ims basics (generic) training

IMS Basics, Version 1.1e T.O.P. BusinessInteractive GmbH of 40

1 Introduction


1.1 The Need for IMS (1/3)...................................................................31.1 The Need for IMS (2/3)...................................................................41.1 The Need for IMS (3/3)...................................................................51.2 UMTS Architecture RAN and CN (1/4) ...........................................61.2 UMTS Architecture RAN and CN (2/4) ...........................................71.2 UMTS Architecture RAN and CN (3/4) ...........................................81.2 UMTS Architecture RAN and CN (4/4) ...........................................91.3 UMTS Architecture Planes (1/3) ...................................................101.3 UMTS Architecture Planes (2/3) ...................................................111.3 UMTS Architecture Planes (3/3) ...................................................121.4 UMTS R99 Architecture (1/9) .......................................................131.4 UMTS R99 Architecture (2/9) .......................................................141.4 UMTS R99 Architecture (3/9) .......................................................151.4 UMTS R99 Architecture (4/9) .......................................................161.4 UMTS R99 Architecture (5/9) .......................................................171.4 UMTS R99 Architecture (6/9) .......................................................181.4 UMTS R99 Architecture (7/9) .......................................................191.4 UMTS R99 Architecture (8/9) .......................................................201.4 UMTS R99 Architecture (9/9) .......................................................211.5 UMTS R4 Architecture (1/2) .........................................................221.5 UMTS R4 Architecture (2/2) .........................................................231.6 UMTS R5 Architecture (1/4) .........................................................241.6 UMTS R5 Architecture (2/4) .........................................................251.6 UMTS R5 Architecture (3/4) .........................................................261.6 UMTS R5 Architecture (4/4) .........................................................271.7 UMTS R6 Aspects........................................................................281.8 UMTS R7 Aspects........................................................................291.9 NGN Aspects................................................................................301.10 Involved Standardization Bodies (1/10) ......................................311.10 Involved Standardization Bodies (2/10) ......................................321.10 Involved Standardization Bodies (3/10) ......................................331.10 Involved Standardization Bodies (4/10) ......................................341.10 Involved Standardization Bodies (5/10) ......................................351.10 Involved Standardization Bodies (6/10) ......................................361.10 Involved Standardization Bodies (7/10) ......................................371.10 Involved Standardization Bodies (8/10) ......................................381.10 Involved Standardization Bodies (9/10) ......................................391.10 Involved Standardization Bodies (10/10) ....................................40


1.1 The Need for IMS (1/3)

The new communication paradigm is about networking Internet Protocol (IP)-based mobiledevices. These terminals have built-in cameras, large, high-precision displays and plentyfulapplications resources. They are always-on-always-connected application devices.



This redefines applications. Applications are no longer isolated entities exchanginginformation only with the user interface. The next generation of more exciting applications arepeer- to- peer entities, which facilitate sharing: shared browsing, shared whiteboard, sharedgame experience and shared two-way radio session (i.e. push to talk).



True integration of voice and data services increases productivity and overall effectivenesswhile the development of innovative applications integrating voice, data and multimedia willcreate demands for new services. These will include, presence, multimedia chat, push to talkand conferencing. The ability to combine mobility and the IP-network will be crucial to servicesuccess in the future and will require more capable networks than the present 3G mobilenetworks.

Nauman Khan

Stamp

Nauman Khan

Stamp


1.2 UMTS Architecture RAN and CN (1/4)

The Radio Access Network (RAN) and Core Network (CN) were concepts developed toovercome the problem of compatibility between the many fixed and wireless network typesbeing developed throughout the world.



The basis of the concept is that the RAN part of the fixed network architecture takes care ofthe radio aspects of the radio link for the mobile station. The aspects covered by the RANinclude handover, power control, random access and radio bearer load control.



In its ultimate form, there will be many different types of RAN (GERAN, UTRAN, UMA,WLAN) as well as many types of CN (IP, Narrow-band-ISDN, Broadband-ISDN, UMTS Corenetwork, Public Data Networks).



The UMTS Release 99 architecture will focus on the UTRAN connected to a GSM CN. I.e.,the Network Subsystem NSS for circuit Switched (CS) services, and the GPRS network forPacket Switched (PS) services. Both parts together create the UMTS Core Network CN.


1.3 UMTS Architecture Planes (1/3)

Another concept introduces the two logically independent planes - the control and userplanes - used to separate control messages, i.e., signaling from user information flow,i.e.,content between user equipment (UE) and the CN.



The user plane in the UMTS system, for instance, will transport all of the user data trafficbetween the UE and the CN. The user data can be any type of user data associated with aCircuit Switched (CS) or a Packet Switched (PS) connection, e.g., voice and e-mail or Webaccess data.



The Control Plane is used to pass control messages between the UTRAN / CN and the UE.The control messages are varied and include signaling messages which relate to functionslike location updates, handover and call setup.


1.4 UMTS R99 Architecture (1/9)

The UMTS network architecture is divided into three subsystems, with well definedinterfaces:The Radio Access Network (RAN) - the Core Network (CN) – and the Operations SupportSystems (OSS).The functions of each subsystem are similar to those of GSM. This makes it very easy foroperators to upgrade their existing networks.



UMTS Release 99 introduces the UTRAN, the radio access network for UMTS, and providesall transmission and control functions needed for area wide radio coverage.

The Air-Interface Uu connects the mobile user equipment UE to the UTRAN.

The CN provides the main switching and subscriber handling functions and connects to theUTRAN via the Iu interface. The OSS is responsible for the management of the wholenetwork.

In the following steps we will introduce the network elements of the different subsystems andshow the network structure will develop within future releases.



The new UMTS Terrestial Radio Access Network (UTRAN) consists of the base stationNode-B and the RNC.

The base station is responsible for the W-CDMA transmission on the air interface, andconnects to the Radio Network Controller RNC via the Iub Interface The main tasks of RNCare radio resource management, mobility management and radio network supervision.



The UMTS Core Network consists of two logical independent domains:

The CS circuit switched domain and the PS packet switched domain.

Already familiar from GSM NSS and GPRS, the CS and the PS domain are based on thesame main elements:

CS consists mainly of MSC, VLR and Gateway-MSC, while PS is based on GPRS corenetwork elements like the GGSN, also introducing a 3G SGSN.

The CS and PS domains share HLR, AC and EIR as common network elements.



The Iu interface between the UTRAN and the CN is divided into two separate functional partsaccording to the services supported: For the circuit switched services the RNC is connectedto a Media Gateway / MSC tandem via the Iu-CS interface.The Media Gateway functionalitywill be explained later. For the packet switched services the Iu-PS interface is used betweenRNC and SGSN. RNCs can also be interconnected via the Iur-interface.



Now let‘s have a look at the combination of the UMTS network with an existing GSM andGPRS structure. The 2G Base Station Subsystem BSS , consisting of BTS and BSC, isconnected to the CS domain via the A interface towards the MSC, whereas the connection tothe PS domain takes place via the Gb interface between the BSC and the 2G-SGSN.



Apart from the new W-CDMA structure on the air interface, there are two more majordifferences between GSM and UMTS networks:

1. In GSM, transmission is based on circuit switching of TDMA timeslots, but in UMTStransport takes place via the network ATM cell transmission.

2. 2. In 2G, transcoding is defined as a BSS functionality, whereas in UMTS it is part of theCS-domain of the CN.



As a consequence, with Rel. 99 a new network element is introduced in the CS-domain: theMultimedia Gateway.With the help of the MGW an existing MSC can be reused. This combination of MSC andnew MGW is also called a 3G-MSC.The main function of the MGW is to provide UTRAN interworking functions for CS servicestowards MSC.At the user plane the MGW takes care of both UMTS transcoding functionality and theATM/TDM conversion.



On the control plane the MGW is responsible for interface signalling conversion between thedifferent protocols, BSSAP protocol stack defined on A-interface and RANAP signalling stackon the Iu-CS interface.In the same manner, in the PS domain a new ATM-based 3G SGSN takes responsible forthe 3G packet oriented traffic on the Iu-PS interface, while timeslot based 2G SGSN is still inuse.



The purpose of Rel. 4 is to offer higher flexibility and better efficiency of transport resourcesin the core network.Thus the user plane and the control plane are strictly separated. As a consequence thenetwork elements in the circuit switched domain 3G MSC, VLR and GMSC are replaced byMSC Server (MSS) that either provides IP or ATM backbone connectivity.



The MSC user plane switching functions are brought to the Multimedia Gateway for MSSwhich is responsible for bearer control. Thus, calls can be switched at MGW sites withoutbeing routed to the MSC server site.The MSC Server handles all control plane functions for CS-call control and mobility controlparts from MSC and VLR. The Release 4 architecture allows a centralization of call controlfunctions to relatively few MSC servers.In the interest of convergence with the packet switched domain the telephony core is basedon an ATM and IP backbone.



The major changes in the Rel.5 network will be the IP Multimedia Subsystem enablingsimpler service integration. UTRAN will be improved by the High Speed Uplink & DownlinkPacket Access (HSUPA / HSDPA) for enhanced uplink data rates at a max. of 1.8Mbps anddownlink data rates at a max. of 10Mbps. 2G BSC and 3G RNC will be directly connected bya new Iur-g Interface. The whole network will become an All-IP network, meaning IPtransport in the core network as well as in the UTRAN, offering End-to-end IP services.



By introducing the IP Multimedia Subsystem (IMS) network operators can offer an universalall-IP backbone network that is able to support any kind of wireless and wire-line accessnetworks.

Thus, network operators can offer seamless services, operate all network subsystems moreeasily and benefit from an utmost flexibility in services creation and network extension.



IMS introduces new categories of server-based network components with dedicatedfunctionality:

Session Management and routing servers, e.g., the Call Session Control Function Data bases, e.g., the Home Subscriber Server which is an evolved HLR Interworking Elements, e.g., Media Gateways Support Entities Charging Entities and Application Servers

We'll present all entities in detail later.



The latter requires an enhancement of our plane model that we discussed earlier. AnApplication Plane is added for bearer-independent content presentation and handling.


1.7 UMTS R6 Aspects

During its ongoing specification work 3GPP has shifted some original UMTS R5 features to alater introduction date. R6 main features related to IMS will include:

Full interworking with circuit-switched networks, WLANs and other IP networks Multiple registration Emergency sessions Usage of Public Key Infrastructure Presence services, group management, conferencing


1.8 UMTS R7 Aspects

Furthermore, 3GPP continues with R7 specification work including a set of IMS features like

Multiple Input Multiple Output antennas (MIMO) Improvements to the Radio Interface, i.e., UMTS at 900 / 1,700 / 2,600MHz PS domain and IMS impacts for the support of IMS Emergency calls Location Services enhancements Advanced Global Navigation Satellite System (A-GNSS) concept System enhancements for fixed broadband access to IMS WLAN 3GPP IP Access Voice over IMS bearer related enhancements

3GPP work items are always subject to modification, this list provides the status as of April2006.


