What are the types of Transfer Modes in HDLC?

HDLC (High-Level Data Link Control) is a bit-oriented protocol used for communication over point-to-point and multipoint connections. This protocol implements the structure of ARQ (Automatic Repeat Request) for reliable data transmission.

The HDLC protocol is standardized by ISO and is designed for high-speed communication of large amounts of data. It was developed based on IBM's SDLC (Synchronous Data Link Control) protocol and can transmit data as bit strings of any required length. The basic unit of data communication in HDLC is called a frame.

A transfer mode in HDLC defines the relationship between two devices in a network and determines who controls the communication link. HDLC defines three types of stations: primary (controlling station), secondary (controlled station), and combined (equal stations).

Types of Transfer Modes

HDLC defines three different transfer modes that determine how stations communicate:

Normal Response Mode (NRM)

In NRM, the primary station has complete control over the communication link. The secondary station can only transmit data after receiving explicit permission from the primary station. Once the secondary station completes its transmission, it must wait for another permission grant before sending additional frames. This mode is typically used in centralized network configurations where one master station controls multiple slave stations.

Asynchronous Response Mode (ARM)

ARM provides more flexibility than NRM by allowing the secondary station to initiate transmission without waiting for explicit permission from the primary station. However, the secondary station still cannot send commands to the primary station. This mode is useful when the secondary station needs to send unsolicited responses or data. ARM can operate in both half-duplex and full-duplex configurations, with full-duplex providing better link utilization.

Asynchronous Balanced Mode (ABM)

ABM treats all stations as equal peers, eliminating the primary-secondary relationship. In this mode, any combined station can initiate communication, send commands, and respond to other stations without requiring permission. This mode is commonly used in point-to-point connections where both endpoints need equal communication privileges, such as in modern network protocols and peer-to-peer communications.

Comparison of Transfer Modes

Conclusion

HDLC's three transfer modes − NRM, ARM, and ABM − provide different levels of control and autonomy in data communication. The choice of mode depends on the network topology and the required level of centralized control versus distributed communication flexibility.