IMS Centralised Services

Voice services migration for a leading European telecom organization

Bachelor Thesis, 2013

54 Pages, Grade: 1,0


Table of Contents

1 Introduction
1.1 Thesis structure
1.2 Motivations behind IMS
1.2.1 Motivations from the operator’s perspective
1.2.2 Motivations from the user’s perspective

2 Overview of the IMS architecture
2.1 Drivers
2.2 IMS horizontal architecture

3 Evolution 2G/3G to IMS
3.1 2G/3G technology features
3.2 Towards IMS
3.3 Intelligent Network (IN) limitations

4 From circuit-switched to IMS-based voice services
4.1 Development path 2013 - 2018
4.1.1 Initial state
4.1.2 Intermediate state based on SCP and IM-SSF Service delivery platform and the Service Broker Service Broker benefits Intermediate state setting
4.1.3 Target state 1 based on the SIP AS and rIM-SSF
4.1.4 Target state 2 based on the SIP AS and the I2 interface
4.2 Analysis criteria for IMS-based Voice services
4.2.1 LTE coverage
4.2.2 Handset capabilities
4.2.3 Service consistency
4.2.4 Roaming
4.2.5 Standardisation
4.3 SWOT analysis
4.3.1 Refarming and the Digital Dividend
4.3.2 Further considerations

5 Major mobile market characteristics
5.1.1 Consolidation trends
5.1.2 Regulation and competition

6 Conclusio


List of Figures

List of Tables

List of Abbreviations

A: Oracle Communications Service Broker functional architecture


This bachelor thesis aims at providing an overview of the evolution process from the current circuit-switched (CS) voice services to those based on the IP multimedia subsystem (IMS). The timeline in question comprises the years 2013 to 2018 and takes into consideration three possible scenarios: one intermediate state and two target states, each of them presenting a particular setting in respect to the CS & IP domain mediation.

Those states are, subsequently, evaluated according to certain predefined criteria (standardisation, service consistency, etc.). In addition, a complementary framework is given by the SWOT analysis which will consequently lead to the final recommendations for one of the leading European telecom organizations (the company in question is not disclosed throughout this thesis on purpose). Within this context, the major characteristics of the Austrian as well as European mobile markets are examined. Finally, the expected key assumptions and developments for the given timeframe are presented together with the particular findings of the topics previously described.

1 Introduction

1.1 Thesis structure

This thesis aims at providing an overview of the evolution process from the current circuit-switched (CS) voice services to those based on the IP multimedia subsystem (IMS). The timeline in question comprises the years 2013 to 2018 and takes into consideration three possible scenarios: one intermediate state and two target states, each of them presenting a particular setting in respect to the CS & IP domain mediation.

Those states are, subsequently, evaluated according to certain predefined criteria (standardisation, service consistency, etc.). In addition, a complementary framework is given by the SWOT analysis which will consequently lead to the final recommendations for the telecom company in question. Within this context, the major characteristics of the Austrian as well as European mobile markets are examined. Finally, the expected key assumptions and developments for the given timeframe are presented together with the particular findings of the topics previously described.

As a starting point, the basic IMS architecture is presented together with its major features in respect to other current technologies. In a further step and based on the previous information, the key motivations for the transition from CS to IMS-based voice services will be discussed. In this sense, a service consistency delivered by centralized services on an access agnostic basis is assumed.

For this purpose, the initial state is evaluated regarding its three relevant levels: the transport/endpoint, session/control and application/service layers. Next, a more comprehensive approach, the intermediate state, is evaluated: here, the legacy SCP supports the CS & IP domain mediation via the IM-SSF. Moreover, two other target states are assessed involving the SIP AS using a reverse IM-SSF and the L2 interface respectively.

Having highlighted the significant characteristics of each (future) state, attention is then drawn on the various criteria for IMS-based voice services such as roaming, handset capabilities and the like. Hence, a SWOT analysis is presented addressing the question ‘Is the deployment of the intermediate state from a technical and economic perspective recommendable? Finally, the major mobile market characteristics in Europe as well as the expected developments for the timeframe 2013 to 2018 will be described. Furthermore, the concluding recommendations are derived from the key findings throughout this thesis.

1.2 Motivations behind IMS

The adoption of broadband access networks (wireless as well as wireline) has dropped the entry barrier for VoIP service providers to a new low level, enabling them to offer inexpensive telephony services to private and corporate customers. As a consequence, the voice’s ARPU (average revenue per user) for telecom service providers is decreasing, forcing them to look for alternative IP-based multimedia communication applications to counteract the loss of profit. In fact, for the communications industry, the (activities’) focus has been altered: from digital telephony to more media rich applications.

