XON/XOFF is a protocol for controlling the flow of data between devices in a serial communication link, such as between a computer and a modem. It’s a software-based flow control method that uses specific control characters to start and stop the flow of data.
Here’s a summary of XON/XOFF:
Purpose: It’s designed to let a receiving device communicate to a sending device when it’s ready for more data or when it needs the data flow to temporarily stop, usually because its buffer is full.
Control Characters:
- XON: Represented by the ASCII control character 17 (0x11 in hexadecimal) and often referred to as
^QorCtrl-Q. It signals the transmitting device to resume sending data. - XOFF: Represented by the ASCII control character 19 (0x13 in hexadecimal) and often referred to as
^SorCtrl-S. It signals the transmitting device to pause or stop sending data.
Operation:
- When the receiving device’s buffer approaches being full (indicating it might soon be unable to store incoming data), the device sends the XOFF character to the transmitting device to tell it to pause its data transmissions.
- Once the receiving device has processed enough data from its buffer to handle more incoming data, it sends an XON character to the transmitting device, indicating that it can resume sending data.
Advantages:
- Simplicity: XON/XOFF is a straightforward protocol, easy to implement, and doesn’t require any additional hardware or dedicated signal lines.
- Flexibility: Being software-based, it’s adaptable to various communication settings and can be used over simple serial connections.
Disadvantages:
- Potential Data Confusion: If the actual transmitted data contains XON or XOFF characters, it could interfere with the flow control. Techniques like byte stuffing can be used to manage this.
- Reduced Throughput: Since it operates at the character level and because the sending device must pause frequently, it can reduce the efficiency of the communication link, especially if the receiver’s buffer size is small or if it processes data slowly.
- Usage: Historically, XON/XOFF was more prevalent when hardware resources were limited. As technology advanced, hardware-based flow control methods, like RTS/CTS, became more common due to their higher efficiency and reliability.
In summary, XON/XOFF is a software-based flow control mechanism in serial communication that utilizes control characters to manage data transmission between devices.