Asynchronous vs. Synchronous Communication Protocols

Asynchronous protocols rely on start and stop bits for each character transmitted. Synchronous protocols, on the other hand, utilize two or more SYN (synchronization) characters at the beginning of a transmission. A prime example of an asynchronous protocol is the Xmodem protocol. Conversely, IBM’s Bisync protocol serves as a classic example of a synchronous protocol. Let us understand these protocols before we dive into differences between them.

Asynchronous protocol

These protocols use character based transmission. Here each character is framed by start and stop bits. Hence shared clock is not required for communication.

• Example : Xmodem - Simple file transfer protocol used over serial connections. Utilizes half duplex stop and wait mechanism

Synchronous protocol

These protocols transmit data in continuous stream mode and uses shared clock signal for synchronization. SYN characters are transmitted at the beginning to align clocks of both sender and receiver.

• Example : Bisync - Uses two SYN characters at the beginning. Frame consists of SYN characters, SOH, STX, data payload and ETX.

Comparison

Summary

Asynchronous protocols are simple and preferred for low speed intermittant communication. Synchronous protocols offer higher efficiency and are suitable for high speed continuous data communication.