It is within this context that a shift in user preferences and economic trends becomes noticeable. First, the services offered are no longer limited to voice due to the worldwide deployment of CS (circuit-switched) cellular networks and the increasing subscriber base. Second, a shift from fixed to mobile communication has taken place leading to greater demand for services.

As a consequence, applications are expected to be access-agnostic and support, at the same time, any type of network they are connected to. An additional influencing factor is the Internet with its widespread data network based on PS (packet-switched) technology. By contrast, the telecom network (e.g. voice network) relies on the circuit-switched technology.

From the abovementioned points, it is reasonable to expect the certain implementation of a single network capable not only of bridging the gap between the CS and PS technologies, but also able to deliver a common set of services independent of time and location. Based on that, two main issues arise involving the future network architecture and the resulting network operator’s position: the convergence of the Internet and telecom worlds could relegate the operator’s infrastructure to simple “bits-carrying pipes” (Cuevas et al., 2006).

The 3rd Generation Partnership Projects (3GPP and 3GPP2) have taken these developments into account though the design of the IP-based Multimedia System (IMS): it is the significant element in the 3G architecture making possible the delivery of global cellular access to all internet services. Furthermore, the Internet protocol runs a shared foundation offering consumers seamless access to any service in a location-agnostic and access-diagnostic manner. For instance, full convergence is motivated by the deployment of wireless broadband technologies like UMTS/HSPA, WLAN, etc. along with IPv6, HTTP/SIP and VoIP.

1.2.1 Motivations from the operator’s perspective

IMS provides an improved approach through the definition of a horizontal architecture (see Figure 2). Here, multiple applications may reuse common functions as well as service enablers. Particularly, the horizontal architecture is well integrated within the current data/voice networks and still provides several advantages coming from the IT domain. In this sense, IMS specifies not only interoperability and roaming, but offers bearer control, charging and security. Hence, IMS can be regarded as a key enabler for the fixed-mobile convergence.

A migration to an all-IP system is at this time taking place within the telecommunications sector. Clearly, the efforts towards cost reduction or the essential need to produce novel services with a positive profit margin represent some of the key drivers. In fact, these objectives apply equally for the fixed and mobile communication. In the same manner, the creation of new business perspectives is fostered since IMS supports the creation as well as deployment of operators’ innovative services (or those coming from third parties).

Consequently, the telecommunications industry may benefit from IMS in the following ways:

- A reduced time to market due to the faster development of (IMS) services.
- Rise of more lucrative services: the combination of several services in one session, unified billing as well as the SSO (single sign-on) are expected to ‘awake’ user’s interest and thus increase revenue opportunities and customer loyalty.
- A more profitable approach for new service deployment: the reduction of the investment threshold becomes feasible based on the uniform service delivery platform.
- The combination of applications/equipment from multiple vendors becomes viable for operators.
- Migration to new platforms is further flexible and cost-effective.

These advantages are, nevertheless, coupled with challenges for those operators that are progressively transitioning a network. The issue of creating a seamless experience for customers should be given special consideration since the modification of familiar or basic services directly influences the turnover derived from the subscriber base. Furthermore, the issue of how to get more out of existing services while simultaneously finding new revenue sources (based on an improved user experience) should be likewise addressed.

1.2.2 Motivations from the user’s perspective

Expectations from cost-conscious consumers regarding the innovative capabilities their communication services may offer in the near future, is increasing. In fact, there is a vivid interest in services beyond voice. It is on this basis that a multimedia service evolution for mobile and fixed operators is motivated.

Some advantages IMS brings to users are:

- Suitable billing schemes based on the operators’ increased awareness of products being used by customers.
- Mobile users are able to access their personal set of services regardless of the handheld (i.e. access agnostic) or which network they are connected to.
- Users have access to all applications and services, whenever and wherever they are needed, with a SSO.
- Reduced expenses for retrieving licensed content from different types of networks or devices.

Figure 1 depicts a comparison between the multiple modes of communication in a ‘Pre-IMS’ and an ‘IMS supported’ communication environment.

Abbildung in dieser Leseprobe nicht enthalten

Figure 1: User benefits based on the IMS supported communication (right-hand side).

Source: AT&T, IMS Benefits

2 Overview of the IMS architecture

2.1 Drivers

For any operator at present, IMS is one of the most significant strategic questions. The IMS key feature is the state-of-the-art multimedia service delivery over fixed and mobile networks based on open standards. In this sense, IMS addresses central concerns such as service creation, service interconnection, delivery convergence as well as open standards. What is more, IMS allows an operator to keep its present business models or develop innovative ones.