1.9 NGN Aspects

Earlier, we said that IMS is access network-independent, although 3GPP has focussed onmaking sure that the radio access networks are ready for IMS services. IMS services fromfixed broadband networks such as Asymmetric Digital Subscriber Lines (ADSL) are alsoreferred to as Next Generation Networks (NGN). We will look at VoIP-based NGN aspects ina later module.


1.10 Involved Standardization Bodies (1/10)

As we have seen already, IMS was developed by 3GPP from UMTS network and serviceconcepts. To achieve better global interoperability other major standardization bodiescontribute to the IMS specifications. We will now have a look at some of them.



The Internet Engineering Task Force (IETF) is a standardization body that assumes the taskof developing and evolving the Internet and its architecture, as well as ensuring its smoothand secure operation. The IETF is made up of network designers, academics, engineers andresearchers from many companies. IETF participation does not require membership and isopen to any individuals who share the same interests.



3GPP and IETF work closely together. 3GPP adopts protocols developed at the IETF asneeded (e.g. SIP, SDP, RTP, DIAMETER). 3GPP generates requirements for a specificproblem and then contacts the IETF for a possible solution to its requirements. The IETFevaluates the 3GPP requirements and provides 3GPP with a protocol that satisfies thoserequirements.



In June 2002 the mobile industry set up a new, global organization called the Open MobileAlliance (OMA). OMA has taken its place as the leading standardization organization fordoing mobile specification work. OMA’s role is to specify different service enablers, such asdigital rights management or push to talk over cellular service (PoC).



OMA has recognized that it is not beneficial for each service enabler to have its ownmechanism for security, quality of service, charging, session management, etc. On thecontrary, service enablers should be able to use an infrastructure like the IMS that providesthese basic capabilities in a very efficient way.



Therefore, OMA and 3GPP will increase their cooperation in the future. OMA might graduallytake overall responsibility for the invention and design of applications and services, while3GPP continues to develop the core IMS.



The Third Generation Partnership Project 2 (3GPP2) is a collaborative project for developing3G systems for the ANSI (American National Standards Institute) community. Like its sisterproject 3GPP, 3GPP2 cooperates with several important organization like ARIB (Associationof Radio Industries and Businesses), CCSA (China Communications Standards Association),TIA (Telecommunications Industry Association) and market representation partners, e.g.,CDMA Development Group and the IPv6 Forum.



3GPP2’s role in IMS standardization lies in specifying IMS as part of the Multimedia Domain.Multimedia Domain and the CDMA2000 Access Network together form 3GPP2’s 3G All-IPCore Network. In turn, 3GPP contributes with its Release 5 IMS specifications.



However, there are differences between 3GPP IMS and 3GPP2 IMS Release 5 solutions dueto different underlying packet and radio technology.



Additionally, both IMS approaches have defined further additions or limitations. The mostsignificant differences are

IP version 4 is also supported in 3GPP2 IMS, whereas 3GPP IMS exclusivelysupports IP version 6

3GPP2 specifics no default codec. There are differences in the charging solutions. 3GPP2 does not support a Universal Integrated Circuit Card which could contain IMS

access parameters. 3GPP2 does not support Customized Applications for Mobile network Enhanced

Logic-related functions.

IMS Basics, Version 1.1e T.O.P. BusinessInteractive GmbH Page 1 of 29

2 Entities & Functions


2.1 Overview ........................................................................................32.2 Proxy-Call Session Control Function (P-CSCF) (1/2) .....................42.2 Proxy-Call Session Control Function (P-CSCF) (2/2) .....................52.3 Serving-Call Session Control Function (S-CSCF)...........................62.4 Interrogation-Call Session Control Function (I-CSCF) ....................72.5 Home Subscriber Server (HSS) (1/6) .............................................82.5 Home Subscriber Server (HSS) (2/6) .............................................92.5 Home Subscriber Server (HSS) (3/6) ...........................................102.5 Home Subscriber Server (HSS) (4/6) ...........................................112.5 Home Subscriber Server (HSS) (5/6) ...........................................122.5 Home Subscriber Server (HSS) (6/6) ...........................................132.6 Policy Decision Function (PDF) (1/2)............................................142.6 Policy Decision Function (PDF) (2/2)............................................152.7 Multimedia Resource Function Controller (MRFC) .......................162.8 Multimedia Resource Function Processor (MRFP) .......................172.9 Media Gateway Control Function (MGCF)....................................182.10 Signaling Gateway (SGW)..........................................................192.11 Breakout Gateway Control Function (BGCF)..............................202.12 IMS Multimedia Gateway Function (IMS-MGW) .........................212.13 Security Gateway (SEG) ............................................................222.14 SGSN and GGSN.......................................................................232.15 Application Servers (1/3) ............................................................242.15 Application Servers (2/3) ............................................................252.15 Application Servers (3/3) ............................................................262.16 Summary (1/3)............................................................................272.16 Summary (2/3)............................................................................282.16 Summary (3/3)............................................................................29


2.1 Overview

In the previous chapter, we presented the 6 main categories of server-based IMS networkentities including:

Session Management and routing servers, e.g., a Call Session Control Function Data bases, e.g., the Home Subscriber Server which is an evolved HLR Interworking Elements, e.g., Media Gateways Support Entities Charging Entities and Application Servers

We will now have a closer look at each entity and its key functionality.


2.2 Proxy-Call Session Control Function (P-CSCF) (1/2)

The Proxy Call Session Control Function (P-CSCF) is the first contact point for users withinthe IMS. All SIP signaling traffic from or to the UE go via the P-CSCF. It validates therequest, forwards it to selected destinations and processes and forwards the response. Anoperators network can contain one or many P-CSCFs.


2.2 Proxy-Call Session Control Function (P-CSCF) (2/2)

Furthermore, the Proxy Call Session Control Function (P-CSCF) provides IPSec or ESP(Encapsulating Security Payload) for SIP signaling and interacts with the PDF (PolicyDecision Function) for media policing purposes. Finally, it contributes to the charging processby sending accounting-related info to the CCF (Charging Collection Function).


2.3 Serving-Call Session Control Function (S-CSCF)

The Serving CSCF (S-CSCF) is the brain of the IMS. An operator’s network can includemultiple S-CSCFs with different functionalities. Key functions include:

Handling registration requests and user de-registration when needed Mutual authentication between the user and the network Download of user information and service-related data from the HSS Routing of mobile-terminating traffic to the P-CSCF and mobile- originated traffic to

the I-CSCF, the Breakout Gateway Control Function (BGCF) or the application server(AS).

Translation of standard telephone numbers according to ITU‘s E.164 numberingscheme into a SIP universal resource identifier (URI). A DNS (domain name system)translation mechanism is used for this purpose, also referred to as ENUM service.

Media policing, a process that checks the content of the user payload to find outwhether it contains media types or codecs, which are not allowed for a user

Accounting-related information on the Charging Collection Function (CCF) for offlinecharging purposes and to the Online Charging System (OCS) for online chargingpurposes.

To summarize here are all S-CSCF functions at a glance. Just click on the text to see the filmagain.


2.4 Interrogation-Call Session Control Function (I-CSCF)

The Interrogating-CSCF (I-CSCF) is the contact point within an operator’s network for allconnections made to a subscriber of that particular network operator.

The functions performed by the I-CSCF are:

Contact with the HSS to obtain the name of the S-CSCF that is serving a user, and S-CSCF assignment

Forwarding of SIP requests or responses to the S-CSCF Provisioning of accounting-related information to the CCF Provisioning of a hiding functionality. An optional integrated Topology Hiding Inter-

network Gateway (THIG) can be used to hide the configuration, capacity andtopology of the network from outside an operator’s network.

For scalability and redundancy reasons an operator’s network may contain multiple I-CSCFs.


2.5 Home Subscriber Server (HSS) (1/6)

The Home Subscriber Server (HSS) is the main data storage for all IMS subscriber andservice-related data. The data stored in the HSS includes user identities, registrationinformation, access parameters and service triggering information.



User identities consist of two types: private and public user identities. The private useridentity is a user ID that is assigned by the home network operator and is used for purposeslike registration and authorization. It can be compared to the International Mobile SubscriberIdentity (IMSI) in GSM networks.

The public user identity is the ID that other users can use for requesting communication withthe end user. It serves a similar purpose as the TMSI, Temporary Mobile Subscriber Identity,in GSM.



A special mechanism called the Subscription Locator Function (SLF) is implemented within I-CSCF, S-CSCF and Application Servers. When separately addressable HSSs have beeninstalled within the network, it is used as a resolution mechanism to find the proper addressof the HSS that holds the subscriber data for a given user identity.



IMS access parameters are used to set up sessions and include parameters like userauthentication, roaming authorization and allocated S-CSCF names.

Service-triggering information enables SIP service execution. The HSS also provides user-specific requirements for S-CSCF capabilities. This information is used by the I-CSCF toselect the most suitable S-CSCF for a user.



There can be more than one HSS in a home network depending on the number of mobilesubscribers, the equipment capacity and the organization of the network. Communicationbetween different HSS functions is not standardized.


2.6 Policy Decision Function (PDF) (1/2)

The Policy Decision Function (PDF) makes policy decisions based on session and media-related information obtained from the P-CSCF. In Release 5 it is an integral part of the P-CSCF, it acts as a policy decision point for Service Based Local Policy control.


2.6 Policy Decision Function (PDF) (2/2)

Service Based Local Policy (SBLP) provides parameters for:

Session identification, e.g., IP addresses, port numbers, bandwidths, etc.. Session authorization to PDF and GGSN depending on the requested bearer, e.g.,

Packet Data Protocol or PDP context Session maintenance (PDP context modification or re-establishment) Session charging by passing an IMS-charging identifier to the GGSN and a GPRS-

charging identifier to the P-CSCF


2.7 Multimedia Resource Function Controller (MRFC)

The Multimedia Resource Function Controller (MRFC) is needed to support bearer-relatedservices, such as conferencing, announcements to a user or bearer transcoding. The MRFCinterprets SIP signaling received via S-CSCF and uses the Media Gateway Control Protocol(MEGACO) or H.248 instructions to control the Multimedia Resource Function Processor(MRFP). For charging purchasing, the MRFC is able to send accounting information to theCCF and OCS.


2.8 Multimedia Resource Function Processor (MRFP)

The Multimedia Resource Function Processor (MRFP) allocates user-plane resources asinstructed by the MRFC.


2.9 Media Gateway Control Function (MGCF)

The Media Gateway Control Function (MGCF) connects IMS communication to legacy CSusers. All incoming CS call control signaling, i.e., ISDN User Part (ISUP) or the BearerIndependent Call Control (BICC) messages are converted into SIP and vice versa.


