Evolution from Fixed Softswitch to IMS

    The Softswitch (SS) concept was gradually developed on the basis of the IP phone system in the late 1990s, and was perfected in the transition of telecommunication networks from narrowband to broadband and from the circuit switched to the packet switched. It inherited the centralized control mechanism and reliable signaling technology of telecommunication networks, and employed the layered structure to enable the separation of call control and media processing.

    After the emergence of the SS concept, China Communications Standards Association (CCSA) promptly introduced the “SS” technical term, and has actively been involved in the standardization work on the SSrelated devices and protocols. Through several years of development driven by equipment vendors and operators, SS has found a wide range of commercial use in operators like China Telecom, China Mobile and China Netcom. These operators have used the SS technologies to implement the evolution to NGN from PSTN, and to maintain a continued growth of business revenue and lower the total operation cost through the introduction of new services such as Color Ring Back Tone (CRBT) and Unique and  Universal Access Number (UAN).

    The IP Multimedia Subsystem (IMS) was originally proposed for the core network by the 3rd Generation Partnership Project (3GPP) Release 5 (R5), and was further improved and expanded by the 3GPP Release 6 (R6). One important feature of IMS is the complete separation of Call Session Control Function (CSCF) and Media Gateway Control Function (MGCF) at the control layer, and a more open and flexible network architecture will accordingly be provided. Another important feature of IMS is that the Session Initiation Protocol (SIP) is used as the call control protocol. Being a subsystem oriented to multimedia applications and put forward by the 3GPP, IMS takes the mobility support and multimedia applications into full consideration from the beginning. Driven by the current trend towards network convergence, the Telecoms and Internet converged Services and Protocols for Advanced Networks (TISPAN), an NGN research organization of the European Telecommunications Standards Institute (ETSI), accepted IMS in its NGN Release 1 (R1) as the core technology for NGN fixed applications, and is prepared to emphasize the IMSrelated specifications in the 3GPP R6. Similarly, the Focus Group on Next Generation Networks (FGNGN) of the International Telecommunication Union  Telecommunication Standardization Sector (ITUT) used and developed the IMSbased NGN architecture. The FGNGN has rolled out IMSbased PSTN Emulation/Simulation Subsystem (PES/PSS) standards, and is now conducting a research on IMSbased IPTV architecture.

    The IMS architecture is not only accepted as the NGN core network technology by international mainstream standardization organizations such as the ITUT, 3GPP and ETSI, but also supported by international leading equipment vendors and operators. All the equipment vendors claim that they will develop NGN systems based on the IMS architecture; operators are also  involved in IMS testings and trials[13].

1 Comparison of IMS and SS
Being functional entities of the NGN core control, both SS and IMS can provide the call and connection control functions for realtime services. They are the core of call and control in the NGN. The SSbased NGN and the IMSbased NGN have quite similar system architectures, which consist of the service, control, access and bearer layers. Figure 1 shows an architecture comparison between the IMS and SS systems.

    At the service layer, both SS and IMS can separate call control from service application, and have an important feature of providing open service interfaces oriented to the third party.

    SS offers new valueadded services via the Application Programming Interface (API) and the application server. Besides, provisioning basic call services and PSTN supplementary services is logically integrated into SS. Through the Intelligent Network Application Protocol (INAP), SS can work with the existing Service Control Point (SCP) in the Intelligent Network (IN) to deliver conventional IN services. In order to support PSTN intelligent services, SS can be connected to the Smart Home Location Register (SHLR) via the Mobile Application Protocol (MAP) interface to implement centralized management of user service data.

    The IMS architecture defines an IMS Service Control (ISC) interface as the service control reference point for interconnection to the SIPbased application server. The ISC interface enables service applications to be individual or combined, and facilitates interoperability between different service application servers. Accordingly, it ensures interoperability of the entire network and service terminals. IMS itself doesn’t integrate any service logics. PSTN service logics are completed by independent PES/PSS Application Servers (APP) via the ISC interface. Moreover, IMS itself doesn’t manage any user data. To implement unified management of user profiles and service data, all user data are connected to the Home Subscriber Server (HSS), which is also called the User Profile Server Function (UPSF) by TISPAN, via the Diameter interface.      

