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?
The main benefit is its flexibility and adaptability to varying data loads and user inputs.
How does asynchronous transmission handle errors?
Basic error detection can be handled using parity bits included in each data unit.
Is asynchronous transmission still relevant today?
Yes, it is widely used in various communication systems, including serial communications and internet protocols.
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.
