Solutions to Convergence of Mobile and Fixed NGNs

1 Development Trend of Communication Networks

The development of communication networks from PSTN to NGN has become a hot topic in the telecom industry in the last two years. On the one hand, ZTE and well-known foreign equipment suppliers such as Ericsson and Lucent have all presented their NGN solutions; on the other hand, leading telecom operators in the world are deploying their Softswitch-based NGN commercial networks, and China Telecom and China Netcom have also initiated testing projects for their NGN trial networks. These are the prelude to the evolution of communication networks from PSTN to NGN.

In Europe, ETSI once took NGN as a concept to define and deploy the network. NGN´s Softswitch-based control system ensures the application flexibility, while its IP-based switching system enables the convergence of multiple services. Therefore, NGN is the basis of realizing the convergence of the telecom, Internet and broadcasting networks. As far as network operation and market competition are concerned, NGN contributes to the realization of less fixed assets investment and lower operation cost, therefore, it can meet the demand of quick network deployment and service diversification in the future.

Like the development of fixed network technologies, open services access system (OSA) of 3G, as a concept aiming at services similar to NGN, has been put forward. OSA enables services independent of the network, and ensures the network provides a standardized application platform. OSA, on the one hand, enhances the flexibility of service development; on the other hand, it provides standardized access means, such as the OSA application-programming interface (API), for the employment of services. Moreover, with the development strategy of R99-R4-R5, the 3G network has been definitely positioned as mobile NGN based on IP.

2 Factors Driving Convergence of Mobile and Fixed NGNs

Providing diversified service applications is the main driving force for the development of the fixed NGN and also the original intention to develop it. However, the current key application of the fixed network is still the voice service, and it is perfectly performed on the existing PSTN. Therefore, PSTN still has a long life cycle. Although the milestone achievements of NGN commercial and trial tests have been attained, the standardized API on the available NGNs is not satisfying, and the services NGN currently provides are still the basic and supplementary services of PSTN and Centrex, IN services and simple multimedia services, all of which can be realized on the current PSTN. At present, NGN with the characteristic of service driving is not able to present its power to provide diversified applications. The development of NGN is facing two main problems: lacking massive successful experiences and without distinct value-added services.

In addition to these facts, in the field of mobile communications, China Mobile´s rapid development in recent years has attracted a lot of attention, and become the brightest light on the Chinese telecom market. The products of “Monternet” and “Uninet” are evidence of the gradual formation of a virtuous circle of mobile data services. As a matter of fact, the flexibility of mobile communications makes the birth of diversified services more rapid and extensive.

With the telecom reform in China, a few local giant telecommunication operators have come into being. Although they are presently not all-service operators, the nation will deliver 3G licenses in the future, which indicates that fostering all-service operators will be a policy guideline. It is expected that the emergence of all-service  operators will be the most forceful factor driving the convergence of mobile and fixed NGNs.

The converged NGN makes it possible for operators´ wired and wireless access systems to work on a common NGN core network, which will greatly reduce the investment in the construction of NGN. It will also enable operators to integrate the application platforms of mobile and fixed services to improve their business flexibility and enhance their overall competence. Therefore, for an all -service operator, to develop a unified NGN is a certain trend.

Terminal users also benefit from the convergence by enjoying communication anytime and anywhere, and more flexible communication with more choices. Due to the prosperity of the mobile market, the convergence will bring more advantages to the development of fixed NGN. In a word, the convergence of mobile and fixed NGNs is the development trend of communication networks.

3 Solutions to Convergence of Mobile and Fixed NGNs

With a technical view, NGN is not the patent of the fixed network. The primary difference between the mobile and fixed networks is their different access modes which, however, are all included in  the multiple access modes of NGN. There is great dissimilarity between the networks because of their different access modes. The roaming of mobile network users determines that, when compared with the fixed Softswitch, the mobile Softswitch as the core control layer of mobile NGN has not only to handle call control, but also to take charge of such special functions as location management, database management and handoff control. However, fixed and mobile Softswitch are essentially the same, both of them embodying the idea of making the service/control part separate from the transmission/access part.

Softswitch is the key technology for NGN. Its core principle is to use software to realize the functions of hardware, that is to say, to realize PSTN functions such as controlling, transferring and service handling by software. All entities are connected and communicate via standardized protocols, which helps the quick realization of various complicated protocols and conveniently provide services. Therefore, technically, the convergence of mobile and fixed NGNs can be realized.

As shown in Figure 1, the convergence of mobile and fixed NGNs should be realized at all levels of mobile and fixed networks:

• Converged core control—the unified Softswitch platform.
• Converged service applications—the unified mobile and fixed service platform.
• Converged core transmission networks—the unified platform based on IP.
• Converged access networks—the basis of NGN convergence.

