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Diagnostic Session : Function 2 - Part 2 - Support Session layer requirements of OSI layer

We have seen a concept of P2 Timer and P2 * Timer in the blog –
https://thevehiclediagnostics.blogspot.com/2020/01/diagnostic-session-function-1-maintain.html


So it is time to know how Service $ 10 – Diagnostic Session Control is having on a function related to these timers.


Session Layer requirement Overview of P2 Timer :
  •  P2 timer and P2 * timer are mentioned as a performance parameters
  •   Standard values for these parameters are defined in ISO 14229 Part -2
  •  But an ECU may change these timings in non-default sessions and Client needs to adapt them dynamically (some conditions are applicable).
Explanation:

Though there is defined value of P2 timer, this timing can be still improved. As these are marked as performance requirement, if the ECU can give better performance, then these timings can be reduced. ECU can change these values in non-default session as per the performance.
           So, as we have seen before, a communication relies on these values and hence a client needs to be made aware of these changes if any. Then a question arise, how this is done?
       
For this purpose, Service $10 plays a major role.
Let us see how:

  • Whenever a client requests a session change below communication takes place.
  • A client sends a request with expected session value and waits for a response from server.
  • A server embeds some values in the response of the service $10 and send it back to client.
Refer the diagram below for detailed explanation.

              



So each time a session is change is requested by a client, server embeds values of P2 and P2 * timer applicable for requested session only .

In this way a client is made aware of the timing values for this session. Client needs to adapt these adapt values dynamically in case there is any change.

We need to refer ODX or CDD file to know the conversion of these hex values to values in miliseconds.  

So this is how service $ 10 plays an important role in following Session Layer requirements for a reliable and uninterrupted diagnostic communication between client and server.

    There are some more requirements regarding these. I hope, you to read them thoroughly in and come up to add it up here for better understanding.

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