Which phase of SDLC we write test case

In which phase of SDLC we can write test cases?
Actually in all phases we can write test cases.

The test activities happens at every phase of software development life cycle.
As we see in the V shape diagram above, system testing case can be prepared when business analyst carrying out requirement analysis, integration testing plan can be drafted when high level design is completed. Unit testing cases can be written when low level design approved.