another. In this way, data bits are received in-sync with one another.
What is parallel transmission?
When data is sent using parallel data transmission, multiple data bits are transmitted over multiple channels at the same time. This means that data can be sent much faster than using serial transmission methods.
Example of Parallel Data Transmission
Given that multiple bits are sent over multiple channels at the same time, the order in which a bit string is received can depend on various conditions, such as proximity to the data source, user location, and bandwidth availability. Two examples of parallel interfaces can be seen below. In the first parallel interface, the data is sent and received in the correct order. In the second parallel interface, the data is sent in the correct order, but some bits were received faster than others.
Example of Parallel Transmission – Data Received Correctly
Example of Parallel Transmission – Data Received Incorrectly
Advantages and Disadvantages of Using Parallel Data Transmission
The main advantages of parallel transmission over serial transmission are:
- it is easier to program;
- and data is sent faster.
Although parallel transmission can transfer data faster, it requires more transmission channels than serial transmission. This means that data bits can be out of sync, depending on transfer distance and how fast each bit loads. A simple of example of where this can be seen is with a voice over IP (VOIP) call when distortion or interference is noticeable. It can also be seen when there is skipping or interference on a video stream.
When is parallel transmission used to send data?
Parallel transmission is used when:
- a large amount of data is being sent;
- the data being sent is time-sensitive;
- and the data needs to be sent quickly.
A scenario where parallel transmission is used to send data is video streaming. When a video is streamed to a viewer, bits need to be received quickly to prevent a video pausing or buffering. Video streaming also requires the transmission of large volumes of data. The data being sent is also time-sensitive as slow data streams result in poor viewer experience.
QUANTIL provides acceleration solutions for high-speed data transmission, live video streams, video on demand (VOD), downloadable content, and websites, including mobile websites. If you want to know more about how we deliver data, you can tweet your questions to our team at @Team_QUANTIL.
Bin Ni is the VP of Engineering of QUANTIL. He and his team is working on all technical aspects of all QUANTIL products.