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Whenever the call button of a mobile phone is pressed, a great deal of information is exchanged in the form of a signaling message among the various network elements of a mobile communications network. In the case of a mobile‐initiated call, the signaling message flow starts from the mobile station to the core network until the subscriber disconnects the voice call. This is further illustrated through a typical GSM MO call flow diagram shown in Figure 2.8. In Figure 2.8, the text inside the bracket shows the corresponding 3GPP technical specification number (described in the subsequent sections), where the definition and more details about the concerned signaling messages can be found.

Figure 2.8 Illustration: GSM MO call flow.

In the typical GSM call flow example shown above, the following network elements are involved to provide an end‐to‐end MO voice call facility to the user.

 Mobile device or station

 BSS, comprising the BTS and the BSC

 MSC

Once the user initiates a voice call, it goes through several phases as summarized below. All these phases involve the exchanging of various signaling messages among the protocol layers of different network elements of the AN and CN domains.

 Establishes a Radio Resource Connection between the MS and BSC to transport the call‐related information to the MSC.

 Allocates the necessary signaling connection and traffic channel by the BSC.

 MSC authenticates the mobile device/station.

 The Radio Resource Connection phase is completed and a connection between the MS and MSC is established.

 MS further sends the call setup message to the MSC. The MSC reserves the necessary resources and connects with the called party; thus, a voice/speech path has been setup between the mobile users. The voice is now in the conversation phase. Once the conversation ends and the user presses the disconnect button, the call enters into the Call Release phase where the BSS, as well as the MSC, frees the allocated radio resources. Similar steps take place whenever a mobile user tries and place a call to a PSTN landline telephone user. However, in this case, the MSC also establishes a signaling connection with the fixed PSTN.