The Mobile Telephone Switching Office (MTSO) acts like an ordinary switching node on the Public Switched Telephone Network (PSTN) or Integrated Services Digital Network (ISDN) and provides all the functionality needed to handle a mobile subscriber, such as registration, authentication, location updating, handoffs, and call routing to a roaming subscriber. All cell phones have special codes associated with them:

• Electronic Serial Number (ESN) Aunique 32-bit number programmed into the phone when it is manufactured.
• Mobile Identification Number (MIN) A10-digit number derived from the mobile phone number.
• System Identification Code (SID) Aunique 5-digit number that is assigned to each carrier by the FCC.

While the ESN is considered a permanent part of the phone, both the MIN and SID codes are programmed into the phone on purchase and activation with a service plan. When the mobile phone is powered up, it listens for the network operator’s SID on the control channel. The control channel is a special frequency that the phone and base station use to talk to one another about such functions as call setup and channel changing.

If the phone cannot find any control channels to listen to, it assumes that it is out of range and displays a “no service” message. When it receives the SID, the phone compares it to the SID programmed into the phone. If the SIDs match, the phone knows that the cell it is communicating with is part of its home system. Along with the SID, the phone also transmits a registration request.

The MTSO then knows the location of the phone, which is recorded in a database so that it knows which cell to target when it wants to ring that phone for an incoming call. When the MTSO gets the call, it looks up the location of the phone in its database. The MTSO picks a frequency pair the phone will use in that cell to take the call. The MTSO 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.

As the mobile phone moves toward the edge of a cell, that cell’s base station notices that its signal strength is diminishing. Meanwhile, the base station in the cell the mobile phone is moving toward is listening and measuring signal strength on all frequencies and sees that the approaching phone’s signal strength increasing. The two base stations coordinate with each other through the MTSO, and at some point, the phone gets a signal on a control channel telling it to change frequencies.

This hand off switches the phone to the new cell. If the SID on the control channel does not match the SID programmed into the mobile phone, then the phone assumes that it is roaming. The MTSO of the cell that the phone is roaming in contacts the MTSO of its home system, which then checks its database to confirm that the SID is valid. The home system verifies the phone to the local MTSO, which then tracks it while it is moving through its cells.

The local wireless cellular network consists of a Mobile Telephone Switching Office (MTSO) with cell sites scattered throughout a geographic serving region. T-carrier or fiber lines are typically leased from the local carrier to interconnect the cell sites with the MTSO. These lines also provide the MTSO with connectivity to a local central office switch so that calls can be completed between the wireless network and the PSTN.