Well before SS7 – Signalling System Number 7 – the main Signalling was MFC R2. MFC stands for Multi-Frequency Compelled and this means that for a request there must be a response. It is a Request / Response signalling protocol. A R2 request will always need an acknowledgement (Ack). Thanks to R2 MFC we have the terms Forward and Backward signalling.
Before the arrival of digital Stored Program Control (SPC) and SS7, R2 MFC was used as a Register signalling system for Analog Telephone Exchanges. Unlike SS7 (out of band), R2 MFC
is an inband system that works within the range 300-3400MHz (or more).
At the time of writing (2019), MFC R2 is a legacy signalling that is used on several International circuits.
The bulk of CSPs – Communication Service Providers – are using SS7 / ISUP / Sigtran (IP).
Reason why SS7 is faster than R2 is that it is an out of band signalling system that is “geared” up for ISDN Services, i.e. Telephony and Data. You cannot launch Advanced Network services based on R2 MFC signalling.