NGN migration scenarios can also be classified into four categories:
1. Soft Switch installation at international and high national transit levels;
2. Class 4 Switch replacement at transit level;
3. Class 5 Switch replacement at access level;
4. IMS Overlay.
From a technological point of view, NGN migration scenarios can be divided into two broad categories:
1. Scenarios where NGN is used to carry voice at transit or access levels over a packet-based transport network.
When undertaken by a fixed-line voice incumbent, the transition is generally transparent to end users in terms of the services offered by the legacy telephony network. However, when voice telephony service is offered over an alternative access such as cable, DSL, fibre or Wireles Local Loop (WLL), the Soft Switch may not need to have exactly the same capabilities as those of legacy telephony networks. A specific scenario with LES allows a migration to new VoIP-enabled voice telephony services at the access level without necessitating the introduction of a Class 5 Soft Switch, allowing the existing legacy Class 5 switches to be kept.
2. Scenarios where an NGN IMS-based architecture is used In this case, the same Multimedia Soft Switch, which is functionally equivalent to a Call Session Control Function (CSCF) in IMS terminology, is used both for multimedia and voice communication services. These scenarios necessarily involve a migration of the user to broadband access. The IMS infrastructure can be introduced either as a full replacement of the telephony network, such as in the case of BT’s 21 CN, or more often in overlay mode, where the existing telephony network is kept for legacy voice services.
NGN migration scenarios can also be analyzed on the basis of three service aspects:
1. Continuity of legacy voice telephony service;
2. Availability of new end-user VoIP enabled telephony services;
3. Possibility to offer new multimedia communication services.
In analyzing NGN migration paths, it is important toconsider that existing legacy networks represent significant sunk capital investments to many operators. This may prevent their complete replacement in the immediate future. As such,a phased approach is usually preferred for the migration to NGN. ITU-T Recommendation Y.2261 describes some possible ways of evolving PSTN/ISDN to NGN. Both IMS-based and call-server-based approaches are described. The Recommendation also describes some factors that need to be considered including the evolution of transport, management, signalling and control parts of PSTN/ISDN to NGN.
Source: ITU
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