---
title: "Short note: HDLC"
subject: "Computer Networks and Security"
chapter: 3
key: "semester_7/computer_networks_and_security:hdlc-note"
source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/hdlc-note
site: BCT Engineering
---

# Short note: HDLC

Computer Networks and Security · Chapter 3: Data Link Layer

- Asked in: 2073 Shrawan, 4 marks
- Video: [High-Level Data Link Control (HDLC)](https://www.youtube.com/watch?v=N2tgsPUPEBE) — Neso Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/hdlc-note
- Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-3

## Question

Write a short note on 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.

## Answer

**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
```text
| 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. HDLC is:
   A. Byte-oriented asynchronous
   B. Bit-oriented synchronous data link protocol
   C. Network layer protocol
   D. Physical layer protocol
   Answer: B. HDLC is a bit-oriented synchronous ISO protocol using bit stuffing. (Key point: HDLC nature)

2. Which frame carries link setup such as SABM, DISC, UA?
   A. I-frame
   B. S-frame
   C. U-frame
   D. F-frame
   Answer: C. U-frames are unnumbered and manage the link. (Key point: Frame types)

3. What do S-frames do?
   A. Carry user data
   B. Provide flow and error control (RR, RNR, REJ, SREJ) without data
   C. Set up the link
   D. Hold the FCS
   Answer: B. Supervisory frames control flow and errors. (Key point: S-frame)

4. In NRM when does a secondary send?
   A. Anytime
   B. Only when polled by the primary
   C. Never
   D. After a token
   Answer: B. NRM is poll-driven. (Key point: Modes)