1. The 3GPP IP Multimedia Subsystem Chapter 4
The Higher Institute of Industry Postgraduate Program
The 3GPP IP Multimedia Subsystem Chapter 4
Course Instructor
Dr. Majdi Ali Ashibani

2. Next Generation Networks (NGN) course agenda
Introduction
PSTN
Mobile IP
GPRS/UMTS
4G mobile networks
VOIP
VOIP QoS issues
Multimedia Control Protocols
H323
H324
Session Initiation Protocol (SIP)
Soft Switching
Convergent Networks
Service Delivery Platforms (SDP)
IP Multimedia Subsystem (IMS)
OSA/Parlay
Next Generation Billing Systems
Ad Hoc Networks
Ad hoc routing issues
Ad hoc network security

3. IP Multimedia Subsytem
The IP Multimedia Subsystem (IMS) as defined by the 3G Partnership Program (3GPP), the body developing standards for 3G mobile networks, has been recognized as an architecture that will revolutionize service delivery, not just for mobile users but across all networks.
The adoption of IMS will allow common applications to be delivered to fixed and mobile endpoints and will eventually lead to single unified infrastructures for the support of both fixed and mobile networks.
This Fixed to Mobile Convergence (FMC) will create a communications environment where end devices can "roam" between fixed and mobile networks, and where end users will be able to access their personalized services from any device, in any location.

4. IMS Motivation
Even though Minutes of Use (MOU) are increasing significantly, average revenues per user (ARPUs) from voice services continue their downward trend.
So, operators needed a way to provide more attractive packet-switched services to attract users to the packet-switch domain, and build new revenue streams.
That is, the mobile Internet needed to become more attractive to its users.
In this way the IMS (IP Multimedia Subsystem) was born.

5. IMS Motivation
By 2004 there will be more mobile subscriptions than fixed mainlines. One year later wireless Internet users will outstrip fixed users.

6. What are Next Generation Services
The primary enabler of a unified network domain is a concept called “fixed-mobile convergence”.
This topic is currently one of the most intense focus areas for service providers and for Siemens worldwide … we have major pilots underway around the world …
Some of you here in the U.S. may have heard the announcement two weeks ago of Siemens winning a large portion of the 3G buildout of Cingular Wireless – the largest wireless provider in the U.S. – and fixed-mobile is a big aim of theirs.
So what is it?
It’s a both a strategy to achieve service convergence for applications and services as well as a horizontally layered architecture designed for application flexibility, infrastructure stability and access independence for users and their devices.

7. Network convergence and Universal SDPs

8. Fixed Mobile Convergence: What Is It?
Service Convergence Is The Goal
Focus on application layer convergence for SIP multimedia and real-time service
Focus on extending services across fixed and mobile access and terminal devices
Not a softswitch, not just a “SIP server”, but a horizontally layered architecture designed for …
Application flexibility: clear separation of application layer from network infrastructure
Infrastructure stability: all-IP core network with common denominators
SIP session control
unified database (single sign on, single number)
media gateway control & media gateway (network interfaces)
Access independence: access-agnostic support of wireline and wireless technologies
GSM, GPRS
UMTS, W-CDMA
WLAN
xDSL
HFC, …

9. Fixed-Mobile Convergence Control: Key to Unifying Access, Content & Applications
Operator services and applications
3rd party applications and content
Composed of all network elements providing users, radio
access to UMTS
RAN options
- GSM EDGE RAN (GERAN)
- Universal Terrestrial RAN (UTRAN)
Content & Applications
Access
- Push and Talk
- Instant Messaging/Chat
- Teen Line - 2nd line voice
- Presence Services
- …
Network Infrastructure
- Movies
- Music
- Information
- Infotainment
- …
IP-based Multimedia Subsystem
IMS
IMS
Control
But to execute on a fixed-mobile strategy, we must have centralized control over the network infrastructure, the access to that infrastructure, and the services, applications and content that are delivered to end users.
The 3GPP group has defined that control through its IP Multimedia Subsystem specification, which describes how 3G operators can bring new applications to wireless users and has emerged as a way to control the convergence of fixed-mobile networks.
UTRAN
DSL
GERAN
WLAN
Fixed line

10. The IP Multimedia Subsystem (IMS)
The IP Multimedia Subsystem (IMS) defined by the 3rd Generation Partnership Projects (3GPP) represents today the global service delivery platform standard for providing multimedia applications in Next Generation Networks (NGN).
It defines an overlay service architecture that merges the paradigms and technologies of the Internet with the cellular and fixed telecommunication worlds.
The IMS as part of the 3GPP Release 5 specification defines an overlay architecture on top of the 3GPP Packet Switched (PS) Core Network for the provision of real time multimedia services, such as voice over IP and video conferencing.

