Overview#Mobile Switching Center (MSC) is the primary service delivery node for GSM/CDMA, responsible for routing voice calls and SMS as well as other services (such as conference calls, FAX and circuit switched data).
The Mobile Switching Center sets up and releases the end-to-end connection, handles mobility and hand-over requirements during the call and takes care of charging and real time pre-paid account monitoring.
In the GSM mobile phone system, in contrast with earlier analogue services, fax and data information is sent directly digitally encoded to the Mobile Switching Center. Only at the Mobile Switching Center is this re-coded into an "analogue" signal (although actually this will almost certainly mean sound encoded digitally as PCM signal in a 64-kbit/s timeslot, known as a DS0 in America).
There are various different names for Mobile Switching Centers in different contexts which reflects their complex role in the network, all of these terms though could refer to the same MSC, but doing different things at different times.
The Gateway MSC (G-MSC) is the MSC that determines which "visited MSC (V-MSC)" the subscriber who is being called is currently located at. It also interfaces with the PSTN. All mobile to mobile calls and PSTN to mobile calls are routed through a G-MSC. The term is only valid in the context of one call since any MSC may provide both the gateway function and the Visited MSC function, however, some manufacturers design dedicated high capacity MSCs which do not have any BSSs connected to them. These MSCs will then be the Gateway MSC for many of the calls they handle.
The anchor MSC is the MSC from which a handover has been initiated. The target MSC is the MSC toward which a Handover should take place. A mobile switching center server is a part of the redesigned MSC concept starting from 3GPP Release 4.
The Mobile Switching Center connects to the following elements:
- Home Location Register (HLR) for obtaining data about the SIM and mobile services ISDN number (MSISDN; i.e., the telephone number).
- Base Station Subsystem (BSS) which handles the radio communication with 2G and 2.5G mobile phones.
- UMTS Terrestrial Radio Access Network ([UTRAN) which handles the radio communication with 3G mobile phones.
- Visitor Location Register (VLR) provides subscriber information when the subscriber is outside its home network.