3.1 Convergence on Core Control Layer

The Softswitch server, being classified into the user Softswitch server and the tandem Softswitch server, realize the functions of the core control layer. The requirements of mobile NGN on the tandem Softswitch server are similar to that of fixed NGN, which makes it possible to use a common tandem Softswitch server. However, the user Softswitch server of mobile NGN functions differently from that of fixed NGN, with two special functions: mobility management and handoff.

Thereby, two steps can realize the convergence at the service control layer. The first step is the realization of a common tandem Softswitch network, but the user Softswitch servers are still separated which, however, are required to be mutually accessible. Also, the Softswitch servers are requested to provide uniform interfaces to application layers. Then the convergence of the user Softswitch servers follows.

The Softswitch convergence plays a key role in the convergence of mobile and fixed NGNs, which ensures true convergence.

3.2 Convergence on Application Management Layer

NGN requires the separation of service operation from network operation, which means that the service provider is independent of the network provider and an open value chain is built up. Various parties on the chain together provide and employ services, and make profits. The open and standard API-Parlay API is the key technology realizing such a separation. Based on Parlay API, 3GPP proposed the OSA architecture for 3G. OSA provides service providers with open, standard and uniform network application programming interfaces. With OSA, personalized services are available to mobile terminal users. Moreover, with the separation of service deployment and network operation, any third-party service provider is able to take part in a fair competition, which is helpful in creating a multi-vendor environment and quick launch of new services.

It is obvious that both fixed and mobile NGNs aim to satisfy the same service demands. Via API, mobile and fixed NGNs are able to realize the convergence on the application layer, which is beneficial for all-service operators to make the most of their advantages and enhance their core competitiveness.

The convergence of mobile and fixed services is the ultimate goal of network convergence, and also the main profit-making mode for all-service operators in the future.

3.3 Convergence on Core Switching  Layer

Having the same requirements on the transmission network, fixed and mobile NGNs can share a common ATM/IP backbone transmission network.

With the gradual convergence of mobile and fixed-line services and the emergence of more diversified services provided by third parties, a unified packet core network based on IP/ATM becomes the basis of multi-service networks. It can optimize network resources, improve bandwidth utilization ratio, and consequently reduce the operation cost and expenditure, and enhance operators´ edge in competition.

3.4 Convergence on Edge Access Layer

The access technologies such as PSTN, IAD, xDSL and LAN are used in the fixed network, while GERAN and UTRAN are used in the mobile network. Besides, the function requirement of the mobile media gateway (MGW) greatly differs from that of fixed MGW, each performing distinct functions, such as user line monitoring in a fixed network and handoff and docking to a radio network subsystem (RNS) in a mobile network.(Functions of mobile MGW such as support for handoff and docking to RNS, which are in accordance with special characteristics of the mobile network, are greatly different from those of fixed MGW such as user line monitoring, which is in accordance with special characteristics of the fixed network.) Due to their different coding modes on the bearing layer, mobile and fixed NGNs require code conversion at the access gateway to realize interworking.

Two steps to realize the convergence of access gateways are suggested. The first step is to unify the MGW interfaces of both networks to support a common coding mode. In this way, the MGWs of both networks can be interconnected and make for easy access to a common transmission network. The second step is to realize the convergence of MGWs.

4 Conclusion: A Long-term Course to Realize Convergence

With the opening of the telecom market and all-service operation and participation of new operators, the market has become more and more competitive. However, building up an all-service NGN with a high starting point can effectively enhance operators´ core competitiveness. As to traditional operators, with more mature technologies available and users´ demand on diversified and personalized services, a development strategy from PSTN to NGN is inevitable. However, operators have to simultaneously operate PSTN and the packet network based on IP in the long term because PSTN is still their main profit source at present. At the same time, they must resolve the problems of network interworking and service interoperating between PSTN and IP networks, in order to complete the smooth transition from the traditional circuit-switching network to NGN. Therefore, the transition is a long-term and gradual course.

Similarly, WCDMA also has a gradual development course from R99 to R4, then to R5. The convergence of fixed and mobile NGNs will be truly realized only when both NGN technologies, network operations and business modes have properly matured.

As a famous equipment supplier in the telecom industry, ZTE has gone deep into the research on NGN and 3G. Based on its deep understanding of the trend of NGN convergence, ZTE will quicken the research and development in NGN convergence. So far, the unified development of fixed and mobile NGNs based on the NGN software platform has been in progress, which lays a foundation for producing converged NGN products in the future. ZTE´s mobile NGN hardware platform is compatible with its WCDMA R99 equipment, so the R99 equipment can be smoothly upgraded to R4, which will not only protect operators´ return of investment but also improve the maturity of software and hardware.