SS7 is Signallng System Number 7.
In a nutshell, Signalling allows two communicating entities, e.g. SPC Time-Space-Time Switch, to initiate, maintain and clear a telecommunication connection.
SS7 is related to a Digital Network. SS7, e.g. ISUP, is used beteen two Digital SPCs. R2 MFC (Multifrequency Compelled) is an analogue Voice Frequency signallin that is used between Analog nodes. This was very popular during the 1950s to the 1970s. Most Networks are ss7 / IP / Sigtran orientated.