Trends and Prospects of Network Convergence

In the middle of 1990s, a notion of the convergence of telecommunications, computer, and broadcast and TV was addressed in the industry along with the popularization of IP applications. However, the convergence of the three networks has not evolved substantially because of technical and administrative system factors. At the beginning of  2000s, the softswitchcentered Next Generation Network (NGN) architecture was addressed in the telecommunication industry based on the idea of the separation of IP phone gateway functions, which is the early NGN.

    The revolutionary contribution of NGN is that it uses a hierarchical architecture in which service, control, bearer and access functions are separated from each other. It replaces the traditional vertical service provisioning mode with the horizontal service provisioning mode, which greatly facilitates the fast development and effective provisioning of new services, with the support for multimedia services including voice, data and video. The early softswitchcentered NGN architecture is suitable for the traditional fixed networks such as Public Switched Telephone Network (PSTN) to evolve to NGN. The 3rd Generation Partnership Project (3GPP) introduced the concept of mobile softswitch in 3GPP Release 4 based on the concept of fixed network softswitch, and addressed IP Multimedia Subsystem (IMS)-centered Third Generation (3G) mobile communication network architecture in 3GPP Release 5. The fact that IMS will replace softswitch and become the core control technology for the NGN service layer is widely recognized in the industry. The IMSbased NGN represents the future trend of Fixed Mobile Convergence (FMC).

1 Concept and Content of Network Convergence

1.1 Concept of Network Convergence
Convergence is an inevitable trend of network development in the future， covering a wide range of subscribers, carriers, network technologies, devices and solutions. For subscribers, they wish to be provided with personalized services anywhere. For carriers, they wish to provide the subscribers with fullservice operations in a universal way to increase their market shares and profits.

    Network convergence uses general and open technology to achieve the merge or convergence of different networks or network elements[1]. Convergence allows full leverage of network resources and reduction of operational cost, thus increasing the competitive force. Convergence also allows the provisioning of various services and onestop service for the subscribers. Subscribers can access the same services whether they are in a fixed or mobile environment. In addition, convergence brings carriers opportunities to increase revenues and lower risks in providing new services; it also allows them to be fullservice carriers quickly.

1.2 Content of Network Convergence
Network convergence includes the convergence of service, core network, access network, terminal, and operation and maintenance.

    Service convergence means the carrier binds multiple separated services together to provide subscribers with onestop service at lower prices. Service binding provides a commercial convergence or business operationlayer convergence that does not involve a lowerlayer network technology. Services are related to access network and terminal. The benefit is to fully leverage the existing network resources to meet the subscribers’ demand for the unified communication service without the need to reconstruct the existing network’s architecture. Such convergence began a long time ago. American carrier, Bell, launched binding service in 2002 which bound the local call, longdistance call, Internet and mobile services together at a discount using one number and one account.

    The convergence of the core network involves multiple layers, including core bearer layer, core control layer and service layer.

• The convergence of core bearer layer allows a unified IP/MPLSbased core bearer network to become a multiservice converged network which supports all of the wired and wireless, mobile and fixed, voice, data and multimedia services. The IP/MPLSbased convergence can simplify the hierarchical network architecture and save costs of network operation and maintenance. In addition, the IP network using MPLS can provide high Quality of Service (QoS) and security assurance.
• The convergence of core control layer provides various multimedia services and terminal users with unified session control and management functions by using softswitch or IMS technology. However, the trend is to achieve the convergence, and thus provide the futureoriented nextgeneration converged network via IMS due to the large differences of functions and protocols between the mobile and the fixed softswitch networks.
• At the service layer, the open service environment using NGN allows service provisioning to be independent of the lowerlayer network resources, and thus provide unified convergence services to the users accessing from various Access Networks. The widely accepted NGN architecture, in which service and control are separated, and the widely supported Parlay/Open Service Architecture (Parlay/OSA) Application Programming Interface (API) provide technical assurance for the service convergence, making service development easier, faster and more economical. Actually, service convergence is the ultimate goal of the network convergence, where service types can be simple services or tariffcombined services, or deeply converged services, such as the PushtoTalk over Cellular (PoC), based on presence service.
  

