saurabh,
The SMAC supplies you with primitives to send and receive data, but it does not like 802.15.4 have an acknowlegdement scheme build in.
This is up to you to implement if it is required by your application.
As you are using a wireless link, packets can be lost. so if you can not afford loosing packets you should implement an acknowlegdement/retransmisison scheme in your applications
Br,
Mads