Questions on Payroll outsourcing

Hi Gurus,

   I would like to know, what are the different procedures for payroll outsourcing to ADP or any third party?

   Is calculating gross payroll mandatory if we use PU12 OTMU for the master data payroll outsourcing?

   Difference between OTMU and IDoc HROT_USM? Can some one specify the adv/dis-adv for the following?

   If we calculate only gross payroll in SAP do we transfer only the master data, deduction wage types and time wage types?

Thank you