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.
The various fields of the frame format are:
- Preamble – It is used for bit synchronization. It is a 1-byte field.
- Start Delimiter – These bits mark the beginning of the frame. It is a 1-byte field.
- Frame Control – This field specifies the type of frame – data frame and control frames. It is a 1-byte field.
- Destination Address – This field contains the destination address. It is a 2 to 6 bytes field.
- Source Address – This field contains the source address. It is a 2 to 6 bytes field.
- 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.
- Checksum – This field contains the checksum bits which are used to detect errors in the transmitted data. It is 4 bytes field.
- End Delimiter – This field marks the end of a frame. It is a 1-byte field.
Ring topology has the following advantages:
- Data collisions are less likely because each node sends out a data packet after receiving the token.
- Under heavy traffic, token passing makes ring topology perform better than bus topology.