    At the control layer, SS uses one functional entity to complete such functions as call processing and control, access protocol adaptation, service interface provision, interoperability and interworking, user data management, and PSTN service logic. However, IMS fulfills the mentioned functions by more independent functional entities including the Call Session Control Function (CSCF), Media Gateway Control Function (MGCF), Access Gateway Control Function (AGCF), Media Resource Control Function (MRCF), and Breakout Gateway Control Function (BGCF). The core CSCF is further divided into Proxy CSCF (PCSCF), Serving CSCF (SCSCF) and Interrogating CSCF (ICSCF). The PCSCF is the first contact point for the user terminal within IMS. The SCSCF actually manages the session states in the network. The
ICSCF is the contact point within an operator’s network for all IMS connections destined to a subscriber of that network operator, or a roaming subscriber currently located within that network operator’s service area.

    At the access layer, SS and IMS can both support the separation of access and control. Their access devices are quite similar, and can even interoperate with each other. The H.248 Trunk Gateway/Signaling Gateway (TG/SG) and the user Access Gateway (AG) in SS can completely correspond to the Media Gateway/Signaling Gateway (MGW/SGW) and the Access Gateway (AGW) in IMS. Furthermore, the Media Server and Media Resource Function Processor (MRFP) have basically the same function. In IMS, there are some mature standards for access of mobile SIP terminals, while the standards for access of PSTN subscribers are still in improvement.

    At the bearer layer, IMS proposes an endtoend allIP architecture. To enable endtoend QoS, it also introduces the Network Attachment Subsystem (NASS), and the Resource and Admission Control Subsystem (RACS), also called the Policy Definition Function (PDF). However, SS puts emphasis on the IP architecture of the core network to ensure evolution from the existing network to an allIP network; thus, the QoS guarantee depends on the IP network itself.

2 Strategies for Evolution from SS to IMS
SS has been widely applied in existing networks, while IMS has been accepted in the industry as a future fullservice network target based on the Fixed Mobile Convergence (FMC). Therefore, it is an inevitable trend for SS to evolve into IMS. Due to the restrictions of IMS technical development and existing network conditions of operators, appropriate strategies should be chosen for the evolution. 

    (1) Upgrade SS to IMS Subset 
    Operators with mobile services as their main business can build new IMS networks by introducing multimedia services at the mobile side. The existing SS can be upgraded to the AGCF, MGCF, SGF, and PES/PSS APP in the IMS domain to implement PSTN services and interoperability. 

    (2) Upgrade SS to the IMS Full Set 
    Fixed network operators with PSTN services as their main business have a demand of business expansion to mobile and multimedia services. They can select SS, in an urgent need of the expansion, to be first upgraded to the IMS full set, and gradually implement the access to mobile and multimedia services while leveraging all the existing SSbased services. When the traffic of mobile and multimedia services grow steadily, all the SSs can be upgraded to the IMS full set.

3 Scenarios and Steps for Evolution from SS to IMS
A certain Chinese mainstream fixed network operator is here taken as an example to introduce the scenarios and steps for the evolution from SS to IMS.

3.1 Status Quo of SS Network
The fixed network operator started the research and testing of SS devices in 2001. Now it has widely adopted the SS technologies at each layer of its network, and completed the upgrade and improvement of the original PSTN network, as shown in Figure 2. The SS technologies are chiefly applied for: 

    (1) The broadband service platform and VIP customer platform, used for offering broadband multimedia services and interregional VIP customer services;  

    (2) The fixed intelligent tandem to make the local network intelligent;

    (3) The Toll Tandem and International Gateway (DC1).

3.2 Upgrade of SS on Broadband Service Platform or VIP Customer Platform
       to IMS
Currently, the SS network on the broadband service platform has relatively few subscribers. As the SS network is independent, subscribers are basically carried over the broadband IP network. IMS has a stronger support of scalability of the multimedia capability and followup standards than SS. Therefore, the current SS devices on the broadband service platform or VIP customer platform can be upgraded to IMS in the current stage. This IMS network can provide the subscribers with featurerich fixed broadband multimedia services, and gradually offer mobile multimedia services.

