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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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Long Term Evolution (LTE) Tutorials

Here are a few hand picked links to LTE tutorials. Click here for the complete list.

LTE video tutorial

LTE video presentations

LTE physical layer

OFDM and SC-FDMA Signal Chains

LTE link layer design

data flow through PDCP, RLC, MAC and PHY layers of LTE

This article describes the LTE link-layer protocols, which abstract the physical layer and adapt its characteristics to match the requirements of higher layer protocols.The LTE link-layer protocols are optimized for low delay and low overhead and are simpler than their counterparts in UTRAN. The state -of-the-art LTE protocol design is the result of a careful crosslayer approach where the protocols interact with each other efficiently. This article provides a thorough overview of this protocol stack, including the sub-layers and corresponding interactions in between them, in a manner that is more intuitive than in the respective 3GPP specifications.

Introduction to LTE Architecture

This article provides an overview of the LTE radio interface, together with a more in-depth description of its features such as spectrum flexibility, multi-antenna transmission, and inter-cell interference control. The performance of LTE and some of its key features is illustrated with simulation results.

This article provides a high-level overview of LTE and some of its key components: spectrum flexibility, multi-antenna transmission, and ICIC. Numerical simulations are used to show the performance of the first release of LTE, as well as assess the benefit of the key features. Indeed these contribute strongly to LTE meeting its performance targets. An outlook of the evolution of LTE toward LTE-Advanced and full IMT-Advanced capabilities complete the article. Clearly, LTE offers highly competitive performance and provides a good foundation for further evolution.

LTE Protocol Stack

Click here for a more LTE tutorials that cover the entire spectrum of LTE development.

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IMS subscriber to PSTN subscriber call flow

IP Multimedia Subsystem is an IP based signaling system for setting up and tearing down multimedia sessions. SIP based signaling is used to setup these sessions.

We have covered call flows for an IMS to IMS and PSTN to IMS calls. We now look at the call flow for a IMS to PSTN subscriber call.

The call is routed via the BGCF (Border Gateway Control Function) to the MGCF (Media Gateway Control Function). The MGCF uses one context with two terminations in IM-MGW (Media Gateway). The termination RTP1 is used towards IMS Core network subsystem entity and the bearer termination TDM1 is used for bearer towards PSTN CS network element.

The call flow is complex and it is analyzed with multiple diagrams. Some of the diagrams are presented here:

IMS to PSTN sequence diagram

A detailed call flow describing all message interactions in a IMS to PSTN call

IMS to PSTN high level flow

A high level view that abstracts out individual component details and just presents the flow between the UE, IMS Core and the PSTN.

IMS to PSTN UE collaboration diagram

Here we examine the call flow from the UE point of view. The call flow is represented as a collaboration diagram.

Link: IMS subscriber to PSTN subscriber call flow

All documents:

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PSTN subscriber to IMS subscriber call flow

IP Multimedia Subsystem is the new IP based signaling system for setting up multimedia sessions. We have already covered the call flow for an IMS subscriber calling another IMS subscriber. Here we will look at the call flow of a regular PSTN subscriber calling an IMS user.

PSTN to IMS call flow

PSTN subscriber to IMS subscriber call flow

This call flow covers the handling of a CS network originated call with ISUP. In the diagram the MGCF requests seizure of the IM CN subsystem side termination and CS network side bearer termination. When the MGCF receives an answer indication, it requests the IM-MGW to both-way through-connect the terminations. Originating and terminating end initiated call releases are also covered.

The following sequence is covered:

  1. ISUP IAM Handling and Initial IM-MGW and MGCF (Mn) Interactions
  2. Initial Handshake between MGCF and IMS CSCF Servers
  3. Mn Interactions for Codec selection
  4. ISUP ACM related interactions on Mn interface
  5. IMS Answer to ISUP ANM Handling
  6. Conversation Mode
  7. Call Release:
    • Calling PSTN Subscriber Initiated Call Release
    • Called Subsciber Initiates Call Release

Link: PSTN subscriber to IMS call flows

Focus on different aspects of the call flow

PSTN to IMS call is a very complex flow. The main sequence flow is supplemented with flows that focus on a particular aspect of the flow. A snapshot of one such diagrams is shown here:PSTN subscriber to IMS subscriber - Terminating S-CSCF interactions

Link: PSTN subscriber to IMS call flows


IMS User to IMS User Call Flow

Call from an IMS user to another IMS user

A detailed IMS subscriber to IMS subscriber is presented here. The preconditions for this call flow are:

  • The calling IMS subscriber is currently roaming outside the home network.
  • The called user is registered in the home network.
  • The caller and the called user require resource reservation before the call can be setup.

The major steps in the call flow are:

  • IMS routing of the initial SIP INVITE.
  • IMS routing of the first response to the INVITE.
  • Resource allocation via PDP context activation.

IMS (IP Multimedia Subsystem) user calls another IMS user

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IMS registration sequence diagram

This sequence diagram details the message interactions involved in IMS registration. IMS registration from a visited IMS network is covered. The interactions covered are:

  • Terminal attaching to the GPRS Network.
  • Terminal activates a PDP context.
  • An unauthenticated IMS registration is attempted.
  • IPSec security association is established between the User terminal and the P-CSCF.
  • Finally the terminal performs an IPSec protected registration that completes the authentication of the network and the terminal.

IMS registration sequence diagram