Introduction
In modern networking, data must move fast, but it must also move correctly. A small error on a link can corrupt a file, break a session, or cause retries that waste bandwidth. That is why the Data Link layer still matters, even when we talk about cloud and high-speed backbones. HDLC in computer network is a classic Data Link layer protocol that helps devices send data in a clean and reliable way over serial and WAN links. It uses a frame-based format, adds error checking, and supports flow control so the sender does not overload the receiver.
HDLC (High-Level Data Link Control) is known for being bit-oriented and synchronous, which makes it a good fit for stable point-to-point links. Understanding HDLC helps you understand how framing, error detection (CRC), and link control really work in real networks.
Before getting into more details, let us first understand what HDLC in computer network is.
What is HDLC in Computer network?
HDLC in computer network is a Data Link Layer protocol that defines how data is transmitted using frames over a communication link. HDLC full form is High-Level Data Link Control. HDLC is mainly described as a bit-oriented, synchronous protocol. “Bit-oriented” means it treats data as a stream of bits and uses techniques like bit stuffing to keep frames clear and unambiguous. “Synchronous” means the sender and receiver use a shared timing approach on the link.
HDLC focuses on reliable delivery by defining how frames begin and end, how devices control the link, and how errors are detected using an FCS field (CRC). It can be used for point-to-point and multipoint communication depending on the mode and station setup.
How Does HDLC works?
Here is the simple workflow:
Step 1: Data is turned into frames
The Network layer (like IP) gives data down to Layer 2. HDLC wraps that data into an HDLC frame. This frame has clear fields like Flag, Address, Control, Information, and FCS.
Step 2: Flags mark the start and end
HDLC uses a special 8-bit flag pattern 01111110 (often shown as 0x7E) to mark where a frame starts and ends. This helps the receiver find boundaries even on a continuous serial stream.
Step 3: “Data transparency” is achieved
A big problem is this: what if the data itself contains the same bits as the flag? HDLC avoids confusion using bit stuffing (also called zero insertion). In simple terms, it changes the transmitted bit stream in a predictable way so the flag pattern does not appear inside the data by accident. On the receiver side, the extra bits are removed to restore the original data.
Step 4: Control field manages flow and reliability
The Control field tells what kind of frame it is (data frame vs control frame) and can carry sequence/ack information for reliable transfer and flow control. This is where HDLC “supervises” communication, not just sends data blindly.
Step 5: FCS checks for errors (CRC)
Before sending, HDLC computes an FCS (Frame Check Sequence) using CRC. The receiver recomputes the CRC and compares it. If it does not match, the frame is treated as damaged, and the sender can retransmit (ARQ behavior is commonly discussed with HDLC-style reliability).
Step 6: The receiver accepts or rejects
If the frame is correct, it is accepted and passed upward (after removing the HDLC header/trailer). If not, the link can request recovery using supervisory/control behavior.
In short, HDLC in computer network is not just “framing.” It is framing + control + error detection working together.
How is HDLC used in IT networks?
Even though many networks today use Ethernet in LANs, HDLC is still important in WAN and serial-link contexts and as a concept that shaped other link-layer designs.
Common real-world uses and places you will hear about HDLC are:
- WAN serial links (router-to-router): HDLC has been widely used for synchronous serial links and WAN connections, especially in point-to-point scenarios.
- Default serial encapsulation in some Cisco environments: Many Cisco routers historically used HDLC encapsulation by default on serial interfaces. You may verify it using interface commands such as show interfaces serial
- Cisco HDLC vs standard HDLC: Standard HDLC is often described as single protocol in some contexts, while Cisco’s HDLC variant can add a Protocol field to support multiple Layer 3 protocols on the same link. This is why mismatched encapsulation types can break a link between different vendors.
- Foundation for other standards: HDLC concepts influenced other link standards and “HDLC-like” framing in other protocols.
Why IT professionals still care?
- If you troubleshoot WAN links, serial encapsulation, legacy systems, or protocol mismatches, understanding HDLC protocol framing helps.
- If you study for network exams, HDLC is a clean example of Layer 2 design: framing, error detection, and control logic.
HDLC Transfer Modes
HDLC defines different ways devices behave on a link. Your prompt asks for two main transfer modes: NRM and ABM.
