IMS Registration-Networks And IP MultiMedia Subsystem-Lecture Slides, Slides for IP Multimedia Subsystems
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IMS Registration-Networks And IP MultiMedia Subsystem-Lecture Slides, Slides for IP Multimedia Subsystems

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This lecture was delivered by Samarth Jilesh at Birla Institute of Technology and Science for Networks And IP MultiMedia Subsystem course. Its main points are: IMS, Registration, Set, Session, Initiation, Packet, Data, P...
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Microsoft PowerPoint - Lectures 5 & 6 IMS Concepts

2

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 GPRS

attach 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 the

registration First, the UE sends a SIP REGISTER request to the

discovered P-CSCF This request contains an identity to be registered and a

home domain name (address of the I-CSCF)

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IMS Registration

P-CSCF processes the REGISTER request and uses the provided home domain name to resolve the IP address of the I-CSCF

The I-CSCF, in turn, will contact the HSS to fetch the required capabilities for S-CSCF selection

After S-CSCF selection the I-CSCF forwards the REGISTER request to the S-CSCF

The S-CSCF realizes that the user is not authorized and, therefore, retrieves authentication data from the HSS and challenges the user with a 401 Unauthorized response

Second Phase, the UE calculates a response to the challenge and send another REGISTER request to the P- CSCF.

Again the P-CSCF finds the I-CSCF and the I-CSCF, in turn, will find the S-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 is correct, downloads a user profile from the HSS and accepts 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 serving the UE.

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

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

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

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6

Implicit Registration

An implicit registration set is a group of public user identities that are registered via a single registration request

When one of the public user identities within the set is registered, all public user identities associated with the implicit registration set are registered at the same time

Similarly, when one of the public user identities within the set is de-registered, all public user identities that have been implicitly registered are de-registered at the same time

Public user identities belonging to an implicit registration set points to different service profiles

To get implicitly registered public user identities the UE must send a SUBSCRIBE request for a registration event package to the S-CSCF

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Implicit Registration Set

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Implicit Registration

When the S-CSCF receives the SUBSCRIBE request it returns the implicitly registered public user identity with a NOTIFY request. For example, a user has four public user identities that are

grouped in two implicit registration sets When Joe sends a REGISTER request containing

[email protected] as an identity to be registered The S-CSCF downloads the service profiles that are associated

with the public user identities belonging to the implicit registration set (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 it returns the implicitly registered public user identity, tel:þ358501234567, within NOTIFY

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Session Initiation When User A wants to have a session with User B

UE A generates a SIP INVITE request and sends it via the Gm reference point to the P-CSCF

P-CSCF processes the request i.e. decompresses the request verifies the originating user’s identity forwarding the request via the Mw reference point to the S-CSCF

S-CSCF processes the request i.e. executes service control which may include interactions with ASs determines the entry point of the home network of User B based

on User B’s identity in the SIP INVITE request I-CSCF receives the request via the Mw reference point

It contacts the HSS over the Cx reference point to find the S-CSCF that is serving User B

The request is passed to the S-CSCF via the Mw reference point

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High-Level IMS Session Establishment

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Session Initiation S-CSCF takes charge of processing the terminating

session 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 SIP INVITE request to UE B

UE B generates a response 183 Session Progress – which traverses back to UE A following

the route that was created on the way from UE A UE B P-CSCF S-CSCF I-CSCF S-CSCF P-

CSCF UE A After few more round trips

both sets of UE complete session establishment able to start the actual application

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Identification – Private User Identity

Private user identity is a unique global identity defined by the home network to uniquely identify the user It identifies the user’s subscription It is mainly used for authentication purposes It is possible to utilize private user identities for accounting and

administration purposes as well

IMS imposes the following requirements for private user identity [3GPP TS 23.228, TS 23.003]: private user identity will take the form of a Network Access

Identifier (NAI) defined in [RFC2486] e.g. [email protected] It will be contained in all registration requests passed from theUE

to the home network It will be authenticated only during registration of the user

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Identification – Private User Identity S-CSCF will need to obtain and store the private user identity on

registration 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 the

home 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 records

based on network policies

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Identification – Public User Identity User identities in IMS networks are called public user

identities They are the identities used for requesting communication with

other users Public identities can be published (e.g., in phone books, Web

pages, business cards) To be reachable from the CS side, the public user identity must

conform to telecom numbering (e.g., þ358501234567) To communicate with Internet clients, the public user identity must

conform to Internet naming (e.g., [email protected])

IMS imposes the following requirements for public user identity [3GPP TS 23.228, TS 23.003]: The public user identity/identities will take the form of either a SIP

Uniform Resource Identifier (URI) or a telephone Uniform Resource Locator (tel URL) format

