- 2073 Shrawan · 4 marks
Short note: HDLC
Short answer
HDLC is a bit-oriented data link protocol using frames delimited by flag 01111110, bit stuffing, and three frame types (I, S, U). It provides reliable communication with sliding window flow control, ARQ error control, and supports point-to-point and multipoint configurations.
Write a short note on HDLC.
HDLC (High-level Data Link Control) is a bit-oriented, synchronous data link layer protocol standardised by ISO, derived from IBM SDLC. It provides reliable, point-to-point and multipoint communication using frames, error control and flow control. PPP, LAPB (X.25) and LAPD (ISDN) are derived from it.
Station types and modes
- Primary (sends commands), Secondary (sends responses), Combined (both).
- NRM (Normal Response Mode): secondary sends only when polled by primary (multipoint).
- ABM (Asynchronous Balanced Mode): combined stations, both can start transmission (point-to-point).
Frame format
| Flag | Address | Control | Information | FCS | Flag |
| 8 | 8 | 8/16 | variable | 16 | 8 |
- Flag: 01111110; bit stuffing keeps the pattern out of the data.
- Control field defines the frame type:
- I-frame (information): carries data, the send and receive sequence numbers N(S) and N(R) (N(R) is a piggybacked acknowledgement), and the P/F bit.
- S-frame (supervisory): flow and error control (RR, RNR, REJ, SREJ), no data.
- U-frame (unnumbered): link setup and management (SABM, DISC, UA).
- FCS: CRC-16 (or CRC-32) for error detection.
Features
- Uses sliding window flow control (window 7, or 127 in extended mode) and ARQ.
- Transparent data using bit stuffing.
- Full duplex with piggybacked acknowledgements.
✓ Answer checked by Claude Opus 5.5 (high effort)