    The upgraded IMS network interoperates with the existing PSTN as well as with the fixed intelligent SS network.

    The evolution steps are described below:
    (1) The CSCF/MGCF and PSS APP functions are added to the SS on the service platform. The original SS supplementary service module is upgraded to the PES/PSS APP to implement PSTN supplementary services. The original H.248 module is upgraded to the MGCF to interoperate with the PSTN network. The CSCF module function (such as P/I/SCSCF) is implemented by extending boards and adding configurations of SS equipment, and makes unified call control for fixed and mobile multimedia subscribers.

    (2) A new HSS is added to store broadband user data and trigger services via the SCSCF to the PES APP. The standard Diameter interface is adopted between the SCSCF and the HSS. This new HSS may be integrated into the 3G Home Location Register (HLR) during the building of a 3G network.

    (3) Software for the existing SS service platform is upgraded to support IMS service capabilities. So the platform can provide new IMSbased broadband multimedia services while maintaining the original service offerings.

    (4) A certain SS board in the upgraded IMS equipment can be used to handle PES services. The PES services delivered by the SS board are completely not affected by IMS.

    (5) The upgraded IMS has a capability of delivering multimedia services in the mobile domain. It enables introduction of 3G network services such as video sharing, Push to Talk over Cellular (PoC), and Soft Digital Assistant (SoftDA), as well as of 2.5G General Packet Radio Service (GPRS) such as SoftDA Instant Messaging (IM) and Presence Service (PS).

    (6) Streaming media services such as IMSbased IPTV and Global Eeye can be introduced at a later stage.

3.3 Upgrade of SS on Fixed Intelligent Tandem to IMS
After the PSTN network of the operator was improved with intelligence, it has fully satisfied the service demand of current narrowband subscribers. However, with the increase of broadband subscribers and the commercial deployment of 3G mobile networks, multimedia services will grow rapidly, and the operator is changing itself from a single PSTN service provider to a fullservice provider with PSTN services as its main business. However, the maintenance of multiple networks will certainly increase the cost of the operator. In order to improve market competitiveness and reduce the operation and maintenance cost, it is necessary for the operator to integrate fixed, mobile, narrowband voice and multimedia services into one network. It is a feasible way for the operator to upgrade the existing backbone SS to the IMS network.       

    The evolution steps are described below:

    (1) SS is upgraded to IMS to offer a small quantity of multimedia services at the initial stage when multimedia users and services are introduced. The introduction of IMS can be implemented by expanding the capacity of the processing boards and frames on the premise of keeping the original SS processing boards and their functions. After the upgrade, a new IMS architecture is employed for broadband services, while the external network management and billing interfaces for narrowband voice services remain unchanged. The SS and IMS processing boards have no impact on each other because the SS processing board is completely separated from the IMS board. Moreover, external interfaces of the upgraded device, including interoffice routing and offsite disaster tolerance, remain unchanged and no alternation is made to other SSs connected to the device. The
newlyadded IMS network adopts the IMSbased offsite disaster tolerance. At the initial stage, the HSS can coexist with the upgraded SHLR on the same physical platform. 

    (2) The service platform is inherited and evolved. The SS narrowband service logic is evolved into the PES/PSS functional module that can fully inherit the existing intelligent services through interworking with the service exchange functional module of the SCP. The original unified SS service platform is smoothly evolved, inheriting and further expanding multimedia applications. 

    (3) With the introduction of 3G mobile networks and increase of fixed multimedia subscribers, the traffic of multimedia services rapidly increases, while narrowband voice services appear to shrink. Under this situation, the idle SS processing boards may be upgraded into the relative network elements of IMS to enable smooth device migration. As the SHLR, PHS HLR, HSS and 3G HLR can be completely integrated on one platform and share one database, services such as Number Portability (NP) can be delivered through data correlation.      

    (4) Streaming media services such as IPTV are gradually introduced in the IMS architecture.

