Token Bus

IEEE 802.4 is a standard that defines the token bus network access method, which is a type of token passing protocol used in local area networks (LANs). The standard specifies the rules for transmitting data over a shared medium in a token bus network, where a token is passed from one node to another to grant the right to transmit data.

Token bus networks were widely used in industrial control systems and other mission-critical applications where a high level of reliability was required. The IEEE 802.4 standard was first published in 1985 and has since been revised several times to improve the performance and functionality of token bus networks. The basic idea behind the Token Bus protocol is that a token is passed from one station to another station on the network, allowing that station to transmit data. Here’s how it works:

1. Token generation: A token is generated by the network interface controller (NIC) of the station that has the highest priority in the network. This station is called the token holder.
2. Token transmission: The token is transmitted onto the network using a broadcast method, so that all stations can see it. The token includes information such as the address of the token holder and a unique identifier.
3. Data transmission: When a station wants to transmit data, it waits for the token to come to it. When it receives the token, it changes the token’s status to “busy” and sends its data onto the network. Other stations on the network can see that the token is now busy and will not attempt to transmit data until the token becomes free again.
4. Token release: After a station has finished transmitting its data, it changes the status of the token back to “free” and sends it back onto the network.
5. Token passing: The token continues to pass from one station to the next in a circular fashion. This allows each station on the network to have an equal chance to transmit data.
6. Error detection: Token Bus uses a cyclic redundancy check (CRC) to detect errors in the data being transmitted. If an error is detected, the data is retransmitted.

The key features of IEEE 802.4 include:

• Token passing protocol: The token bus network access method is a type of token passing protocol that ensures fair access to the shared medium by granting the right to transmit data to one node at a time.
• Priority levels: The standard defines several priority levels for transmitting data, allowing high-priority messages to be transmitted with minimal delay.
• Collision detection: The standard specifies a collision detection mechanism that allows nodes to detect and recover from collisions that occur when two or more nodes attempt to transmit data simultaneously.
• Physical layer: The standard defines the physical layer specifications for token bus networks, including the type of cable and signaling methods used to transmit data.
• Network management: The standard includes provisions for network management, including methods for adding and removing nodes from the network and monitoring network performance.

However, despite its reliability, token bus networks fell out of favor with the widespread adoption of Ethernet and other faster and more flexible networking technologies. The IEEE 802.4 standard is now considered obsolete, and most token bus networks have been replaced with more modern alternatives.