IP Multimedia Subsystem (IMS) – detailed

This is a folow up on my previous post on IMS which described what is IMS and why it came into existing. Also how IMA can benifits with its rich feature set and huge OPEX savings. To read the previous post click here .

This post described IMS architecture and core concepts in detail.

 

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IMS Home Network

SIP call between 2 SIP  User agents or from SIP UA to PSTN endpoint.

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  • HSS ( Home subcriber subsystem ) contains all of the subscriber information.
  • AS ( Application Servers ) conatin applications for example, be originating services or terminating services.
  • Filtering for applications for users is loaded into the S-CSCF and activated when the subscriber registers with the network.
  • DNS is used to identify elements use in the session set up.
  • The CSCFs manage the session control: registration, set up, tear down, feature activation.
    • The P-CSCF is first point of interaction with the User Agent. It also manages Quality of Service and other conditions specific to a UA.
    • The I-CSCF is used in network to network signaling. The I-CSCF hides the network topology from an external network.
    • The S-CSCF is the primary signal processing engine in IMS. It manages registration, checks for triggers for services and performs routing .
  • Media Resources may be conference services, IVRs or other network services.
  • If a call must egress to the PSTN the BGCF selects the appropriate Media Gateway that can be used.
  • Media Gateways control the conversion from IP to PSTN TDM signaling. Media Gateway Control Functions control the signaling between IMS and the PSTN (e.g. IP to SS7).

IMS Vistited Network <–> Home Network

Home Network to visited Network connectivity

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  • If call orignates from Visited network the P-CSCF and S-CSCF in visited network itself , process the origination of the call and select the destination network.
  • Since call receiver of the call is in Home network the I-CSCF receives the call signaling from the Visited network, chooses the appropriate S-CSCF to process the call and the call is completed with RTP flow, depicted with blue line in diagram.

UE registration

IMS registration is where the subscriber requests authorization to use the IMS services in the IMS network. The CSCF and HSS in core IMS network authenticates and authorizes the user .

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  • The UA/UE initiates the registration process .
  • The SIP registration is passed to the S-CSCF.
    • For a user in Home network , the registration request is passed via P-CSCF to S-CSCF.
    • If the user is roaming in visiting network then the P-CSCF in the Visited network would pass the registration to the S-CSCF in the Home network through a I-CSCF.
    • Users are always registered in the Home network.
  • The S-CSCF forwards the request to the HSS via the Multimedia Auth Request (MAR) message to 1) download authentication data via the Multimedia Auth Answer (MAA) message and 2) inform the HSS that this S-CSCF is in control and any other queries to the HSS should be returned to this S-CSCF.
  • The S-CSCF creates a SIP 401 Unauthorized response that includes a challenge that the IMS terminal should answer.
  • The IMS terminal sends a new Register that contains the response to the challenge.
  • The S-CSCF validates the user and sends a Session Auth Request (SAR) message to the HSS informing it that the user is now registered and requesting the user profile, including services, that come in a Session Auth Answer message (SAA).

Subcription Changes

A registered user requires to be notified of his state changes. For example,

  • registration may be valid for a fixed period of time and then the network requires the user to register.
  • user or network element may go out of service and need to inform the other of some state change.

UA/UE subscribes to the registration state also P-CSCF serving the UA/UE subscribes so it can be informed.

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  • When the IMS terminal has completed registration the P-CSCF sends a Subscribe request for the registration event. The request is directed at the S-CSCF (which is in the Home network).
  • The S-CSCF receives the request and installs that subscription, i.e. the S-CSCF takes the role of a notifier. The S-CSCF sends a Notify request to the P-CSCF. This request includes Public User Identities and the registration state.
  • When the IMS terminal has completed registration it sends a Subscribe request for the registration event. The request is directed at the S-CSCF (which is in the Home network).
  • The S-CSCF receives the request and installs that subscription, i.e. the S-CSCF takes the role of a notifier. The S-CSCF sends a Notify request to the user. This request includes Public User Identities and the registration state.

In case the S-CSCF has to shutdown or there is some other stimulus the S-CSCF will inform the user (and the P-CSCF) of the event.

Call

IMS multimedia calls sample –
voice call originated by User A
video call originated by User B
data call originated by User A

The voice call originates from user A and enters the IMS network X at the P-CSCF
P-CSCF passes the call to the S-CSCF
S-CSCF interrogates the Application Server for originating services
S-CSCF forwards the call to the I-CSCF of network Y.
I-CSCF interrogates the HSS to determine the S-CSCF and passes the call to it.
S-CSCF interrogates the Application Server for terminating services.
S-CSCF passes the call to the P-CSCF assigned for the user and the voice call is completed.
A video call is set up from User B to User A and the signaling path is reversed.

Finally, User A sets up a data call to User B using the same signaling path.

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