Mobile Telephone Switching Office

The Mobile Telephone Switching Office (MTSO) is the mobile equivalent to a PSTN Central Office. The MTSO contains the switching equipment or Mobile Switching Center (MSC) for routing mobile phone calls. It also contains the equipment for controlling the cell sites that are connected to the MSC.

The systems in the MTSO are the heart of a cellular system. 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.

All cellular systems have at least one MTSO which will contain at least one MSC. The MSC is responsible for switching calls to mobile units as well as to the local telephone system, recording billing data and processing data from the cell site controllers.

The MSC is connected to a close telephone exchange by a trunk group. This provides an interface to the (Public Switched Telephone Network) (PSTN). It also provides connectivity to the PSTN. The region to be served by a Cellular Geographic Serving Area(CGSA) is split into geographic cells. These cells are ideally hexagonal in shape and they are initially laid out with their centers about 4 to 8 miles apart from each other. Other MTSO equipment, the cell site controllers provide control functions for a group of cell sites and actions of mobile phones through command and control data channels. To achieve this, there has to be a method of connectivity between the MTSO and the cell site. This may be by DS1, DS3, OCn or Ethernet circuits.

The MSC also plays a major role in call routing. 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. If it finds a control channel to listen to, receives the SID and then compares it to the SID programmed into the phone. If both SIDs match then it knows that it is communicating with a cell in its home system.

...
Wikipedia