During the set-up of the migration plan, many of technical views and considerations would be carefully examined because it will impact to the whole of communication infrastructure including related regulations as well. ITU-T Recommendation Y.2261 identified that the PSTN/ISDN is the prime candidate for evolution to the NGN and the Recommendation Y.2261 recommend all aspects should be carefully examined and appropriate measures should be taken. In PSTN/ISDN, most of the functionalities are located in a single exchange and may use proprietary protocols. But, in the NGN, functionalities may be distributed amongst several elements. 
For better and smooth migration, followings are recommended as consideration points:
Growth of voice service market and dilemma to grow with legacy PSTN/ISDN or with the NGN;
Seamless continuity of services as well as backward compatibility of voice services;
Inter-working with existing legacy equipment;
Provision of new communication services to emerging users in addition to existing network services;
Possibilities offered through NGN migration to grow new markets (broadband, triple/quadruple-play) and enrich infrastructure competition between distinct access platforms;
OPEX with legacy networks vs. CAPEX for new NGN infrastructure;
Timelines for the whole process of migration, and;
Decision to begin replacement of infrastructure;
A significant portion of users switches to NGN services, the other direction, reduction of true PSTN/ISDN usage visible;
Replacement parts of the infrastructure (e.g. local switch) by new infrastructure, without forcing all users to migrate.
Special consideration should be addressed for service expansion in rural remote areas. Migration to the NGN in the rural areas should not be abrupt, and both the older and the newer technologies should co-exist for a reasonable period of time.
In general, PSTN/ISDN is comprised of the following entities, each with one or multiple functionalities:
Transport entities (including switching, transmission and physical mediums): physical PSTN/ISDN infrastructures with forming access and core networks;
Control and signalling: identify communication end points and forming control center for the communications;
Management: monitor, maintain and repair of communication services and networks to keep operations and provision properly;
Service: mainly voice or speech class of services including voice-band data and in the case of ISDN narrow-band information services are also available.
Transport is the most significant and fundamental parts to be considered whenever think about the migration or transition. There are two distinct domains: access network and core network (including switching entities).
Core network is recognized rather soft than other domains because it takes a role to transport bundle of traffics from one node to other relevant node(s) without much concerns about the service aspects. Thus there are fewer impacts for migration. However core network during the migration process should be prepared providing managed capabilities in terms support of QoS, security and mobility, but the level of capabilities should be identified according to the given conditions of each country or operator.
Figure 24 shows an example of structural configuration of core network during the migration process. It is expected that physical medium should be aimed using optic fibre based infrastructure whether SDH, IP, Ethernet, WDM will be used on top of the fibre together or not. In the case of switching capabilities, it is expected to use softswitch which are core part of call-server based NGN to support voice oriented services. In the case of supporting content oriented services such as IPTV and VoD (Video on Demand), it is required to use relevant servers.
Figure 24 – Core network structure during migration
Access network, the most difficult part, has very complicated situations in terms of technologies as well as geographical coverage and distributions. Many of different technologies have been developed and used in access networks. Fortunately most of technology provides IP connectivity which is the critical technical feature to meet the NGN requirements. In fixed access networks, xDSL is mainly used for broadband access. xDSL gives the opportunity to use existing copper based access infrastructure as much as possible for deploying broadband infrastructure in economic way, but with limited capacity (maximum few 10s Mbps). Fibre optics is a kind of target technology in the area of fixed networks with its unlimited capacity. Only concerns are related to the cost and construction difficulties. Both concerns will be faced by the quick development of the technology. In any case, it is not recommended to choose only one specific technology to replace any legacy access network systems. It is rather recommended to consider harmonization among different technologies to cover customer requests in more flexible and economical way. Therefore it is recommended to use both xDSL and Fibre together in the access network as a preparation of migration to the NGN including preparation of enough broadband capability.
Following Figure 25  shows an example how access networks in fixed networks be migrated taking consideration of service enhancements such as voice only, double play and triple play.
Note: HG (Home Gateway), RSU (Remote Service Unit), DSLAM (Digital Subscriber Line Access Multiplexer),
MSAN (Multi-Services Access Node), GB (GigaBit), MDF (Main Distribution Frame)
Figure 25 – Migration in MDF site
It is useful and easy to explain access network migration based on the migration of MDF (Main Distribution Frame) as shown in Figure 25. A MDF is a signal distribution frame or cable rack used in telecommunication to interconnect and manage telecommunication wiring. The MDF connects equipment inside a telecommunications facility to cables and subscriber carrier equipment. Every cable that supplies services to user subscriber lines ends up at an MDF and is distributed through MDF to equipment within local exchanges.
Legacy networks, especially using traditional local loop (i.e. twisted pair cable), generally connected through RSU (including termination function of digital subscriber loops) to the local switch. This will be replaced with DSLAM if need to provide broadband access connectivity which forms also fundamental requirement of NGN and internet. If considering the triple-play which provides legacy services as well as content based services such as IPTV and VoD, it should be supported by MSAN jointly with HG.
Signaling and control aspects [b-ITU-T Y.2261]
PSTN/ISDN uses signalling systems such as analogue line signalling, channel associated signalling (CAS) like signalling systems R1 [Q.310-Q.332], R2 [Q.400-Q.490], and common channel signalling (CCS), like SS7 or Digital Signalling System No. 1 (DSS1) [Q.931]. All these signalling systems are for the circuit switched networks. Since NGN transport is packet-based (and call and bearer are decoupled), other suitable types of signalling (e.g., BICC, SIP-I [Q.1912.5], etc.) may be required. Also, the signalling function and call control function may reside in more than one NGN element.
Since the NGN has to communicate with the PSTN/ISDN and other networks, interworking between the NGN signalling systems and the legacy network signalling systems is required.
The NGN naming, numbering and addressing schemes, in accordance with ITU-T Recommendation Y.2001 shall be able to interwork with the existing E.164 numbering scheme. During PSTN/ISDN migration to the NGN process, it should be ensured that the country code numbering, naming, addressing and identification plans, is fully maintained. Also, as a minimum, support should exist for Internet IP addressing schemes including E.164 Telephone uniform resource identifiers (TEL URIs), e.g., tel: +98 765 4321 and/or SIP Uniform Resource Identifiers (SIP URIs), e.g., sip:firstname.lastname@example.org.
Management aspects 
PSTN/ISDN management is comprised of activities from a core exchange network, access network, intelligent network and the operations support system (OSS). ITU-T Recommendation M.3400 and M.3010 provide management principles for PSTN/ISDN. An NGN management system is comprised of three planes, namely the network management plane, the network control plane and the service management plane. Each of the three planes implements corresponding management functions to each layer in the NGN layered model.
Migration of PSTN/ISDN management (i.e., operations, administration and management) systems requires the ability to support the transition of PSTN/ISDN through intermediate stages towards the NGN.
Operation, administration and maintenance (OAM) functionality is used to verify network performance, and to reduce operational expenses by minimizing service interruptions, service degradation and operational downtimes. As a minimum, when performing PSTN/ISDN migration to the NGN, the ability to detect faults, defects and failures such as lost, errored or mis-inserted packets, should be provided. Additionally, there should be mechanisms to indicate connectivity status and provide support for performance monitoring.