Normal Response Mode (NRM)
NRM is an “unbalanced” style mode. It uses a primary station and one or more secondary stations.
- The primary sends commands.
- The secondary responds, and typically sends only when the primary allows it (polling style).
- This can support point-to-point and multipoint setups.
This mode matches older “host controls terminals” designs and multipoint lines where order matters.
Asynchronous Balanced Mode (ABM)
ABM is a “balanced” mode, usually used in point-to-point links.
- Both ends are treated more equally (often called combined stations).
- Either side can send commands and responses.
- ABM is widely associated with efficient point-to-point communication because it avoids strict polling overhead.
Note: Some materials also mention ARM (Asynchronous Response Mode) for completeness, but your requested focus is NRM and ABM. (You can see an overview of station concepts and modes in academic notes too.)
Standard HDLC Frame Structure
The strength of HDLC in computer network is that it uses a clear and repeatable frame format. A standard HDLC frame is commonly explained with these key fields:
1). Flag
- Size: 8 bits
- Bit pattern: 01111110 (often written as 0x7E)
- Purpose: Marks start and end of the frame.
2). Address Field
- Typical size: 8 bits, but can extend depending on needs.
- Purpose: Identifies the receiver (like the secondary station address in primary/secondary setups). It can also support group/broadcast addressing in some descriptions.
3). Control Field
- Size: 8 or 16 bits
- Purpose: This is the “brain” of the frame. It indicates the frame type and carries flow/error control information (sequence and supervision behavior).
4). Payload / Information Field
- Size: variable
- Purpose: Carries the actual user data from the Network layer.
Note: This field may be missing in some control-only frames (like many supervisory frames).
5). FCS (Frame Check Sequence)
- Size: 16 or 32 bits (often shown as 2 bytes or 4 bytes)
- Purpose: Error detection using CRC. The receiver checks this value to detect corruption during transmission.
This is why the HDLC protocol is often taught as a “model” Data Link layer protocol.
Types of HDLC Frame
HDLC frames are commonly grouped into three types. The type is determined by the Control field.
1). Information Frames (I-frames)
I-frames carry the real user data (payload) from the Network layer.
- They also support flow/error control info that can be piggybacked with data.
- In many references, the I-frame is identified by the first bit in the control field being 0.
Use I-frames when you actually want to send data, not just manage the link.
2). Supervision Frames (S-frames)
S-frames (supervisory frames) are used when you need flow control or error control, but you are not sending user payload.
- Typically, S-frames do not carry an information field.
- They help with acknowledgments, readiness, and recovery actions.
- Many descriptions identify S-frames with the first two bits of the control field as 10.
Think of S-frames as “traffic signals” for the link.
3). Unnumbered Frames (U-Frames)
U-frames are used for link management and other control tasks that are not simple data transfer.
- Depending on the specific U-frame function, it may or may not include an information field.
- Commonly identified by the first two bits of the control field as 11.
Think of U-frames as “setup and management messages” for the connection.
Frequently Asked Questions
Q1. What are the three types of frames in HDLC?
HDLC defines three frame types: Information I frames carry user data; Supervisory S frames manage flow and errors; Unnumbered U frames handle control signals overall.
Q2. How to use HDLC?
To use HDLC, establish a synchronous link, agree on a mode, send frames with address and CRC, use bit stuffing, and acknowledge promptly each time.
Q3. What is the HDLC method?
The HDLC method is a bit-oriented data link procedure that frames traffic, uses sliding windows, and provides robust flow control, error detection, and recovery.
Q4. What does HDLC mean?
HDLC means High-Level Data Link Control, an ISO standard protocol for point-to-point or multipoint communication, used on serial WAN links in practice.
Conclusion
HDLC in computer networks is one of the clearest examples of how the Data Link layer keeps communication reliable. The HDLC full form (High-Level Data Link Control) reflects its goal: control the link at a high level using structured frames.
The HDLC protocol defines a standard frame with Flag, Address, Control, Information, and FCS. It also defines how devices behave using transfer modes like Normal Response Mode (NRM) and Asynchronous Balanced Mode (ABM). Finally, its frame types, I-frames, S-frames, and U-frames, separate data transfer from supervision and link management.
