Asynchronous Transmission

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Asynchronous transmission stands as a cornerstone in the realm of serial data communication, particularly for modems and various telecommunication devices. Characterized by its method of transmitting data as a continuous stream segmented by start and stop bits, this transmission mode differentiates itself from its counterpart, synchronous transmission, which relies on a synchronized timing mechanism to regulate data flow between devices. Commonly known as start/stop transmission, asynchronous transmission offers a unique approach to data exchange.

In this article:

  1. What is Asynchronous Transmission?
  2. Operational Principles of Asynchronous Transmission
  3. The Significance of Start and Stop Bits
  4. Asynchronous Transmission vs. Synchronous Transmission
  5. Error Detection and Correction in Asynchronous Transmission
  6. Implementing Asynchronous Transmission
  7. Applications of Asynchronous Transmission
  8. References
Asynchronous Transmission: the essence of data transmitted in intervals.

1. What is Asynchronous Transmission?

Asynchronous transmission is a fundamental method of data communication that facilitates the exchange of information between devices without requiring the data to be sent at regular intervals. This method stands in contrast to synchronous transmission, where data is sent in a steady stream synchronized by a clock signal. Asynchronous transmission is characterized by its flexibility and efficiency in environments where data transmission occurs sporadically or over varying intervals.

Asynchronous transmission is also referred to as start/stop transmission.

Key Characteristics of Asynchronous Transmission:

  • Start and Stop Bits: Unlike synchronous transmission, which relies on a continuous stream of data framed by timing signals, asynchronous transmission frames each piece of data (or character) with distinct start and stop bits. These bits signal the beginning and end of the data packet, respectively, ensuring that the receiver can correctly interpret the data even in the absence of a fixed timing reference.
  • No External Clock: Asynchronous transmission does not require a shared clock between the sender and receiver. This absence of a synchronized clock means that the sender and receiver rely on the start and stop bits to determine the timing of each data packet.
  • Variable Data Rates: The transmission speed in an asynchronous system can vary because it is not dictated by a shared clock. Devices can negotiate speeds based on their capabilities and current network conditions, making asynchronous transmission adaptable to a wide range of communication scenarios.
  • Efficiency in Low-Speed Communications: Given its character-by-character transmission approach, asynchronous transmission is particularly suited to environments where data is transmitted intermittently or where transmission speed is not the primary concern.

2. Operational Principles of Asynchronous Transmission

Asynchronous transmission operates on a simple yet effective principle: data is transmitted in small packets, typically one character at a time, with each packet framed by start and stop bits. This section delves into the mechanics of how asynchronous transmission works, emphasizing the role of these framing bits.

In asynchronous communication, only about 80 percent of the transmitted bits actually contain data, while the other 20 percent contain signaling information in the form of start and stop bits. Each data frame starts with a start bit and ends with a stop bit, with data bits in between. When the receiving station receives a start bit, it knows that pure data will follow. When a stop bit is received, it knows the data frame has ended and waits for the next one.

Asynchronous Transmission

Asynchronous transmission is essentially character-based with additional bits between characters to enable synchronization and error correction. An optional parity bit for error checking can be located immediately before the stop bit in each frame. With parity correction, an 8-bit character requires 3 bits of control information (start, stop, and parity bits), which means an actual overhead of 3/8 or 38 percent.

The Role of Start and Stop Bits

  • Start Bit: At the beginning of each data packet, a start bit signals to the receiver that a new piece of data is about to be transmitted. This bit is usually a transition from a high voltage level (representing a 1) to a low voltage level (representing a 0), which is easily detectable by the receiver.
  • Data Bits: Following the start bit, the actual data payload is transmitted. The data bits represent the information being sent, encoded in a binary format. The number of data bits can vary but typically ranges from 5 to 8 bits per character.
  • Parity Bit (Optional): After the data bits, an optional parity bit may be included for error checking. The parity bit can be set to make the total number of 1s in the packet either even (even parity) or odd (odd parity), providing a simple form of error detection.
  • Stop Bit(s): Each data packet is concluded with one or more stop bits, signaling the end of the packet. The stop bits return to a high voltage level, preparing the receiver for the potential start of the next packet. The duration of the stop bit is longer than that of the start and data bits, ensuring a clear demarcation between packets.

Asynchronous communication is not synchronized by a timer mechanism or clock, and asynchronous devices are not bound to send or receive information at an exact transmission rate. Instead, the sender and receiver negotiate transmission speeds based on hardware limitations and the need to maintain a reliable flow of information. Asynchronous transmission is mainly suitable for low-speed transmission, but speeds can be increased by using data compression.

Synchronization and Error Correction

Despite the lack of a shared clock, the start and stop bits provide a form of synchronization at the character level, allowing the receiver to accurately determine the timing of each bit within the packet. The optional parity bit offers a rudimentary form of error detection, enabling the system to identify some types of transmission errors.

Asynchronous transmission’s simplicity and flexibility make it a viable option for many communication scenarios, particularly where data transmission is sporadic or where the cost and complexity of establishing a synchronous system are prohibitive.

3. The Significance of Start and Stop Bits

In asynchronous communication, start and stop bits are crucial for the accurate transmission and reception of data. These bits frame each character or data packet, serving as indicators for the beginning and end of the data, respectively. Their importance in asynchronous transmission cannot be overstated, as they enable the sender and receiver to maintain data integrity without a shared clock or continuous data stream.

Role of Start Bits

The start bit signals the onset of a new data packet. It acts as a synchronization point for the receiver, indicating that the following bits constitute a character or data packet. Typically, this transition from idle (a high voltage level or a ‘1’) to the start of communication (a low voltage level or a ‘0’) alerts the receiver to prepare for the incoming data.

