This is a scenario to deploy new NGN infrastructure overlay to the existing legacy networks which means two infrastructures both NGN and legacy networks exist together. NGN will take a role to provide advanced services while legacy networks keep existing services. Depending on the growing trend of the new services reflecting end users needs, the expansion speed of the NGN could be decided (also decided the level of shrink the legacy networks).
In any case, communication between users, at least universal services such as voice service, should be provided whether the user belongs to legacy or the NGN, thus it is essential to use interworking functions between two different networks. In addition, ADF function also needed providing the NGN connectivity to the legacy terminal devices.
Following Figure 21 shows high level configuration of this scenario.
Figure 21 – Overlay scenario
This scenario uses mixed two NGN capabilities, simulation and emulation. NGN simulation has used for providing PSTN/ISDN-like services to the NGN users with advanced NGN features. NGN emulation widely uses to support voice oriented services but keeping the legacy terminal such as legacy telephone, thus end user could not recognize the change of technology behind their terminal. Therefore combining of these two capabilities will give flexible ways to migrate legacy networks according to the environments such as end user needs for advanced features, status of legacy network systems and business strategy of the operators as well as service providers (may include national vision for upgrading legacy infrastructure).
In this scenario, overlay NGN will be deployed first where there are needs for the advanced services. This overlay NGN takes a role to invite NGN users with advanced service features as well as deploying NGN terminal equipment. This will be a fundamental infrastructure for the future transformation of legacy networks. In addition, NGN access networks in this overlay NGN also install where there is not have enough PSTN/ISDN infrastructure which means lack of connectivity to support voice services. In this case, NGN access network should have emulation capability, so it easily incorporates existing legacy terminals. Consequently this is a replacement of legacy access network into the NGN. According to the growth of end user needs as well as status of legacy network capabilities, this replacement should be expanded.
Through this scenario, current users even using PSTN/ISDN have continuous support without any change of their terminal and operator will stop their deployment of PSTN/ISDN but will need other investment for replacement to the NGN. For this, operator will provide ADF (Adaptation Function) to the current PSTN/ISDN users to provide continuous usage of voice services.
Following Figure 22 shows high level configuration for the replacement scenario.
Figure 22 – Replacement scenario
9Considerations identify the scenario
As explained in the previous section, each scenario has their own specifics, thus it is useful to take into account various issues to be considered. This section introduces various issues for choosing scenarios for migration of legacy networks into the NGN.
There are many of issues to be considered when building migration plan from one to another. Legacy networks are not simple objective for replacement, because they consisted many of portions of telecommunication infrastructures as well as cover significant and fundamental communication services such as voice services. Therefore it is highly required to consider many of things from various perspectives during the set up of migration plan carefully. By considering this, there may neither a single way nor the best way in global and/or overall sense, because migration should be based on each country situation such as national vision, user needs, politic & regulatory environments, technology trends and business environments such as given condition by operators and industries.
High level aspects such as national vision, user needs, policy and regulations including business aspects should be investigated and analyzed as soon as possible before identify the scenario(s) for the migration, because they have a lot of impacts for the whole process of migration. And it is noted that understanding the exact status of such high level aspects generally requires in-depth study with wide knowledge of that country.
Identified scope of each subject areas are summarized as followings and show in Figure 23 which described the relationship among them:
User demands: this is a fundamental point to know the status of end user needs for telecommunication services including any new requirements and analyze potentials for the future developments. This is also contributed to identify NGN as a social infrastructure from user point of view;
Government Policy: this should cover translation of the national vision of ICT and telecommunications into government policy including relevant strategies and milestones for the developments as well as identify the roles and responsibilities of players who will involve migration process;
Regulation Environments: this provides regulatory guidance and preparation of necessary regulations to help and support various ways of migration (may be included for improvements or updates) of legacy telecommunication infrastructures into the NGN;
Business Environments: this is for knowing the status of environments and practical capabilities as a business potential of the country.