Evolution from Fixed Softswitch to IMS

Tu Jiashun Yang Wei Gao Yi

(Network Division of ZTE Corporation, Nanjing

210012, China）

Abstrac t: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 SS-based Public Switched Telephone Network Emulation Subsystem(PES)service functions.The core control layers of the Next-Generation 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 IMS-based full-service NGN through the evolution,which not only saves the investment and operation costs,but also reduces the risk of network alteration.

T he Softswitch(SS)concept was gradually developed on the basis of the IPphone 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 controlmechanism 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 SSconcept,China Communications Standards Association(CCSA)promptly introduced the“SS”technical term,and has actively been involved in the standardization work on the SS-related devices and protocols.Through several years of development driven by equipment vendors and operators,SShas found a wide range of commercial use in operators like China Telecom,China Mobile and China Netcom.These operators have used the SStechnologies 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 IPMultimedia 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 3GPPRelease 6(R6).One important feature of IMSis 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 IMSis 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,IMStakes 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 IMSin its NGN Release 1(R1)as the core technology for NGN fixed applications,and is prepared to emphasize the IMS-related specifications in the 3GPPR6.Similarly,the Focus Group on Next Generation Networks(FGNGN)of the International Telecommunication Union-Telecommunication Standardization Sector(ITU-T)used and developed the IMS-based NGN architecture.The FGNGN has rolled out IMS-based PSTN Emulation/Simulation Subsystem(PES/PSS)standards,and is now conducting a research on IMS-based IPTVarchitecture.

The IMSarchitecture is not only accepted as the NGN core network technology by international mainstream standardization organizations such as the ITU-T,3GPPand 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 IMStestings and trials［1-3］.

▲Figure 1. Comparison of IMS and SS in system architecture.

Being functional entities of the NGNcore control,both SSand IMScan provide the call and connection control functions for real-time services.They are the core of call and control in the NGN.The SS-based NGN and the IMS-based NGN have quite similar system architectures,which consist of the service,control,access and bearer layers.Figure 1 shows an architecture comparison between the IMSand SS systems.

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

SSoffers new value-added 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),SScan 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 IMSarchitecture defines an IMS Service Control(ISC)interface as the service control reference point for interconnection to the SIP-based 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.IMSitself doesn't integrate any service logics.PSTN service logics are completed by independent PES/PSSApplication Servers(APP)via the ISC interface.Moreover,IMSitself 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 controllayer,SSuses one functional entity to complete such functions as call processing and control,access protocoladaptation,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 CSCFis further divided into Proxy CSCF(P-CSCF),Serving CSCF(S-CSCF)and Interrogating CSCF(I-CSCF).The P-CSCFis the first contact point for the user terminal within IMS.The S-CSCFactually manages the session states in the network.The I-CSCFis 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,SSand IMScan 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 SIPterminals,while the standards for access of PSTN subscribers are still in improvement.

At the bearer layer,IMSproposes an end-to-end all-IParchitecture.To enable end-to-end QoS,it also introduces the Network Attachment

1 Comparison of IMS and SS

Subsystem(NASS),and the Resource and Admission Control Subsystem(RACS),also called the Policy Definition Function(PDF).However,SSputs emphasis on the IParchitecture of the core network to ensure evolution from the existing network to an all-IPnetwork；thus,the QoSguarantee depends on the IPnetwork itself.

2 Strategies for Evolution from SS to IMS

SShas been widely applied in existing networks,while IMShas been accepted in the industry as a future full-service 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 IMStechnicaldevelopment and existing network conditions of operators,appropriate strategies should be chosen for the evolution.

(1)Upgrade SSto IMSSubset

Operators with mobile services as their main business can build new IMS networks by introducing multimedia services at the mobile side.The existing SScan be upgraded to the AGCF,MGCF,SGF,and PES/PSSAPPin the IMSdomain to implement PSTN services and interoperability.

(2)Upgrade SSto the IMSFull 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 IMSfull set,and gradually implement the access to mobile and multimedia services while leveraging all the existing SS-based services.When the traffic of mobile and multimedia services grow steadily,all the SSs can be upgraded to the IMSfull 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 SSto IMS.

3.1 Status Quo of SS Network

The fixed network operator started the research and testing of SSdevices 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 VIPcustomer platform,used for offering broadband multimedia services and inter-regional VIPcustomer services；

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

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

▲Figure 2. Networking and application of the existing Softswitch network of a fixed network operator.

