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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

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LTE Attach and Default Bearer Setup Sequence Diagram

LTE Attach and Default Bearer Setup

Click to open the PDF sequence diagram describing LTE attach and default bearer setup

This call flow describes the signaling involved in LTE attach and default bearer setup. The sequence diagram covers the following phases:

  1. Random Access Procedure: The UE uses a slotted aloha procedure to access the eNodeB. The UE sends a preamble on a random access channel, the network replies with corrected timing and resource assignments.
  2. RRC Connection Establishment: The UE and the eNodeB then interact to establish a Signaling Radio Bearer (SRB). The SRB will be used for the signaling between the eNodeB and the UE.
  3. Attach and Authentication: The UE now registers with the Core Network. Session contexts are setup at the MME and the Serving Gateway. This step also results in the authentication of the UE and the Network.
  4. Default Radio Bearer Setup: Finally, the default bearer for data transfer is established. Default bearer session is established at the UE, eNodeB, MME, Serving GW and PDN Gateway. User data sessions is exchanged once the default bearer is setup.

The sequence diagram also contains links to access further details about the messages. Click on the messages with blue title for details about the message.

RRC Connection, LTE Attach and Default Radio Bearer Setup


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Convert trace logs to sequence diagrams

Browsing through the code traces is an exercise in frustration. Applications produce huge logs that take hours to analyze. The Python scripts presented here let you visualize the traces as sequence diagrams. Interactions between objects are presented visually.

Python scripts and EventStudio work together to transform the following trace…

[16-Apr-2011 03:05:02.111][Messaging][MessageHandler.cpp:563]received RandomAccessMessage(signalQuality=15) from DSP_01
[16-Apr-2011 03:05:02.111][Messaging][MessageRouter.cpp:94]called MessageRouter::OnRandomAccessMessage()
[16-Apr-2011 03:05:02.111][Messaging][MessageRouter.cpp:90]decoded RRCConnectionRequest(cause = Mobile Originated Data, s_tmsi=5)
[16-Apr-2011 03:05:02.111][Dispatcher][MobileManager.cpp:92]called MobileManager::OnRRCConnectionRequest()
[16-Apr-2011 03:05:02.112][Dispatcher][MobileManager.cpp:94]created Mobile(id=5)
[16-Apr-2011 03:05:02.112][UE][Mobile.cpp:110]called Mobile::OnRRCConnectionRequest()
[16-Apr-2011 03:05:02.112][UE][Mobile.cpp:89]moved-to Setting up RRC Connection
[16-Apr-2011 03:05:02.112][UE][Mobile.cpp:100]begun RRC Connection Setup
[16-Apr-2011 03:05:02.112][UE][Mobile.cpp:105]allocated TrafficDSP(id=23)
[16-Apr-2011 03:05:02.112][UE][Mobile.cpp:141]called Mobile::SendRRCConnection()
[16-Apr-2011 03:05:02.112][UE][Mobile.cpp:220]sent RRCConnectionSetup(s_tmsi=5) to DSP_23
[16-Apr-2011 03:05:02.112][UE][Mobile.cpp:118]returned from Mobile::SendRRCConnection
[16-Apr-2011 03:05:02.112][UE][Mobile.cpp:117]started AwaitRRCConnection
[16-Apr-2011 03:05:02.112][UE][Mobile.cpp:118]returned (val=5) from Mobile::OnRRCConnectionRequest

…sequence diagrams and collaboration diagrams.

This slideshow requires JavaScript.

The Python scripts lets you customize the mapping from your traces to sequence diagrams with regular expression definitions.

The example below shows how named regular expression groups are mapped to sequence diagram generation language (FDL).

 # Regular expression for parsing the function/method entry trace body
    invokeRegex = '(?P\w+)(\.|::)(?P\w+)\s*(\((?P\w+)\))?'

    # FDL mapping template for function/method entry
    invokeTemplate = '{method} {params} :{caller} -> {called}'

Trace Log to Sequence Diagram Conversion


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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Introducing a sequence diagram based system modeling tool

Model object and message interactions in text. Visualize with sequence diagrams and context diagrams.