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Token Bus (IEEE 802.4)

Last Updated : 03 May, 2023
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Token Bus (IEEE 802.4) is a popular standard for token passing LANs. In a token bus LAN, the physical media is a bus or a tree, and a logical ring is created using a coaxial cable. The token is passed from one user to another in a sequence (clockwise or anticlockwise). Each station knows the address of the station to its “left” and “right” as per the sequence in the logical ring. A station can only transmit data when it has the token. The working of a token bus is somewhat similar to Token Ring

The Token Bus (IEEE 802.4) is a standard for deploying token rings in LANs over a virtual ring. The physical medium uses coaxial cables and has a bus or tree architecture. The nodes/stations form a virtual ring, and the token is transmitted from one node to the next in a sequence along the virtual ring. Each node knows the address of the station before it and the station after it. When a station has the token, it can only broadcast data. The token bus works in a similar way as the Token Ring.

The above diagram shows a logical ring formed in a bus-based token-passing LAN. The logical ring is shown with the arrows. 

Frame Format: 

The various fields of the frame format are: 

  1. Preamble – It is used for bit synchronization. It is a 1-byte field. 
  2. Start Delimiter – These bits mark the beginning of the frame. It is a 1-byte field. 
  3. Frame Control – This field specifies the type of frame – data frame and control frames. It is a 1-byte field. 
  4. Destination Address – This field contains the destination address. It is a 2 to 6 bytes field. 
  5. Source Address – This field contains the source address. It is a 2 to 6 bytes field. 
  6. Data – If 2-byte addresses are used then the field may be up to 8182 bytes and 8174 bytes in the case of 6-byte addresses. 
  7. Checksum – This field contains the checksum bits which are used to detect errors in the transmitted data. It is 4 bytes field. 
  8. End Delimiter – This field marks the end of a frame. It is a 1-byte field.

Ring topology has the following advantages:

  1. Data collisions are less likely because each node sends out a data packet after receiving the token.
  2. Under heavy traffic, token passing makes ring topology perform better than bus topology.


  1. Bus Topology: Token Bus uses a bus topology, where all devices on the network are connected to a single cable or “bus”.
  2. Token Passing: A “token” is passed around the network, which gives permission for a device to transmit data.
  3. Priority Levels: Token Bus uses three priority levels to prioritize data transmission. The highest priority level is reserved for control messages and the lowest for data transmission.
  4. Collision Detection: Token Bus employs a collision detection mechanism to ensure that two devices do not transmit data at the same time.
  5. Maximum Cable Length: The maximum cable length for Token Bus is limited to 1000 meters.
  6. Data Transmission Rates: Token Bus can transmit data at speeds of up to 10 Mbps.
  7. Limited Network Size: Token Bus is typically used for small to medium-sized networks with up to 72 devices.
  8. No Centralized Control: Token Bus does not require a central controller to manage network access, which can make it more flexible and easier to implement.
  9. Vulnerable to Network Failure: If the token is lost or a device fails, the network can become congested or fail altogether.
  10. Security: Token Bus has limited security features, and unauthorized devices can potentially gain access to the network.

Here are, some of the advantages and disadvantages of Token Bus are:


  1. Token Bus provides a fair access mechanism, which ensures that each device gets an equal opportunity to transmit data.
  2. It supports a large number of nodes (up to 72 nodes), which makes it suitable for use in large networks.
  3. Token Bus is a deterministic protocol, which means that the time required for a device to access the network is predictable.
  4. It provides a high level of reliability and fault tolerance, as a single point of failure (such as a broken cable) does not affect the entire network.
  5. Token Bus is a standardized protocol, which ensures interoperability between devices from different vendors.


  1. Token Bus has a relatively low data transfer rate compared to other LAN protocols, such as Ethernet.
  2. It requires a strict physical layout of the network, with a maximum length of 2500 meters and a maximum of 8 taps (devices) between any two active nodes.
  3. Token Bus has a complex protocol, which requires a high level of expertise to design and implement.
  4. It is susceptible to collisions, as multiple devices may try to access the network at the same time.
  5. Token Bus requires a dedicated token, which can result in increased latency and decreased network performance.

Overall, Token Bus has some advantages and disadvantages depending on the specific requirements of the network. It is suitable for use in some situations where reliability and deterministic behavior are important, but may not be the best choice for high-speed, high-bandwidth applications.

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