2.10 Signaling Gateway (SGW)

For the lower signaling layers, a Signaling Gateway (SGW) converts standard SS7 MTP orIP-based SCTP (Stream Contol Transmission Protocol) messages into the TCP or UDPformat and forwards them to the MGCF.


2.11 Breakout Gateway Control Function (BGCF)

The Breakout Gateway Control Function (BGCF) determines where a signaling breakout tothe CS domain occurs. If the breakout occurs in the same network, the BGCF selects anMGCF in the same network to convert SIP signaling into ISUP/BICC signaling to the CSdomain. If the breakout occurs in another network, the BGCF selects another BGCF in adifferent network. Then the BGCF selects its own MGCF to convert SIP signaling intoISUP/BICC signaling to the CS domain.


2.12 IMS Multimedia Gateway Function (IMS-MGW)

The MGCF also controls an IMS user plane entity for the breakout to legacy CS networks: It'scalled the IMS Multimedia Gateway Function (IMS- MGW). It terminates the bearer channelsfrom the CS networks and media streams from the backbone network and converts themaccordingly. This includes transcoding and signal processing for the user plane whenneeded. Furthermore, the IMS-MGW provides tones and announcements to CS users.Obviously the MGCF must be able to report account information to the CCF.


2.13 Security Gateway (SEG)

To protect signaling traffic between different security domains, traffic will pass through asecurity gateway (SEG). In many cases, a security domain will be identical to a networkoperated by one network operator. According to the operator’s security policy the SEG maybe defined for interaction towards all reachable security domain destinations or it can bedefined for only a subset of all those destinations.


2.14 SGSN and GGSN

Finally, we should not forget the standard GPRS network elements like SGSN and GGSN. InIMS they used in the same way for UE mobility management, session management andinterconnection to other IP networks.


2.15 Application Servers (1/3)

So far we have gained a broad overview of IMS entities related to user plane and controlplane functions. As we learned in the first module, IMS provides a third layer that caters forapplication that provides value-added multimedia services in the IMS.



The Application Plane does not need to belong to the IMS operator. Hence, independentvalue-added service providers may contribute to the mobile business with multimediapresence, conference or messaging services. SIP Application Servers host these servicesbased on three basic functionalities:

Processing and impact an incoming SIP session received from the IMS Originating SIP requests Sending accounting information to the CCF and the OCS.



Application can be provided in three different ways:

as core SIP services that can directly interact with the S-CSCF using the Open Service Architecture philosophy and an appropriate Application

Programming Interface for IMS interworking or using the legacy Intelligent Network approach with CAMEL services developed with a

CAMEL Service Environment. To connect it to the SIP-based IMS a special IMSService Switching Function is implemented using the CAMEL Application Part forinternal signaling.


2.16 Summary (1/3)

As we come to the end of this module let us now try to put all entities together for a fulloverview of the IMS architecture. In the User Plane we have 3 elements that providesconnectivity to the different wireless and wire-line access networks: SGSN, GGSN and IMS-MGW. The MRFP allocates the necessary IMS-internal resources.


2.16 Summary (2/3)

In the Control Plane we have a number of signaling-relevant entities like HSS, the 3 CSCFswith PDF and THIG, MRFC, MGFC and SGW, BGCF and SEG. The Charging CollectionFunction (CCF) and the Online Charging System (OCS) complete the list of networkelements.


2.16 Summary (3/3)

Finally, the Application Plane hosts Application Servers for value-added multimedia services.

Remember, all these components are not necessarily stand-alone entities. Moreover, theseelements are functions or processes that can be implemented on common server platforms.


3 NGN Aspects


3.1 IMS and NGN: General Aspects (1/3).............................................33.1 IMS and NGN: General Aspects (2/3).............................................43.1 IMS and NGN: General Aspects (3/3).............................................53.2 Layers, Sub-systems and Functions...............................................63.3 Transport Layer ..............................................................................73.4 Service Layer (1/5) .........................................................................83.4 Service Layer (2/5) .........................................................................93.4 Service Layer (3/5) .......................................................................103.4 Service Layer (4/5) .......................................................................113.4 Service Layer (5/5) .......................................................................123.5 IMS and NGN: Network Entities (1/9) ...........................................133.5 IMS and NGN: Network Entities (2/9) ...........................................143.5 IMS and NGN: Network Entities (3/9) ...........................................153.5 IMS and NGN: Network Entities (4/9) ...........................................163.5 IMS and NGN: Network Entities (5/9) ...........................................173.5 IMS and NGN: Network Entities (6/9) ...........................................183.5 IMS and NGN: Network Entities (7/9) ...........................................193.5 IMS and NGN: Network Entities (8/9) ...........................................203.5 IMS and NGN: Network Entities (9/9) ...........................................21


3.1 IMS and NGN: General Aspects (1/3)

Earlier, we said that IMS is access network-independent, although 3GPP and 3GPP2 havefocussed on their radio access networks being ready to accept IMS services. Fixedbroadband networks using Digital Subscriber Lines (xDSL) will also offer IMS services. Theywill be referred to as Next Generation Networks (NGN). We will now talk about VoIP-basedNGN aspects with the focus on architectural details.



NGN architecture has a different layered approach we met with IMS. In IMS we consideredUE interaction with 3 planes representing application, control and user aspects.



An NGN is divided into two main layers, namely the service layer and the transport layer.Each layer is composed of a number of subsystems and a number of common functions.


3.2 Layers, Sub-systems and Functions

The NGN architecture allows for any distribution of the elements and the subsystems indifferent networks. As such, it provides for an access network, a visited network, and a homenetwork, each one providing a different type of service.

The transport layer provides the layer 2 connectivity, IP connectivity, and transport control.


3.3 Transport Layer

The transport layer is divided further into the Network Attachment Subsystem (NASS), theResource and Administration Control Subsystem (RACS), and a number of common transferfunctions.

NASS is responsible for supplying the terminal with configuration parameters, e.g. IPaddresses, authentication at the IP- layer, authorization of network access and accessconfiguration based on users’ profiles. It's also the location manager at the IP layer.

RACS provides resource management, gate control functionality, policy enforcement, andadmission control based on user profiles.

Common transfer functions incorporate a number of functional elements, e.g., mediagateways and border gateways. They are used and controlled by the different NASS orRACS entities.


3.4 Service Layer (1/5)

The service layer contains a number of subsystems that provide the platform for enablingservices to the user according to the concepts of PSTN/ISDN emulation and PSTN/ISDNsimulation. Let us have a look at these two service concepts.



The term PSTN emulation is used to refer to an NGN that implements the same services astodays PSTN and ISDN networks. Users will have exactly the same services as in legacynetworks, and will keep using their existing phnones. They will not be aware that an NGN isacutally delivering the service.



The term PSTN/ISDN simulation is used to refer to an NGN that provides servicescompatible with the PSTN/ISDN, but which need not be necessarily exactly the same. Theconcept indicates a replacement of the PSTN/ISDN by NGN, and of legacy terminals bybetter performing user equipment.



The PSTN/ISDN Emulation Subsystem (PES) implements the PSTN/ISDN emulationconcept. PES is typically implemented as a monolithic softswitch.

The core IMS that we discussed earlier implements the PSTN/ISDN simulation concept.Besides legacy speech services the core IMS enables SIP-based multimedia services toNGN terminals, e.g., presence, instant messaging, etc.



The service layer in NGN also provides for the existence of applications, typicallyimplemented in Application Server Functions (ASF).

A number of common functions provide functional services to several subsystems. This is thecase of the User Profile Server Function (UPSF), a database that contains user- specificinformation, like the HSS for the IMS.


3.5 IMS and NGN: Network Entities (1/9)

NGN is largely based on the 3GPP IMS specifications, but it considers only SIP networkelements such as CSCFs, BGCF, MGCF, and MRFC. Applicatin Servers, MRFP, MGW, userdatabases, etc., are considered part of the common functions or of the transport layersubsystems.



The P-CSCF contains an IMS Application Level Gateway (IMS-ALG) that provides control forthe network address and port translator functions, these are located in the TransitionGateway (TrGW). IPv4-IPv6 translation is sometimes necessary, e.g., when the localcustomer network implements a private IPv4 addressing scheme.



The I-CSCF, S-CSCF, BGFC, and MRFC are the same as in IMS. The MGCF in NGN keepsthe same functionality as in the IMS, but it has additional functions that provide appopriateinterworking with circuit-switched networks beyond the basic call.



Common functions represent NGN architecture elements, like the User Profile ServerFunction (UPSF), the Subscription Locator Functino (SLF), the Interconnection BorderControl Function (IBCF), the Interworking Function (IWF), and Application Server Functions(ASFs).



The UPSF in NGN is similar to the HSS in IMS. Since HSS is an evolution of the GSM HLR itwould also include HLR/ AUC pair to provide mobility management. As this is not required inNGN, the UPSF is limited to the IMS-specific parts of the HSS.



When several UPSFs are used in NGN a Subscription Locator Function (SLF) is available asin IMS. It identifies the UPSF to be addressed by the Service Layer entities.

The Interconnection Border Control Function (IBCF) is a new functional entity that acts as aseparator between two different IMS domains. It performs IP version interworking and insertsan Interworking Function (IWF) in the communication path, if needed. In addition the IBCFmay hide some SIP headers that an operator may consider dangerours to expose externally.



The Interworking Function (IWF) provides interworking between SIP and other protocols,e.g., H.323.

Application Server Functions (ASFs) execute IMS services, such as presence, instantmessaging, video conferencing or document collaboration.



NGN identitfies two types of ASFs. ASF Type 1 may interact with some Transport Layerfunctionality when providing a service to the user. ASF Type 2 merely relies on the callcontrol protocol to provide the service and is equivalentfunctionally to the AS in the IMS.



Finally, a Charging Collection Function (CCF) has the same function in NGN, as in IMS.

IMS Basics, Version 1.1e T.O.P. BusinessInteractive GmbHPage 1 of 22

4 Reference Points


4.1 IMS Reference Points: Overview.................................................... 34.2 IMS Reference Points: Gm............................................................. 44.3 IMS Reference Points: Mw ............................................................. 54.4 IMS Reference Points: Cx .............................................................. 64.5 IMS Reference Points: Dx .............................................................. 74.6 IMS Reference Points: ISC............................................................. 84.7 IMS Reference Points: Sh .............................................................. 94.8 IMS Reference Points: Si ............................................................. 104.9 IMS Reference Points: Dh ............................................................ 114.10 IMS Reference Points: Ut ........................................................... 124.11 IMS Reference Points: Mm......................................................... 134.12 IMS Reference Points: Mg.......................................................... 144.13 IMS Reference Points: Mi ........................................................... 154.14 IMS Reference Points: Mj ........................................................... 164.15 IMS Reference Points: Mk.......................................................... 174.16 IMS Reference Points: Mr .......................................................... 184.17 IMS Reference Points: Mp.......................................................... 194.18 IMS Reference Points: Mn.......................................................... 204.19 IMS Reference Points: Go.......................................................... 214.20 IMS Reference Points: Gq.......................................................... 22


4.1 IMS Reference Points: Overview

Let us now have a look at the interfaces of the various IMS elements to better understandtheir functionality. Because the network structure is so complex, we will set some limitationsfor clarity:

We will describe interfaces to Charging Functions and Security Gateway separately We will not differentiate between different types of Application Servers nor show the

interfaces between Application and User Plane.