Additionally, two forces are shaping the IMS architecture:

- The requisite to offer services for a mass market having global reach like SMS or telephony.
- The aspiration to exploit the dynamics the IP community has to offer to operators and ultimately to users.

On the one hand, standardized services may arise supported by a great variety of terminals that interact within the international operator community. Availability, scalability and performance are some features of these mass-market services. On the other hand, operators may deliver non-standardized services on an individual basis to their customers: here, the fast time to market and flexibility apply to those services.

2.2 IMS horizontal architecture

Based on this architecture, operators are able to diverge from the traditional vertical implementation of novel services that are indeed expensive and complex to build and maintain (Figure 2). For every service in a ‘Pre-IMS’ network this would imply the setup of separate implementations for each layer. Moreover, this structure is replicated from the terminal via the core network to the other user’s terminal.

Abbildung in dieser Leseprobe nicht enthalten

Figure 2: Vertical service compared to horizontal service implementations

Source: Ericsson, IMS - IP Multimedia Subsystem. The value of using the IMS architecture

By contrast, IMS comprises a set of common functions such as directory, provisioning or group/list management, characterized by their generic structure as well as implementation. Hence, the reuse by practically all services in the network is possible (e.g. less development work is required). Thanks to this approach, operators benefit from reduced operational and capital expenditures (OPEX/CAPEX), particularly in areas like billing, operations and maintenance, customer care and service provisioning. Furthermore, the necessary operations competence across services becomes more general, giving further importance to service-specific knowledge.

In the same manner, the IMS architecture has been conceived to facilitate operators the provision of comprehensive packet-based, real-time services. Besides, the tracking of user preferences and behaviour render, in addition to the well-known time-based invoicing, the packet/service-based charging possible.

Figure 3 depicts the IMS network architecture together with its relevant layers. Although IMS was conceived for mobile networks, it can similarly be used to deliver services for fixed networks simultaneously, meaning a distinctive service combination with transparency for consumers. Furthermore, it is important to remark that the IMS architecture comprises a pool of functions linked by standardized interfaces.

The IMS architecture comprises three layers:

a) Transport/Endpoint Layer

This layer cares for the initiation and termination of SIP sessions as well as for the conversion of data transmitted between analog/digital and IP packet formats. IMS terminals are, in fact, able to connect to the IP network through a variety of transmission media such as DSL, WiFi, GPRS, SIP or WCDMA. Furthermore, IMS devices are able to make and receive calls from PSTN or other CS networks thanks to the PSTN gateway.

b) Session/Control Layer

The orchestration of logical connections between several network components is performed in this layer. What is more, it provides routing of SIP messages, registration of end-points as well as the overall coordination of media and signalling resources.

At the core of this layer are located the Home Subscriber Server (HSS) database and the Call Session Control Function (CSCF).The HSS maintains an exclusive service profile for all users (e.g. preferences, registration information, voicemail options, etc.). By centralizing users’ data in the HSS, integrated personal directories and centralized user data management are enabled across all IMS services.

The CSCF, a SIP server, processes SIP signalling in the IMS.Different types of CSCF provide specific functionalities.

- P-CSCF: In the signalling plane, the Proxy-CSCF represents the first point of contact between the IMS device and (IMS) network. In other words, all requests originated by the IMS device or meant for it pass through the P-CSCF.
- I-CSCF: The interrogating-CSCF interfaces the Application Servers (AS) so as to (re)route requests destined to services rather than to users.In addition to the SIP proxy server functionality, the I-CSCF maintains an interface (based on the Diameter protocol) to the user databases: Subscriptor Locator Function (SLF) and HSS. Therefore, The I-CSCF acquires user location data in order to forward the SIP request to the proper target.
- S-CSCF: the Serving-CSCF, basically a SIP server, performs session control and provides SIP routing services. Furthermore, by keeping a link between the user location and its Public User Identity (e.g. the user’s SIP address), the S-CSCF operates as a registrar.

Abbildung in dieser Leseprobe nicht enthalten

Figure 3: IMS layered architecture

Source: Dialogic, The Architecture and Benefits of IMS

What is more, The S-CSCF inspects every SIP message and decides whether the SIP signalling, sent or received by the IMS terminal, has to visit one or more AS on the way to its final destination (In fact, these AS may provide a service to the user).

c) Application/Service Layer

The AS, a SIP entity that hosts and executes services, is located on top of the IMS architecture.For instance, the two other layers discussed above ensure an integrated (standardized) network platform enabling service providers to offer a great variety of multimedia services in this layer.This implies that AS are not only responsible for service hosting and execution, but also provide the interface against the control layers (S-CSCF and I-CSCF using the SIP protocol) and HSS (based on the Diameter protocol).

