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IP Multimedia Subsystem (IMS) Call Flows

IP Multimedia Subsystem (IMS) is the next generation platform for IP based multimedia services. Detailed IMS call flow diagrams for the following scenarios are covered here:

IMS Registration Sequence Diagrams

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 to IMS 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.

PSTN (ISUP) to IMS Interworking 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.

IMS to PSTN (ISUP) Interworking Call Flow

This call flow describes the call setup from an IMS subscriber to ISUP PSTN termination. 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.

SDP Codec Selection and QoS Signaling in an IMS Call

Session Description Protocol (SDP) specifies a format for exchanging streaming related parameters between SIP subscribers. The following sequence diagram focuses on the SDP interactions between two IMS subscribers. The flow covers two phases of the SDP negotiation:

  • Codec selection between the calling and call IMS subscribers.
  • SDP signaling involved in exchanging quality of service information.

IMS Presence Subscription and Notification Flow

IP Multimedia Subsystem (IMS) provides a framework and building blocks for building advanced telecom services. One such service is network wide publication and subscription of presence information. Users can subscribe to presence information for their contacts. If the contact accepts their request, the subscriber will be registered for presence notification. Whenever the friend publishes presence information, the IMS presence framework will notify the subscribed users.

IMS Presence Resource List Subscription and Notification Flow

IP Multimedia Subsystem (IMS) provides a framework and building blocks for building advanced telecom services. One such service is network wide publication and subscription of presence information. Users can subscribe to presence information for their contacts. If the contact accepts their request, the subscriber will be registered for presence notification. Whenever the friend publishes presence information, the IMS presence framework will notify the subscribed users.

IMS presence information communication can put a lot of load on the network. One way to reduce this load is to predefine the list of friends and family. When you subscribe to the "friends and family" list, a Resource List Server (RLS) in the IMS subscribes to the individual subscribers (presentities) on your behalf. Once the RLS completes the registrations, it collates the individual presence status into a single NOTIFY message.

IMS Conference Call

This sequence diagram shows an IMS user creating a conference by using a conference-factory URI. The conference is created at a MRFC/AS of the users home network. The steps involved in the conference are:

  • The conference initiator UE uses the conference factory URI to initiate a conference with the MRFC/AS (Multimedia Resource Function Control/Application Server).
  • The MRFC/AS assigns a conference URI to the conference and configures the MRFP (Multimedia Resource Function Processor).
  • The conference call is setup and the RTP data begins flowing between the conference initiating UE and the MRFP.
  • The conference initiator then uses the refer procedure to add more users to the conference. The new users establish a call to the conference URI passed in the refer message.
  • When the conference is in progress, RTP media streams are being mixed and propagated to all the participants.
  • The conference user drops out of the conference. All users are notified for this exit from the conference.

IMS Push-to-talk over Cellular (PoC) Pre-established Session

Push-to-talk over Cellular (PoC) service allows cell phones to used as walkie-talkies. A group of users in a PoC session can communicate by simply pressing a button and speaking when the phone indicates it is OK to do so. The user releases the button when he or she is done speaking.

When a user begins to speak, the PoC server allocates resources and notifies other users in the PoC session that the user is speaking. The PoC server then delivers the speech packets to all the users in the session.

PoC is resource efficient as it allocates resources only when a user is actually speaking. This makes it suitable for applications where there are long gaps between individual session participants speaking.

This flows covers the case where PoC Client A invites PoC Client B to a Pre-established Session by sending SIP REFER request to PoC Server A.