1. Call Flow Process When a Call is Destined to a Mobile Station

• Scanning of forward control channels

Whenever a mobile phone is switched on, it first scans the group of forward control channels to determine the strongest one. Then it keeps on monitoring the same channel until the signal level drops below a minimum level after which it again starts to scan. Every mobile phone repeats this process as long as it is kept switched on. The signal level of the current control channel to which the phone is locked will be displayed on the handset. For each cellular system, the control channels make up to about 5 $\%$ of the total control channels. The other 95 $\%$ are reserved for the voice and data traffic for the end users.

• Broadcasting of MIN

When a telephone call is placed to a mobile user, the MSC dispatches the request to all Base Stations in the cellular system. The mobile identification number ( MIN number) which is the subscribers telephone number is then broadcasted as a paging message over all of the forward control channels throughout the cellular system as shown in Figure 8.

• Transmission of SCM and ESN

All mobile stations will receive this page and each mobile matches its MIN with the received MIN. The MS whose MIN matches with the recived MIN then acknowledges it's receipt and sends its Electronic serial number (ESN) number and Station Class Mark Number (SCM) on its Reverse Channel. SCM is a 4 digit number corresponding to the power levels. ESN is a unique number 32 bit number assigned to every mobile station. Figure 9 illustrates transmission of SCM and ESN by target mobile station to Base Station.

• Verification and Authentication by MSC

The Base Station then relays the acknowledgement sent by the mobile and informs the MSC about the target mobile. The MSC verifies that that the mobile has a valid MIN-ESN pair and authenticates the mobile. It instructs the BS to allot an unused voice channel pair.

• Allocation of voice channel pair

The Base Station then transmits a message on Forward control channel to mobile to move to a specific voice channel number.

• Voice transmission and reception

The mobile receives the message and tunes to the specific voice channel. The MSC then connects the mobile with the calling party on the PSTN. Voice transmission and reception begins on the Forward Voice Channel and Reverse Voice Channel.

2. Call Flow Process When a Call is Initiated by a Mobile

When a mobile user originates a call, a call initiation request is sent on the reverse control channel. With this request, the mobile unit transmits it telephone number (MIN), Electronic serial number (ESN) and telephone number of the called party. The mobile also transmits its own SCM.

The Cell Base station receives this data and sends it to the MSC. The MSC then validates the request, makes connection to the called party through the PSTN and instructs the BS and the mobile to tune to a unused pair of voice channel.