2G GSM cellular networks were initially designed for voice and circuit-switched services. For that reason, the architecture of such networks was comparatively simple and comprises two main parts:
With the emergence of IP and Web services, 2G GSM networks eventually evolved to efficiently support packet data transmission:
The CS domain is composed of a MSC/VLR (Mobile Switching Center/Visitor Location Register) responsible for end-to-end call setup and in charge of maintaining user location information (such information is typically used to page a user terminal in order to establish user-terminated communication sessions). The GMSC (Gateway MSC) is a specific type of MSC, as being the gateway switch responsible for PSTN inter-working.
The PS domain is composed of the SGSN (Serving GPRS Support Node), which basically plays the role of a MSC/VLR for the packet domain, and the GGSN (Gateway GPRS Support Node), which is equivalent to the GMSC for inter-working with external packet networks.
PS and CS domains may possibly be linked together in order to maintain consistent user location information between the two domains and therefore reduce the amount of radio and network signalling.
In addition to the domain-specific nodes, the Core Network also contains the HLR (Home Location Register), accessed by both the CS and PS domains. The HLR is a key part of the network architecture, containing all information related to user subscription.