Thank you for the prompt reply.
but on which criteria we decide that which signal should be on Top layer & which signal should be in Inner layer?
Pl hel me.
Which signals go on which layer is up to you and how difficult it will be to route. Your main consideration is impedance and coupling of high-speed signals. You may need to route a shield around fast signals. You may want to investigate microstrip and strip-line design, your layer stack could possibly be reduced.