11. The IP Multimedia Subsystem (IMS)
There are four additional key functionalities that mark the IMS as the future technology in a comprehensive service and application oriented network:
The IMS provides easy and efficient ways to integrate different services, even from third parties. Interactions between different value added services are anticipated.
The IMS enables the seamless integration of legacy service and is designed for consistent interactions with circuit switched domains.
The IMS supports for a mechanism to negotiate Quality of Service (QoS) for each media components in the multimedia session.
The IMS provides appropriate charging mechanisms. thus different business models can be realized to charge users for specific events using an appropriate scheme.

12. The IP Multimedia Subsystem (IMS)
IMS is a subsystem of the Core Network (CN) that depends on the Packet Switch (PS) domain

13. IMS within the 3G Network Technologies
IMS Core Idea:
Define an IP multimedia overlay-network over GPRS (for session control based on Internet protocols!)
Data (Media) transport (as well as signaling transport) via GRPS.
Circuit domain (GSM) becomes obsolete over time.

14. High Level Requirements
The following high level requirements shall be supported for IP multimedia applications [3GPP TS ]:-
Negotiable QoS for IP multimedia sessions both at the time of a session establishment as well as during the session by the operator and the user
Support for interworking with the Internet and circuit-switched networks.
Support of roaming, negotiation between operators for QoS and for Service Capabilities is required.
Support for strong control imposed by the operator with respect to the services delivered to the end-user.
Support for rapid service creation without requiring standardisation.
Support for access independence of the IMS.

15. IMS Services & Architecture
Some of the service that can be provided over IMS are:
voice and video telephony,
rich telephony calls,
presence services,
instant messaging,
Multipart gaming,
chat rooms.

16. IMS Services & Architecture
While possible services are numerous, they are all based on a small set of capabilities:
Endpoint identities, including telephony numbers and Internet name.
Media description capabilities, including coding formats and data rates.
Person-to-person real-time multimedia services, including voice telephony.
Machine-to-person streaming multimedia services, including TV channels.
Generic group management, enabling chat rooms and messaging.
Generic group communication, enabling voice and video conferencing.

18. IMS Services & Architecture
To maximize flexibility, the IMS organize its functionality in three layers:
Transport and endpoint layer
Session control layer
Application server layer

19. Application server Layer Transport & Endpoint Layer
Legacy
SCPs
Application
Servers
CAMEL, ANSI-41
INAP, TCAP
Parlay API
Application server Layer
Non-Telephony
Services
Telephony
Services (TAS)
IM-SSF
Supplemental
Telephony
Services
OSA-GW
SIP-ISC
Session Control Layer
HSS
CSCF
DAL, , GPRS, CDMA
MRFC
MGCF
Transport & Endpoint Layer
SIP
Media
Server
Media
Gateway
PSTN

20. IMS Services & Architecture
Transport & Endpoint Layer
Initiates & terminates the signaling needed to setup & control sessions.
Provides bearer services between the endpoints.
Media gateways are provided to convert from/to analog/digital voice telephony formats to/from IP packets using RTP.
IMS signaling is based on SIP on top of IPv6

21. The session control layer
Provides functionality that allows endpoints to be registered with the network and sessions to be setup between them.
It also contains the functions that control the media gateways and servers so as to provide the requested services.
In this layer multiple transport services may be combined in a single session.

22. IMS Services & Architecture
The application server layer
Allows sessions to interact with various AS entities.
In this layer multiple sessions may be coordinated to provide single application.

23. IMS Architecture IMS Other IP/ IMS network UTRAN Legacy/ PSTN
HSS Cx
I-CSCF
Other IP/
IMS network Mm
I-CSCF
P-CSCF
S-CSCF Mw
Traffic Plane
Control Plane Mk
MRF Gi Mr Mi
PDF Go Gq Go Gi Mg
BGCF Mj
MGCF
UTRAN
IMS-
MGW
SGW Mn Gi
Legacy/
PSTN UE
SGSN
GGSN
PS Domain

24. IMS Elements IMS consists of the following elements:
HSS (Home Subscriber Server), SLF (Subscriber Location Function).
P-CSCF (Proxy Call Session Control Function)
I-CSCF (Interrogating CSCF)
S-CSCF (Serving CSCF)
AS (Application Server)
MRFP (Multimedia Resource Function Processor)
MRFC (Multimedia Resource Function Controller)
SGW (Signalling Gateway)
MGW (Media Gateway)
MGCF (Media Gateway Control Function)
BGCF (Breakout Gateway Control function)