4.2 IMS Reference Points: Gm

The Gm interface connects the UE to the IMS. Please note: Whatever RAN is used, Gmtraverses it transparently! Gm carries all SIP signaling messages between the UE and theIMS counterpart which is the P-CSCF. 3 main Gm procedures can be identified:

Registration / De-registration Session control and Transactions, i.e., message exchange without previous dialog definition between the

entities.


4.3 IMS Reference Points: Mw

The Mw interface then takes over the responsibility for the procedures. P-CSCF, I-CSCF andS-CSCF process Registration / De-registration, session control and transaction messagesrespectively.


4.4 IMS Reference Points: Cx

HSS permanently stores subscriber and service data that is needed by the I-CSCF and theS-CSCF when the user registers or receives sessions. Therefore, there must be a referencepoint (Cx) between the HSS and these two CSCFs. In contrast to the previously mentionedinterfaces Cx uses the Diameter protocol. This is an evolution of the familiar RADIUSprotocol (Remote Authentication Dial-In User Service) and will be described later. Cxprocedures can be divided into three main categories:


4.5 IMS Reference Points: Dx

When multiple and separately addressable HSSs have been deployed in a network, I-CSCFor S-CSCF will need help to know which HSS to contact. This is the task of the SLF, a fuctionintegrated to either I-CSCF or S-CSCF. A dedicated Dx interface is defined and is alwaysused in conjunction with the Cx reference point. It also works using the DIAMETER protocol.As the SLF knows which HSS will host the requested subscriber data it informs therequesting network element to re-direct the Cx messages to the appropriate HSS.


4.6 IMS Reference Points: ISC

Let us now involve the Application Plane with an AS that hosts and executes services, e.g.,presence, messaging and session forwarding. An interface is needed to send and receiveSIP messages between the I-CSCF, S-CSCF and the AS. This reference point is called theIMS Service Control (ISC) interface. The selected protocol is SIP. ISC procedures can betriggered by either the CSCFs or the AS.


4.7 IMS Reference Points: Sh

An AS may need subscriber data or need to know which S-CSCF to address. Thisinformation is stored in the HSS. Hence, a reference point between the HSS and the ASmust be established: it's the Sh interface, again using the DIAMETER protocol. Shprocedures are divided into two main groups:

data handling and subscription notification

The HSS maintains a list of ASs per user which are allowed to obtain or store data.


4.8 IMS Reference Points: Si

When the AS provides the service using CAMEL it is the AS-inherent IM-SSF thatcommunicates with the HSS. Instead of Sh interface, the Si reference point is used totransport CAMEL subscription information including triggers from the HSS to the IM-SSF.The protocol used is SS7 Mobile Application Part (MAP).


4.9 IMS Reference Points: Dh

Again we might encounter the problem of multiple HSSs in the network where the AS cannotknow which HSS it needs to contact. Once more the SLF will help out being contacted firstvia the Dh reference point. As Cx and Dx reference points are always used in conjunction Dhand Sh are always used together by DIAMETER messages. To get an HSS address, the ASsends the Sh request which is aim for the HSS to the SLF. On receipt of the HSS addressfrom the SLF, the AS sends the Sh request to the HSS.


4.10 IMS Reference Points: Ut

The Ut reference point links the UE and the AS. It enables users to surely manage andconfigure their network services-related information hosted on an AS. HTTP is used for Utmessage transfer, hence any protocol chosen for an application that makes use of Ut mustbe based on HTTP.


4.11 IMS Reference Points: Mm

For communicating with other IP multimedia networks, an interface is needed between bothnetworks. The Mm reference point allows I-CSCF to receive a session request from anotherSIP server or terminal. Similarly, the S-CSCF uses the Mm reference point to forward IMSUE-originated requests to other multimedia networks.


4.12 IMS Reference Points: Mg

The Mg reference point links the circuit-switched edge function, i.e., the Media GatewayControl Function, to the I-CSCF. This interface allows MGCF to forward incoming sessionsignaling from any CS domain to the I-CSCF. MGCF is responsible for converting incomingSS7 ISUP or Bearer Independent Call Control (BICC) signaling to SIP. This is the protocolused on Mg.


4.13 IMS Reference Points: Mi

When the S-CSCF discovers that a session needs to be routed to a CS domain it uses the Miinterface to forward the session to a Breakout Gateway Control Function, this in turn knowshow to route the signaling to the destination circuit-switched network.


4.14 IMS Reference Points: Mj

In order to address the destination CS network BGCF forwards the session to MGCF via theMj reference point using SIP. Again MGCF converts SIP into SS7 ISUP or BICC.


4.15 IMS Reference Points: Mk

If the destination CS network has to be reached via another IMS the Mk interface betweenthe two BGCFs involved takes care of signaling message transfer using SIP.


4.16 IMS Reference Points: Mr

When the S-CSCF needs to activate bearer-related services, e.g., an announcement, itactivates the Media Resource Function Controller (MRFC) via the SIP Mr interface.


4.17 IMS Reference Points: Mp

When the MRFC needs to control media streams it addresses the Media Resource FunctionProcessor (MRFP) within the User Plane using ITU’s H.248 protocol. This might be the caseif connections for conference media must be established or released.


4.18 IMS Reference Points: Mn

H.248 is also used for signaling between MRFC and the IMS Media Gateway for generaluser plane resource management.


4.19 IMS Reference Points: Go

In order to provide the GGSN with the allowed service characteristics for a particular usersession, the Policy Decision Function submits the needed QoS level information via the Goreference point. Charging correlation information can also be added. The protocol used hereis the Common Open Policy Service (COPS) protocol.


4.20 IMS Reference Points: Gq

PDF information about session parameters is also needed in the network element that actsas the primary contact point of the UE. That is the P-CSCF. The Gq reference point catersfor this task, using the Diameter protocol.


5 Signaling Protocols


5.1 Overview ........................................................................................ 35.2 Session Control Protocols .............................................................. 45.3 Session Initiation Protocol (SIP) ..................................................... 55.4 Diameter......................................................................................... 65.5 Common Open Policy Service (COPS) .......................................... 75.6 Media Gateway Control Protocol (MEGACO) ................................. 85.7 Real Time Transport Protocol (RTP) .............................................. 95.8 Real Time Transport Control Protocol (RTCP) ............................. 105.9 Resource Reservation Protocol (RSVP) ....................................... 115.10 Session Description Protocol (SDP) ........................................... 125.11 Domain Name Service (DNS)..................................................... 135.12 Transport Layer Security (TLS) .................................................. 145.13 Internet Protocol Security (IPSec) .............................................. 155.14 Dynamic Host Configuration Protocol for IPv6 (DHCPv6)........... 165.15 XML Configuration Access Protocol (XCAP) .............................. 175.16 Conference Policy Control Protocol (CPCP)............................... 185.17 Summary (1/2)............................................................................ 195.17 Summary (2/2)............................................................................ 20


5.1 Overview

While discussing IMS reference points we mentioned some of the protocols used. Let us nowget a more complete overview of those protocols used within and between the IMS planes.


5.2 Session Control Protocols

IMS protocols involved in session control are all based on IP. Bearer Independent CallControl, developed by ITU under the Q.1901 standard and used in UMTS Rel. 99 networkswas an initial candidate for a Session Control Protocol.Another candidate was ITU's H.323 used in legacy VoIP installations.

Finally, IETF’s SIP (Session Initiation Protocol) was chosen as the future-proof solution forsignaling between IMS network entities.


5.3 Session Initiation Protocol (SIP)

IP is a text-based protocol using the same end-to-end working principles as the successfulSimple Mail Transfer Protocol (SMTP) and Hypertext Transfer Protocol (HTTP). It is easy toextend and debug and services can be built easily using HTTP frameworks, e.g., CGI(Common Gateway Interface) and Java applets.It is the standard protocol for signaling messages exchanged between IMS entities within thecontrol plane and between the application, control and user planes.


5.4 Diameter

IETF’s DIAMETER has been selected as the IMS protocol for Authentication, Authorizationand Accounting. It is an evolution of the Remote Access Dial-up User Service that is used toestablish a fixed network connection with a standard Internet service provider.

DIAMETER uses a set of “applications” for specific interactions with other protocols, e.g.,with SIP for session set-up or credit control accounting.

It's used for signaling message exchange

on Cx / Dx interfaces between CSCFs and HSS /SLF on Sh interface between HSS and ASs on Gq interface between P-CSCF and PDF


5.5 Common Open Policy Service (COPS)

A Common Open Policy Service protocol is used for policy administration, configuration andenforcement. For these purposes, IETF designed COPS to use a simple query / responsemodel between clients (users) and policy servers. It is used on the Go interface betweenPDF and GGSN.


5.6 Media Gateway Control Protocol (MEGACO)

H.248 or Media gateway Control protocol (MeGaCo) was developed jointly by IETF and ITU.It provides standard capabilities to control media resources in the IMS User Plane, especiallyon the Mp interface between the MRFC and the MRFP, and on the Mn interface betweenMGCF and IM-MGW.


5.7 Real Time Transport Protocol (RTP)

RTP (Real-time Transport Protocol) is optimized for IMS media transport, i.e., audio andvideo signals for a proper end-to-end delivery. RTP identifies the codec used, ensuressequence numbering, time stamping and delivery monitoring. RTP does not support anyQuality of Service mechanisms. It is mainly used between User Plane entities.


5.8 Real Time Transport Control Protocol (RTCP)

This is where RTCP (Real-time Transport Control Protocol) comes in. It ensures Quality ofService monitoring and provides information about media session participants. For thispurpose, RTCP packets are inserted periodically into an RTP bit stream.

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5.9 Resource Reservation Protocol (RSVP)

Only RSVP (Resource Reservation Protocol) ensures defined Quality of Service levels formedia transport. It is based on COPS and used on the Go interface between PDF andGGSN.


5.10 Session Description Protocol (SDP)

The text-based Session Description Protocol (SDP) is part of the SIP application layer usedto describe multimedia sessions between two enduser entities. Reception capabilities, mediaformats and reception address / port are announced during the session set-up process orwhile the session is in progress.


5.11 Domain Name Service (DNS)

Domain Name services (DNS) is a legacy database that stores alphanumeric addresses andtheir corresponding IP addresses (a.O.). It has existed for a long time in public TCP/IPnetworks like the Internet and is used in the same way in IMSs. Divided into n-level domains,the DNS can resolve IP addresses and even pinpoints to a dedicated device in a company’sdepartment in a specific country.


