It is responsible for interconnecting calls with the local and long distance landline telephone companies, compiling billing information (with the help of its CBM/SDM), etc.
It also provides resources needed to efficiently serve a mobile subscriber such as registration, authentication, location updating and call routing.
Its subordinate BSC/RNC are responsible for assigning frequencies to each call, reassigning frequencies for handoffs, controlling handoffs so a mobile phone leaving one cell (formally known as BTS's) coverage area, can be switched automatically to a channel in the next cell.
When a mobile phone is turned on, it listens for the network operator's SID (System Identification Code) on the control channel.
If it cannot find any control channels to listen to then it assumes it is outside the range and displays a message indicating no service.
Anytime a MSC successfully registers a subscriber the HLR record is also updated.
As a mobile unit engaged in a call moves away from a cell site or formally known as Base transceiver station and its signal weakens, the BSC(GSM) or RNC(3G UMTS) will automatically instruct it to tune to a different frequency, one assigned to the newly entered BTS.
The BSC/RNC determines when handoff should take place by analyzing measurements of radio signal strength made by the present controlling cell site and by its neighbors.
Highly populated areas require more BTSs and BSCs/RNCs, which will in turn reduce the geographic coverage of a DMSC.