Asynchronous transmission is a data communication method where data is sent independently with variable intervals between each transmission. Unlike synchronous transmission, where data flows in a continuous stream coordinated by clock signals, asynchronous transmission allows for data to be sent as individual units or blocks (often bytes or characters), without the need for a steady timing signal between them. This results in more flexibility and adaptability, especially in scenarios with varying workloads and unpredictable user inputs.
Key Characteristics
- Independent Data Units: Data is sent as discrete units, such as bytes or characters.
- Variable Intervals: There are variable and sometimes unpredictable intervals between the data transmissions.
- Start and Stop Bits: Each data unit typically starts with a start bit and ends with one or more stop bits, indicating the beginning and end of the transmission unit.
- Error Detection: A parity bit might be included in each unit for basic error detection.
- Flexibility: Adaptable to varying data loads and user inputs.
Types of Asynchronous Transmission
Character-Based Asynchronous Transmission
In this type, characters (usually 8 bits) are sent individually, with start and stop bits framing each character. For example:
- Start Bit: Signals the beginning of the data unit.
- Data Bits: Represent the actual character data.
- Parity Bit: Used for error detection (optional).
- Stop Bits: Signal the end of the data unit.
Byte-Based Asynchronous Transmission
Similar to character-based but typically deals with data in bytes. This is commonly used in modern serial communication protocols.
Special Considerations
- Latency: Asynchronous transmission methods can experience higher latency compared to synchronous transmission, due to the start and stop bits required for each data unit.
- Compatibility: Devices communicating asynchronously must agree on the frame format (number of data bits, parity bit usage, number of stop bits).
Examples
- Serial Ports: Asynchronous communication is widely used in serial ports (RS-232, for instance) for transmitting data between computers and peripheral devices.
- Internet Communications: Many internet protocols (like HTTP) inherently use asynchronous transmission methods to handle varying loads and user requests.
Historical Context
Asynchronous transmission has been a foundational technology in data communications since the advent of early telegraph systems. It allowed for efficient communication over long distances, even with limited and unstable connections.
Applicability
Advantages
- Flexibility in handling different types and loads of data.
- Simpler hardware requirements, as there is no need for precise synchronization.
Disadvantages
- Lower throughput due to the overhead of start and stop bits.
- Potentially higher latency and less efficient error handling compared to synchronous transmission methods.
Comparisons with Synchronous Transmission
- Synchronous Transmission: Uses continuous data streams synchronized by clock signals, typically more efficient with less overhead but requires precise timing.
- Asynchronous Transmission: Data sent independently in discrete units, more adaptable to varying conditions but with higher overhead due to start and stop bits.
Related Terms
- Synchronous Transmission: A method where data is sent in a continuous, synchronized stream.
- Serial Communication: Transmission of data one bit at a time, often using asynchronous methods.
- Parity Bit: An error detection mechanism usually employed in asynchronous transmissions.
FAQs
What is the main benefit of asynchronous transmission?
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References
- Stallings, W. “Data and Computer Communications.” Pearson Education, 2014.
- Tanenbaum, A. S., and Wetherall, D. J. “Computer Networks.” 5th ed., Pearson, 2010.
- Kurose, J. F., and Ross, K. W. “Computer Networking: A Top-Down Approach.” Pearson, 2017.
Summary
Asynchronous transmission is a versatile and adaptable method for data communication, characterized by its use of independent data units and variable intervals between transmissions. It provides flexibility, accommodates varying workloads, and remains a fundamental component of modern communication systems.
