1. Schutzvermerk nach DIN 34 beachten Ethernet Deterministics

2. Schutzvermerk nach DIN 34 beachten Definitions What does realtime mean ? –The term „realtime“ means that there will be a punctual reaction to occurring events under all circumstances. What does „deterministic“ mean ? –Deterministic means that occurring delays can be precalculated.

3. Schutzvermerk nach DIN 34 beachten Which difficulties are there ? delay 20 %40 %30 %10 %50 %60 %70 %80 % Network load low highHigh Throughput Commencing difficulties Overload Accumulation of collisions -> unforseeable delays. High load:

4. Schutzvermerk nach DIN 34 beachten Measures for improving the realtime capabilities Smaller collision domains with the help of bridges and switches Higher transmission rate Full duplex operation High speed backbone Message prioritization Restrictive operating conditions for the stations

5. Schutzvermerk nach DIN 34 beachten Smaller collision domains Within a collision domain four factors influence the timing behaviour: –Number of network stations Many stations increase the collision probability –Extension of the network Increased signal travelling times; collisions will be detected later –Frame length Increased transmission period; possible jam of stations willing to send –Amount of data traffic Increase of delays corresponding to the network load

6. Schutzvermerk nach DIN 34 beachten Smaller collision domains 1 2 3 4 5 6 7 8

7. Schutzvermerk nach DIN 34 beachten Smaller collision domains Spanning Tree Algorithm Brigde Network 1 Network 2 Network 3 Multiple path between two networks

8. Schutzvermerk nach DIN 34 beachten Smaller collision domains Switched LAN Shared LAN SwitchHub data traffic

9. Schutzvermerk nach DIN 34 beachten Higher transmission rate © Prof. Dr. Ing. Jörg Wollert Uiversity of applied sciences Bochum User data frame data efficiency Transmisision rate Net data rate

10. Schutzvermerk nach DIN 34 beachten Full duplex operation FDX –Simultaneous sending and receiving –Two stations –Suitable medium required 100 MBit/s

11. Schutzvermerk nach DIN 34 beachten Backbone

12. Schutzvermerk nach DIN 34 beachten Message prioritization 802-1 p/Q –IEEE standard (Insitute of Electrical and Electronics Engineering) –Accelerated Processing of time-critical messages in bridges & switches –Dynamic registration of the time-critical messages –Restriction on multicast packets –Ethernet Header is extended by four Bytes –Compatibility problems ? –Handling of the additional information is left to the switches & bridges

13. Schutzvermerk nach DIN 34 beachten Message prioritization

14. Schutzvermerk nach DIN 34 beachten Operating conditions for the stations Rules for the stations Avoid collisions Create a set of conditions for the collision treatment algorithm so that the system‘s performance will not deteriorate

15. Schutzvermerk nach DIN 34 beachten Load restricting factors Message rate –Max. average number of messages per station –Max. average length of messages per station Intermessage Gap –Min. time interval between two messages of the same station Intermessage Gap Ethernet

16. Schutzvermerk nach DIN 34 beachten Realization of the load restrictions Application Layer Presentation Layer Transport Layer Network Layer Data Link Layer Physical Layer Session Layer Application TCP IP Ethernet Network (1) (2) The above mentioned load restrictions can be realized either directly in the application programm of each controller or in an additional layer between application program and TCP layer (1) or between IP layer and Ethernet (2).

17. Schutzvermerk nach DIN 34 beachten What else has an influence on the timing behaviour ? Program... send... Program... send... Socket interface TCP protocol IP protocol Socket interface TCP protocol IP protocol Ethernet Program... receive... Program... receive... Total transmission time

18. Schutzvermerk nach DIN 34 beachten Summary The Ethernet is not deterministic due to the access mechanism CSMA/CD and the Back-Off-Algorithm, which may lead to an accumulation of collisions. Nevertheless it is suitable for a lot of application areas in the automation business, since a sufficient performance can be achieved by applying the appropriate measures (e.g. switching, low network load).