hi the Msc is the main switch witch swtchs speesh calls only but if thers is any other kind of data (packet switching for exemple)we need to pass from other equipements thus we go via the MGW of the packet switching
we use the media gateway to go for un other network like pstn…etc.
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.