    The convergence of access network is also an important part of the overall network convergence. The future access network includes various wired, wireless, broadband and narrowband access networks that ensure network coverage maximally. Therefore,  the key points of the access network convergence are constructing IPbased public bearer platform, achieving the converged access of broadband/narrowband and wired/wireless services, and the seamless switchover among various access networks. Terminal convergence provides a terminal with support for various access technologies and services, such as the support for both wired and wireless access, as well as for the access of voice, data and multimedia services.

    The convergence of operation and maintenance is mainly required for commercial use. It allows a carrier to use a unified billing and customer care system, and operation and maintenance system for its own fixed and mobile networks. Such unified systems provide users with unified management interfaces such as unified subscription service and unified bill service, as well as allow unified user data storage and management. The convergence of subscriber data will allow more customized convergence services such as mixed telephone number service, number portability, and unified number services.

2 Trend of IMSbased Core Network Convergence
FMC has become a hot topic in the industry in the recent two years. The convergence of core network is the heart of FMC and also the most significant step. It includes convergence in the core bearer layer, core control layer and service layer. The development of MPLS and IMS architecture has paved a way to FMC. MPLS helps to solve QoS, security and traffic engineering problems in IP network services. IMS is widely supported with its advantages, and is regarded as the only way to the convergence of mobile and fixed networks and the NGN network.

2.1 IMS Hierarchy and Characteristics
IMS is a subsystem addressed by 3GPP in 3GPP Release 5 to support IP multimedia services. It is based on the Session Initiated Protocol (SIP) architecture. It creates, manages and terminates various multimedia services using a SIP call control mechanism. It is an accessindependent network architecture based on full IP packet transmission, as shown in Figure 1.

    As a basic architecture for network convergence, the IMS system includes four layers, namely, service layer, core control layer, core bearer layer and access layer, from top to bottom.

    The service layer supports three classes of application servers—Parlay/OSA application server, SIP application server and Customized Applications for Mobile network Enhanced Logic (CAMEL) application server—which are used for different types of applications. The core control layer controls and manages sessions based on SIP, including network terminal registration and SIP signaling message routing. The core bearer layer provides bearer service for various services based on the
IP/MPLS packet network. The bottom layer is the access layer. It provides the IPCAN access function that allows various terminals to access the IMS core network.
Functional entities in IMS include Call Session Control Function (CSCF) which includes ProxyCSCF, InterrogatingCSCF, and ServingCSCF, Breakout Gateway Control Function (BGCF), Media Gateway Control Function (MGCF), Home Subscriber Server (HSS), Subscription Locator Function (SLF), Media Resource Function (MRF), IMSMedia Gateway (IMSMGW) and Signaling Gateway (SGW). IMS ensures the separation of service provisioning from network control by providing open service interfaces between the service layer and the core control layer.

    As the core of the nextgeneration converged network, IMS has the following features:

• Enhanced hierarchical open architecture, making the network architecture clearer and reasonable;
• Access technologyindependent service development, allowing fast service update;
• SIP for unified multimedia service control, achieving real endtoend IP communication;
• Independent of access after further improvement;
• Supports mobility;
• Centralized user data management and maintenance via unified HSS, allowing easy binding of number with user;
• Supports flexible billing modes and provides basic session information including service type, service flow and service time period for the service providers to establish different billing policies.

2.2 Reason for IMSbased Network Convergence
As the softswitch technology emerged, it was considered that it could be used for core control layer convergence. However, softswitch was an evolution solution developed originally for the fixed network. It was retrofitted for the mobile network later, and a mobile softswitch based GSM network evolution solution was addressed. Although the architectures are basically similar, the fixed and mobile softswitches have considerable differences with regard to function and protocol, which lowers the possibility for softswitch to become the focus of the core control layer convergence.

