oose coupling ensures services operate independently across all network infra- structures. The NGN defines a technology-neutral reference model to visualize ser- vice and transport independence. The reference model is shown in Figure 2.1 and is modified from [5] to include reference points, discussed in Section 2.2.1. The reference model depicts the separation between services and transports using two strata. The network stratum contains the complex physical network resources. Also, the stratum contains network-oriented functions that abstract access and usage of both legacy and new transports and systems. These functions abstract capabilities offered by network databases, signaling gateways, and OSS/ BSS. The network stratum intersects with the customer domain, thereby abstract- ing customer terminal capabilities. Also, the stratum intersects with other network transports, thereby illustrating transport network convergence between the telco, the enterprise, and Internet networks. The service stratum contains the NGN service platform that uses network stra- tum functions. The service platform manages the service-oriented capabilities of the NGN. It enables various types of services to be developed, deployed, and managed independently of the underlying networking infrastructure. As a result, services are portable across SDP implementations. The separation between services and network is achieved by abstracting access to technology dependent network functions using service enablers. The reference model also shows the service stra- tum intersecting with the customer domain. This enables customers to access and use services offered by the service platform. In addition, the service stratum inter- sects with other network operators' service platforms. This allows the telco to expose its service enablers to external sources, such as service developers. This exposure also supports standardized interworking between SDP implementations when dif- ferent telcos want to interwork or federate across services and service enable