Tds on service tax

If an entitity makes payment towards professional fees or any other payment towards services received & the threshld for making TDS is crossed, then does the payer is required to deduct tax only on the net amount before service tax charged by teh service provider or on the gross amount inclusive of ST.  Also if teh threshols is crosed because of charging Service tax,  then also TDS is required to be made or not?

Will this not amount to Tax on Tax