Calling

 Calling:

Upon powering up the mobile terminal, try to initially register the mobile with the desired network. The SIM card has the information of the home network of the subscriber. The SIM card is first checked for authenticity of the user.

Once mobile acquires the home network then the mobile device sends a connection request otherwise it tries to connect to the last network before it was switched off. All this data is present in the SIM.

On arriving at a new location, the mobile device searches all the frequencies within the band to establish a connection to the existing network i.e. The mobile device looks for a strong BCCH signal that includes FCCH and SCH.

To understand how the traffic and control channels are used, we will consider the case of call establishment from a mobile.

The subscriber unit should be synchronized to a nearby base station as it observes the BCH.

By receiving the FCCH, SCH, and BCCH messages, the subscriber will be locked on the system and correct BCH.

To originate the call, the user dials the intended digit combination and then presses the send button on the phone. The mobile transmits a burst of RACH data. A channel is requested on RACH. It can be in contention with other mobile using slotted ALOHA.

The base station then responds with an AGCH message. On receiving the ad grant, the mobile proceeds with call setup on the allocated dedicated control channel by sending setup messages to the network. The network accepts the call establishment on SDCCH.

When the called party alerting is initiated, an alerting message is sent to the mobile over the FACCH. A ringing tone can be generated by the network and sent to the mobile.

When the call has been accepted at the remote end, a connect message is transferred to the mobile that indicates that the connection is established in the network.

The mobile station responds by sending the connect acknowledge message. It enters in active state.

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The table gives the detailed activities during call establishment from a mobile.

Call establishment to a mobile :

When calls are originated from the PSTN, the process is similar.

A paging message PCH is routed to the traffic area in which the mobile is registered and transmitted on the paging channel.

When an access grant is received from the base station, the mobile responds with the call confirmed message on the dedicated control channel. A traffic channel (TCH) is then allocated and the call proceeds.

The below table shows the activities in call establishment to a mobile.