5.12 Transport Layer Security (TLS)

Transport Layer Security (TLS) provides confidentiality and data integrity between two endpoints on a reliable transport layer such as TCP. It is split into two sub-layers: TLS Recordprotocol uses symmetric key cryptography for ciphering and a keyed message authenticationchecksum. It encapsulates the second sub-layer called TLS Handshake Protocol (a.o.) that,in turn, carries the client-server authentication.


5.13 Internet Protocol Security (IPSec)

In contrast to TLS, Internet Protocol Security provides encryption between two infrastructureentities to cypher upper layers. It is available in two versions, IPv4 and IPv6. IPSec transportmode only offers limited protection to IP headers whereas in IPSec tunnel mode the entire IPdatagram is protected.


5.14 Dynamic Host Configuration Protocol for IPv6 (DHCPv6)

Dynamic Host Configuration Protocol for IPv6 is a client-server protocol that allows for deviceconfiguration including management information. A dynamically assigned address is issuedper session by a DHCP server. In addition, it allows SIP clients to identify their outbound SIPservers by a SIP server domain name list or by a list of 128 bit IPv6 addresses.


5.15 XML Configuration Access Protocol (XCAP)

XML Configuration Access Protocol uses HTTP to upload and read information set by usersto enable the network to provide services to the user, e.g., messaging, presence or push-to-talk over cellular. The XCAP server forwards the user settings to the correspondingapplication server. XCAP is used on Ut interface between UE and ASs and between ASs.

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5.16 Conference Policy Control Protocol (CPCP)

The Conference Policy Control Protocol can be seen as a specific XCAP type used by IMSusers to manipulate rules for an IMS conference. The Conference Policy Server defines theconference lifespan, the parties which are included or excluded and the conference parties’roles and responsibilities.


5.17 Summary (1/2)

At the end of this description of signaling protocols’ let us now put things together in twodifferent ways. The first way shows how the different protocols are built upon the lower layersof the IMS signaling protocol stack. The Physical Layer and Data Link Layer are provided bythe wireless access media, e.g., GPRS, W-CDMA or WLAN. IPv4 or IPv6 take care ofNetwork layer functions. DHCP, DNS, RTP and RTCP rely on the UDP for Transport Layerfunctions. TCP is used by SIP, TLS and HTTP and, under certain circumstances, RTP andRTCP as well. In most cases, RSVP is directly supported by the IP layer, under somecircumstances it resides upon the UDP. RTP, RTCP and RSVP provide services for MediaEncoding whereas SIP is used by SDP and XCAP and CPCP by HTTP capabilities.


5.17 Summary (2/2)

The second overview relates protocol used to the IMS network entities involved. SIP is themost used protocol between the P-, S- and I-CSCFs. BGCFs use it to communicate with S-CSCF and MGCF, P-CSCF with the UE. The S-CSCF addresses AS and MRFC using SIP.DIAMETER is used between AS and HSS, P-CSCF and PDF, I-CSCF, S-CSCF and HSS.H.248 or MeGaCo is used by MRFC to control the MRFP and IMS-MGW. COPS is onlyfound between PDF and the GGSN. Finally, HTTP, TLS, XCAP and CPCP are used betweenthe UE and the AS.


6 Media Encoding and Transport


6.1 Overview ........................................................................................36.2 Adaptive Multi-Rate Codec (1/4).....................................................46.2 Adaptive Multi-Rate Codec (2/4).....................................................56.2 Adaptive Multi-Rate Codec (3/4).....................................................66.2 Adaptive Multi-Rate Codec (4/4).....................................................76.3 AMR-WB Codec (1/4).....................................................................86.3 AMR-WB Codec (2/4).....................................................................96.3 AMR-WB Codec (3/4)...................................................................106.3 AMR-WB Codec (4/4)...................................................................116.4 Video Codecs (1/2).......................................................................126.4 Video Codecs (2/2).......................................................................136.5 H.263 Codec (1/3) ........................................................................146.5 H.263 Codec (2/3) ........................................................................156.5 H.263 Codec (3/3) ........................................................................166.6 H.261 Codec ................................................................................176.7 MPEG Codecs (1/3) .....................................................................186.7 MPEG Codecs (2/3) .....................................................................196.7 MPEG Codecs (3/3) .....................................................................206.8 Text Encoding (1/3) ......................................................................216.8 Text Encoding (2/3) ......................................................................226.8 Text Encoding (3/3) ......................................................................236.9 Save Media Transport (1/2) ..........................................................246.9 Save Media Transport (2/2) ..........................................................256.10 Real-Time Transport Protocol (1/4) ............................................266.10 Real-Time Transport Protocol (2/4) ............................................276.10 Real-Time Transport Protocol (3/4) ............................................286.10 Real-Time Transport Protocol (4/4) ............................................296.11 Real-Time Transport Control Protocol (1/3) ................................306.11 Real-Time Transport Control Protocol (2/3) ................................316.11 Real-Time Transport Control Protocol (3/3) ................................32


6.1 Overview

In this module we will have a closer look how IMS media, i.e., speech, video and text areencoded to assure a high quality when transmitted over the air interface.


6.2 Adaptive Multi-Rate Codec (1/4)

3GPP defined two speech codecs for IMS terminals: AMR (Adaptive Multi-Rate) speechcodec is the mandatory speech codec. It co-operates with legacy GSM terminals usingEnhanced Full-Rate speech codecs. Then we have the Adaptive Multi-Rate Wide-Bandspeech codec for those UEs supporting wide-band services.



AMR consists of eight different codecs, each with a different bandwidth. Their differentbandwidths are 12.2, 10.2, 7.95, 7.40, 6.70, 5.90, 5.15, and 4.75 kbps. These are alsoreferred to as AMR modes. The 12.2, 7.40, and 6.70 kbps AMR modes are also known asGSM-EFR (Enhanced Full Rate) codecs.



The AMR modes were designed initially to be used in GSM networks that provide a fixed ratefor circuit-switched voice calls. This rate is split into channel coding and speech coding.When radio quality is poor due to high air interface interference the terminals use low-bandwidth AMR modes, e.g., 4.75 kbps well protected by a large channel coding element.When the air interface interference is low terminals use high AMR bandwidth modes with12.2 kbps and only a few bits for channel coding.



The AMR codec itself is able to switch modes on a frame-by-frame basis. That is, one 20 msspeech frame can be encoded using one particular AMR mode and the next speech frameusing another one.


6.3 AMR-WB Codec (1/4)

3G networks using WCDMA access do not use AMR modes in the same way as GSMnetworks. WCDMA uses fast power control and does not perform mode adaptation at thechannel-encoding level. WCDMA networks use low-bandwidth modes to gain capacity whenmany users make voice calls at the same time.



When AMR is transported over a packet-oriented path the overhead introduced by the RTP,UDP and IP headers is fairly large. Thus, it only allows for low-bit rate speech coding. IPheader compression can be used to gain some extra payload capacity to use high-bandwidthAMR modes.



The AMR-WB codec as defined in 3GPP Technical Specification 26.190 encodes voice using16,000 samples per second, instead of 8,000 samples per second used by AMR. This highersampling frequency allows AMR-WB to encode a wider range of frequencies. Consequently,AMR-WB encodes speech with higher quality than the codecs we described earlier.



AMR-WB consists of a family of codecs which encode audio using the following bandwidths:23.85,23.05, 19.85, 18.25, 15.85,14.25, 12.65, 8.85, and 6.60 kbit/s. AMR-WB is themandatory codec for 3GPP IMS terminals that provide wideband services. Thus, IMSnetworks provide better speech quality due to higher bit rates per speech sample for bothhigh and low air interface interference situations.


6.4 Video Codecs (1/2)

Video encoding relies on single image encoding. Video transmission uses a set of encodedstill images taken with a short interval between them. If the subsequent presentation of thestill image is fast enough the human eye perceives this succession of images as a movingimage. A standard sequence used in TV systems issues 25 pictures per sec, i.e., each 40msa new picture is presented.


6.4 Video Codecs (2/2)

IMS video encoding formats are well defined by international standardization bodies.Examples include: ITU’s H.263 which is mandatory for IMS or H.261 an additional option.MPEG (Motion Picture Experts Group) developed several standards that can also be usedfor IMS services.


6.5 H.263 Codec (1/3)

H.263 evolved out of H.261, the previous ITU standard for video compression and theMPEG-1 and MPEG-2 standards.

H.263 was developed for the ITU-T H.324 multimedia framework used with low bit ratecommunications. It supports five formats to encode images: Sub-QCIF (Quarter CommonIntermediate Format), QCIF, CIF, 4CIF and 16CIF. All of these formats encode the color ofthe pixels using a luminance component and two chrominance components, but supportdifferent resolutions.


6.5 H.263 Codec (2/3)

The luminance components is the black-and-white component of the image and defines howdark or light each pixel is. The chrominance components provide the color of a pixel inrelation to the colors red and blue.


6.5 H.263 Codec (3/3)

Image resolution formats used by H.263 are related to the four defined formats and vary inresolution. Luminance resolution can vary between 128 x 96 pixels and 1408 x 1152 pixels.The chrominance resolution varies between 64 x 48 pixels and 702 x 576 pixels.Please note the 2:1 relationship between the figures. This results from the fact that thehuman eye is more sensitive to luminance information than to chrominance information.


6.6 H.261 Codec

ITU-T’s H.261 standard defines the video codec used by the H.320 video-teleconferencingframework. This codec was designed to be used over ISDN lines and therefore, producesbandwidths that are multiples of 64kbps ranging from 64 to 1984 kbps.

The data rate of the coding algorithm was designed to be able to operate between 40 kbpsand 2 Mbps. The standard supports CIF and QCIF video frames with luminance resolutionsof 352x288 and 176x144 respectively. The chrominance resolutions are a 176x144 and88x72. It also has a backward-compatible trick for sending still picture graphics with 704x576luminance resolution. This was added in a later revision in 1994.


6.7 MPEG Codecs (1/3)

The MPEG video standards are used for both media storage and for videoconferencing.MPEG-1 was developed to encode audio and video at rates about 15Mbps. It also includesthe popular Layer 3 (MP3) audio compression format.



MPEG-2 was developed to encode audio and video at rates between 4 and 80 Mbps which ishigher than MPEG-1, also providing higher quality. While the VCD (Video CD) format isbased on MPEG-1, DVD (Digital Video Disk), DVB (Digital Video Broadcasting) and somedigital satellite and cable television systems are all based on MPEG-2.



Initially, MPEG-3 was designed for High-definition TV, but it was abandoned when it wasdiscovered that MPEG-2 with extensions was sufficient for HDTV. MPEG-4 expands MPEG-1 to support video / audio "objects", 3D content, low bitrate encoding and Digital RightsManagement. The DivX and XviD formats are based on MPEG-4.


