Service tax query

MRIDUL (DGM) (32 Points)

06 January 2013  
An amount received from the government in installments by Public sector undertaking for implementing e-governance project in Govt. departments. PSU further assigns that work to specialized agency to execute the work. In this process, PSU earns some percentage as income like 5%.
 
Brain storming Question :-
1. What is the service tax liability for PSU?
2. Whether Service tax is applicable on the Income part only or the gross receipt amount received from Govt.?
3. Service tax applicability at the time of receiving advance from Govt. or at the time of billing to sub-contractor.?
 
Thanks in advance!!!