Token Bus (IEEE 802.4) Network

Token Bus (IEEE 802.4) is a standard for implementing token ring over the virtual ring in LANs. The physical media has a bus or a tree topology and uses coaxial cables. A virtual ring is created with the nodes/stations and the token is passed from one node to the next in a sequence along this virtual ring. Each node knows the address of its preceding station and its succeeding station. A station can only transmit data when it has the token. The working principle of the token bus is similar to Token Ring.

Token Passing Mechanism in Token Bus

A token is a small message that circulates among the stations of a computer network providing permission to the stations for transmission. If a station has data to transmit when it receives a token, it sends the data and then passes the token to the next station; otherwise, it simply passes the token to the next station. This is depicted in the following diagram −

Frame Format of Token Bus

The frame format is given by the following diagram −

The fields of a token bus frame are −

• Preamble: 1 byte for synchronization.

• Start Delimiter: 1 byte that marks the beginning of the frame.

• Frame Control: 1 byte that specifies whether this is a data frame or control frame.

• Destination Address: 2-6 bytes that specifies address of destination station.

• Source Address: 2-6 bytes that specifies address of source station.

• Payload: A variable length field that carries the data from the network layer.

• Checksum: 4 bytes frame check sequence for error detection.

• End Delimiter: 1 byte that marks the end of the frame.