Token bus

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Token Bus in the TCP / IP protocol stack :
application HTTP IMAP SMTP DNS ...
transport TCP UDP
Internet IP ( IPv4 , IPv6 )
Network access Token bus

Token bus is a form of the token passing access method in a computer network and is defined in the IEEE standard 802.4 .


The token bus is based on the token that is forwarded in the network from one station to the neighboring station. With Token Bus (in contrast to Token Ring ), the neighboring station is determined using an address. To do this, the sending station increases its node ID by the value 1 in order to address the neighbor.

The name bus results from the fact that the token is sent over the entire network and received by all stations. Only the station with the next higher node ID is allowed to receive the token. If the incoming token is not required, a new token with the neighboring address is created and forwarded.

The token bus network is initialized by all stations listening to the bus. If no information is received from a station within a certain time interval, it tries to claim the token and then to set up the logical ring.

For ring expansion, each station periodically asks whether stations with addresses between their own address and that of the successor, to whom the token is otherwise passed, want to be included in the logical ring. If this is the case, the station that owns the token passes it on to the new station and thus allows it to be incorporated into the logical ring.

If a station wants to leave the logical ring, it waits until it receives the token. If it has the token, it informs its predecessor station of the address of its successor and then passes the token on to the successor. The logical ring is thus retained.

A typical application of Token Bus is ARCNET or the Profibus field bus .

Benefits of Token Bus

With the token bus, too, access is controlled via token passing, but the network has a bus or tree structure. So here we have the case that a logical ring structure is based on a physical bus structure. The method is e.g. B. used in ARCNET and in industrial automation in Manufacturing Automation Protocol (MAP). Therefore, the order of the stations is not done by the hardware connection, but purely logically by the address assignment. The tokens are passed on from the station with the highest address to the one with the next lower address. The station with the lowest address closes the logical ring by addressing it to the highest address.


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