ZTE TDM Switch´s Seamless Evolution Strategy to NGN


1. Telecom Network  Evolution Tendency

From its existing equipment and network conditions, the traditional fixed network has the following problems: aging of PSTN equipment, single access means, isolated networks, single service and inconvenience to provide new services.

Compared with traditional network problems, Next Generation Network (NGN) should have the following features: optimized network structure, strong service provision capability, capability of integrated access, effective operating and supporting system and high reliability and safety. NGN has become the trend of telecom development.

2. Network Evolution    Step

Network development is a gradual process. In fixed-line network, its development roadmap is from current TDM switch to TDM+IP Gateway, then to all-IP core network.

Currently, PSTN switch is the kind of TDM switch. It occupies fixed 64K time slot voice communication, so it has high voice quality. Whereas, with booming multimedia services and cheap VoIP services, TDM switch´s disadvantages arise.

In early phase of NGN, NGN has no large-scale commercial applications´ conditions, data network development is limited and current investment on PSTN should be protected, so current network access should be considered. Access equipment from PSTN to NGN is media gateway, so media gateway has TDM+IP double bus structure. ZTE has TDM switch ZXJ10. It can serve as local/tandem/toll/international offices. It has witnessed wide applications all over the world and total lines running in service are 80 million. The outstanding feature of ZXJ10 is that it can evolve to NGN media gateway ZXMSG 7200/ZXMSG 9000 seamlessly. In our Fixed 3G @ NGN architecture, media gateways after seamless evolution from TDM switch can be connected with PSTN and NGN. All the user information is stored in SHLR.

With the development of IP technology, it is widely believed that the coming backbone network should be the All IP network. While the NGN network will gradually replace the traditional PSTN network to become the main fixed network. Although the PSTN network will still play a role in the near future, it will be finally integrated into NGN. The IP Multimedia Subsystem (IMS) proposed by 3GPP is regarded as the blueprint of the future All IP network. It is a multimedia or call control platform based on PS domain, and it enables the PS domain some CS functions such as delivering voice, video phone and other real-time services using the QoS and enhanced billing function provided. IMS can also be regarded as a platform for all kind of mobile multimedia services, and it is the target network in the evolution of service providing system facing All IP Network.

3. ZTE TDM Switch-ZXJ10 Evolution     Strategy

3.1 NGN Evolution Plan for TDM Switch -ZXJ10

3.1.1 Integrated Evolution Plan

The local network may typically evolve to NGN by the following two approaches: one is to evolve to NGN on the level of local offices which will evolve to AG or MSAG and another is to evolve to NGN on the level of tandem offices which will evolve to TG while local offices remain unchanged.

The evolution to NGN on the level of local offices creates a clear network structure and complies with the standard Softswitch network structure. The data management and service for former PSTN subscribers can be supported by Softswitch network that can provide various new services for such subscribers. However, the complete NGN evolution on the level of end offices is not realistic under the current conditions because there are a great number of end offices, which requires enormous network modifications and huge amount of expenses on such modifications. On the other hand, local offices will be useless after modification, contributing to a waste of investment, and some old equipment is not capable of transiting to AG equipment. As a result, only a certain number of local offices can evolve to NGN.

The evolution to NGN on the level of tandem offices can reduce the PSTN modification cost and workload while maintaining the fixed investment by operators on a large number of end offices. Therefore, it is feasible to evolve to NGN on the level of tandem offices. The existing ZXJ10 Exchange evolves to TG in the NGN architecture on the level of tandem offices, featuring: clear network structure, standard Softswitch network structure, less network construction and modification and lower investment on network equipment.

3.1.2 Evolution Plan for Stand-alone Offices

Seamless evolution of TDM Switch  -ZXJ10 means just add VoIP processing part, routing part and network management board. Digital trunk part, control part, subscriber part and switching network part will be reserved in MSG7200, so telecom carrier´s investment on current TDM switch ZXJ10 will be protected in the future.

3.1.3 Evolution Plan for Multi-module Offices

If there is a large-capacity and multi-module office, Central module-CM64/CM256 (just one rack) will be altered to MSG9000 and all the former switching modules of ZXJ10-SM8/SM16 will be reserved to protect former investment.

3.1.4 Switching Module Evolves to Media Gateway

Multi-module networking using ZXJ10 is often adopted in the large capacity offices. High capacity single T network such as 64K x 64K or 256K x 256K served as central switching module (CM), SM8 or SM16 with 8K or 16K switching network is used as switching module (SM). Every SM8 or SM16 can access up to 15360 subscribers or 7680 digital trunks. All the switching modules or part of them can evolve to media gateway separately. If part of them evolves to media gateway, its evolution method is the same with stand-alone media offices and previous central module and other switching modules are reserved.

3.2 Key Evolution Technology Description

In light of NGN commercial conditions and telecom carrier network´s safety, media gateway takes into consideration the ratio of narrow-band interfaces and broadband interfaces. The ratio can be modified freely according to telecom carriers´ requirements. Narrowband interface boards are DTI or MDT. Broadband interface boards are MIPS. DTI and MIPS can be inserted in one frame. MDT and MIPS can be inserted in one frame. Changing number of DTI/MDT and MIPS can change the ratio of narrowband and broadband interfaces.

Currently, evolved media gateway supports hairpin function, which reduces  the investment cost of media gateway and the time delay of media transfer, and improves quality of calls. Hairpin function is that media flow can be switched directly via TDM switching board and doesn´t need to be switched via IP switching network, when two subscribers in one media gateway make a call. At this time, signaling is processed by MGC function in Softswitch equipment.

In the future, media gateway of ZTE can support self-switching function. This function is that a call between two subscribers in one media gateway can be processed normally by media gateway, when communication between media gateway and Softswitch equipment is abnormal. This function will have higher safety and reliability.

4. Summary

R&D roadmap of ZTE equipment ZXJ10 is a consecutive course. It considers not only satisfying current service requirements, but also evolving to media gateway in NGN on one hardware platform. It´s the kind of seamless evolution. In current phase, it can serve as local/tandem/gateway offices. In the future, just adding some boards can evolve to media gateway to enjoy all the new services of Softswitch network.

(Tang Jing, Zhong Xin)

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Next: Build a Unified Subscriber Management Platform By Using the SHLR
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