ZTE´s Softswitch-Based Reconstruction and Optimization Solution for Evolution of PSTN to NGN

ZTE´s NGN experts believe, the leading strategy for a traditional PSTN network evolving to NGN is to use the Softswitch technology to reconstruct and optimize different layers of the PSTN network, implementing its integration with the NGN network and finally complete the evolution to NGN.

    ZTE´s Softswitch-based solution for the evolution of the PSTN to the NGN focuses mainly on four parts:

• Reconstruction and optimization for end offices and module exchanges of the PSTN
• Reconstruction and optimization for tandem switches of the PSTN
• IP distribution of the toll traffic of the PSTN
• Reconstruction and optimization of the access network of the PSTN.

More information at:
http://www.zte.com.cn/English/03product/list1.jsp?CateID=281

    With the high-speed development of the mobile communication and Internet technologies, modern communication patterns, like mobile voice, IP voice, Instant Messenger, have made a huge impact on traditional Public Switched Telephone Network (PSTN) voice services. The traditional business model with voice as dominant services is being threatened. The PSTN carriers now face stern challenges and new value orientation requirements for both network and telecommunication operation modes. The development of Next Generation Network (NGN) has already brought new operation ideas and development for them. In recent years, NGN obtained a rapid development in the world. New NGN services penetrated into various aspects of the daily life. Now, voice is just a basic service of carriers, and the application and development of NGN value-added services is an essential requirement for carriers to survive from intense competitions. The global traditional PSTN carriers are seeking for the transformation of their networks, and expect to obtain new strengths to win competitions by getting their current networks evolving to NGNs. So, with the network development, it´s inevitable for traditional PSTN networks evolving to NGNs.

    ZTE´s NGN experts believe, the leading strategy for a traditional PSTN network evolving to NGN is to use the Softswitch technology to reconstruct and optimize different layers of the PSTN network, implementing its integration with the NGN network and finally complete the evolution to NGN. From the network architecture aspect, ZTE´s NGN solution for the PSTN network reconstruction and optimization focuses mainly on four parts: reconstruction and optimization for PSTN end offices and module exchanges, reconstruction and optimization for PSTN tandem switches, IP distribution of the toll traffic of PSTN, reconstruction and optimization of the access network. From the services aspect, ZTE´s NGN solution for the traditional PSTN network reconstruction and optimization includes using Softswitch technologies to provide NGN value-added service platforms for the entire network, providing universal NGN value-added services for all subscribers (the existing PSTN subscribers, PHS subscribers and mobile subscribers) in the network, but this aspect will not be discussed here.

1 Reconstruction and Optimization of End Offices of PSTN
A traditional PSTN has a large number of end offices with equipment from different vendors, and their service provisions are also very different from each other. In such a situation, it´s very difficult to provide intelligent services for the end offices of the PSTN. With its Softswitch technology, ZTE provides a complete solution to optimization and evolution of the end offices of the PSTN to NGN. The solution is implemented by directly replacing all the end offices with the NGN access gateways.

    The essence of this solution is, based on the Softswitch network, to use ZTE´s latest NGN integrated Media Gateway with powerful functions to substitute the C5 end offices. Therefore, the network overlaps with the existing PSTN network. The integrated Media Gateway, providing interfaces for twisted-line, V5 access networks, remote modules, and more, makes full use of the existing PSTN network resources to provide access to subscribers.

    Meanwhile, ZTE´s integrated Media Gateway also has integrated multimedia access capabilities to offer voice-and-data-integrated access services to subscribers. The toll/tandem services at reconstructed end offices can be connected to the toll/tandem of the PSTN through the integrated Media Gateway and Signaling Gateway of the Softswitch network. The solution is demonstrated in Figure 1.

    This solution has the following advantages:

• It uses the integrated Media Gateways to substitute for the end offices of the PSTN. The uplink access of the Softswitch network is implemented through the IP interfaces. Multiple ways such as the twisted-line, V5, E1 and Multi-Service Transport Platform (MSTP) are supported downlink. Therefore, this solution supports the integrated access of voice, the Integrated Services Digital Network (ISDN), the Digital Data Network (DDN) and the Digital Subscriber Line (DSL) services. This solution makes full use of the existing access resources of the end offices.
• After the replacement of the end offices with the integrated Media Gateways, the subscriber data in the old switches will be transferred to the Softswitch platform or to the iHLR (the centralized subscriber attributes database) to have a centralized management. Therefore, the true intelligence and mobility of the PSTN network are realized.
• The subscriber is able to enjoy various new value-added services that are provided by the NGN integrated service platform.
  

2 Reconstruction and Optimization of Tandem Offices of PSTN
The PSTN network has a large number of end offices with various functionality levels, so the evolution of the end office layer to NGN should be implemented step by step. However, how do the subscribers of the unchanged end offices access NGN seamlessly and enjoy NGN services at the initial stage of the evolution? ZTE suggests reconstructing and optimizing the tandem offices of the PSTN at the same time as the end offices are evolved to the NGN gateways.

    The solution uses ZTE´s excellent trunk media gateways to substitute for the existing tandems step by step. The media gateway is put at the location of the tandem offices. The end office equipment is connected to the trunk gateway, and the trunk gateway provides the function of tandem. New offices may adopt the NGN trunk gateways directly.

    The solution is shown in Figure 2.

