telecom • networking • design

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Voice Over LTE (VoLTE) Originating Call

Voice over LTE (VoLTE) is the standard for voice call setup in LTE networks. When VoLTE is deployed, phones will not need to fallback to 3G for voice calls.

VoLTE uses IMS SIP signaling to set up voice calls. The following VoLTE call flow describes the IMS call setup and release. An example of sending an SMS over IMS is also included. Sample RTP and RTCP messages are also shown in the flow.
Voice over LTE Call Flow

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GSM Terminating SMS Call

SMS terminating call flow

In this call flow we will look at how a terminating SMS is handled in GSM. Setting up a terminating
SMS session is a multi-step process.

  1. Interrogate the MSC to locate the subscriber
  2. Setting SMS session setup and acquiring radio resources
  3. Sending the SMS.
  4. Releasing the session and associated radio resources.

GSM Mobile Terminated SMS Sequence Diagram


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GSM Mobile Originated SMS

GSM Mobile Originated SMS

This scenario describes the session setup for a GSM originating SMS. This sequence diagram describes the SMS signaling and data transfer between the mobile subscriber and the SMS service center.

SMS is implemented by sending Short message transported via a GSM SDCCH (Standalone Dedicated Control CHannel) signalling channel. Thus they can be received while the user is talking. The MS establishes an SDCCH using RR establishment procedure.

GSM Mobile Originated SMS Signaling Flow Diagram