In the same manner, the IMS architecture supports a considerable assortment of services based on the Session Initiation Protocol(SIP). SIP was defined by the IETF as a protocol to establish and manage multimedia sessions over IP networks.Moreover, these services can be retrieved from different devices via the IP network or the classic telephony system.

Three types of AS can be distinguished:

- SIP AS: represents the native AS which hosts and executes IP multimedia services based on SIP.

- OSA-SCS (Open Service Access–Service Capability Server): provides an interface to the OSA framework AS, permitting the securely IMS access from external networks. This node performs as an AS (interfacing the S-CSCF with SIP) as well as an interface (between the OSA-AS and the OSA-API).

- IM-SSF (IP Multimedia Service Switching Function): based on this AS, CAMEL services, originally developed for GSM in the IMS, can be reused. Concisely, The IM-SSF operates in two ways:

o As an AS when interfacing the S-CSCF with SIP

o As a SSF when interfacing the GSM Service Control Function with a protocol based on CAP (CAMEL Application Part).

For further details on the IMS architecture and explicit component functionalities, please refer to the literature provided at the end of this thesis.

3 Evolution 2G/3G to IMS

3.1 2G/3G technology features

Concerns regarding higher capacity, global standard and multimedia represent major drivers of 3G systems.First, the issue of link capacity required to be addressed so as to tackle the quick development of mobile communications.Second, establishing a global standard became ever more significant since users expect location-independent data access.An additional reason was the fact that 2G system standards are, to some extent, regional standards.Third, the data transfer rate in 2G systems is indeed slow (about 9.6 kbps) if rich data like images/text are to be accessed.

Table 1 shows some 2G characteristics.

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Table 1: 2G technology: Capacities, advantages, and disadvantages of digital signals

Adapted from: Wireless Network Evolution 2G to 3G, Pearson, 2002

The development from 2G mobile/fixed networks towards 3G networks did not happen in a single step.The International Telecommunication Union’s(ITU) IMT-2000 global standard for3G represents an essential move, allowing a blend of novel and emerging mobile access technologies to cohabit with existing fixed-access as well as wireless technologies.It is within this context that industrialized and developing countries are capable to offer a large variety of data, voice and Internet services in a cost-effective way.

IMT-2000 (International Mobile Telecommunications) embodies the worldwide coordinated definition of 3G, involving central topics like technical standards and frequency spectrum use.Furthermore, IMT-2000 is the product of the group effort of entities such as ITU-R, ITU-T, 3GPP, 3GPP2. For instance, various radio technology options are incorporated in the IMT-2000 standard, thus permitting a seamless service development based on diverse 2G (mobile) standards broadly implemented over the world.

Some key characteristics of IMT-2000 (Source: ITU, Cellular Standards for the Third Generation) are:

- Flexibility: considering the internationalizations and consolidations within the mobile industry, operators were reluctant to maintain different technologies and interfaces.The IMT-2000 standard deals with this issue by providing a flexible system able to support a huge variety of applications/services: five radio interfaces can actually be accommodated based on CDMA, FDMA and TDMA.

- Compatibility: IMT-2000 services need to guarantee compatibility with available systems (e.g. 2G) based on seamless migration paths.

- Affordability: 3G systems must be reasonably priced so as to promote their acceptance by operators and consumers.

- Modular Design: IMT-2000 systems are required to be by far expandable with the goal of encouraging the expansion of coverage areas, subscriber base as well as innovative services.

All this leads to IMT-2000 systems supporting symmetrical/asymmetrical data transmission and worldwide roaming.Besides the high transmission rates for outdoor/indoor operations, a voice quality similar to the wireline quality has to be facilitated. Moreover, support should be given for CS, PS (e.g. IP traffic) and multimedia services.On this basis, economies of scale become viable through the adoption of open global standards.

Table 2 complements the information provided in Table 1 by outlining some features of the 2G and 3G technologies

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Table 2: Comparison of 2G-3G technologies

Based on:Taiwan 4G Networks, Evolution of Mobile Technology


Excerpt out of 54 pages


IMS Centralised Services
Voice services migration for a leading European telecom organization
University of Applied Sciences Technikum Vienna  (Electronic Engineering)
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ISBN (Book)
IP Multimedia Subsystem, IMS evolution path, IMS centralised services, IMS voice service migration
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MSc. Omar Amoretti (Author), 2013, IMS Centralised Services, Munich, GRIN Verlag,


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