6.8 Text Encoding (1/3)

Text already consists of digital information, so there is no need to perform the analog-to-digital or digital-to-analog conversions which are needed for audio and video. There are twotypes of textcommunication: Instant messages and real-time text.



Instant messages convey a whole message, such as: "How are you?" That is, the sendertypes the message, edits it if necessary, and sends it. This way the receiver only gets thefinal version of the message.



Real-time text consists of transferring keystrokes instead of text. If the sender writessomething and then deletes it to write something else, the receiver will see how thesechanges are performed. That is, the receiver gets letters and commands (e.g., carriagereturns or delete characters) one by one as they are typed. The most common format forreal-time text is ITU-T’s Recommendation T.140.


6.9 Save Media Transport (1/2)

For a safe media transport, the critical question is whether or not the payload data cantolerate a certain amount of packet loss or not. Audio and video messages will loose qualitydramatically if packets are lost whereas web browsing or instant messaging are more robustapplications.

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6.9 Save Media Transport (2/2)

Hence, the type of media should choose within IMS depending on the question whether it isgoing to be transported on reliable connections like TCP or unreliable ones like UDP.

As UDP is widely used to transport all kinds of media in IP networks some additionalmeasures must be taken to ensure a proper media transport. The IETF has specified theReal-time Transport Protocol and its sister protocol, the Real-time Transport Control Protocolto reside upon UDP for this purpose.


6.10 Real-Time Transport Protocol (1/4)

Applications using RTP are less sensitive to packet loss, but typically very sensitive todelays, so UDP is a better choice than TCP for such applications. Services provided by RTPinclude:

Payload-type identification - Indication of what kind of content is being carried Sequence numbering - PDU sequence number Time stamping - presentation time of the content being carried in the PDU Delivery monitoring - packet can still delivered out of order

RTP does not provide mechanisms to ensure timely delivery.

Nor does it give any Quality of Service (QoS) guarantees. These things must be provided by some other mechanism.



RTP’s major benefit is to allow receivers to play out media at a proper pace even if the IPnetwork does not keep the time relationship of the transported data.

As we know IP networks can produce jitter, that allows the jitter signal to arrive earlier thanthe original signal. Thus, another scheme is needed by the receiver to re-establish a certaininformation order - this is provided by the RTP timestamps.



The receiver stores all incoming data in a buffer according to their timestamps and thenstarts playing them. If a certain packet is still missing because of delay an interpolation of thepresent signal is played instead. It might even be a simple replay of the whole sample. If thepacket arrives afterwards it is discarded.

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t is obvious that the receiver should not wait too long before starting to play, nor should itstart too early. Field trials have proven that a majority of bits arrive 50ms after they weresent. A waiting time of approx. 100ms should be a good interval before packets are played.

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6.11 Real-Time Transport Control Protocol (1/3)

RTCP, which stands for Real-time Transport Control Protocol, provides out-of-band controlinformation for RTP flow. It partners RTP in the delivery and packaging of multimedia data,but does not transport any data itself.



It is used periodically to transmit control packets to participants in a streaming multimediasession. The primary function of RTCP is to provide feedback on the quality of service beingprovided by RTP.

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RTCP gathers statistics on a media connection and information such as the bytes sent, thepackets sent, lost packets, jitter, and round trip delay.

An application can use this information to increase the quality of service perhaps by limitingflow, or using a low compression codec instead of a high compression one.

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7 Procedures


7.1 IMS Registration (1/13)................................................................... 47.1 IMS Registration (2/13)................................................................... 57.1 IMS Registration (3/13)................................................................... 67.1 IMS Registration (4/13)................................................................... 77.1 IMS Registration (5/13)................................................................... 87.1 IMS Registration (6/13)................................................................... 97.1 IMS Registration (7/13)................................................................. 107.1 IMS Registration (8/13)................................................................. 117.1 IMS Registration (9/13)................................................................. 127.1 IMS Registration (10/13)............................................................... 137.1 IMS Registration (11/13)............................................................... 147.1 IMS Registration (12/13)............................................................... 157.1 IMS Registration (13/13)............................................................... 167.2 IMS Session Scenarios (1/2) ........................................................ 177.2 IMS Session Scenarios (2/2) ........................................................ 187.3 Mobile Originated Call (non-roaming) (1/10)................................. 197.3 Mobile Originated Call (non-roaming) (2/10)................................. 207.3 Mobile Originated Call (non-roaming) (3/10)................................. 217.3 Mobile Originated Call (non-roaming) (4/10)................................. 227.3 Mobile Originated Call (non-roaming) (5/10)................................. 237.3 Mobile Originated Call (non-roaming) (6/10)................................. 247.3 Mobile Originated Call (non-roaming) (7/10)................................. 257.3 Mobile Originated Call (non-roaming) (8/10)................................. 267.3 Mobile Originated Call (non-roaming) (9/10)................................. 277.3 Mobile Originated Call (non-roaming) (10/10)............................... 287.4 Mobile Terminated Call (roaming) (1/2) ........................................ 297.4 Mobile Terminated Call (roaming) (2/2) ........................................ 307.5 PSTN-Originated Call (1/6)........................................................... 317.5 PSTN-Originated Call (2/6)........................................................... 327.5 PSTN-Originated Call (3/6)........................................................... 337.5 PSTN-Originated Call (4/6)........................................................... 347.5 PSTN-Originated Call (5/6)........................................................... 357.5 PSTN-Originated Call (6/6)........................................................... 367.6 PSTN-Terminated Call (1/2) ......................................................... 377.6 PSTN-Terminated Call (2/2) ......................................................... 387.7 Call Release by Mobile (1/5)......................................................... 397.7 Call Release by Mobile (2/5)......................................................... 407.7 Call Release by Mobile (3/5)......................................................... 417.7 Call Release by Mobile (4/5)......................................................... 427.7 Call Release by Mobile (5/5)......................................................... 437.8 Call Release by PSTN (1/4) ......................................................... 44


7.8 Call Release by PSTN (2/4) ......................................................... 457.8 Call Release by PSTN (3/4) ......................................................... 467.8 Call Release by PSTN (4/4) ......................................................... 477.9 Addition of Media Resources (1/2) ............................................... 487.9 Addition of Media Resources (2/2) ............................................... 49


7.1 IMS Registration (1/13)

Let us start with the presentation of an IMS registration procedure.



At first, we will look at the entire procedure which consists of 22 major steps. In this samplescenario we will assume that the UE performs the registration process in a roaming situation.



This first part of the registration procedure represents the GPRS specific stages, i.e., PDPContext Activation. An IP address will be allocated by the GGSN which can be used as thehost address for the duration of the PDP context.. Next, the UE must discover which P-CSCFto address in the visited network. Normally, the UE obtains the IP address of the P-CSCFduring the PDP Context Activation procedure as the GGSN has a list of available P-CSCFs.The UE might also resolve the IP address using GGSN’s DHCP and Domain Name Service,if able to do so.



After the UE has discovered the P-CSCF address, it can now start to construct the initialRegister request. This SIP register request contains the SIP Universal Resource Identifier(URI), i.e. [email protected]. It also contains the UE‘s IP address and the P-CSCF‘s IP address.



Now the P-CSCF realizes that the request URI points to Tom‘s home network. So the P-CSCF acts as an outgoing proxy and has to locate the I-CSCF in the home network. Toachieve this, a DNS lookup using the request URI is performed that will return the address ofthe I-CSCF.



The P-CSCF determines which message transport type to take: Either UDP, TCP or secureTCP. In this example the P-CSCF selects UDP and forwards the Register request to the I-CSCF in the home network using UDP and port 5060.



The I-CSCF is the entry point to Tom‘s home network. It receives every Register Requestsent from Tom‘s UE. To determine the serving S-CSCF the I-CSCF queries the HSS which isassigned to the registering user. Once the appropriate S-SCSF has been found, the I-CSCFforwards the SIP Register to the S-SCSF.



The S-CSCF needs to authenticate the UE and so it requests an Authentication Vector fromthe HSS. This request procedure is based on the Diameter protocol on the Cx interfacebetween HSS and S-CSCF. The HSS returns a set of Authentication vectors, eachcontaining the familiar parameters of the respective GSM scenario.



The S-CSCF selects an Authentication Vector from the received set and forwards theparameters needed back to the UE using a “401 UNAUTHORIZED” response message.



Upon reception, the UE extracts the relevant authentication parameters for the authenticationresponse calculation. Furthermore, it calculates the numerous keys used for ciphering.Lastly, if all have passed, the UE returns the authentication response in a standardREGISTER request.



Again a DNS query is performed at the P-CSCF level to locate the I-CSCF to forward theREGISTER request containing the authentication response. The I-CSCF interrogates theHSS to locate the S-CSCF to forward the REGISTER request message. In this case, theHSS returns the S-CSCF name that was selected previously in step 5.



The S-CSCF checks the authentication response received from the UE against its owncalculated value. lf the two values match the UE has been successfully authenticated.Registration on the S-CSCF can now take place.



The S-CSCF notifies the HSS that the UE registration was successful. In turn, the HSSprovides the S-CSCF with the complete user profile. The S-CSCF returns a SIP “200 OK”response to the I-CSCF which is then sent to the P-CSCF and then to the UE.


7.2 IMS Session Scenarios (1/2)

IMS session control involves all 3 known planes, i.e., User Plane, Control Plane andApplication Plane. For simplicity we will focus on session control procedures on the ControlPlane level.


7.2 IMS Session Scenarios (2/2)

Let us have a closer look at 7 typical IMS session scenarios:

Mobile Originated Call in a non-roaming situation Mobile Terminated Call while one UE is roaming PSTN Originated Call PSTN Terminated Call Call Release by Mobile Call release by PSTN and, finally, a sample multimedia session flow integrating a second media to an

existing call.


7.3 Mobile Originated Call (non-roaming) (1/10)

Let us start with a Mobile Originated Call in a non-roaming situation. To initiate an IMS call aSIP INVITE message is used. The INVITE message contains a number of importantparameters, e.g.,

the Uniform Resource Identifiers (URI) of the calling party and the URI of the called party QoS parameters Media information

it is forwarded to the P-CSCF of the originating party.



Next, a ”100 Trying” SIP message is returned to UE-1 to indicate a provisional response. P-CSCF1 adds some routing information to the INVITE message before forwarding it to S-CSCF1.



S-CSCF1 acknowledges receipt of the INVITE message and evaluates the Media Criteria forthe calling party that were received from the HSS with the service profile. They define the"triggers" that will cause the S-CSCF1 to send the message to the appropriate ApplicationServer (AS).



