- 2070 Asar · 4 marks
Basic data transfer mechanisms in USB
Short answer
USB defines four transfer types: control for configuration and status with reserved bandwidth; bulk for large bursts with no time guarantee; interrupt for small data with bounded latency; isochronous for real-time data with guaranteed bandwidth and no retry.
Explain the basic data transfer mechanisms used in USB standards.
USB defines four basic transfer types. The transfer type is fixed per endpoint and is declared in its endpoint descriptor.
| Type | Purpose | Direction | Guarantees | Error handling | Examples |
|---|---|---|---|---|---|
| Control | Configuration, commands, status | Both | Reserved bandwidth (10% low/full speed, 20% high speed) | CRC and retry | Enumeration, SET_ADDRESS |
| Bulk | Large bursts of data | One way per endpoint | No time guarantee, uses spare bandwidth, lowest priority | CRC and retry (ACK/NAK) | Printers, scanners, flash drives |
| Interrupt | Small data, bounded latency | Mostly IN | Guaranteed maximum polling interval (1-255 ms) | CRC and retry | Keyboard, mouse |
| Isochronous | Real-time, constant rate | One way | Guaranteed bandwidth and latency | CRC detection, no retry, no handshake | Audio, video camera |
- Control transfer stages: SETUP (token + 8-byte data + ACK), optional DATA stage (IN/OUT), STATUS stage (zero-length packet in the opposite direction).
- Bulk packets are 8/16/32/64 bytes at full speed and 512 bytes at high speed.
- The host controller divides each 1 ms frame between the types: periodic transfers (isochronous and interrupt) are scheduled first and bulk and control take the remaining time.
✓ Answer checked by Claude Sonnet 5.5
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