The exact infrastructure differentiates between the generations. Let’s begin with a general overview.
Mobile Device <-Air-> Access Network <-Wire-> Core Network <-Wire-> PSTN/Internet
The access network works as the front end to provide access to the core network.
The core network works as the back end to realise the logic for the communication.
I’ve choosen the 3rd generation for a more detailed look. 3G is also called UMTS, which is based on GSM.
| Radio Access | Core Network |
|------------------|--------------------|
| | Telephone call |
UE <-|-> NodeB <--\ /-|-> MSC <--> GMSC <-|-> PSTN
| ↓ ↓ | ↓ CS |
UE <-|----\ RNC |---HLR--------------|
| ↓ ↑ ↑ | ↑ PS |
UE <-|-> NodeB <--/ \-|-> SGSN <--> GGSN <-|-> Internet
| | IP communication |
UE ... User Equipment (Mobile device with (e)SIM) -------------------------------------------------------------------------- NodeB (Cell site / Base station) RNC ... Radio Network Controller (Base station controller) -------------------------------------------------------------------------- CS ... Circuit Switching (Only one communication on one channel) MSC ... Mobile Switching Center (auth. UE + sets up connection) GMSC ... Gateway Mobile Switching Center (Gateway to the PSTN) PSTN ... Public Switched Telephone Network (Telephone line network) -------------------------------------------------------------------------- PS ... Packet Switching (Multiple communications on one channel) SGSN ... Serving GPRS Support Node (auth. UE + sets up connection) GPSN ... Gateway GPRS Support Node (Gateway to the internet (NAT)) -------------------------------------------------------------------------- HLR ... Home Location Register (Database of registered users)