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By Wu Xiaochuan

As the use of traditional PSTN switches is approaching expiration, plans are being considered for PSTN reconstruction, and the two available options are softswitch and IMS-based PSTN/ISDN Emulation. In order to select which solution is preferable, detailed analyses and judgments must be made regarding service needs and relative technological maturity. At present, the activities of two major European operators – BT and FT – propose different solutions and provide case studies for evaluation.


The Reconstruction of PSTN is all but Started

PSTN has a positive history in terms of generating profits for fixed network operators, but the impending obsolescence of running PSTN switches has seen a decline in their production and equipment suppliers opt for breakthrough solutions. Accompanied by dynamic PSTN technology upgrades, many fixed network operators are faced with the serious challenge of how to maintain, optimize and expand the capacity of legacy PSTN switches.

Telecom experts have pointed out, this is not a simple task given that PSTN network reconstruction and updating terminals are still in their infancy stages. The prevailing economic climate is exacerbating problems for operators since profits are declining sharply, necessitating each to produce a systematic reconstruction plan using measured and expedient solutions.

After several years' development, Call Server has proven its worth in terms of both realizing PSTN and ISDN services and the ability to provide C4/C5 over broadband in place of traditional PSTN switches. In contrast with mobile network application, softswitch is not regarded as the ideal network architecture for fixed network, but this solution has nevertheless accomplished a qualitative change in PSTN by separating the bearer and control layers. IMS, as defined by 3GPP, has further improved system logic and session control separation, while also providing concentrated subscriber data management, flexible network architecture and conveniently combining services.

When viewing the evolution from current networks to broadband IMS, IP-supporting SIP multimedia intelligent terminals are not a viable short-term option, although the future will no doubt see the incorporation of IMS with PSTN circuit switched domain terminals. However, limits on adaptive potential and the capability of terminals means that traditional terminals are confined to supporting supplementary PSTN/ISDN services, specifically the subset of voice services provided by an IMS network.


The Introduction of PSS and PES

Both ITU-T and TISPAN have adopted IMS architecture to define the NGN broadband multimedia domain. FT (France Telecom) and BT (British Telecom) are leading the way in formulating a new standard for IMS-based fixed network architecture. Based on this foundation, PSTN services can be simulated on IP multimedia terminals that do not require PSTN's call model and signaling protocol. TISPAN terms this the PSS (IMS-based PSTN Simulation Subsystem).

Currently, though, PSS services are only applicable to IP terminals over NGN networks, such as SIP Phone. Subscribers would have to use comparatively expensive IP multimedia terminals, which are cost prohibitive for most users. PSS services with IMS are therefore focused mainly on business subscribers to whom services such as IP Centrex, video conferencing, electric whiteboard and file sharing are available.

To realize PSTN/ISDN network reconstruction and smooth service evolution, TISPAN intends to define the PES (PSTN/ISDN Emulation Subsystem) service model. After the PSTN/ISDN core network is replaced by PES, the network changes are unapparent to original terminal subscribers. PES would share other network elements with IMS when original terminals, user network interfaces and service levels are kept.

A requirement of PES is that it technologically inherits PSTN/ISDN's original services and experiences, although some fixed network operators are of the view that the provision of all the traditional services via IMS architecture is unnecessary. FT, for example, states that only some top supplementary services are sufficient.

In order to satisfy wider application requirements, equipment manufacturers view that PES should inherit all existing PSTN/ISDN service features, user terminals and user network interfaces. As displayed in Fig.1, TISPAN contentiously defined the network architecture of IMS-based PES following pressure from members such as FT. Draft ETSI TS 02030 can be referred to for more detailed information.

Fig.1 IMS-based PES architecture defined by TISPAN

IMS-based PES architecture requires that a new network node AGCF is added to the IMS core network. Located in the same place as P-CSCF, it connects to the interconnecting protocol conversion unit in order to accomplish conversion between the access equipment's control protocol (such as H.248) and the SIP protocol. AGCF also provides agent access for POTS and ISDN subscribers, which provides subscribers with IMS's unified management, and offers IMS-based PSTN/ISDN services. The functions of other IMS network nodes defined by 3GPP remain the same.