Function of Stop Bits

Following the data and optional parity bits, the stop bit(s) signify the end of the data packet. The stop bit ensures a clear separation between consecutive data packets, allowing the receiver’s timing mechanisms to reset in preparation for the next start bit. The use of one or more stop bits creates a necessary pause, ensuring that the start bit of the next character is clearly recognized as such.

Importance in Asynchronous Communication

The start and stop bits are essential for asynchronous transmission’s flexibility and adaptability. They allow for variable data transmission rates and ensure that data can be accurately transmitted and received across devices with differing capabilities and in various conditions. This system does not require the sender and receiver to be synchronized beyond the duration of each character’s transmission, significantly simplifying the communication process.

4. Asynchronous Transmission vs. Synchronous Transmission

Key Differences:

  • Timing Synchronization: Synchronous transmission relies on a shared clock signal between the sender and receiver or the data stream itself, ensuring data is sent in a continuous, timed manner. Asynchronous transmission, conversely, does not require a shared clock, using start and stop bits for timing synchronization on a per-character basis.
  • Data Framing: In synchronous communication, data is transmitted in large, continuous blocks or frames, often with timing information included. Asynchronous transmission sends data in smaller packets, typically one character at a time, framed by start and stop bits.

Advantages of Asynchronous Transmission:

  • Simplicity: Easier to implement and manage, particularly for low-speed or intermittent data transmission.
  • Flexibility: Can handle variable data transmission speeds without the need for a shared clock or continuous connection.

Disadvantages of Asynchronous Transmission:

  • Overhead: The addition of start and stop bits introduces overhead, reducing the effective data transmission rate.
  • Limited Speed: Generally slower than synchronous transmission, making it less suitable for high-speed data transfer requirements.

Advantages of Synchronous Transmission:

  • Efficiency: More efficient for continuous, high-speed data transmission as it lacks the start/stop bit overhead.
  • Capacity: Better suited for high-volume or real-time data transfer.

Disadvantages of Synchronous Transmission:

  • Complexity: Requires more sophisticated hardware and software to manage timing and synchronization.
  • Continuous Connection: Less suited for intermittent data transmission due to the need for continuous synchronization.

5. Error Detection and Correction in Asynchronous Transmission

Error detection and correction are pivotal in maintaining data integrity in asynchronous transmission. One commonly used mechanism is the parity bit, an additional bit appended to each data packet to help detect errors.

Parity Bits: The parity bit is set to either make the total number of 1s in the packet even (even parity) or odd (odd parity). This simple form of error checking allows the receiver to detect if the data has been altered during transmission, such as through electrical interference or other forms of corruption.

Error Detection: Upon receiving a data packet, the receiver calculates the number of 1s in the packet, including the parity bit. If the total does not match the expected even or odd parity, the receiver knows that an error has occurred in one of the bits.

Limitations of Parity Bits: While parity bits can detect single-bit errors, they cannot correct them or detect more complex errors, such as when two bits have been altered. For more sophisticated error detection and correction, additional mechanisms, such as cyclic redundancy checks (CRC) or more advanced error-correcting codes, may be employed.

The use of parity bits in asynchronous transmission provides a basic level of error detection, enhancing communication reliability, especially in noisy environments. However, the simplicity of this method also limits its effectiveness to relatively simple error scenarios.

6. Implementing Asynchronous Transmission

Technical Considerations

When implementing asynchronous transmission, several key technical aspects must be considered to ensure effective communication:

  • Data Rate Selection: Determine the optimal data transmission rate based on the capabilities of both the sender and receiver, as well as the quality of the communication medium.
  • Start and Stop Bit Configuration: Configuring the correct number of start and stop bits is critical for aligning the sender’s and receiver’s data framing.
  • Parity Checking: Decide whether to use parity for error detection and which type (even or odd) best suits the application’s reliability requirements.

Hardware Requirements

  • Transmitter and Receiver: Devices capable of encoding and decoding data into asynchronous format, including start/stop bits and optionally parity bits.
  • Communication Medium: A suitable medium for transmitting data, such as twisted pair cables, fiber optic cables, or wireless channels.
  • Interface Devices: Hardware interfaces (e.g., UARTs—Universal Asynchronous Receiver-Transmitters) that facilitate asynchronous communication between devices and networks.

Setup Processes

  1. Configure Communication Parameters: Set the data rate, parity checking, and start/stop bits on both the transmitter and receiver to match.
  2. Establish Physical Connection: Connect the devices using the chosen communication medium and verify the connection’s integrity.
  3. Test Communication: Send test data packets to ensure that data can be transmitted and received accurately, making adjustments as necessary.

7. Applications of Asynchronous Transmission

Asynchronous transmission finds its application in a variety of scenarios where simplicity, flexibility, and low-speed communication are prioritized:

  • Serial Ports in Computers: For connecting peripherals such as keyboards and mice, where data is sent sporadically.
  • Telemetry and Remote Sensors: Transmitting data from remote sensors where data flow is intermittent and does not require high-speed transmission.
  • Point-of-Sale (POS) Terminals: In retail environments for scanning barcodes and processing payments, where transactions do not demand continuous data streams.
  • Modems: Early modems utilized asynchronous transmission for dial-up internet connections, valuing its simplicity for low-speed data exchange.

8. References

  1. Data and Computer Communications by William Stallings – Provides a comprehensive overview of communication systems, including detailed discussions on asynchronous and synchronous transmission.
  2. RFC 1662: PPP in HDLC-like Framing – Offers insights into packet framing and error handling in protocols that can be adapted for asynchronous transmission.
  3. Serial Port Complete” by Jan Axelson – A guide to serial communication, with a focus on implementing asynchronous transmission in practical applications.

See also:

  • Synchronous Transmission (this article also explains the difference between Synchronous and Asynchronous Transmission)