The Q.39xx-series of Recommendations relates to testing methods and specifications for model networks. The model network concept was created in 2006 due to the complexity of next generation network (NGN) equipment over digital networks, the unlimited services set in principle, the level of guaranteed quality of service (QoS) for separated QoS classes, and so on. The global interoperability concept was also created as a next step in testing methodology. Global interoperability includes the interoperability of technical means, of services, and of QoS classes and parameters.