ZTE´s Softswitch-Based Equipment for 3G Core Networks

ZTE is the first Chinese telecom equipment vendor to do research on Softswitch technology and to develop Softswitch-based products. With great efforts for several years, ZTE and its Softswitch products have got a general recognition in the industry. Its Softswitch products are playing an important role in NGN construction for fixed networks, which lay a solid basis for the wide application of Softswitch in the future.

  ITU has approved three 3G systems: WCDMA, CDMA2000 and TD-SCDMA. Due to the greater popularity of GSM and with some of its special characteristics, WCDMA has gained worldwide support. Smooth transition from 2G/2.5G is a highlight of CDMA2000, and it has had a lot of practical applications in China. As a standard with a proprietary intellectual property right owned by China, TD-SCDMA is strong in the air interface.

  In fact, these 3 systems differ from one another mainly in the Radio Access Network (RAN) part. Comparatively speaking, there is very little difference in their Core Network (CN) part, so it is totally possible to adopt a unified technology to implement the CNs of the 3 systems.

1 Softswitch Is a Basic Technologyfor 3G Core Networks

1.1 Application Modes of Softswitch on 3G  Networks
WCDMA R4 has adopted the mode of gateway separation in its Circuit Switching (CS) domain. Its Gateway Mobile Switching Center (GMSC) server and Media Gateway (MGW) function entity implement the functionalities of the Mobile Switching Center (MSC). R5, with all-IP structure， further improves such an implementation model. Although research into R6 has just begun, it is estimated that R6 would be used in Packet Switching (PS) to make the whole CN based on the gateway separation mode.

  The development stages of CDMA2000 are not as clear as those of WCDMA, but the all-IP Legacy Mobile Station Domain (LMSD) of CDMA2000 has a similar functionality as the CS of WCDMA R4. It also adopts the gateway separation mode, and implements the functions of MSC via the GMSC emulator and MGW. The IP Multimedia Subsystem in future will do so in a similar model.

  Gateway separation (or separation of control and media) is the most important technical basis of Softswitch. All the systems based on gateway separation can be regarded as applications of the Softswitch technology. Therefore, R4 and LMSD are definitely implemented by the Softswitch technology.

  The Softswitch technology has two applications in 3G. One is in basic networks, and the other in value added services.

1.2 Application of Softswitch in Basic 3G Networks
R4 and LMSD will be analyzed here. Their network models are separately shown in Figure 1 and Figure 2.

  Analyzing the R4 model, we can find that R4 divides the GMSC in the CS of R99 into two function entities: GMSC Server and MGW. The GMSC Server mainly implements control of signaling and calling, while the MGW deals with media flows. The H.248 protocol is used between the GMSC Server and MGW, while BICC that has nothing to do with bearing is used between GMSC Servers. The GMSC Server implements interconnection with the Public Switched Telephone Network (PSTN) and Public Land Mobile Network (PLMN) via the Signal Gateway (SGW). As for LMSD, a similar analysis can be done.

1.3 Application of Softswitch in Value  Added Services of 3G
There are 3 ways to implement value added services in 3G. One is implemented directly by MSC
Server/MSCe (and the support of the Radio Access Network (RAN) and MGW is needed in certain cases). Another is by interconnection with the Service Control Point (SCP). And the third is by using open service systems (and Parlay is specified for use in 3G).

  The first way has the highest efficiency but worst in terms of openness. It is actually the service implementation of traditional switches, and mainly fit for implementation of simple services closely related to basic calls and without independent logic themselves.

  The second way protects the past investment in mobile intelligent networks. MSC Server/MSCe implements the functionalities of the Service Switching Point (SSP) as well as interconnection with SCP via Customized Applications for Mobile network with Enhanced Logic (CAMEL)/Wireless Intelligent Network (WIN). It can well inherit existing intelligent voice services.

  The last one is a main implementation model of 3G. It is suitable for implementation of comprehensive services that converge voice, data and video services and have a similar operation mode with the Internet. It is usually implemented by providing a totally open and programmable service environment. 3G networks, however, are still dominated by voice services, so this way cannot be broadly applied until a real all-IP era comes.

  With the gradual commercialization of NGN, traditional telecommunication business operation modes will be greatly changed. Individual Service Vendors (ISV) will participate in market competition more and more actively. Therefore, the last way is expected to be used widely.

2 ZTE’s 3G Core Network Equipment Based on Softswitch
Along with the quickening of 3G applications at home and abroad, ZTE has successfully applied the Softswitch technology into 3G networks. ZTE’s Softswitch-based 3G core network equipment includes two series: the R4 series and the LMSD series.

  During ZTE’s product design and development, Softswitch is regarded as an important basic technology. This has been applied to all network equipment of ZTE’s CN subsystems, especially in its MSC Server/MSCe. According to related standards issued by 3GPP/3GPP2 and ITU, ZTE has made functionality enhancement and system optimization for its Softswitch products along with inheriting the characteristics and strengths of fixed NGN.

  Specifically speaking, on the basis of its fixed network control equipment, ZTE mainly changes its MSC Server/MSCe products with their protocol interfaces and accordingly deletes some functions:

• Adding mobility management functions including location update management, handover management and roaming management.
• Adding Visitor Location Register (VLR) and data support for new protocols.
• Adding the Radio Access Network Application Protocol/Base Station Subsystem Application Part Protocol for 3G base stations (at Iu/Alp interfaces) and 2G base stations (at A/A1 interfaces).
• Adding the BICC protocol for interconnection with equal network entities (only available for the MSC Server).
• Adding CAMEL/WIN protocols for communication with the SCP of mobile intelligent networks.
• Adding MAP for communication with database servers such as HLR.
• Adding support for H.248 mobile packets that control MGW (only available for MSC Server).
• Adding bearing protocol stacks of ATM (only for MSC Server) and TDM.
• Screening the H.323 protocol that is used for communication with existing VoIP networks. But it can be put into use again if necessary.
• Screening the Media Gateway Control Protocol (MGCP) that controls MGW. But it can be put into use again if necessary.
• Screening the Intelligent Network Application Protocol (INAP) for communication with the SCP of fixed intelligent networks.
• Introducing the Session Initiation Protocol for Telephones, this is only available for MSCe at present, but may meet the requirements of R5 in the future.

  3G is undoubtedly an organic part of NGN. Therefore, as a main solution for NGN, Softswitch will get reasonably broad applications in 3G. Meanwhile, it is just because of Softswitch, a common foundation that convergence of NGN of fixed networks and 3G networks becomes possible in practice. Accordingly, it may effectively promote the shaping of a Softswitch-based unified communication network.

Manuscript received：2004-07-06