What is an ARC NET in the Computer Network?

ARC NET (Attached Resources Computing Network) is a local area network topology that uses coaxial cables and token passing protocol for data transmission. Developed by Datapoint Corporation in the late 1970s, it was one of the earliest commercial networking technologies, predating both Ethernet and IBM Token Ring.

ARC NET uses a 93-ohm RG-62 A/U coaxial cable, which is incompatible with thin Ethernet's 50-ohm cables despite their similar appearance. The network operates at 2.5 Mbps using analog signaling and can support up to 255 nodes (not 55 as often misunderstood) with proper configuration.

Token Passing Mechanism

Although ARC NET uses a physical bus topology, it implements a logical ring for token circulation. Each station has a unique node ID (1-255), and the token passes sequentially based on these numerical addresses, not physical positioning.

When a station receives the token, it can transmit data for a predetermined time slice. After transmission or if no data is queued, the station passes the token to the next node in numerical sequence. This deterministic access method eliminates collisions and provides predictable network performance.

Technical Specifications

Frame Format

ARC NET frames contain eight fields with a maximum frame size of 508 bytes for data payload. The frame includes start delimiters, source and destination addresses, system code, length field, data, and frame check sequence for error detection.

SOH | SID | DID | COUNT | DATA (0-508 bytes) | CRC1 | CRC2 | EOT

Advantages and Disadvantages

Advantages: Deterministic access, no collisions, simple implementation, and reliable performance under heavy loads. ARC NET was cost-effective and easier to install than Token Ring.

Disadvantages: Limited bandwidth (2.5 Mbps), proprietary technology, lack of standardization, and poor scalability compared to Ethernet. The analog signaling made it susceptible to electrical interference.

Applications and Legacy

ARC NET was primarily used in industrial automation, factory control systems, and small office environments where deterministic access was more important than high speed. It found success in embedded systems and real-time applications requiring predictable network behavior.

Conclusion

ARC NET was an early networking technology that used token passing over a physical bus topology with logical ring organization. While superseded by faster technologies like Ethernet, it played an important role in industrial networking and demonstrated the viability of token-based access methods.