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IMS Subscriber to IMS Subscriber Call Flow

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 to IMS Call Flow pdf                                 IMS to IMS Call Flow Poster (11x17) pdf

The call flow is explained below by focusing on a different aspects of the SIP call signalling:

Call Overview
The following diagram provides a single sheet overview of the message interactions. Interaction details and remarks are omitted from the diagram.

 IMS to IMS Call Flow Overview pdf

UE - Network Interactions
This diagram treats the whole IMS network as a black box and focuses on the interactions between the user equipment (UE) and the network.

 IMS to IMS call flow with focus on UE and Network Interactions pdf

IUE, Visited and Home Network Interactions
The following diagram adds more details. Interactions between the UE, Visited and Home network are covered. The individual networks are treated as black boxes.

 IMS to IMS Call Flow that focuses on interactions between the UE, the visited and the home IMS networks. pdf

Originating User Equipment (UE) Role
The originating UE initiates the call by sending a SIP INVITE. The major steps are:
  • Route the invite to originating P-CSCF.
  • Accept the response from the originating P-CSCF.
  • Inform the called subscriber when the local PDP context activation has been completed.

Origination UE Role Sequence Diagram pdf                  Originating UE Role Collaboration Diagram pdf

Originating P-CSCF Role
The Originating P-CSCF resides in the visited network. It acts as the Originating UE's gateway into the home network. The Originating P-CSCF is responsible for:
  • Routing the SIP INVITE to the Originating P-CSCF.
  • Routing the "183 Session Progress" to the Originating UE after obtaining a media authorization token.
  • Routing all the messages over the SIP dialog established in the initial handshake.

Originating P-CSCF Role Sequence Diagram pdf         Originating P-CSCF Role Collaboration Diagram pdf

Originating S-CSCF Role
The Originating S-CSCF resides in the home network of the caller. It is responsible for:
  • Locating the terminating UE's network and forwarding the SIP INVITE to the terminating I-CSCF.
  • Participating in the SIP dialog.

Originating S-CSCF Role Sequence Diagram pdf        Originating S-CSCF Role Collaboration Diagram pdf

Terminating I-CSCF Role
The terminating I-CSCF resides in the home network for the called subscriber. It acts as the front-end of the called IMS network. The I-CSCF is responsible for:
  • Locating the called subscriber's S-CSCF by querying the HSS.
  • Routing the SIP INVITE to the terminating S-CSCF.
  • Participating in the SIP dialog for the call.

Terminating I-CSCF Role Sequence Diagram pdf         Terminating I-CSCF Role Collaboration Diagram pdf

Terminating S-CSCF Role
The Terminating S-CSCF resides in the home network of the called subscriber. It is responsible for:
  • Routing the initial SIP INVITE to the terminating P-CSCF.
  • Participating in the SIP dialog to either the I-CSCF or the originating S-CSCF.

Terminating S-CSCF Role Sequence Diagram pdf        Terminating S-CSCF Role Collaboration Diagram pdf

Terminating P-CSCF Role
The Terminating P-CSCF resides in the home network. It acts as the Terminating UE's proxy into the home network. The Terminating P-CSCF is responsible for:
  • Routing the SIP INVITE to the Originating P-CSCF after obtaining a media authorization token.
  • Routing the "183 Session Progress" to the Terminating S-CSCF .
  • Routing all the messages over the SIP dialog established in the initial handshake.

Terminating P-CSCF Role Sequence Diagram pdf         Terminating P-CSCF Role Collaboration Diagram pdf

Terminating User Equipment (UE) Role
The originating UE initiates the call. The major steps are:
  • Respond to the SIP INVITE with "183 Session Progress".
  • Activate the local PDP context.
  • Ring the subscriber when the originating end has completed PDP context activation.

Terminating UE Role Sequence Diagram pdf                  Terminating UE Role Collaboration Diagram pdf

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