[ns] how to trigger the error model in the middle of the simulation?

[Farzaneh Razavi Armaghani]

Dear friends,
   
  I want to run an error model from middle of my simulation not beginning (at a specific time). How do I trigger the error model in the middle of the running simulation?
  Thanks for your response.
   
  Regards,
  Farzaneh.

       
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