Skip to content

IEEE 802.4 Token Bus in Detail

IEEE 802.4 is a standard that defines the operation of a token bus network. Token bus is a LAN architecture that combines the bus topology with a token-passing protocol to control access to the shared medium. This standard was developed primarily for industrial environments where predictable communication times are critical.

Key Features of IEEE 802.4 Token Bus

  1. Deterministic Access:
    • Token bus networks provide deterministic access to the network, meaning that the maximum time a device must wait to transmit data is predictable.
  2. Token-Passing Protocol:
    • The network uses a token-passing protocol where a token, a special data packet, circulates around the network. A device can only transmit data if it holds the token, ensuring that only one device transmits at a time and preventing collisions.
  3. Bus Topology:
    • Physically, the network is configured as a bus topology, where all devices are connected to a single communication line.
  4. Logical Ring:
    • Although the physical layout is a bus, the network operates logically as a ring with the token being passed in a predefined order among devices.

Frame Structure

The frames used in IEEE 802.4 Token Bus networks are similar to those in other token-based networks and typically include the following components:

  1. Preamble:
    • Used for synchronization.
  2. Start Frame Delimiter (SFD):
    • Indicates the start of the frame.
  3. Frame Control:
    • Defines the type of frame and its control information.
  4. Destination Address:
    • The address of the recipient device.
  5. Source Address:
    • The address of the sender device.
  6. Data/Payload:
    • The actual data being transmitted.
  7. Frame Check Sequence (FCS):
    • Used for error detection using a cyclic redundancy check (CRC).

Token-Passing Mechanism

  1. Token Circulation:
    • A token circulates through the network, passed from one device to the next in a predetermined sequence.
  2. Transmission Rights:
    • When a device receives the token, it gains the right to transmit data. After transmitting, it passes the token to the next device.
  3. Token Holding Time:
    • Each device can hold the token for a specified maximum time (Token Holding Time, THT) to ensure fair access for all devices.
  4. Token Loss and Regeneration:
    • If the token is lost or corrupted, the network has mechanisms to detect the loss and regenerate the token, ensuring continued network operation.

Physical Layer

  1. Cabling:
    • Typically uses coaxial cable, though other types of cabling like twisted pair or fiber optic can be used.
  2. Connectors:
    • Uses BNC connectors for coaxial cables.
  3. Topology:
    • Physical bus topology with terminators at both ends of the bus to prevent signal reflections.

Logical Layer

  1. Logical Ring:
    • Devices are logically arranged in a ring for token passing, despite the physical bus layout.
  2. MAC Addresses:
    • Each device has a unique MAC address for identification and communication.

Error Handling

  1. Error Detection:
    • Frames include an FCS field for detecting transmission errors.
  2. Token Recovery:
    • If a token is lost, a new token can be generated by a designated monitor station.

Advantages of Token Bus

  1. Deterministic Access:
    • Predictable access times are ideal for real-time applications.
  2. Collision-Free:
    • The token-passing protocol ensures that only one device transmits at a time, eliminating collisions.
  3. Fair Access:
    • Each device gets an equal opportunity to transmit, avoiding issues of network fairness.

Disadvantages of Token Bus

  1. Complexity:
    • More complex to implement and maintain compared to simpler protocols like CSMA/CD.
  2. Token Management:
    • The need to manage the token and handle token loss adds overhead.
  3. Scalability:
    • Less scalable compared to Ethernet due to the overhead of token management and the need for a logical ring structure.

Applications

  • Industrial Automation:
    • Used in factory automation and industrial control systems where real-time communication and deterministic access are critical.
  • Manufacturing Networks:
    • Employed in environments where precise timing and coordination are required.

Conclusion

IEEE 802.4 Token Bus is a networking standard designed for environments where predictable communication times and collision-free transmission are essential. By combining the physical bus topology with a logical ring and a token-passing protocol, it provides deterministic network access, making it suitable for industrial and real-time applications. However, its complexity and token management overhead have led to its decline in favor of simpler and more scalable technologies like Ethernet in many applications.