GATE CS 1992 Q02 — Concept of Layering: OSI and TCP/IP Protocol Stacks

In asynchronous serial communication, there is no shared clock signal between the transmitter and the receiver to coordinate the transfer of data. Instead, synchronization is achieved on a character-by-character basis using start and stop bits:

• Start Bit: Signals the receiver that a new character frame is arriving. This transition (typically from a high idle state to a low state) prompts the receiver to start its internal clock to sample the incoming data bits at the correct intervals.
• Stop Bit: Signals the end of the character frame and returns the transmission line to the idle state (high), ensuring that the next start bit (low) can be clearly detected.

Therefore, these bits are used for synchronization.

Why other options are incorrect:

• Error detection: Typically handled by parity bits or checksums (like CRC).
• Error correction: Handled by error-correcting codes (like Hamming codes) or retransmission protocols.
• Slowing down the communications: While start and stop bits introduce overhead and reduce the effective data rate, this is an undesirable side-effect, not their intended purpose.