Token bus
| 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 .
technology
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
- For production techniques such as B. Manufacturing engineering and process engineering you need deterministic response times and data rates , i. H. the maximum response times and minimum data rates must be guaranteed. Ethernet , according to IEEE 802.3, cannot guarantee the response time and is therefore unsuitable. Only the real-time variants of Ethernet, e.g. B. RTnet , PROFINET , EtherCAT or SERCOS III are able to do this.
- When a station is disconnected, the ring is no longer functional with Token Ring . However, this risk is significantly lower when using multistation access units . This problem does not arise with Token Bus if the bus is not severed.
- The line length is shorter than with Token Ring because there is no closed ring. Above all, this has cost advantages.
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.
literature
- Hans-Peter Messmer, Klaus Dembowski: PC hardware book. Structure - Functionality - Programming, 7th edition, Pearson Education, Munich 2003, ISBN 978-3-8273-2014-8 .
- Christoph Meinel, Harald Sack: Internetworking. Springer-Verlag, Berlin / Heidelberg 2012, ISBN 978-3-540-92940-6 .
- Dirk Larisch: The beginners' seminar on network technology. 3rd edition, hjr-Verlag, Heidelberg 2010, ISBN 978-3-8266-7515-7 .
- Dieter Conrads: data communication. Procedure - Networks - Services, 2nd edition, Friedrich Vieweg & Sohn Verlag, Wiesbaden 1993, ISBN 978-3-528-14589-7 .
- Dirk Traeger: LAN practice of local networks . Springer Fachmedien, Berlin / Heidelberg, ISBN 978-3-519-06189-2 .
See also
Web links
- Information on access procedures (including token bus)
- Token Bus (IEEE-802.4) (accessed August 4, 2017)
- Process automation technology communication networks (accessed on August 4, 2017)
- Token Bus (accessed August 4, 2017)
- Performance analysis of token bus protocol with maintenance functions (accessed August 4, 2017)