Relationships Among NGN, Softswitch and lMS

Xu Heyuan

（Institute of Communications Standards Research, CATR, MII, Beijing 100045, China）

Abstract:The Next Generation Network（NGN）has been a term frequently quoted in the hot topics of Telecommunication industry.The Softswitch and the IP Multimedia Subsystem（IMS）are proposed to implement the NGN services.It is imperative to reach a consensus on positioning and planning their future directions.However,it is rather difficult to realize smooth network evolution towards IMS as the Softswitch mainly supports the traditional Public Switched Telephone Network（PSTN）and Integrated Services Digital Network（ISDN）services.In addition,the basic and complementary services of Softswitch are offered locally.

Since the concept appearance of Next Generation Network（NGN）,topics about its name,definition,and service range have been intensely discussed.What technology is better for the service layer has also become a focus issue.The original viewpoint considers NGNas a multi-service and integrated network system,and Softswitch is the main network technology for the service layer.Later the IPMultimedia Subsystem（IMS）was introduced and it was considered that IMSwould supersede NGN as the primary network technology.Then,what is the relationship among NGN,Softswitch,and IMS?What is Softswitch oriented for?These questions will be analyzed and discussed in the following paragraphs.

1 Next Generation Network

The literal interpretation of NGN is ambiguous,therefore,for every network following the preceding one can be called the next generation network.Actually,NGNat the current researching stage is a packet-based core network,rather than a time-division-multiplexing network.It provides multiple services in addition to the traditional telecommunications services,and supports several broadband and Quality of Service（QoS）-enabled transport technologies.Service-related functions are independent of transport technologies.According to the choices of the users,it enables unfettered access to the networks,service providers and services.Therefore,the name—NGN—is chosen to emphasize its innovative and revolutionary characteristics,compared with previous networks.

The NGNis capable of providing full-services with various bandwidths via wired or wireless,including voice,data,video,streaming media,Internet access,digital TVbroadcast,mobile,and more.With its open interfaces,different service providers can build and introduce their services.From the user's point of view,NGNshould cover allavailable telecommunications services and applications from the current service environments（such as telephone services,wireless/mobile services,broadcast and distributed services,and Internet）.The services and applications mentioned indicate that NGN is not a revolutionary goal,but a perfect goal.That is,the new technology acts as a substitute not only to implement all services and applications of the existing networks,but also to accommodate all that possible in the future.

2 Softswitch and Its Orientation

The technology which is bearer independent of control is used for the Softswitch in the evolution of telecommunications switching networks.It employs the data transmission capacity of packet data networks to transfer media,and makes callcontrol functions,media bearing functions and service functions of traditional circuit switches independent.Softswitches no longer deal with media streams or service attributes.They are only responsible for the basic call control.Different gateways are used to access different users.

The Softswitch technology is originally aimed to separate bearer from control and control from service,while using transport capabilities of IPnetworks to carry the Voice over IP（VoIP）service.This technology contributes to providing voice-based Value Added Services（VASs）faster and more effectively by operators according to the requirements of different user groups.Thanks to the ability of IPnetwork to transfer integrated services,including the IP-based voice,video and multimedia services,it is realistic to hope to provide multiple services over a single network.Therefore,if Softswitch is able to control these services,it realizes the ideal network convergence and service integration,which is targeted by the telecommunication industry.For the reason mentioned,the Session Initiation Protocol（SIP）users with broadband access are added into the Softswitch controlled network.The SIPusers can acquire broadband video,multimedia,and some data services under the control of Softswitch.

▲Figure 1. Configuration of the next generation network.

However,the emergence of IMSset by NGN blocked the development of the original idea aforementioned.Telecommunication and Internet converged Services and Protocols for Advanced Networks（TISPAN）of European Telecommunications Standards Institute（ETSI）planned to propose four service components of NGN.The four components are the Public Switched Telephone Network（PSTN）/Integrated Services Digital Network（ISDN）emulation component,the IMScomponent,the streaming component,and other multimedia components.In addition,the ITU-T SG13 introduced the configuration of NGN（see Figure 1）,in which the part based on IMSis the main body of NGN,and ITU-Tinsists that IMSis the direction of NGN evolution.It can emulate PSTN/ISDNservices,offer IMS-based broadband services,and enable Fixed Mobile Convergence（FMC）by supporting fixed and mobile broadband users simultaneously.Because the study on IMS-based network convergence was launched not long before,there is much room for improving its maturity and stability,as well as the related standardization.

Presently,quite a few operators are faced with the problem of how to evolve traditional circuit-based PSTN/ISDN into packet-based NGN.At the meeting of ITU NGN in December 2004,it was agreed to initiate the standardization of Call Server（CS）-based PSTN/ISDN Emulation Subsystem（PES）network evolution.The Focus Group on NGN（FGNGN）started the research on PSTN evolution in June 2004,and proposed a CS-based solution with examples for PSTN evolution.The CS-based solution mainly achieves the evolution of circuit switching networks,while IMSprimarily carries out multimedia services.Both of them will coexist for a long term as IMS matures.

Accordingly,evolution from circuit switching into packet switching through Softswitch is an important technicaldrive for the development of telephone networks.The IMSis a potential total architecture to converge fixed and mobile broadband networks with multimedia services in the future.

