An Erlang is a unit of telecommunications traffic measurement. Lots of articles can be found by searching this community.
GSM has a collection of nodes: BTS, BSC, MSC/VLR, HLR, EIR, AuC, etc.. Look at the 3GPP standards for an overview.
MFC R2 – Multi-Frequency Compelled Signalling is a Register to Register signalling system that is based on Voice Frequency. Generally used over a CU (copper). Note that MFC R2 was the main signalling used with Analog switching nodes.
C7 / ss7 – Digital signalling.