To provide Radar separation or effectively radar separation you need to be provided with continuous updates of position information. This is achieved with ADS B by the use of VHF. The difference with ADS C is that you only receive position information at a predetermined interval that is set in the original contract at logon or if you pass a compulsory reporting point. From memory, 27 min maximum for 50nm sep and 14 min for 30 nm sep. This system generally uses satellites (although VHF is also available if in range of a receiver) to rx/tx so it can be very expensive for the airlines if the position reports were to be every 6 seconds to match the update rate of radar.