Saj,
You mean tier and not tire. This might have been a typo, but I wanted to call your attention to it in case it is not.
Do i need to implement transactions in both Service layer as well as DAO layer ?
No. I usually only use transactions in the service layer.
if not is it enough to implemnt in only one layer?
Yes. The advantage of the service layer is it makes the whole service one logical transaction. Which tends to be what you need.
What are the disadvantages im facing if i implement TX in both layers?
It depends on the settings and your requirement. Do you have a requirement that the DAO roll back independently? If not, the setting would be to use the service layer's transaction. And the disadvantage would be added complexity.
What are the techniques i have to follow, i mean, are there specific techniques to implement service level transactions compared to DAO transactions? can i use spring frameowrk?
Yes, Spring is a good way to create a service level transaction.