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2                                        ITU-T Focus Group IMT-2020 Deliverables


            and the access type used. And functional entities are likely to be distributed on-demand basis over the
            infrastructure. Therefore, in the IMT-2020 architectures, there shall be a clear decoupling of service, network,
            and infrastructure, allowing them to be offered separately and to evolve independently. The separation is
            represented by three strata in the figure.

            In  general,  each  stratum  will  have  its  own  set  of  roles,  players  and  administrative  domains  (refer  to
            [b-ITU-T Y.110]). The roles involved in service(s) provision are independent from those involved in network
            connectivity provision. Each stratum needs to be treated separately from a technical point of view. In the
            figure, Multi-domain orchestration refers to the automated management of services and resources in multi-
            technology  (multiple  domains  involving  different  cloud  and  networking  technology)  and  multi-operator
            (multiple administrative domains) environments [b-ITU-T IMT-O-047].

            I.1.1.1    IMT-2020 network stratum
            By  virtue  of  softwarization  and  pervasive  deployment  of  computing  infrastructure,  network  function
            instances  can  be  distributed  on-demand  basis.  This  brings  IMT-2020  network  architecture  a  shift  in
            networking  paradigms:  a  transition  from  deployment  of  network  entities  with  fixed  functionalities
            (e.g., PGW, SGW, etc.) to deployment of network functions as necessary.

            IMT-2020 network stratum provides network services requested by service stratum using network functions
            instantiated at right place. IMT-2020 infrastructure stratum provides required infrastructure resources to
            instantiate the network functions.

            The functional architecture of IMT-2020 network stratum shall provide a complete set of network functions
            required  to  support  all  IMT-2020  services.  Reference  points  and  information  flows  between  interacting
            functions will be defined in the architecture, too. As new services emerge in the future, interconnection
            model between network functions needs to allow easy insertion of new network functions while maintaining
            the existing network function interconnection path without increasing complexity of the system.

            Network operators can provision and operate many different network slices according to their business
            strategies. A network slice is comprised of only necessary network functions. They are collected from a
            complete set of network functions in the IMT-2020 network functional architecture, and orchestrated for the
            particular service and purpose.
            Life  cycle  management  including  orchestration  and  instantiation  of  network  slices  is  performed  by
            management plane network functions (e.g., NSMF). Management plane is also responsible for the life cycle
            management of network functions.
            Network functions in control plane and user plane operate on instantiated network slices, i.e. network slice
            instances. Fundamental control plane network functions include ANCF (Access Network Control Function),
            NSCF  (Network  Slice  Control  Function),  MMCF  (Mobility  Management  Control  Function),  SMCF  (Session
            Management  Control  Function),  PCRF  (Policy  and  Charging  Rule  Function),  and  AAAF  (Authentication
            Function), etc.
            Figure 5-3 shows IMT-2020 network architecture framework. High level functional architecture of IMT-2020
            is defined in clause 6. Each network function is comprised of one or more functional entities.






















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