25. The Databases: the HSS (Home Subscriber Server) and the SLF (Subscriber Location Function).
The HSS is the central repository for user related information.
identification information (user’s telephone number, SIP addresses, IMSI);
security information (secret authentication keys);
location information (current serving GGSN, SRNC, IP address);
user profile information (subscribed services).
The SLF is needed in networks with more then one HSS.
The SLF is a simple database that maps users’ addresses to HSSs
Both HSS and SLF implement the Diameter protocol (RFC 3588) thought the Cx interface to communicate with the CSCFs.

26. P-CSCF: Proxy CSCF
The proxy CSCF is the first point of contact (in the signaling plane) between IMS terminal and IMS network.
It validates the request, forwards it to selected destinations and processes and forwards the response.
It may include a Policy Decision Function) PDF, which authorize media plane resources and manages Quality of Service (QoS) over the media plane.
The P-CSCF also generates charging information toward a charging collection node.
--+An IMS network usually includes a number of P-CSCFs for the sake of scalability and redundancy.--+
The P-CSCF may be located either in the visited network or in the home network.

27. I-CSCF: Interrogating CSCF
The I-CSCF is a SIP proxy located at the edge of an administrative domain.
Beside, it has an interface to the SLF and HSS (based on Diameter protocol), to retrieves user location information and route the SIP request to the appropriate destination (typically an S-CSCF).
The I-CSCF may optionally encrypt parts of the SIP message that contain sensitive information about the domain (number of servers, their DNS names, or capacity) – THIG (Topology Hiding Inter-network Gateway) functionality.
The I-CSCF is usually located in the home network, although in some especial cases (THIG), it may be located in the visited network as well.

28. S-CSCF: Serving CSCF
The S-CSCF is the central node in the signaling plane.
Can behave as a Registrar, and as a SIP server.
Delivery point of services to the user.
Performs registration and security for the client
Interacts with HSS to obtain subscriber profile, authenticate, and register the client. Protocol to HSS is AAA/Diameter
Provides billing information
S-CSCF is always located in the home network.

29. AS: Application Server
The AS is a SIP entity that hosts and executes services.
Three different types:
SIP AS (Application Server): hosts and executes services based on SIP.
OSA-SCS (Open Service Access-Service Capability Server): provides an interface to the OSA framework Application Server.
IM-SSF (IP Multimedia Service Switching Function): allows to reuse CAMEL services that were developed for GSM in the IMS.
The AS can be located either in the home network or in an external third-party network.

30. MRF: Multimedia Resource Function
The MRF is responsible for providing functions such as:
mixing media for video/voice conferencing (conferencing bridge);
providing multimedia announcements;
processing media streams, e.g. audio transcoding.
The MRF functionality is split into:
a control (MRFC);
and a media processing (MRFP) part.
The interface between the two components is controlled using the H.248/MEGACO protocol.
The MRFC receives call control signaling via the SIP protocol (e.g. to establish a Videoconference between a number of parties).
The MRF is always located in the home network.

31. PSTN Interworking Entities
BGCF (Breakout Gateway Control Function)
A SIP server that inclides routing functionality based on telephone numbers.
The main functionality of the BGCF is:
To select an appropriate network where interworking with the circuit-switched domain is to occur.
Or, to select an appropriate PSTN/CS gateway, if interworking is to occur in the same network.
SGW (Signaling Gateway function)
Interfaces the signaling plane of the CS network.
Performs lower layer protocol conversion.
MGCF (Media Gateway Control Function)
Perform interworking to the PSTN i.e. between ISUP and SIP
Controls the MGW for media conversion
Selects the I-CSCF for PSTN originating calls
MGW (Media Gateway)
Terminates media streams from a packet network

32. The PSTN/CS gateway interfacing a CS network

33. Home and Visited Networks
The IMS reused the same concept of having a visited and home network from the GSM.
Home network: using the infrastructure of the network operator.
Visited network: using the infrastructure of other operators (roaming).
The IMS allows two different configurations, depending on the P-CSCF location (home or visited network).

