What Is HDLC & Why Is It So Important for Network Security?

 

It  ensures safe delivery of data frames across a network or communication link. HDLC offers different operations like framing, data transparency, error detection, & correction, and even flow control. 

Primary stations just send commands containing secondary station addresses. The secondary station then just sends responses containing the primary address.

History Of High-Level Data Link Control (HDLC)

HDLC is a communication protocol that helps computers communicate with each other reliably. It started as IBM’s SDLC protocol and was later standardized by organizations like ITU and ANSI. 

You can think of it as a set of rules for packaging data before sending it across networks. It's like an envelope system for digital messages that makes sure they arrive safely and in the correct order.

The best part of HDLC is that it's very flexible. The protocol isn't a straitjacket for all the details, and as such, it allowed many other protocols to be developed using it. You'll find reasons why HDLC features find themselves in a majority of major network technologies:

  • X.25 networks utilized a variation named LAPB.
  • Modems utilize a version named LAPM in V.42.
  • Frame Relay networks use LAPF.
  • LAPD is used on ISDN telephone lines.

PPP, the protocol of the Internet, employs HDLC-type framing on some connections.

HDLC was initially defined in multiple ISO standards documents, but it's now all consolidated into a single standard (ISO/IEC 13239:2002).

HDLC also helped shape other key protocols, such as IEEE 802.2, and derivatives of it are found in telephone equipment and Cisco network devices.

In a sense, HDLC is a building block that has set the pattern for how data is transmitted over numerous different kinds of networks and devices.

How HDLC Works & Where It Is Used?

High-Level Data Link Control (HDLC) is a bit-oriented protocol that is used at the data link layer (Layer 2) of the OSI stack. Basically, HDLC makes reliable data transmission between devices in a network by framing information into structured frames.

High-Level Data Link Control (HDLC) FRame Story

All HDLC communications are initiated and terminated by a flag field—a special pattern of bits (01111110) that marks the beginning and the end of every frame. Between the flags, the frame includes an address field with the identity of which station is supposed to receive the data, a control field that handles the flow of communication, and typically an information field with the actual data. The frame ends in a Frame Check Sequence (FCS), with which the receiver can identify any transmission errors.

HDLC Operational Modes

HDLC can function in a number of different modes based on the relationship between communicating devices. In Normal Response Mode, a primary station dictates when secondary stations may send data. Asynchronous Response Mode permits secondary stations to start communication without direct permission, and Asynchronous Balanced Mode considers all stations equal, with each serving as both primary and secondary.

Types Of Frames

The protocol satisfies different communication requirements using different types of frames.

  • A supervisory frame (S-frame) in HDLC serves the purpose of flow control and error recovery rather than conveying actual data. It facilitates communication by accepting received frames, asking for retransmissions, and making data transmission smooth.
  • Information frames transfer the real user data; supervisory frames handle flow control and error recovery but do not transport data
  • Unnumbered frames help in establishing, sustaining, and terminating device connections.

Error control in HDLC is advanced but effective. The protocol numbers are framed in sequence and demand acknowledgments from the receivers. In case of errors or missing frames, HDLC has provisions to ask for retransmission of individual frames instead of retransmitting all data.

3 main HDLC Operational Modes

HDLC provides multiple methods for network devices to communicate, known as transfer modes. These modes establish relationships between communication stations and set rules for which device can transmit data and when.

Normal Response Mode (NRM)

In this system, we have a hierarchy between devices. The primary station acts as a controller between communication channels, while the secondary stations wait for explicit permission before sending data. 

Let's take an example of a classroom to understand this better. Here, the teacher acts as the primary station, and the students act as secondary stations. The students (secondary stations) can only speak when called upon by the teacher (primary station). The primary station starts all communication by sending commands, and the secondary stations respond only when prompted to do so.

NRM is especially suited to scenarios where centralized management is significant, such as for mainframe-based computing environments or control systems of industrial applications. One of the main benefits of NRM is that it can operate on half-duplex links, in which data can move only one way at a time. 

The main station works around this constraint by scheduling when each device gets to send, preventing collisions on the network.

Asynchronous Response Mode (ARM)

It gives secondary stations more independence. No doubt there is a distinction between a primary and a secondary station, but a secondary station does not need to wait for permission. It can start data transmission without any approval. 

You can take the example of attendees acting as secondary stations and the chairperson as the primary station.

Now think of ARM as a gathering where the attendees (secondary stations) are allowed to voice their opinions as soon as they have something to say, rather than waiting for the chairperson (primary station) to call upon them. The chairperson is still in control of the entire meeting but can't control attendees to share their opinions.

In ARM, the master station is still in charge of making connections, handling error recovery, and governing the general flow of data. The secondary stations, however, are free to transmit whenever they feel the channel is available.

Asynchronous Balanced Mode (ABM)

You can consider it the most fair mode of communication. In ABM, there’s no hierarchy; all the stations are considered equal, with equal status and capabilities.

Each station is free to make a contribution without needing permission from the rest. It is comparable to coworkers freely sharing ideas during a brainstorming session, where anyone can speak up when they have something to say.

ABM is currently the most popular HDLC mode used in today's networking. It is the basis for several essential protocols, such as:

  • LAP-B (Link Access Procedure, Balanced) employed in X.25 networks
  • LAP-D (Link Access Procedure on the D-channel) employed in ISDN
  • LAP-F (Link Access Procedure for Frame Mode Bearer Services) employed in Frame Relay
  • Aspects of PPP (Point-to-Point Protocol) utilized to connect to the Internet

Let’s Sum Up the Debate

High-Level Data Link Control (HDLC) is a general-purpose data link layer protocol designed by ISO for error-free data transfer between network points. It has a defined frame structure, error control mechanism, and effective flow control. 

HDLC operates in three modes and specifies three types of frames: information, supervisory, and unnumbered. HDLC's flexibility enables usage in different types of networks, and several other protocols have been inspired by it. The importance of HDLC is that it is used extensively in telecommunication and networking settings, working at Layer 2 of the OSI model. 

Up to 2025, HDLC is still important in contemporary networking, especially in point-to-point links between routers or network interfaces, because of its flexibility, reliability, and error handling.