    (5) All the SS processing boards are upgraded to the AGCF or MGCF at a proper time: after the IMS architecture standards and interfaces are specified, functional modules are standardized, and the traffic of narrowband voice services gets much smaller than that of multimedia services, and the narrowband voice services are offered by the IMSbased PES. Therefore, the network is unified, which is a good basis for further operation optimization. 

    It is unnecessary to evolve the fixed intelligent SS to IMS promptly, considering the stability and security of the existing SS network, as well as the maturity of IMS. The evolution needs to be taken into account when there are a large number of broadband subscribers, and when IMS gets more mature. The SS should have the ability to smoothly evolve into IMS.

    The fixed network operator now takes fixed voice services as its main business, and will probably introduce mobile network and multimedia services in the future. However, the fixed line services will still dominate for quite a long time. An optimal option for the development of Chinese telecommunication networks is to build fullservice networks based on the backbone fixed line networks. At the present stage, developing multimedia and mobile networks, centered by the existing fixed intelligent networks, is a better way to save investment cost and facilitate network optimization.

3.4 Upgrade of SS on DC1 and International Gateway to IMS
Serving as the PSTN toll tandem network and international gateway, the DC1 SS and international gateway SS implement the transfer of VoIPbased domestic and international toll voice services, and will continue to undertake the transfer function for a certain period of time in the future. However, as the reconstruction of each provincial SSbased tandem exchange in China is completed, the SS network is getting flatter, and the traffic at the DC1 SS will gradually decrease. Accordingly, the idle SS processing capability may be upgraded to the IMS network to provide VIP customers in this SS region with interregional broadband multimedia services. For the interregional and interprovincial DC1 SS, it is recommended to offer nomadic services and multimedia services oriented to interregional VIP customers.    
The evolution steps of the DC1 SS and international gateway SS are similar to those of the fixed intelligent SS.

4 Conclusions
In terms of basic network architecture and target, SS and IMS are completely identical. They are both established on the flat IP network. Moreover, both of them put emphasis on the separation of call control and service bearer, as well as on an open service platform. In terms of standardization and technical maturity, it is more practical to implement PES services based on the SS network because related international standards and devices have been mature for a wide range of commercial use at home and abroad. In terms of technical trends, IMS, which supports mobility and multimedia services, has a high degree of openness and convergence. It represents the future development trend, meets the tendency towards FMC and narrowbandbroadband convergence, and thus enjoys a more promising prospect[4-6]. It can be said that the SSbased network is the primary development stage of NGN, and that the IMSbased network is the advanced stage of NGN. Therefore, it is of great significance for SS equipment vendors and operators to evolve the SS networks into the target IMS architecture.

References
[1] Wei Leping. Strategic Cogitation on NGN [J]. ZTE Communications, 2004,10(1):14.
[2] Mi Zhengkun.Softswitch Networking and Services [M]. Beijing：Posts and Telecommunications Press,2005.
[3] Smooth Evolution of Softswitch to IMS [R]. Shenzhen：ZTE Communications.
[4] Xu Heyuan. Relationships among NGN, Softswitch and IMS [J]. ZTE Communications, 2006,12(5):14.
[5] Softswitch and IMS [R]. Shenzhen：ZTE Corporation.
[6] Zhao Huiling. NGN and Softswitch [J].ZTE Communications, 2005,11(3):3033.

Manuscript received: 20061030

[Abstract] Reasonable steps, measures and strategies should be chosen for the evolution from fixed Softswitch to the IP Multimedia Subsystem (IMS). A smooth evolution to the IMS is possible by gradually introducing and expanding multimedia and mobile services to achieve Fixed Mobile Convergence (FMC) at the core network layer while leveraging the existing SSbased Public Switched Telephone Network Emulation Subsystem (PES) service functions. The core control layers of the NextGeneration Network (NGN), SS and IMS have basically identical architectures. However, SS is oriented to PSTN emulation services while the IMS makes a useful expansion in terms of mobility, security, Quality of Service (QoS) and Open Service Architecture (OSA) to enable FMC and multimedia services. Therefore, the evolution from SS to IMS is technically feasible. Operators who have already used SS can enable IMSbased fullservice NGN through the evolution, which not only saves the investment and operation costs, but also reduces the risk of network alteration.