34. The P-CSCF located in the Visited Network
In the IMS, both the GGSN and the P-CSCF share the same network.
To allow the P-CSCF to control the GGSN over the Go interface , which make its operation simpler.
Visited Network

35. The P-CSCF located in the Home Network
The visited network only provide the radio bearers and the SGSN (no need to support IMS)
The media will rout first to the home network, then to their destination (disadvantage).
Visited Network

36. Identification in the IMS
Public User Identities:
an IMS user is allocated with one or more.
give the ability to:
differentiate personal (private) identities (known to friends and family), from business public user identities (known to colleagues)
Or for triggering a different set of services.
are either a SIP URI (RFC 3261) (e.g.
or a TEL URI (RFC 2806) (e.g. tel: )
Are used as contact information on business cards.
In the IMS, Public User Identities are used to route SIP signaling.
Similar function for IMS as the MSISDN (Mobile Subscriber ISDN Number) for GSM
In the IMS it is possible to register several Public User Identities in one message, saving time and bandwidth.

37. Identification in the IMS
Private User Identities:
each IMS subscriber is assigned one Private User Identity.
It take the format of a NAI (Network Access Identifier – RFC 2486)
Private User Identities are exclusively used for subscription identification and authentication purposes.
Only visible to control nodes inside IMS
Similar function for IMS as the IMSI (International Mobile Subscriber Identification) for GSM
Need not be known by the user (stored on SIM – Subscriber Identity Module)

38. Relation of Private and Public User Identities in 3GPP R5.

39. Relation of Private and Public User Identities in 3GPP R6
Only one Private User Identity is stored in the SIM card, but users may have different SIM cards that they insert in different IMS terminals.

40. Versions of 3GPP Release 1999
IMS
phase 2
3GPP Release 6
IMS
phase 1
3GPP Release 5
3GPP Release 4
3GPP R’99
scope  first drafts  release frozen
 corrections  feedback from implementations
3GPP TSGs
Plenary Meetings
Versions of 3GPP Release 1999 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1
1999
2000
2001
2002
2003
2004

41. 3GPP Releases Release 97 (1997):
Often called 2.5G, this release introduced GPRS for data delivery over GSM (2G).
Release 99 (1999, UMTS R3 3GPP):
First release of the (3G) UMTS standard, in Included W-CDMA.
Release 4 (2001):
Separated the system into Circuit Switched and Packet Switched domains
Release 5 (March 2003):
First IMS Release, introduced the IMS as control structure of the Packet Domain, based on SIP for call control and mandatory IPv6.
Introduced End-to-end QOS and Service-Related Local Policy (SRLP).
Purpose: to enable new applications over the GPRS/UMTS bearer.

42. It is likely that this list will expand.
Release 6 (December 2004 to March 2005):
Includes some leftover IMS issues from Release 5.0, such as QOS Improvements (SRLP Control), and decoupling of PDF and P-CSCF for QOS Policy Control (allows non-IMS application services authorization).
Introduces support for IMS Access Independence, Instant Messaging and Presence Service, Push to talk over Cellular Service, WLAN integration, MMS (Multi-media Messaging Services), plus Enhancements to use SIP, Multicast and Broadcast Service (MBMS), and Event-Based Charging.
Release 7:
Just getting under way in mid 2005, and is currently expected to focus on leftovers from Release 6, as well as defining fixed broadband access via IMS, policy issues, voice call handover between CS, WLAN/IMS and end-to-end QOS.
It is likely that this list will expand.

43. The Signaling Plane in the IMS

44. IMS Complications
One complication is that, while UMTS networks support both IPv4 and IPv6 , the IMS uses exclusively IPv4. Therefore, IPv4 to IPv6 translation gateways are needed at the edge of an IMS network to convert between the different header formats and addresses. Note that this is an issue for all IPv6 networks, nor only for IMS
Another complication is that while SIP is an Internet standard, it has been extended to better handle requirements of the IMS. As a result, when SIP requests are received from or directed to external networks, the S-CSCF will discover that one side does not support the IMS specific extensions. Depending on operator’s policy, the S-CSCF may either refuse to setup sessions with non IMS specific SIP semantics. If media transcoding is also needed, it may performed by MRFP under control of the MRFC.

45. Challenges
Both the GSM and the GPRS RAN were circuit switched, with data packets temporarily using idle circuits. While the same arguments in favour of IP can be made as for the CN, it is still debated in the mobile industry whether to switch to an all-IP RAN or to continue providing IP services over an existing RAN. While an all-IP RAN would simplify interoperation with the all-IP CN, the high overhead of IP packets (such as headers) makes many parties reluctant to use IP, at least over the air. Since bandwidth efficiency is a critical issue, work is under way to provide header and signalling compression for the RAN.
For IMS to be able to replace circuit switched services, strict QoS guarantees have to provided, therefore similar mechanisms will be required in order to provide the required QoS. In addition, these mechanisms will have to interoperate with the mechanisms provided by external IP networks, so as to provide end-to-end QoS.

