Just a follow on to all the excellent answers.
Before 3GPP Release 4 the traditional MSC/VLRs handled both
Signalling and Payload traffic. It was essential a combined function node.
To help with the evolution of GSM (Global System for Mobiles)
to 3G (e.g. WCDMA(Spread-Spectrum) a layered architecture approach is used
where the traditional MSC/VLRs is “repurposed” to a Softswitch. This implies that
the MSC/VLR does not have a “switching fabric”. Instead, it handles pure signalling traffic.
With 3G, MSS – (MSC Server System or Mobile Soft-Switch) – is now
the repurposed MSC/VLR. Next step is to have a node to address the Payload traffic from TDM, etc..
This node is called the Media Gateway (MGw).
MSS communicates with another MSS via BICC (combination of CS-2 and ISUP). MSS uses H.248 to communicate with the MGw.