Token Bus (IEEE 802.4) Network

Token Bus (IEEE 802.4) is a network access method that combines the physical simplicity of a bus topology with the collision-free characteristics of token ring networks. It creates a virtual ring over a physical bus or tree structure, typically using coaxial cables, where stations pass a token in a predetermined logical sequence.

In a token bus network, each station knows the address of its predecessor and successor in the virtual ring. A station can only transmit data when it possesses the token, ensuring orderly access to the shared medium without collisions.

How Token Bus Works

The token passing mechanism operates as follows:

• Token circulation − A special control frame (token) circulates among stations in logical order

• Data transmission − When a station receives the token and has data to send, it transmits its frame(s) then passes the token

• Token forwarding − If a station has no data to transmit, it immediately forwards the token to the next station

Frame Format of Token Bus

The IEEE 802.4 frame structure contains the following fields:

• Preamble − 1 byte for synchronization purposes

• Start Delimiter − 1 byte marking the beginning of the frame

• Frame Control − 1 byte specifying frame type (data or control)

• Destination Address − 2-6 bytes containing the recipient station address

• Source Address − 2-6 bytes containing the sender station address

• Payload − Variable length data field from the network layer

• Checksum − 4 bytes frame check sequence for error detection

• End Delimiter − 1 byte marking the end of the frame

Advantages and Disadvantages

Conclusion

Token Bus (IEEE 802.4) provides deterministic, collision-free access to a shared bus medium by creating a virtual ring topology for token circulation. While offering guaranteed bandwidth and real-time capabilities, its complexity and overhead have limited its adoption compared to simpler protocols like Ethernet.