    IMS is developed for the 3G mobile network, but it uses the same SIP architecture as the fixed network and has advantages including the access independent feature, support for user roaming and centralized user data management. Therefore, it is possible to achieve network convergence using IMS. Specific reasons include:

    (1) IMS follows the trend of IP for network and multimedia for service. IP represents the convergence point of technology convergence. IP can perfectly meet the bearer requirement for multimedia convergence service via the combination of technologies such as MPLS. Multimedia represents the convergence point of service convergence.

    (2) IMS meets the requirement of separating NGN service, control, bearer, and access functions. It is an extension of softswitch. It subdivides control functions based on softswitch to form a more flexible communication control platform. It allows multimedia communications between the user and the content server, as well as multimedia communications between users.

    (3) IMS has been supported by many standardization organizations. It has been developed and improved continuously which greatly accelerates the process of standardization, making it possible for IMS to become the core control technology for unified networks in the future. 3GPP/3GPP2 defined the network components and basic architecture of IMS. Organizations including 3GPP/3GPP2, ETSI Telecommunications and Internet Converged Services and Protocols for Advanced Networking (TISPAN) and ITUT are studying IMSbased NGN convergence solutions, making IMS to be a SIPbased universal control platform with support for various accesses of both fixed and mobile networks, thus achieving overall network convergence. The Internet Engineering Task Force (IETF) defined SIP and Session Description Protocol (SDP) in the IMS framework and their extension. The Open Mobile Alliance (OMA) defined a series of services in the IMS architecture such as Instant Messaging (IM) and PushtoTalk (PTT).

    (4) IMS uses HSS at the core control layer. It supports user roaming and switchover by using the mobility management technology and centralized network databases.

    (5) IMS is located at the core control layer and allows the control and management of sessions. Users can access via any of the IP access networks such as GPRS, Universal Terrestrial Radio Access Network (UTRAN), Wireless Mesh Network (WMN), Wireless Local Area Network (WLAN), Wideband Code Division Multiple Access (WCDMA), CDMA2000, Digital Subscriber Line (DSL) and Cable. All these access networks are called IPCAN. That is, the core control layer provided by IMS is independent of access; therefore, it can achieve real convergence of fixed and mobile networks.

2.3 Architecture of the IMSbased NGN Converged Network
Figure 2 illustrates the architecture of the IMSbased NGN converged network[2]. The goal of IMSbased NGN is to provide a safe and reliable IPbased multiservice and multiaccess network with support for realtime and nonreal time services. It supports centralized storage of user data; service applications are provided in a unified mode.

    In the IMSbased NGN architecture, two service subsystems are defined at the service layer, i.e. PSTN/ISDN emulation subsystem and IMS. This PSTN/ISDN subsystem emulates services for traditional telephone terminals connected to the IP network, which ensures the availability and consistency of all the PSTN/ISDN services. IMS is composed of all the core network units framed on the packet network for providing IP multimedia services.

    At the access layer, the support for the fixed network access is added on the basis of IMS to provide traditional PSTN/ISDN services to traditional terminals. Both the Network Attachment Subsystem (NASS) and the Resource and Admission Control Subsystem (RACS) are developed to enhance the IMS access capability and QoS assurance. NASS functions include IP address allocation, access authentication and authorization, service data configuration and management, and relevant location management. RACS provides the resource and admission control and gateway control functions, and it also checks the authorization of subscription data saved in NASS.

2.4 Key to the IMSbased Core Network Convergence
As viewed from the progress of the current researches, IMS standards for the mobile core network have matured and can provide support for both 2G and 3G mobile accesses. However, the technical standards for IMSbased network convergence are still in the developing stage.

    There are many differences between the wired and wireless networks in different aspects including network bandwidth, terminal authentication, location information and resource management. Hence, the differences between the fixed and mobile networks still exist in the aspects of service capability, architecture, protocol and coding[3]. All these problems will become the key to the NGN Core Network convergence via IMS. Therefore, it’s necessary to extend and amend 3GPP IMS specifications. TISPAN is being extended in these aspects to achieve fixed access.

