IP Multimedia Subsystem - Computer Architecture - Lecture Slides, Slides of Computer Architecture and Organization

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Lecture -5 & 6

IP Multimedia Subsystem (IMS)

Architectural Concepts

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IMS Registration

Before IMS registration

The UE must obtain an IP connectivity bearer

The UE discover an IMS entry point (i.e., the P-CSCF) - For example, in case of GPRS access the UE performs the GPRSattach procedure and activates a Packet Data Protocol (PDP)context for SIP signalling

IMS registration contains two phases

How the network challenges the UE

How the UE responds to the challenge and completes theregistration

First, the UE sends a SIP REGISTER request to thediscovered P-CSCF

This request contains an identity to be registered and ahome domain name (address of the I-CSCF)

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High Level IMS Registration

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IMS Registration

Finally, the S-CSCF checks the response and, if it iscorrect, downloads a user profile from the HSS andaccepts the registration with a 200 OK response

During the registration procedure both the UE and the P-CSCF learn which S-CSCF in the network will be servingthe UE.

It is the UE’s responsibility to keep its registration activeby periodically refreshing its registration

If the UE does not refresh its registration, then the S-CSCF will silently remove the registration when theregistration timer lapses

When the UE wants to de-register from the IMS it sets aregistration timer to 0 and sends a REGISTER request

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Implicit Registration Set

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Implicit Registration

When the S-CSCF receives the SUBSCRIBE request itreturns the implicitly registered public user identity with aNOTIFY request.

For example, a user has four public user identities that aregrouped in two implicit registration sets

When Joe sends a REGISTER request containingjoe.smith@brandnewcar.com as an identity to be registered - The S-CSCF downloads the service profiles that are associatedwith the public user identities belonging to the implicit registrationset (service profile 1)

To obtain the implicitly registered public user identities,Joe’s UE sends a SUBSCRIBE request to the S-CSCF

When the S-CSCF receives the SUBSCRIBE request itreturns the implicitly registered public user identity,tel:þ358501234567, within NOTIFY

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High-Level IMS Session Establishment

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Session Initiation

S-CSCF takes charge of processing the terminatingsession that include:

interactions with ASs

delivers the request to the P-CSCF over the Mw reference point

P-CSCF uses the Gm reference point to deliver the SIPINVITE request to UE B

UE B generates a response

183 Session Progress – which traverses back to UE A followingthe route that was created on the way from UE A

UE B

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P-CSCF

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S-CSCF

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I-CSCF

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S-CSCF

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P-

CSCF

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UE A

After few more round trips

both sets of UE complete session establishment

able to start the actual application Docsity.com

Identification – Private User Identity

S-CSCF will need to obtain and store the private user identity onregistration and on unregistered termination

It will not be used for routing of SIP messages - It will be permanently allocated to a user - It is securely stored in IMS Service Identity Module (ISIM)application - It will be valid for the duration of the user’s subscription within thehome network - It will not be possible for the UE to modify the private user identity - HSS will need to store the private user identity - Private user identity will optionally be present in charging recordsbased on network policies Docsity.com

Identification – Public User Identity

User identities in IMS networks are called public useridentities

They are the identities used for requesting communication withother users

Public identities can be published (e.g., in phone books, Webpages, business cards) - To be reachable from the CS side, the public user identity mustconform to telecom numbering (e.g., þ358501234567) - To communicate with Internet clients, the public user identity mustconform to Internet naming (e.g., joe.doe@example.com)

IMS imposes the following requirements for public useridentity [3GPP TS 23.228, TS 23.003]:

The public user identity/identities will take the form of either a SIPUniform Resource Identifier (URI) or a telephone Uniform ResourceLocator (tel URL) format Docsity.com

Derived Private/Public User Identities

The User’s Identities are stored in an ISIM application

When IMS is deployed there will be a lot of UEs that do notsupport the ISIM applications

In this model, private user identity, public user identity andhome domain name are derived from an InternationalMobile Subscriber Identifier (IMSI)

This mechanism is suitable for UE that has a UniversalSubscriber Identity Module (USIM) application

Self study

Derivation of public and private user identities

Relationship between these identities - Identification of network entities Docsity.com

Identity Modules – ISIM

ISIM is an application residing onthe Universal Integrated CircuitCard (UICC)

The UICC is physically securedevice that can be inserted andremoved from UE - The ISIM itself stores IMS-specificsubscriber data - Stored data can be divided into sixgroups - Most of the data are needed whena user performs an IMSregistration [3GPP TS 31.103] Docsity.com

Sharing Single Identity/Multiple Devices

In CS every single user has her own Mobile StationInternational ISDN (MSISDN) number that is used to reach theuser

It is not possible for a single user to use multiple terminals withthe same MSISDN number simultaneously

Nowadays, users may have more than one item of UE withtotally different capabilities: big/small screen, camera/nocamera, full keyboard and so forth.

Different UE may serve different purposes (e.g., one for gaming,another for ordinary voice and video sessions)

From the user’s point of view, the user should be reachable viathe same identity regardless of the number of UEs that she isusing simultaneously

The IMS makes this feature possible

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Sharing Single Identity/Multiple Devices

Release 6 IMS allows users to register the same public useridentity from a number of items of UE

Different registrations can be differentiated by means of theprivate user identity and the used IP address

In example user has two UEs: one for video sessions andanother for chat and gaming applications

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