1.
Introduction to HDLC
Q1: What does HDLC stand for, and what is its purpose?
A1: HDLC stands for High-Level Data Link Control. It is a synchronous data link layer protocol
used for reliable data transmission between network devices. It ensures error detection, flow
control, and proper synchronization of data frames.
2. HDLC Standardization and History
Q2: Who standardized HDLC, and what are its variations?
A2: HDLC was adopted by the CCITT (now ITU-T) as part of telecommunications standards:
1980: Adopted under the name LAP (Link Access Procedure).
1988: Improved version called LAP-B (Link Access Procedure-Balanced).
Some variations of HDLC include:
LAPB (used in X.25 networks).
Cisco HDLC (proprietary implementation used in Cisco routers).
PPP (Point-to-Point Protocol, which evolved from HDLC and is widely used in
modern networks).
3. Characteristics of HDLC
Q3: What are the key characteristics of HDLC?
A3: HDLC has the following characteristics:
Synchronous Protocol: Uses clock synchronization for precise data transmission.
Bit-Oriented: Uses bit-stuffing to ensure frame integrity.
Connection-Oriented: Establishes logical connections before data transmission.
Three-Phase Communication:
1. Link Establishment
2. Data Transfer
3. Link Termination
4. Types of HDLC Communication
Q4: What are the two modes of communication in HDLC?
A4:
1. Point-to-Point (LAP-B): Direct communication between two devices.
2. Multipoint (multi-drop): One primary device communicates with multiple secondary
devices.
5. HDLC Frame Structure
Q5: What are the main components of an HDLC frame?
A5: An HDLC frame consists of the following fields:
Field Description
Flag (0111 1110) Marks the start and end of a frame.
Address Identifies the destination of the frame.
Control Defines the type of frame and sequencing.
Data Contains the actual information being transmitted.
FCS (Frame Check Provides error detection using CRC (Cyclic Redundancy
Sequence) Check).
6. HDLC Transparency Mechanism
Q6: How does HDLC ensure transparency in data transmission?
A6: HDLC uses a bit-stuffing mechanism to prevent data from being misinterpreted as
control flags:
The flag sequence 0111 1110 is used to indicate the start and end of a frame.
If the data contains five consecutive ‘1’ bits, HDLC automatically inserts a ‘0’ bit after
them.
The receiver removes the inserted bit to reconstruct the original data.
This ensures that the flag sequence is not accidentally present within the data payload.
7. Types of HDLC Frames
Q7: What are the three types of HDLC frames?
A7:
1. I-Frames (Information Frames)
o Used for data transmission.
o Contains sequence numbers for flow control and error handling.
2. S-Frames (Supervisory Frames)
o Used for error recovery and flow control.
o Types:
RR (Receive Ready) → Acknowledges received frames.
RNR (Receive Not Ready) → Indicates the receiver is busy.
REJ (Reject) → Requests retransmission of all frames from a certain
number.
SREJ (Selective Reject) → Requests retransmission of a specific frame.
3. U-Frames (Unnumbered Frames)
o Used for control and link management (e.g., connection setup and
termination).
o Examples: SABM (Set Asynchronous Balanced Mode), DISC (Disconnect), UA
(Unnumbered Acknowledge).
8. HDLC Error Handling
Q8: How does HDLC handle errors?
A8: HDLC employs several mechanisms for error detection and correction:
Frame Check Sequence (FCS): Uses a 16-bit CRC to detect errors in received frames.
Acknowledgment Mechanism:
o RR (Receive Ready) acknowledges correct frames.
o REJ (Reject) requests retransmission of frames after a missing or corrupted
one.
o SREJ (Selective Reject) asks for a specific frame to be retransmitted instead of
all.
Time-out Mechanism: If an acknowledgment is not received, the sender retransmits
the data.
9. HDLC vs. Other Data Link Protocols
Q9: How does HDLC compare to Ethernet and PPP?
A9:
Feature HDLC Ethernet PPP
Type Bit-oriented Byte-oriented Bit-oriented
Error Detection CRC (FCS) CRC (FCS) CRC (FCS)
Connection Point-to-Point, Multipoint Broadcast Point-to-Point
Type (CSMA/CD)
Addressing Uses explicit addresses Uses MAC addresses No addressing
Usage WAN links, serial LAN networks WAN links, dial-
communication up
Standard ITU-T IEEE 802.3 IETF
10. HDLC Implementations and Real-World Use Cases
Q10: Where is HDLC used in real-world applications?
A10:
WAN Communication: Used in serial point-to-point links between routers.
Frame Relay Networks: HDLC serves as an encapsulation protocol for Frame Relay.
ATM Networks: Used for establishing connections before ATM transmission.
ISDN Networks: Implemented as LAP-B for error control in Integrated Services Digital
Network (ISDN).
Satellite Communication: Provides reliable data transmission in satellite links.
11. HDLC Advantages and Disadvantages
Q11: What are the advantages and disadvantages of HDLC?
A11:
✅ Advantages:
Efficient and reliable due to its error detection mechanisms.
Supports both point-to-point and multipoint communication.
Bit-oriented protocol, making it more versatile than character-based protocols like
Ethernet.
❌ Disadvantages:
No built-in authentication (unlike PPP).
More overhead compared to simpler data link protocols.
Cisco HDLC is proprietary, meaning it is not interoperable with non-Cisco devices.
12. Comparison Between HDLC and PPP
Q12: Why is PPP preferred over HDLC in modern networks?
A12:
PPP supports authentication (PAP, CHAP), while HDLC does not.
PPP is more flexible, supporting multiple network protocols (IP, IPX, AppleTalk).
PPP is widely supported across different vendors, while HDLC has proprietary
versions like Cisco HDLC.
PPP can be used for both synchronous and asynchronous connections, whereas
HDLC is primarily synchronous.
