CPB
There are three different flavours of Mode S All Call:
1. Mode S Only All Call - Mode S transponders reply, A/C do not (normally use this one)
2. Mode A/C Only All Call - Mode S transponders do not reply, A/C reply with normal A/C response. (Can use this mixed with 1)
3. Mode A/C/S All Call - Mode S transponders reply with All Call, A/C reply with normal A/C response (not currently proposing to use this one)
For now Mode S stations will probably use a mix of 1,2 and normal A/C interrogations.
The Mode S transponder is fully backwards compatible with Mode A/C and will look like any other A/C transponder to a non-Mode S ground station.
RR