Token Bus (IEEE 802.4)

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.

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