What is HDLC protocol?
Operating at the Data Link Layer (Layer 2) of the Open Systems Interconnection (OSI) paradigm, the High-Level Data Link Control (HDLC) protocol is a crucial and extensively used communication standard. The International Organization for Standardization (ISO) created it, and the ISO/IEC 13239:2002 standard defines it. IBM’s Synchronous Data Link Control (SDLC) protocol serves as the foundation for HDLC.
For data frames to be delivered over communication channels in a dependable and effective manner, HDLC is necessary.
Important Features and Purposes
HDLC is characterized by a number of essential features:
- Bit-Oriented and Synchronous: Data is handled as an uninterrupted stream of bits by this bit-oriented, synchronous communication protocol. For data transmission, a synchronized clock source is necessary.
- Reliability: It offers dependable link transfer and data integrity.
- Use: It can be used for both multipoint and point-to-point communications. These days, point-to-point connections, such those between routers or network interfaces, are where it is mostly used.
- Low Overhead: In contrast to other protocols, such as LAPB, HDLC generates a negligible amount of overhead.
- Key Features: It offers key features including flow control, error detection and correction/control, data transparency, and framing.
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HDLC Frame Structure
Data is arranged by HDLC into “frames,” which are logical units. A Flag Sequence Field (F) must appear at the start and finish of every frame. Up to six fields are normally present in a typical HDLC frame:
| Field Name | Standard Size (bits/bytes) | Description |
|---|---|---|
| Flag Field | 8 bits | Marks the beginning and end of the frame. The bit pattern is the unique delimiter 01111110 (0x7E), which also serves as a synchronization pattern for the receiver. The ending flag of one frame can serve as the beginning flag of the next frame. |
| Address Field | 8 bits or more (1 to several bytes) | Generally includes the HDLC address of the secondary station. A primary address can either be a source or a destination, eliminating the need to include the primary’s address. |
| Control Field | 8 or 16 bits (1-2 bytes) | Used to determine how to control the communication process, primarily managing flow and error control. The interpretation of the bits depends on the frame type (I-frame, S-frame, or U-frame). |
| Information Field | Variable length | Carries the actual user data (payload) from the network layer. It is optional and is not used in some types of HDLC frames. |
| Frame Check Sequence (FCS) Field | 16 or 32 bits (2 or 4 bytes) | Used for error detection. It typically uses CRC16 (16-bit Cyclic Redundancy Check) or CRC32 (32-bit CRC) code, computed over the Address, Control, and Information fields. |
| Closing Flag Field | 8 bits | Identical to the starting Flag Field (01111110). |
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Data Transparency and Bit Stuffing
During synchronous framing, a technique known as bit stuffing is used to make sure that the unique Flag Sequence (01111110) does not inadvertently show up in the frame data (such as the Address, Control, or Information fields) and confuse the receiver.
- Every time five consecutive “1” bits show up in the data stream, the transmitter pauses the data and sends a “0” bit.
- This pattern five “1s” followed by a “0” is recognized by the receiving device, which then removes the inserted “0” bit from the received data (unsnuffing).
- Making sure there are enough signal transitions helps the receiving modem synchronize its clock, which is another crucial secondary function of this stuffing process.
Instead, “control-octet transparency,” sometimes known as byte stuffing, is employed for asynchronous framing. This involves employing a control escape octet (0x7D) to escape certain control octets, such as the frame boundary 0x7E.
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Types of HDLC Frames
The type of frame is determined by the contents of the Control Field. HDLC frames come in three basic varieties:
Information Frames (I-frames):
- Transporting user data from the network layer is the goal.
- Control Field: 0 is the first transmitted, least important bit.
- Function: They also contain information about flow and error control, which is frequently added to user data. They include the sequence numbers for sending (N(S)) and receiving (N(R)).
Supervisory Frames (S-frames):
- Use: Only for flow and error control when piggybacking, or data transmission, is not acceptable or feasible.
- Control Field: Bits 10 are the first two.
- Content: There are no information fields on them. Receive Ready (RR), Receive Not Ready (RNR), and Reject (REJ) are a few examples.
Unnumbered Frames (U-frames):
- Use: For a variety of tasks, including link management (including setting up and disconnecting links).
- Control Field: The initial two bits are 11.
- Content: An information field may be present in certain U-frames. Unnumbered Acknowledge (UA), Disconnect (DISC), and Set Asynchronous Balanced Mode (SABM) are a few examples.
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HDLC Transfer Modes
Various data transport modalities are supported by HDLC depending on the link configuration:
- One primary terminal and one or more subsidiary terminals make up the imbalanced Normal Response Mode (NRM) setup. Secondary stations only give data in response to commands from the primary station, which starts data transfer (sends commands).
- Asynchronous Response Mode (ARM): Although secondary terminals can broadcast without awaiting authorization from the primary terminal, this arrangement is likewise imbalanced.
- The most popular balanced arrangement in contemporary use is called Asynchronous Balanced Mode (ABM), in which all devices are treated equally (combined terminals). The link can be initialized, monitored, error-repaired, and frames sent at any moment by either station.
Cisco’s Proprietary Implementation
HDLC is the standard encapsulation for synchronous serial communications in Cisco routers. Cisco’s HDLC implementation is proprietary, though.
Because its header lacks a “Type” field to identify the encapsulated Layer 3 protocol, the original ISO standard HDLC was not designed to encapsulate numerous Network layer protocols over a single link. HDLC would only allow a single protocol environment if manufacturers did not include a means of communicating the Layer 3 protocol. As a result, Cisco routers employ a proprietary HDLC variant that includes a Type field.
HDLC implementations are typically proprietary across vendors and will not converse with non-Cisco HDLC equipment since almost all vendors have their own methods for recognizing the encapsulated network layer protocol.
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