Mobile Telephone Switching Office
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The Mobile Telephone Switching Office (MTSO) is the mobile equivalent of a
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
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 (
Subscriber Registration
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.
The phone also transmits a registration request along with the SID. If the subscriber has previously registered to a particular
A subscriber's VLR profile has a LAC(Location Area Code - Area server by cluster of BTS/cell sites) CID (Cell ID) as well as a list of allowed and disallowed services/features and other information.
With the subscriber's VLR Profile the MSC can determine the last known LAC/CID for this subscriber and knows which BTS to use when it needs to ring/page that phone for an incoming call. When the MSC gets the call, it checks its database for the location of the phone. Then it picks a frequency pair the phone will use in that cell to take the call. The MSC communicates with the phone over the control channel to tell it which frequencies to use, and once the phone and the tower switch on those frequencies, the call is connected.
Handovers
As a mobile unit engaged in a call moves away from a cell site or formally known as
) 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
See also
- Cellular Network
- Roaming
- Handoff
- Advanced Mobile Phone System
References
- Nathan J. Muller (2003). Wireless A to Z. McGraw-Hill Professional.
- James Harry Green (2005). The Irwin handbook of telecommunications. McGraw-Hill Professional.
- Roger L. Freeman (2004). Telecommunication system engineering. John Wiley and Sons.
- Roger L. Freeman (2005). Fundamentals of telecommunications. John Wiley and Sons.