As the IMS-based PES scheme is based on IMS architecture, many network entity functions can be integrated and shared. In terms of service features, the telephony ASs adopted by PES and PSS are similar. If these can be shared or integrated, operators' CAPEX and OPEX will be lowered.


Two Typical Operation Models

BT and FT have designed two typical models for PSTN reconstruction.

• The FT Model
  
  FT's running networks now possess very outdated switches (called 2G Exchange in FT) for 10 million lines. Over half have been in use for over 22 years and urgently require upgrading, which FT plans to complete by the end of 2007.
  
  FT's network reconstruction is aimed towards POTS subscribers and TDM switches (3G Exchange) offer less pressure in achieving this. FT believes that IMS-based PES architecture provides a better long-term solution than Call Server-based PES, and they eventually intend to adopt IMS-based PES. At the March 2006 21CN forum in London, Mr. Bruno Chatras from the FT R&D Center offered reasons for IMS-based PES selection: "FT thinks that all voice service users will evolve into IMS-based PSS adopting IP Phone and Home Gateway. FT feels there is no great difference between IMS-based PES and IMS-based PSS, so they should not be separated. During network deployment, the basic architecture could be shared."
  
  FT adopts both PSS and 3G exchange for ISDN users. The former describes the PSS plan to change users' SIP intelligent terminals and the latter is used to converge current ISDN users to new 3G TDM switches. Both plans seek to avoid providing ISDN services over PES since this is not a viable short-term option given current technical standards and product maturity levels.
  
  
• The BT Model
  
  To date, BT has 29 million narrowband users, 7 million broadband users and 1,200 exchange offices. Primary profits are generated through call fees, network storage wholesale, narrowband wires, inter-network transport, telecom outsourcing for large-scale enterprises and monthly rental fees from narrowband and broadband subscribers. Over 60% of BT's profit derives from traditional voice services, but the company's PSTN network is outdated and OPEX-lowering reconstruction is desperately needed.
  
  While BT is actively deploying IMS networks, they regard both Call Server and IMS-based PES as workable PSTN reconstruction plans. Call Server was the logical choice due to the lack of IMS-based PES maturity and the fact that network reconstruction must avoid interrupting current service provision. BT favors PSS and regards it as representative of the advantages of IMS architecture.
  
  At the 21CN forum, BT's CTO, Mr. Alan Nunn, stated: "Emulation is best suited where complete service replication is required. Simulation is better suited to deliver next-generation services."

A Perspective on Future Developments

Analysis shows that IMS-based PES supports connections with traditional fixed networks' circuit domain terminals and therefore operators can realize PSTN reconstruction through unified IMS architecture. Future IMS core networks can be integrated to facilitate complete resource sharing and promote service deployment for PSTN users and across broadband and mobile networks. Implementation strengthens operators' competitive edge and ARPU, and the developmental status of IMS-based PES ensures that Call Server remains the current best choice for fixed network operators to realize PSTN reconstruction.

Fig.2 Figuration of PSTN Evolution

Fig.2 illustrates BT's representation that PSTN Emulation offers more or less the same experiences - including terminals - as PSTN and the view that only PSTN Simulation can deliver a richer user experience. Table 1 shows that a given operator will select a specific reconstruction plan based on its own service deployment characteristics and that CAPEX is a governing factor in this process.

Table 1 Comparison of PSTN reconstruction plans

From traditional switches to softswitch, and further to IMS, closed telecom equipments are divided into separated parts. Meanwhile, telecommunication networks are evolving into open architecture ALL-IP systems that incorporate nascent IT technologies. Operators will consequently make decisions based on the system structure of equipment, and all now define IMS networks and PSTN reconstruction as their evolutionary targets.