S-CSCF1 forwards the INVITE message to the I-CSCF. This is always located at the edge ofthe home network for the terminating party. I-CSCF responds with a “100 Trying” message. I-CSCF will now try to locate UE2. To do this, I-CSCF sends a Cx-Location-Request messageto the HSS. The HSS responds with the address of the S-CSCF that is currently serving UE2(S-CSCF-2) and forwards the INVITE message accordingly.



S-CSCF2 performs a similar service control function as S-CSCF1. It checks the serviceprofile of UE2 against the media types requested in the media announcements. In thisexample, it is assumed that UE2 is not allowed to use stereo transmission and so one of theaudio streams is removed from the SDP media list.



S-CSCF2 remembers the address of P-CSCF2 from the registration process that UE2undertook previously. It examines the media parameters and removes any which are notallowed. Finally, P-CSCF2 forwards the INVITE message to UE2. We should not forget the“100 Trying” messages returned to the various transmitting entities.



UE2 checks the received media announcements list against its own abilities beforeresponding to the INVITE with a “183 Session Progress” SIP message. The “183 SessionProgress” message is send back via the whole signaling path causing two mediaauthorization procedures within the two P-CSCFs involved. Both P-CSCFs reserve the mediaresources needed for the IMS session.



Finally, UE1 selects the media needed and confirms its choice to the network entitiesinvolved using a Provisional Resource Acknowledgement message or PRACK. UE2responds to the PRACK message with a “200 OK” SIP message sent back along thesignaling route to UE1.



Now UE1 firmly reserves the chosen media resources and acknowledges this to UE2 usingan „UPDATE“ SIP message. Upon reception, UE2 performs the same task and confirms it toUE1 using a „200 OK“ message again. Once all resources are reserved UE2 will startringing. UE1 will hear the ringing tone as its tone generator is activated by a correspondingRINGING message. The ringing of UE2 is reported back using a PRACK message again.



UE2’s answer is a “200 OK” message. As soon as UE2 answers a final response to UE’sINVITE is sent back to the originating party using a 200 OK message. Both P-CSCFs committo the defined QoS parameters and media flow starts between the two end users. A finalACKNOWLEDGE message is issued by UE1 to terminate the call setup process.


7.4 Mobile Terminated Call (roaming) (1/2)

The next procedure we will look at will be a Mobile Terminated Call assuming that theterminating UE is roaming.


7.4 Mobile Terminated Call (roaming) (2/2)

Fortunately, this scenario does not differ from non-roaming session control. The messagesand entities involved are the same. We just have to bear in mind that I-CSCF, HSS and S-CSCF are located in UE2‘s Home IMS.


7.5 PSTN-Originated Call (1/6)

The next procedure we will look at will be a PSTN Originated Call.



As legacy PSTNs use SS7 signaling an Initial Address Message (IAM) starts the signalingprocedure. The PSTN establishes a bearer path to the MGW, and forwards it to the MediaGateway Control Function.



The MGCF initiates a H.248 or MeGaCo command, to seize the Voice Bearer and to definean IP port. The MGCF initiates a SIP INVITE request containing an initial SessionDescription Protocol offer as seen already with the Mobile Originated Call procedure. Thedestinations media streaming capabilities are returned along the signalling path in the sameway as seen with the MOC procedures. MGCF initiates a H.248 / MeGaCo command tomodify the connection parameters and instruct the MGW to reserve the resources needed forthe session.



MGCF selects the media streams for this session from those offered confirms receipt of theOffer Response and sends a Response Confirmation. The terminating endpoint responds tothe Response Confirmation using the familiar procedure. MGW reserves the resourcesneeded for the session. Upon completion, the MGCF sends the successful ResourceReservation message to the terminating endpoint which, in turn, responds to the successfulmedia resource reservation.



The destination endpoint now performs the alerting procedure. It signals this to theoriginating party by a provisional response indicating Ringing. This message is sent to MGCFwhich issues an Answer Complete Message (ACM) to the PSTN. Additionally, the destinationparty sends a SIP 200-OK final response to MGCF which forwards an Answer message(ANM) to the PSTN.



The MGCF initiates a H.248 / MeGaCo command to alter the connection at the MGW tomake it bi-directional. The MGCF acknowledges the SIP final response with a SIP ACKmessage and media flow starts to transport the user information.


7.6 PSTN-Terminated Call (1/2)

Now we will consider a PSTN Terminated Call.


7.6 PSTN-Terminated Call (2/2)

The entry session control procedure is as we have just seen. Now, it is the MGCF thatterminates the SIP INVITE request which is issued by the S-CSCF.


7.7 Call Release by Mobile (1/5)

The next scenario familiarizes us with a mobile initiated call release.



Let us have a look to a first call release scenario. We will assume that one mobile will hangup which generates a SIP BYE message from the UE to the P-CSCF. The UE initiates therelease of the bearer PDP context. The GPRS subsystem releases the PDP context. The IPnetwork resources which had been reserved for this session are now released by the GGSN.



The P-CSCF removes the authorisation for resources that had previously been issued forthis endpoint and this session.. The P-CSCF then sends a hangup to the S-CSCF of thereleasing party.



S-CSCF1 forwards the hangup to the S-CSCF of the other party, i.e., S-CSCF2. Both S-CSCFs invoke whatever service logic procedures are appropriate for this ending session andpush the hangup message to P-CSCF2. P-CSCF2 removes the authorisation for resourceswhich had previously been issued for this endpoint for this session. This step also results in arelease indication to the GPRS subsystem. Finally, P-CSCF2 forwards the hangup on toUE2.



The mobile responds with an acknowledgement, the SIP OK message (number 200), that issent back to the P-CSC2. In parallel, UE2 initiates the release of the bearer PDP context atthe responsible GGSN. The SIP OK message continues its way to the releasing party via S-CSCF2, S-CSCF1 and P-CSCF1.


7.8 Call Release by PSTN (1/4)

What is the release procedure when the fixed party side initiates the release?



The PSTN party hangs up, which generates an ISUP RELEASE message to the MGCF.MGCF sends a Hangup (SIP BYE message) to the S-CSCF to notify the mobile that theother party has disconnected. The MGCF notes the reception of the RELEASE andacknowledges it with an ISUP RELEASE COMPLETE message.



The MGCF requests the MGW to release the audio codec and ISUP trunk using theH.248/MEGACO Transaction Request. The MGW sends an acknowledgement to the MGCFupon completion of the resource release. The S-CSCF invokes the appropriate service logicprocedures for this ending session and forwards the Hangup to the P-CSCF.



The P-CSCF removes the authorisation for resources that had previously been issued forthis endpoint and this session. This step also results in a release indication to the GPRSsubsystem to confirm that the IP bearers link with the UE2 session have been deleted.Finally, the P-CSCF forwards the Hangup to the UE. The UE responds with anacknowledgement, the SIP OK message (number 200), which is sent back to the P-CSCF.GGSN releases the PDP context Including all IP network resources that had been allocated.The SIP OK message is sent to the S-CSCF which forwards the message to the MGCF.


7.9 Addition of Media Resources (1/2)

At the end of this module let us have a look to an addition of resources within a call.


7.9 Addition of Media Resources (2/2)

In contrast to all existing communication systems, IMS provides easy and standardizedprocedures to allocate multiple media resources per call. First, a standard session controlprocedure establishes the communication link between the two UEs as discussed earlier indetails. The allocation of additional media resources follows the same procedures exceptlocalization and identification of an I-CSCF. As all addresses are known the procedurefocuses on resource reservation, authorization and allocation steps.


8 Services


8.1 Overview (1/2) ................................................................................ 48.1 Overview (2/2) ................................................................................ 58.2 Links & Bearers (1/2)...................................................................... 68.2 Links & Bearers (2/2)...................................................................... 78.3 QoS Aspects (1/3) .......................................................................... 88.3 QoS Aspects (2/3) .......................................................................... 98.3 QoS Aspects (3/3) ........................................................................ 108.4 Key IMS Services ......................................................................... 118.5 Presence Service (1/9) ................................................................. 128.5 Presence Service (2/9) ................................................................. 138.5 Presence Service (3/9) ................................................................. 148.5 Presence Service (4/9) ................................................................. 158.5 Presence Service (5/9) ................................................................. 168.5 Presence Service (6/9) ................................................................. 178.5 Presence Service (7/9) ................................................................. 188.5 Presence Service (8/9) ................................................................. 198.5 Presence Service (9/9) ................................................................. 208.6 Instant Messaging (1/6) ................................................................ 218.6 Instant Messaging (2/6) ................................................................ 228.6 Instant Messaging (3/6) ................................................................ 238.6 Instant Messaging (4/6) ................................................................ 248.6 Instant Messaging (5/6) ................................................................ 258.6 Instant Messaging (6/6) ................................................................ 268.7 Conferencing (1/10)...................................................................... 278.7 Conferencing (2/10)...................................................................... 288.7 Conferencing (3/10)...................................................................... 298.7 Conferencing (4/10)...................................................................... 308.7 Conferencing (5/10)...................................................................... 318.7 Conferencing (6/10)...................................................................... 328.7 Conferencing (7/10)...................................................................... 338.7 Conferencing (8/10)...................................................................... 348.7 Conferencing (9/10)...................................................................... 358.7 Conferencing (10/10).................................................................... 368.8 Push-To-Talk over Cellular (1/15)................................................. 378.8 Push-To-Talk over Cellular (2/15)................................................. 388.8 Push-To-Talk over Cellular (3/15)................................................. 398.8 Push-To-Talk over Cellular (4/15)................................................. 408.8 Push-To-Talk over Cellular (5/15)................................................. 418.8 Push-To-Talk over Cellular (6/15)................................................. 428.8 Push-To-Talk over Cellular (7/15)................................................. 438.8 Push-To-Talk over Cellular (8/15)................................................. 44


8.8 Push-To-Talk over Cellular (9/15)................................................. 458.8 Push-To-Talk over Cellular (10/15)............................................... 468.8 Push-To-Talk over Cellular (11/15)............................................... 478.8 Push-To-Talk over Cellular (12/15)............................................... 488.8 Push-To-Talk over Cellular (13/15)............................................... 498.8 Push-To-Talk over Cellular (14/15)............................................... 508.8 Push-To-Talk over Cellular (15/15)............................................... 51


8.1 Overview (1/2)

In GSM / GPRS and UMTS Rel. 99 networks only a few services can be deilveredsimultaneously. In GSM it is only speech and SMS. In GPRS, class A mobiles support both,speech and slow data transfer at the same time. In UMTS Rel. 99 both speech and someslow motion video can be delivered in parallel as more bandwidth is available on the airinterface.


8.1 Overview (2/2)

If the UMTS network is upgraded by HSDPA, sufficient performance is provided to usespeech and data-intensive applications like a file download in the background. True multi-media that is the support of multiple applications at the same time is only available in IMSnetworks.

