28 August 2011

CALL PROCESSING


Within a cellular telephone system, three types of calls can take place involving mobile cellular telephones : i) mobile to wireline ii) Mobile to mobile iii) Wire line to Mobile

i) Mobile to wireline call procedures :

1. Calls from mobile telephones to wirelines telephones can be initiated in one of two ways:

            a. The mobile unit is equivalently taken off hook (usually by depressing a talk button).
                 After the mobile unit receives a dial tone, the subscriber enters the wireline 
                 telephone number using either a standard Touch-Tone keypad or with speed dialing.
                After the last digit is depressed, the number is transmitted through a reverse control
                channel to the base station controller along with the mobile unit’s unique
                identification number (which is not the mobile unit’s telephone number). 

            b. The mobile subscriber enters the wireline telephone number into the unit’s memory using a standard Touch-Tone keypad. The subscriber then depressed a send key, which transmits the called number as well as the mobile unit’s identification number over a reverse control channel to the base station switch.

2. If the mobile unit’s ID number is valid, the cell-site controller routes the called number over a wireline trunk circuit to the MTSO.

3. The MTSO uses either standard call progress signals or the SS7 signaling network to locate a switching path through the PSTN to the destination party.

4. Using the cell-site controller, the MTSO assigns the mobile unit a nonbusy user channel and instructs the mobile unit to tune to that channel.

5. After the cell-site controller receives verification that the mobile unit has tuned to the selected channel and it has been determined that the called number is on hook, the mobile unit receives an audible call process tone (ring-back) while the wireline caller receives a standard ringing signal.

6. If a suitable switching path is available to the wireline telephone number, the call is completed when the wireline party goes off book (answers the telephone). 

ii) Mobile to Mobile call Procedures :

1. The originating mobile unit initiates the call in the same manner as it would for a mobile to wireline call.

2. The cell site controller receives the caller’s identification number and the destination telephone number through a reverse control channel, which are then forwarded to the MTSO.

3. The MTSO sends a page command to all cell site controllers to locate the destination party (which may be anywhere in or out of the service area).

4. Once the destination mobile unit is located, the destination cell-site controller sends a page request through a control channel to the destination party to determine if the unit is on or off hook.

5. After receiving a positive response to the page, idle user channels are assigned to both mobile units.

6. Call progress tones are applied in both directions (ring and ring-back).

7. When the system receives notice that the called party has answered the telephone, the switches terminate the call progress tones, and the conversation begins.

8. If a mobile subscriber wishes to initiate a call and all user channels are busy, the switch sends a directed retry command, instructing the subscriber’s unit to reattempt the call through a neighboring cell.

9. If the system cannot allocate user channels through a neighboring cell, the switch transmits an intercept message to the calling mobile unit over the control channel.

10. If the called party is off hook, the calling party receives a busy signal.

11. If the called number is invalid, the calling party receives a recorded message announcing that the call cannot be processed.

iii) Wireline to Mobile call procedures:

 1. The wireline telephone goes off hook to complete the loop, receives a dial tone, and then inputs the mobile unit’s telephone number.

2. The telephone number is transferred from the PSTN switch to the cellular network switch (MTSO) that services the destination mobile number.

3. The cellular network MTSO receives the incoming call from the PSTN, translates the received digits, and locates the base station nearest the mobile unit, which determines if the mobile unit is on or off hook (i.e., available).

4. If the mobile unit is available, a positive page response is sent over a reverse control channel to the cell site controller, which is forwarded to the network switch (MTSO).

5. The cell-site controller assigns an idle user channel to the mobile unit and then instructs the mobile unit to tune to the selected channel.

6. The mobile unit sends verification of channel tuning through the cell-site controller. 

7. The cell-site controller sends an audible call progress tone to the subscriber’s mobile telephone, causing it to ring. At the same time, a ring back signal is sent back to the wireline calling party.

8. The mobile answers (goes off hook), the switch terminates the call progress tones, and the conversation begins.