Mobile Telephone Switching Office (MTSO)

- The Mobile Switching Center

The Mobile Telephone Switching Office (MTSO) is the mobile equivalent of 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.

- How MTSO Works

MTSO systems monitor each cell phone user’s relative position by broadcasting signals from all cell phone towers near the user which are received by the user’s cell phone and broadcasted back to each tower. By monitoring data quality as well as the time it took for the signal to return to each tower, the MTSO is able to assess the user’s relative reception. Once the MTSO has determined the relative reception of the user’s cell phone and which tower is closest to the user, it can then automatically adjust the user’s cell phone to a specific tower in order to maximize reception.

[The Mobile Telephone Switching Office (MTSO) - RF Wireless World)]

- Handovers

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. This process is called handoff. 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. The returning instructions for handoff sent during a call must use the voice channel. The data regarding the new channel are sent rapidly (in about 50 milliseconds), and the entire returning process takes only about 300 milliseconds. After handoff, if the SID on the control channel does not match the SID programmed into the phone, then the phone assumes that it is roaming. 

The MSC also performs handovers/handoffs which occurs when a call needs to be handed off to a different BSC/RNC it serves or to completely new MSC. A MSC can serve many BSCs/RNCs which in turn server many BTS. As a result, a MSC can serve a large area typically hundreds of miles. Highly populated areas require more BTSs and BSCs/RNCs which will in turn obviously, will reduce the geographic coverage of a DMSC. The above in a MSC is what's considered as its mobility management. The BTS, BSC/RNC are the RAN/UTRAN (Radio Access Network & UMTS Radio Access Network) subset in the mobile network. The remaining functions of a MSC are identical to a PSTN switch.

- Advantages

MTSO systems are advantageous because they allow cell phone users to experience the maximum amount of cell phone reception available without manually switching to each tower. MTSO systems are also advantageous because they are nearly instantaneous, monitor the user’s cell phone reception at all times, and work without the user ever even knowing about it.

[More to come ...]