    This solution has the following advantages:

• The PSTN and the NGN are completely converged. The subscribers of the end offices are able to enjoy NGN services. Mixed service subscription of the PSTN and the NGN is available. Therefore, the investments of operators can be fully protected.
• Data management and service provision for the subscribers of the end offices are provided by the Softswitch platform. The new services for the subscribers of the old end offices and access networks are triggered and converged to the media gateway. Therefore, they are offered by the Softswitch platform. This solution not only avoids the reconstruction and optimization of the end office but also allow subscribers to use various intelligent services and value-added services offered by the Softswitch platform. Therefore, the reconstruction cost and workload are decreased.
• ZTE´s integrated media gateway supports Time Division Multiplexing (TDM) switching. Therefore, the calls between the end offices it connects will not go through the IP bearing network. This reduces the requirements for the IP bearing network.
• With mixed TG/AG functions, ZTE´s integrated media gateway can be used in combined local/tandem offices. Its downlink interfaces include Plain Old Telephone Service (POTS), Asymmetric Digital Subscriber Line (ADSL), ISDN and more. Therefore, it is able to support integrated access of voice and data services, to connect with remote modules or units to implement subscriber access, and to offer V5 access to support the direct access of the subscribers in the existing access networks to the Softswitch network.
• ZTE´s integrated media gateway provides TDM uplink channels. In the initial phase of NGN operation, TMD backup channels for toll/tenderm services are offered to guarantee the network operation quality.
  

3 Distribution of Toll Traffic of PSTN
Along with the increase of intra-province/inter-province toll traffic in the PSTN network and the aging of toll equipment, the PSTN toll offices are now facing reconstruction and expansion. ZTE´s distribution solution of the toll traffic of the PSTN makes use of carriers´ abundant broadband backbone data network resources. The solution implements the optimization and reconstruction of the toll network of the PSTN and its gradual evolution to NGN.
As shown in Figure 3, ZTE´s Softswitch-based IP voice toll network uses the IP data backbone network of the traditional fixed network carrier as a bearer. The toll TDM media streams of the PSTN are converted into the IP media streams through the media gateway, and sent to the receiver-side media gateway that assigned by the Softswitch core equipment through the IP broadband data network, and then re-converted into the trunk media streams at the circuit side to finally implement the application of the toll voice service.

    The solution has such 4 advantages:

• The solution is able to be implemented in one time, while multiple construction projects are necessary for one PSTN network, such as the construction of local networks, tandem offices, toll offices, signaling networks and synchronization networks.
• Compared with PSTN equipmeot, the Softswitch network elements with much smaller size save equipment room and equipment investment greatly. Therefore, the project implementation is more efficient and convenient. Moreover, future investment in equipment room and maintenance in the following network construction is reduced.
• With the Softswitch technology, the second toll network is easy to be set up. It guarantees the reliability of toll operation by traffic divergence with the original toll network or as a backup of the original one.
• It is easy to provide new value-added toll services through the value-added service platform of the Softswitch-based NGN.
  

4 Solutions for Evolution of  Access Network to NGN
The typical architecture of the narrowband/broadband integrated access network is composed of several Optical Network Units (ONUs) and one or several Optical Line Terminals (OLTs). The ONU implements service access and the first convergence, and then the built-in transmission modules implement the following service transmission. The OLT separates the PSTN/DDN services from data services and connects them with the backbone network through different interfaces.

    By network reconstruction, the traditional access network can directly evolve to the NGN. ZTE provides 2 complete Softswitch-based solutions for the evolution.

(1) Mixed networking by replacing ONU with integrated media gateway
    This solution has perfect integrated media gateway functions. It provides user interfaces such as POTs, VDSL, ADSL, ADSL2+, ADSL+POTS, ATM SHDSL, ISDN (BRA , PRI) and DDS (V.24, V.35). Therefore, the ONU functions can be completely replaced. For the optimization and reconstruction of the access network, the integrated media gateway is able to be used as a substitute for ONU, and provides direct uplink connection to NGN network through IP interfaces. Moreover, the Softswitch system provides abundant NGN services. This mixed networking solution is shown in Figure 4.

    The integrated media gateway may also act as ONU to networking with other ONUs. The voice service is compressed at the media gateway with other broadband services and transmitted to OLT through the MSTP. Then the OLT separates it from the data services. The separated voice service will, on uplink, connect to the Softswitch network through V5 interfaces.

(2) Networking by replacing OLT with integrated media gateway
    Another solution is to use the integrated media gateway to replace OLT. It is able to implement the functions of the trunk gateway. On the uplink, the IP interfaces help to implement direct access of the Softswitch network. The integrated media gateway cascades ordinary subscriber frames, ordinary ONU or Multiple Service Access Network (MSAN). It supports various built-in transmission modes (SDH, MSTP, PON, etc).  Additionally, it supports remote networking. The solution is shown in Figure 5.

    The integrated media gateway may also work as an independent OLT in the existing integrated access network. It connects to other integrated media gateways through V5 interfaces and implements access of the Softswitch network, or of the original PSTN network.

5 Conclusions
Based on full protection of carriers´ existing investments on networks, ZTE´s reconstruction and optimization solutions for evolving the traditional PSTN to the NGN implement the gradual network evolution and convergence. Moreover, with flexible service provision of the NGN, the solutions help carriers conveniently and quickly offer personalized value-added services that integrate data, voice and multimedia services. With successful applications in many countries and regions, the solutions have obtained wide recognition and support from telecommunication carriers worldwide.

Manuscript received: 2005-03-01