- 2068 Baisakh (old course) · 4 marks
Synchronous vs asynchronous transmission
Short answer
Synchronous transmission sends blocks of characters using a shared clock and sync characters, has low overhead, high speed (Mbps), e.g., SPI. Asynchronous transmission sends one character at a time framed by start/stop bits, no common clock, higher overhead (~25–40%), lower speed (kbps), e.g., RS‑232.
Differentiate between synchronous and asynchronous data transmission.
| Point | Synchronous transmission | Asynchronous transmission |
|---|---|---|
| Unit of transfer | Block of characters (frame) | One character at a time |
| Clock | Clock shared between sender and receiver (separate line or embedded in data) | No common clock; each side uses its own, with the same baud rate |
| Synchronisation | SYNC characters / flags at the start of the block | Start and stop bit per character |
| Overhead | Small (header and CRC for a long block) | Large: 2-3 extra bits per 7-8 data bits (about 25-40%) |
| Speed | High (Mbps) | Low (typically up to about 115 kbps for a UART) |
| Idle time | No gaps between characters | Gaps allowed between characters |
| Hardware | Complex: USRT/USART, clock recovery | Simple UART |
| Cost | Higher | Lower |
| Error detection | CRC on block | Parity per character |
| Example | HDLC, SDLC, Ethernet, SPI | RS-232, keyboard, serial mouse |
✓ Answer checked by Claude Sonnet 5.5
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