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Identification – Public User Identity At least one public user identity will be securely stored in an ISIM

application It is not possible for the UE to modify the public user identity It will be registered before the identity can be used to originate IMS

sessions and IMS session-unrelated procedures (e.g. MESSAGE, SUBSCRIBE, NOTIFY)

It will be registered before terminating IMS sessions, and terminating IMS session-unrelated procedures

This does not prevent the execution of services in the network by unregistered users

The network will not authenticate public user identities during registration

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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 not

support the ISIM applications In this model, private user identity, public user identity and

home domain name are derived from an International Mobile Subscriber Identifier (IMSI)

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

Self study Derivation of public and private user identities Relationship between these identities Identification of network entities

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Identity Modules – ISIM ISIM is an application residing on

the Universal Integrated Circuit Card (UICC)

The UICC is physically secure device that can be inserted and removed from UE

The ISIM itself stores IMS-specific subscriber data

Stored data can be divided into six groups

Most of the data are needed when a user performs an IMS registration [3GPP TS 31.103]

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Identity Modules – USIM Universal Subscriber Identity Module (USIM) is required

for accessing the PS domain (GPRS) It identifies a particular subscriber USIM application also resides on the UICC as a storage

area for subscription and subscriber-related information It may also contain applications that use the features

defined in the USIM Application Toolkit USIM contains the following data:

security parameters for accessing the PS domain, IMSI, list of allowed access point names, Multimedia Message Service (MMS) related information [3GPP TS

31.102, TS 22.101, TS 21.111]

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Sharing Single Identity/Multiple Devices In CS every single user has her own Mobile Station

International ISDN (MSISDN) number that is used to reach the user

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

Nowadays, users may have more than one item of UE with totally different capabilities: big/small screen, camera/no camera, 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 via the same identity regardless of the number of UEs that she is using 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 user

identity from a number of items of UE Different registrations can be differentiated by means of the

private user identity and the used IP address In example user has two UEs: one for video sessions and

another for chat and gaming applications

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Sharing Single Identity/Multiple Devices When someone is calling the user – it is the S-CSCF that

makes the decision as to which UE is going to be contacted in the first place

This decision can be done based on the preferences given at the registration phase: for example, if the incoming session contains a video component, then the S-

CSCF could select UE2, based on primary preference for video sessions.

In addition to preference-based routing, the S-CSCF may perform forking. There are two types of forking: sequential forking - different items of UE are contacted one by one parallel forking - different items of UE are contacted at the same

time

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Discovering the IMS Entry Point In order to communicate with the IMS, the UE has to know

at least one IP address of the P-CSCF The mechanism by which the UE retrieves these

addresses is called ‘‘P-CSCF discovery’’ Two dynamic mechanisms for P-CSCF discovery have

been standardized in 3GPP: Dynamic Host Configuration Protocol’s (DHCP) DNS procedure GPRS procedure

Additionally, it is possible to configure either the P-CSCF name or the IP address of the P-CSCF in the UE

In the GPRS procedure the UE includes the P-CSCF address request flag in the PDP context activation request (or secondary PDP context activation request)

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Discovering the IMS Entry Point The UE receives the IP address(es) of the P-CSCF in the

response This information is transported in the protocol

configuration options information element [3GPP TS 24.008]

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Discovering the IMS Entry Point In DHCP DNS procedure, UE sends DHCP query to the IP connectivity

access network (e.g., GPRS), which relays the request to DHCP server According to [RFC3319] and [RFC3315], UE could request either a list

of the SIP server domain names of the P-CSCF(s) or list of SIP server IPv6 addresses of P-CSCF(s)

When domain names are returned, UE needs to perform a DNS query (NAPTR/SRV) to find an IP address of the P-CSCF

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S-CSCF Assignment Three Cases of S-CSCF Assignment:

when a user registers with the network when an unregistered user receives a SIP request when a previously assigned S-CSCF is not responding

Scenario 1: S-CSCF Assignment during Registration When a user is registering with a network the UE sends a

REGISTER request to the discovered P-CSCF, which finds the user’s home network entity – i.e., the I-CSCF

Then I-CSCF exchanges messages with the HSS (UAR and UAA) As a result, the I-CSCF receives S-CSCF capabilities Based on the received capabilities I-CSCF selects suitable S-CSCF Capability information is transferred between the HSS and the I-

CSCF within the Server-Capabilities Attribute Value Pair (AVP)

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S-CSCF Assignment Server-Capabilities AVP contains [TS 29.228 & TS 29.229]:

Mandatory-Capability AVP – this AVP type is unsigned contains the mandatory capabilities of the S-CSCF

Optional-Capability AVP – this AVP is unsigned and contains the optional capabilities of the S-CSCF

Server-Name AVP – this AVP contains a SIP URI used to identify a SIP server

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