    IMS has been recognized as a converged NGN core network technology. Some convergence service providers are performing IMS tests, and hope to achieve the convergence of fixed and mobile networks via IMS. However, IMS faces a series of challenges in aspects of technology, standard and operation policy. The IMSbased network convergence still has a long way to go.

3 Trend of the Convergence of Access Network and Terminal
The access network convergence and terminal convergence are also important parts of the overall network convergence.

    Although the core network has implemented the convergence at the service, control and bearer layers, various useroriented access networks including wired, wireless, broadband and narrowband networks still exist, which ensure network coverage to the maximum extent. As viewed from the wireless access technique, the convergence is implemented as 2G/2.5G/3G access network convergence and terminal convergence, CDMA and WCDMA convergence, GSM/WCDMA and GSM/CDMA terminal convergence, and the convergence of mobile access and fixed wireless access. As DSL/WLAN/WiMAX/WMN is maturing, the future converged network should also support DSL, WLAN, WiMAX and WMN accesses and corresponding services, and solve the problems on user roaming.

    In addition, stream media technologies, like digital television, are used more and more widely; so, the future converged network should support stream media technologies, such as Digital Video Broadcast (DVB), and the provisioning of relevant services. Therefore, the construction of IPbased public bearer platform, the converged access of broadband/narrowband services and wired/wireless services, and the seamless service switchover between various access networks, are the key points of the access network convergence.

    Terminal convergence allows a terminal to support various access technologies and services such as both the wired and wireless access, as well as the access of voice, data and multimedia. Terminal convergence is proceeding to a certain extent as some multimodule terminals (such as 3G/WiFi dualmodule mobile phone) emerge. For the access using converged terminals, the terminals should support seamless switchover between different access networks. This requires relevant devices in the core network, such as the Home Location Register (HLR) in the GSM network and HSS in the 3GPP IMS network, to identify which network a terminal accesses.
Therefore, strictly speaking, terminal convergence will be gradually demanded after network convergence.

4 Conclusions
The ultimate goal of the future network convergence is service convergence. As viewed from the trend of network convergence, 3G services and the other existing services will be carried on a unified IP/MPLS core bearer network. IMS will be used at the core control layer, while a unified, open service provisioning architecture will be used at the service layer. The demarcation among the traditional fixed network, mobile network, broadband Internet, and even the cable television network, will disappear as the network convergence evolves further. IMS will be gradually used in the core network, and diversity and IP will be the trend of the access network, which supports multiple access modes. Terminals will be designed to be
multimodular and intelligent[4], while network carriers will be able to provide full services, which allows a user to access services anytime and anywhere using one number, one terminal, one account and one core network.

References
[1] 3GPP TS 23.228 v6.5.0. IP Multimedia Subsystem (IMS) Stage 2 [S]. 2004.
[2] ITUT Y.NGNFRA. NGN functional requirements and architecture of the NGN [S]. 2006.
[3] Zhang Yuan, Analysis of Key Technical Issues on the IMSbased Network Convergence [N].  People's Posts and Telecommunications News, 20060307.
[4] Zhang Xueli, NGN Service Provisioning Modes and Development Trend [J]. ZTE Communications, 2006, 12 (5): 710.

Manuscript received: 20061205

[Abstract] The concept of the separation of service, control, bearer and access in the Next Generation Network (NGN) provides a unified architecture for the network convergence in the future. Network convergence involves service, core network, access network, terminal, and operation and maintenance convergence. In the future, the converged network’s trend is to use IP Multimedia Subsystem (IMS) at the core control layer; Internet Protocol/Multiple Protocol Label Switching (IP/MPLS) technology at the core bearer layer; unified and open service provisioning architecture at the service layer; as well as support fixed, mobile, narrowband and broadband access technologies and so on at the access layer. Terminals will evolve to be multi-modular and intelligent, and would finally enable users to access full services anytime and anywhere using one number, one terminal, one account and one core network.