GATE CS 1997 Q2.3 — Concept of Layering: OSI and TCP/IP Protocol Stacks

In RS-232 asynchronous serial communication, the transmitter and receiver do not share a common clock signal. Instead, they agree beforehand on a transmission speed (baud rate).

To ensure the receiver samples the incoming data bits at the correct time intervals:

• The transmission line remains in an idle state (typically high voltage, called MARK).
• The beginning of each byte is signaled by a start bit (transition to low voltage, called SPACE).
• This transition alerts the receiver to start its internal clock and synchronize its sampling timing for the incoming bits of that specific byte.

Since this synchronization process is repeated for every individual byte transmitted, the purpose of the start bit is to synchronize the receiver for receiving every byte.