The Softswitch is a way for fixed network operators driven by market and service competitions to build NGN at this stage.The Softswitch can provide C4 services to substitute traditional circuit switching and make trunk call.It can also provide C5 services by connecting different terminals to core service networks via data access.

There are two structures to construct Softswitch networks.One is temporarily named as integrated Softswitch which accesses different wireline users,including narrowband and broadband users.The other consists of narrowband and broadband Softswitch,where Media Gateway Controller（MGC）and SIP server,which are responsible for the access and control of narrowband and broadband users respectively,are cooperatively used for Softswitch services.Softswitch is a preliminary stage of the NGN and it aims to the services related to wireline users.However,its development should provide room for future requirements of NGN.If operators set the CS-based PESnetwork as the target of PSTN evolution at the beginning of the design of NGN,the consideration of the requirements for the construction and evolution of Softswitch networks is a must.Therefore,the narrowband Softswitch can be deployed to meet the demands of the market.It is recommended that integrated Softswitch,rather than broadband Softswitch,should be used for the network construction,although there are requirements for broadband services.

The broadband Softswitch mainly offers voice service on devices similar to IPterminals through the SIPserver.It seems that broadband Softswitch supports mobility of these terminals and meets some requirements of IPvoice,and the SIPserver can evolve into IMS according to its standards.However,there is a big gap between the Softswitch and IMS.Specifically,the broadband Softswitch has no centralized database setting and management,and processing the user and service data via the localserver.Furthermore,there is only the SIPserver in the core control,which is not in line with the IMS specification of dividing functions into Proxy-Call Session Control Function（P-CSCF）,Inquiry-Call Session Control Function（I-CSCF）and Serving-Call Session Control Function（S-CSCF）.However,the SIPserver of product provision platform possibly becomes IMSready and the commercial network organized by present SIP servers is difficult to evolve into NGN IMS.

3 IP Multimedia Subsystem and Its Orientation

The IMSwas originally proposed by the Third Generation Partnership Project（3GPP）in the specifications of Release 5（R5）.The core structure,network element functions,interfaces,and procedures are defined in R5.The IMSservice characteristics,interoperability specifications between IMSand other networks,and access of Wireless Local Area Network（WLAN）are added in R6.The standardization of FMC is strengthened in R7 to impose on IMSthe support of fixed accesses via Digital Subscriber Line（xDSL）,wireline modem and more.Presently the R5 and R6 are on hold.

The TISPANand ITU-TSG13 are studying the NGN-related specifications with the starting point of fixed networks.The IMSis one of service components in NGN architecture defined by TISPAN.The TISPAN IMSexpands function entities and protocols based on the 3GPPR6 IMScore specifications.It can support fixed access.

After the 3G licenses have been granted,each operator has the possibility to be a full-service operator.A traditionalfixed network operator soon will probably build an IMSnetwork under the market competition,even if he has established a R4 3G network with services.At the beginning,this IMS network can only accommodate 3G users,but its network protocols should consider the access of wireline broadband data users.In nature,it is better to plan the two user access requirements as a whole,and construct a convergence network at the very beginning.However,there are differences in service time and service type between wireline and wireless user accesses,which possibly makes the service networks apart.It is predictable that the unanimity of users'service requirements and the full-service operation of the operators will drive the convergence of the two networks.At that time,users can access the converged network through different access technologies or access networks,such as WLAN,Worldwide Interoperability for Microwave Access（WiMAX）and more.

The IMSis a multimedia/call control platform in Packet Switching（PS）domain.It grants PSsome Circuit Switching（CS）functions and supports conversation or non-conversion multimedia services.The IMSis also a generalized service platform for future multimedia applications,such as presence,message,conference,Push-to-talk over Cellular（PoC）,and other typical services.As to traditional mobile network operators,requirements for IMSare actually stimulated by the need of future FMC and current market.Consequently,not only the requirements of mobile users,but also the requirements of VIPs and potential fixed broadband users are worth to consider at the IMSnetwork planning.However,it should be noted that the fixed services mainly imply broadband multimedia services,instead of PSTN/ISDN services.Therefore,only the PSTN Simulation Service（PSS）rather than the PSTN Emulation Service（PES）need to be supported when planning the IMS network.

In the future,the IMSusers of broadband access have demands on multimedia services,and even streaming services.So far,multimedia services can be carried in NGN R1 and it has gotten ready to support streaming media services over IMSin NGN R2.The trigger mechanism of customized services that stored in the home service server is one of IMSadvantages,which contributes to individual streaming media services（or IPTVservice）to be implemented over IMS.

4 Conclusions

The NGN is an evolving network.Though the present research of NGN is focused on providing multiple services over a unified packet network,the likely change in each layer（the service layer,the bearer layer,and the transport layer）or each part（terminals,the client network,the access network and the core network）of NGN needs to be involved in the research area［1-6］.The Softswitch is a current NGN service resolution with little standardization,which has already been applied.On the contrary,IMShas a network framework standard,but having few applications.Considering the device implementation,Softswitch can smoothly evolve into IMS.However,there are difficulties for the network evolution,because Softswitch mainly supports traditional PSTN/ISDN services,while having the basic services and supplementary services supported locally.