3.2 Upgrade of SS on Broadband Service Platform or VIP Customer Platform to IMS

Currently,the SSnetwork on the broadband service platform has relatively few subscribers.As the SS network is independent,subscribers are basically carried over the broadband IP network.IMShas a stronger support of scalability of the multimedia capability and follow-up standards than SS.Therefore,the current SSdevices on the broadband service platform or VIP customer platform can be upgraded to IMSin the current stage.This IMS network can provide the subscribers with feature-rich fixed broadband multimedia services,and gradually offer mobile multimedia services.

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

The evolution steps are described below:

(1)The CSCF/MGCFand PSSAPP functions are added to the SSon the service platform.The original SS supplementary service module is upgraded to the PES/PSSAPPto implement PSTNsupplementary services.The original H.248 module is upgraded to the MGCFto interoperate with the PSTNnetwork.The CSCF module function(such as P/I/S-CSCF)is implemented by extending boards and adding configurations of SSequipment,and makes unified call control for fixed and mobile multimedia subscribers.

(2)Anew HSSis added to store broadband user data and trigger services via the S-CSCFto the PESAPP.The standard Diameter interface is adopted between the S-CSCFand the HSS.This new HSSmay 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 IMSservice capabilities.So the platform can provide new IMS-based broadband multimedia services while maintaining the originalservice offerings.

(4)Acertain SSboard in the upgraded IMSequipment can be used to handle PESservices.The PESservices delivered by the SSboard are completely not affected by IMS.

(5)The upgraded IMShas 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 IMS-based IPTVand Global E-eye 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 full-service 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 SSto the IMSnetwork.

The evolution steps are described below:

(1)SSis upgraded to IMSto offer a small quantity of multimedia services at the initialstage when multimedia users and services are introduced.The introduction of IMScan 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 SSand IMSprocessing boards have no impact on each other because the SSprocessing board is completely separated from the IMSboard.Moreover,external interfaces of the upgraded device,including inter-office routing and off-site disaster tolerance,remain unchanged and no alternation is made to other SSs connected to the device.The newly-added IMSnetwork adopts the IMS-based off-site disaster tolerance.At the initial stage,the HSScan coexist with the upgraded SHLRon the same physical platform.

(2)The service platform is inherited and evolved.The SSnarrowband service logic is evolved into the PES/PSS functionalmodule that can fully inherit the existing intelligent services through interworking with the service exchange functional module of the SCP.The original unified SSservice 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,PHSHLR,HSSand 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 IPTVare gradually introduced in the IMS architecture.

(5)Allthe SSprocessing boards are upgraded to the AGCFor MGCFat a proper time:after the IMSarchitecture 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 IMS-based PES.Therefore,the network is unified,which is a good basis for further operation optimization.

It is unnecessary to evolve the fixed intelligent SSto IMSpromptly,considering the stability and security of the existing SSnetwork,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 IMSgets 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 willstill dominate for quite a long time.An optimal option for the development of Chinese telecommunication networks is to build full-service 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 tolltandem network and international gateway,the DC1 SS and international gateway SSimplement the transfer of VoIP-based 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 SS-based tandem exchange in China is completed,the SS network is getting flatter,and the traffic at the DC1 SSwill gradually decrease.ccordingly,the idle SSprocessing apability may be upgraded to the IMS etwork to provide VIPcustomers in this Sregion with nter-regional broadband multimedia ervices.For the inter-regional and nter-provincial DC1 SS,it is ecommended to offer nomadic services nd multimedia services oriented to nter-regional VIPcustomers.

The evolution steps of the DC1 SS nd international gateway SSare similar to those of the fixed intelligent SS.

4 Conclusions

In terms of basic network architecture and target,SSand IMSare completely identical.They are both established on the flat IPnetwork.Moreover,both of them put emphasis on the separation of call controland 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 SSnetwork because related international standards and devices have been mature for a wide range of commercial use at home and abroad.In terms of technicaltrends,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 narrowband-broadband convergence,and thus enjoys a more promising prospect［4-6］.It can be said that the SS-based network is the primary development stage of NGN,and that the IMS-based network is the advanced stage of NGN.Therefore,it is of great significance for SSequipment vendors and operators to evolve the SSnetworks into the target IMSarchitecture.