46. Session Setup
The various CSCF elements provide session control for subscribers
accessing services within the IMS CN

47. IMS Quality of Service QoS architecture
The 3GPP has adopted the layered QoS architecture.

48. IMS Quality of Service
UE is decomposed into two pieces, the Terminal Equipment (TE) and the Mobile Station (MS), where the MS is the 3G device and the TE is a laptop, a PDA or a mobile phone.
While IMS sessions are setup using SIP signaling and various CSCF entities, the session data are exchanged directly between the UEs. Therefore, as shown in the figure, the path from the UE goes through the UTRAN, the SGSN and the GGSN, and from there enters the other party’s network.

49. IMS Quality of Service
An application that requires a certain QoS is mapped to an appropriate Bearer Service that describes how a given network defines and provides QoS in its layer.
Each Bearer Service relies on the QoS-enabled services of the lower layers.
The End-to-End Bearer Service is the service seen by the user, which is not standardized as it lies partly outside the 3G network.
Here we are concerned only with the UMTS bearer service, which is what the 3G network provides.
The UMTS Bearer Service consists of two parts:
the Radio Access Bearer Service for the cellular wireless network,
the Core Network Bearer Service for the backbone network.
3GPP networks provide four different QoS classes with different characteristics:
The conversational class is used for real time applications such as voice and video conferencing, and it has the most stringent delay requirements.
The streaming class is used for applications such as video streaming, which can accept some delay variation.
The interactive class is used for services requiring some assured throughput in order to provide good response time, such as web browsing.
Everything else can use the background class, for example , which has the lowest priority.

50. IMS Quality of Service

51. IMS Quality of Service Policy-based QoS control
A UMTS network implements the policy-based architecture is outlined in Figure
QoS handling is split between two entities:
the Policy Control Function (PCF)
and the Policy Enforcement Point (PEP).
When a call request is received specifying a particular QoS profile, the PCF retrieves the network policy rules from a Policy Repository to see whether the request can be granted or not. If the request can be granted, the PCF translates the policy rules into specific configuration actions for a QoS mechanism and sends them to the PEP, which actually implements the QoS mechanisms. This procedure decouples the policies of the network from the specific mechanisms used to achieve them.
While both the PCF and the PEP may be separate entities, it is reasonable to combine the PCF with the P-CSCF in each network, since the P-CSCF handles all call setup signalling for the network and is therefore ideally suited for resource authorization. On the other hand, the actual data follow a different path from the signalling, always passing through the network’s GGSN where the PDP context resides, therefore it is reasonable to put the PEP there so as to implement the required mechanisms. The PEP and the PCF communicate via the COPS-PR protocol, which is an extended version of the IETF COPS protocol.

52. IMS Quality of Service

53. IMS Quality of Service
The role of the PCF is to intercept the SIP requests from UEs as they arrive to the P-CSCF, examine the description of the media for the call and decide whether the request should be accepted or not based on policy rules.
If the call is to be accepted, the PCF creates an authorization token that is sent to the UE.
This token will be used later to persuade the PEP that the resources requested have been approved by the PCF.
The role of the PEP is to control resource usage in the GGSN. When the UE attempts to create a PDP context for a new session its request is sent to the GGSN, where it will be intercepted by the PEP.
The PEP will use the authorization token to confirm with the PCF that the call has been indeed approved.
If so, the PDP context will be created and the PEP will start classifying incoming packets and handling them based on the reserved resources. The procedure of allowing IP packets from a session to pass through the PEP is called opening the gate, while the dropping of unauthorized IP packets is called closing the gate.
In order for the PCF and PEP to work, they must first establish a session between them. The PCF starts by downloading policy configuration information to the PEP. The PEP informs then the PCF about its capabilities. Finally, the PCF instructs the PEP which policies it should configure. At the conclusion of this process, the PCF can start authorizing IMS calls since it knows what can be supported at the PEP, while the PEP may start creating PDP contexts after asking the PCF whether to accept them or not.

54. IMS Quality of Service
QoS Setup
A wireless network requires strict authorization of UEs so that network resources are not abused. Once authorized and approved, the network must guarantee that these resources are made available to the legitimate users. Therefore an IMS session must go through the following steps during setup.
Authorization of resources.
Reservation of resources.
Commitment of resources.

55. IMS Quality of Service