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8.2 Links & Bearers (1/2)

Each service requires a bearer that enables end-to-end communication between the UE andthe distant party. The bearer is more precisely called a Radio Access Bearer (RAB) as itprovides services to the Core Network via Wireless Access Network . It faciliates the transferof user plane information between the UE and the Core Nework.

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8.2 Links & Bearers (2/2)

Each RAB is supported by one or more links. A Link is a physical connection between twoentities, e.g., the UE and a single R5/R6 UTRAN access point, if W-CDMA is the wirelessaccess network. Each radio link is defined by a set of parameters, e.g., its RF frequency,spreading code and scrambling code.


8.3 QoS Aspects (1/3)

To assure a adequate bandwidth for each service Quality of Service parameters are definedby the network operators.

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Quality of service (QoS) defines the ability to satisfy traffic and service requirements across anetwork. This quality is measured in terms of delay, delay variation (jitter), throughput anderrors. Inter-connected networks require defined QoS on an end-to-end basis and not justwithin a specific network.



The QoS parameters are considered in relation to four different types of traffic classes. Theywill be used to define the basic delay characteristics of the service and other quantities likepriority and scheduling. It is obvious that for speech services we require a low delay, which isguaranteed. Streaming activities do not need a low delay but it must be defined. Interactivegaming needs instantaneous reactions hence these signals must be handled with priority.And, finally, an e-mail download as a background service is never subject to any delayaspects.

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8.4 Key IMS Services

Let us have a closer look at 4 typical key IMS services, i.e., presence service, instantmessaging, conferencing and push-to-talk.

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8.5 Presence Service (1/9)

In 2001, the SIMPLE working group was formed within IETF to develop a suite of standardsfor presence and instant messaging applications over the Session Initiation Protocol (SIP).SIMPLE stands for „SIP for Instant Messaging and Presence Leveraging Extensions“. Itspecifies extensions to the SIP protocol which deal with a publish and subscribe mechanismfor presence information and sending instant messages.



In November 2002, the Open Mobile Alliance (OMA) released the first version of the XML-based OMA Instant Message and Presence Service (IMPS). IMPS defines a systemarchitecture, syntax and semantics for representation of presence information. It alsoprovides a set of protocols for the four primary features:

presence instant messaging chat shared content

Presence is the key enabling technology for IMPS.



At present, OMA has adopted IETF‘ s SIMPLE standard for presence services includingpersence services in IMS.



Presence service facilitates a variety of additional information on the status and technicalcapabilities of the user‘s terminal. Presence information is generally considered to cover atleast the following examples:

User‘s communication availability, e.g., available, not available User’s communication preferences (voice call, SMS, MMS, IM, Push to talk)



User‘s context (in a meeting, on vacation) User‘s terminal information and communication capabilities (MMS capable, PoC

capable) User‘s availability for different applications (logged in to IM, available for PoC,

available for online gaming)



With presence service we distinguish between two different parties: The publisher can definewho can see his or her presence information and what presence information is visible toothers. The watcher is a person who is interested in another user’s presence information.



Let us describe these roles with an example. As a Publisher, Jill can publish and update herpresence status and control who can see her presence information.

Network elements and services are able to update user’s presence information. Subscribedusers - the Watchers - can have Jill’s presence information updated in the background andalways up-to-date on their mobile devices.

Users can subscribe to changes in her presence status.



The central element in the Presence Service is a Presence Server belonging to the IMSapplication plane. It stores and distributes the presence information previously provided bythe publisher.



Let us put things together now to see how a subscriber’s presence information updatesequence is handled by the system.

Mobile terminal user A publishes his or her Presence information on the PresenceServer.

Users B and C subscribe to user A’s Presence information. Presence information can be updated on the Presence Server by the terminal or by

the network. The Presence information is updated in the background to those who have

subscribed to it. Others can also fetch user A’s Presence information when needed on a one-time

basis without subscribing to the changes. Contact lists are used for authorisation. User A may, for example, authorise user D to

subscribe to his or her private Presence information.


8.6 Instant Messaging (1/6)

Instant Messaging is a typical text-based user of presence services as previously discussed.In contrast to e-mails or SMS, the parties know if the peer is available from the publishinginformation offered by a presence service. Thus, they can exchange messages in near real-time.



IMS supports three different types of IMS messaging:

Immediate messaging Session-based messaging Deferred delivery messaging

A crucial requirement to IMS is, of course, the simple interworking of all messaging types.



In immediate messaging, the UE generates a SIP MESSAGE request and inserts the contentto be submitted. It typically consists of text but can also contain multimedia elements, e.g.,sound and images. The sending UE adds the Uniform Address Indicator (URI) of therecipient and routes it through the IMS in the familiar way.



The recipient, UE2 might answer this IM in the same way. However, there is no protocolsession involved between the two UEs, the transactions are completely independent fromeach other.



Session-based messaging relates to the IETFs Internet Relay Chat (IRC) and uses textmessages that are exhanged in an established session as discussed earlier.Using any conferencing functionality, session-based mesaging can turn into a multi-partychat conference.



Deferred delivery messaging in IMS stage 1 is the same as Multimedia Messaging (MMS) in legacyGPRS / UMTS networks. It uses the MMS-related network components such as the MultimediaService Center (MMS), and the Serving and Gateway GPRS Support Nodes.


8.7 Conferencing (1/10)

A conference establishes a communication between multiple participants. In IMS, aconference is not limited to audio media but can also involve video or whiteboard sharing inreal time.

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We can differentiate between two types of conferences. An ad-hoc conference is set upsponteanously by sub-sequently integrating the parties.

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A scheduled conference uses a conference policy which might include a fixed start and endto the conference, parties to be included and even parties that might be excluded if theysponteanously attempt to join.

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There is always a conference master that controls the conference. It is referred to as a„focus“. Both, the focus and a conference factory, i.e., an Application Server (AS) supportingthe conference will initiate all signaling to integrate all conference parties.

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Ad-hoc conferences uses legacy SIP signaling between all user equipment, CSCFs, MediaResource Function Controllers (MRFC) and Application Servers.



User A, the focus starts the conference with a SIP INVITE message. The MFRC / ASresponds with a SIP 183 Message (Session Progress Message) that invoke a PRACKmessage (Provisional Acknowledge) as in a standard call set-up. A multiple exchange of SIP200 OK, UPDATE and ACKknowledge terminates the call set-up for the focus.



The other parties are integrated using SIP REFER messages followed by a set of 202(ACCEPTED), NOTIFY and SIP 200 OK messages.



A scheduled conference uses the CPCP (Conference Policy Control Protocol) that wediscussed earlier. It is used on the Ut interface between the UE and the AS (ConferenceServer) to establish the rules for the upcoming conference.



At the scheduled time, the AS executes the policy. AS creates the focus, in our case UE1that subscribes to the conference. It discovers that UE2 is on the dial-out list and integrates itwith the standard SIP message flow. UE3 is on the dial-in list, hence he must initiate a call toget integrated. In both cases, UE1 is notified when UE2 and UE3 are on board.



The AS allocates the scheduled media, e.g., audio and video codecs to be used. At the endof the conferencing session, AS takes care of the proper release of all resources andconnections using SIP BYE messages.


8.8 Push-To-Talk over Cellular (1/15)

Push to Talk over Cellular (PoC) provides a direct walkie-talkie-like one-to-one and one-to-many voice communication service in legacy cellular networks.

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PoC brings new commercial opportunities in the domain of real-time voice communications.It allows users to make Push to Talk calls between two people and a group of people overnationwide networks and even across them. Typical business users include:

Service and repair Hotels Retail, distribution Couriers, Taxi, limousine services Public transportation Airports, Harbors Manufacturing, Industrial plants Utilities, public services Construction companies



In contrast to standard voice calls which are full-duplex, dialogue-oriented, PoC is a half-duplex service, i.e., only one speaker can speak at a time while the other parties arelistening. The receivers doesn’t have to answer the call. Standard mobile voice service needsa certain time for call set-up whereas PoC supports instantaneous call set-up by pushing justone button.



Thanks to IP technology, the Push to Talk service is ‘always-on’. It uses cellular access andradio resources more efficiently than circuit-switched cellular services. This is thanks to shortspurts of talking used to communicate.



Up to now, PoC voice services have been implemented in “2G / 3G networks using networkoperator proprietary implementations. In IMS, the Open Mobile Alliance PoC standard isused for its Push-to-Talk service. This allows other media like video and whiteboard sharingto be integrated in the PoC session.



OMA defines four categories of services:

One-to one PoC, i.e., a PoC session between two users Ad-hoc PoC Group: a user selects a set of users in an ad-hoc fashion, e.g., from a

presence service buddy list Pre-arranged PoC Group: a service like the scheduled conference service we

discussed earlier. It links pre-selected users Chat PoC Group: an ad-hoc PoC service that allows other parties to join in

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Let us now have a closer look at the IMS PoC architecture using the well-known layeredstructure of the core network. The PoC UE contains two logical entities: The PoC client andthe XML Document Management Client XDMC.



The PoC client communicates with P-CSCF in the Control Plane using SIP on the Gminterface and RTP or TBCP, i.e., the Talk Burst Control Protocol) to address the MRFP andIM-MGW in the User Plane via the Mb interface. TBCP is a RTCP-based protocol whichdefines which user can speak at any given time.



The XML Document Management client communicates with S-CSCF in the Control Planeusing SIP on the ISC interface. It uses XCAP to address an AS, the PoC Aggregation ProxyApplication Plane via the Ut interface.



The PoC Aggregation Proxy Application allocates SIP URIs to the members of a PoC grouplist. It also takes care of authentication and XDMC message routing towards other PoCApplication Servers, e.g., the PoC XML Document Management Server (PoC XDMS) and theShared XML Document manageant server (Shared XDMS). The protocol used is XCAP.



The PoC XML Document Management Server (PoC XDMS) manages documents that arespecific to PoC services, e.g., a list of PoC group members. The Shared XDMS providesinformation needed by PoC services which might be shared with other IMS services, e.g.,presence related info.



Both, the PoC XML Document Management Server and Shared XML DocumentManagement Server use the SIP ISC inteface to communicate with the S-CSCF on updatesof PoC infomation details.



Finally, the PoC Server interacts with the S-CSCF for PoC session handling, media mixing,flow control and policy enforcement. It uses a SIP ISC interface for these purposes. It alsoconnects to other IMSs as the PoC group members can be distributed over several networks.



Each PoC session party is controlled by a so-called participating PoC server. One of themcan act as the controlling PoC server combining the participating PoC server functions forthis particular group member.

Nauman Khan

Stamp

Nauman Khan

Stamp



PoC session establishment does not complicate things with new messages but usesstandard SIP INVITE requests and 100 TRYING answers to integrate the different PoCgroup members. First, the controlling PoC server is addressed, which in turn triggers theparticipating PoC servres. Please note that for simplification purposes the other IMS networkentities involved in session control are not presented.

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