Yes Wec can write test cases at the design phase

At the time of designing we should be ready with test cases

NO U r wrong...
For design we don't write TCs..
TCs are only for functionality test
Design or req testing is done by review and work 
through..for which TC is not written u may refer some check 
lists...

which one is true ,which one is wrong anybody tell me plz..

we can write test cases at design level.
Here we test the alignment of controls, Consistency, 
spelling etc.

We can write Test cases before designing phase.but its not 
mandatory.
some time we write test cases before designing phase and 
some times
we prepare test cases after designing phase.
we know SDLC levels
1)requirements gathering(BRS) 
2)Analysis(SRS)
3)Designing phase (HLD AND LLD)
4)Coding(unit,INTEGTATION TESTING(STUB,DRIVER))
5)Testing
6)Relese and maintanance.
We can prepare test cases before designing phase.
but execution starts from after designing.that means,
after coding.
in coding developers conducts integration testing.on that 
I.T deve.release s/w build to testing team.
After receives build from developers then only we go for 
test execution side