What are the types of stations for the HDLC Protocol?

High-Level Data Link Control (HDLC) is a bit-oriented protocol developed by the International Organization for Standardization (ISO) under standards ISO 3309 and ISO 4335. It defines a packetization standard for serial connections and has been widely adopted worldwide due to its versatility in supporting both half-duplex and full-duplex communication, point-to-point and multi-point networks, and switched or unswitched media.

HDLC provides different modes of operation, including a sliding window mode for reliable delivery and an unnumbered data mode for unreliable delivery used by some Internet applications where higher-level protocols like TCP handle retransmission.

Types of Stations

HDLC defines three types of stations based on their control authority and communication capabilities:

Primary Station

The primary station acts as the controlling station on the link with complete authority over all other stations (typically secondary stations). It issues commands and is responsible for organizing the data stream, managing the link, and handling error recovery at the data link level (Layer 2 of the OSI model).

The primary station maintains independent logical connections with each secondary station on the link and coordinates all communication activities.

Secondary Station

A secondary station operates under the complete control of the primary station and has no direct authority for controlling the connection. It only responds when requested by the primary station and cannot initiate communication independently.

The frames sent by secondary stations are called responses, and these stations can only acknowledge or respond to commands from the primary station. Secondary stations depend entirely on the primary station for communication coordination.

Combined Station

A combined station combines the capabilities of both primary and secondary stations. It can send and receive both commands and responses without requiring permission from other stations on the link.

Each combined station operates independently and maintains complete control over itself. No other station can control a combined station, making this configuration suitable for peer-to-peer communication where stations have equal authority.

Station Configuration Comparison

Conclusion

HDLC defines three station types that determine the control hierarchy and communication patterns on the link. Primary stations control the link, secondary stations respond to commands, while combined stations operate as independent peers capable of both commanding and responding.