Fig: Mobile Cellular Call controlled by single MTSO:

Steps in a typical call between two mobile users within an area controlled by a single MTSO:

• Mobile unit initialization: When the mobile unit is turned on, it scans and selects the strongest setup control channel used for this system . Cells with different frequency bands repetitively broadcast on different setup channels. The receiver selects the strongest setup channel and monitors that channel. The effect of this procedure is that the mobile unit has automatically selected the BS antenna of the cell within which it will operate.2 Then a handshake takes place between the mobile unit and the MTSO controlling this cell, through the BS in this cell. The handshake is used to identify the user and register its location. As long as the mobile unit is on, this scanning procedure is repeated periodically to account for the motion of the unit. If the unit enters a new cell, then a new BS is selected. In addition, the mobile unit is monitoring for pages, discussed subsequently.

• Mobile-originated call: A mobile unit originates a call by sending the number of the called unit on the preselected setup channel shown in fig b. The receiver at the mobile unit first checks that the setup channel is idle by examining information in the forward (from the BS) channel. When an idle is detected, the mobile unit may transmit on the corresponding reverse (to BS) channel. The BS sends the request to the MTSO.

• Paging: The MTSO then attempts to complete the connection to the called unit. The MTSO sends a paging message to certain BSs depending on the called mobile unit number (Figure 10.6c). Each BS transmits the paging signal on its own assigned setup channel.

• Call Accepted: The called mobile unit recognizes its number on the setup chan- nel being monitored and responds to that BS, which sends the response to the MTSO. The MTSO sets up a circuit between the calling and called BSs. At the same time, the MTSO selects an available traffic channel within each BS's cell and notifies each BS, which in turn notifies its mobile unit shown in fig d . The two mobile units tune to their respective assigned channels.

• Ongoing call: While the connection is maintained, the two mobile units exchange voice or data signals, going through their respective BSs and the MTSO shown in Figure e .

• Handoff: If a mobile unit moves out of range of one cell and into the range of another during a connection, the traffic channel has to change to one assigned to the BS in the new cell shown in Figure f. The system makes this change without either interrupting the call or alerting the user.