MSC and MGW

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Normal Assignment
  The normal assignment procedure for a MS terminated call i initiated by the MSC. This is shown in the figure below. The MSC sends a layer 3 call control set up sms to the MS, including the bearer service and tele-service to be used for the call. The MSC can indicate more than one bearer service. The MS checks this sms. If it can accept the call, it sends a call confirmation message which can contain a bearer capability parameter indication which bearer service is preferred.
The BSS performs the physical context and channel assignment. This is described in normal assignment. Once the TCH is assigned, the MS alerts the user and sends and Alerting message to the MSC. When the MS user answer, the MS send a connection message to the MSC. The MSC sends a connection acknowledgment message to the MS and connects the call. All these message are layer 3 call control messages, and are transparent to the BSS.

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