This ITU-T Recommendation contains signalling requirements for the Network-to-Network Interface (NNI) protocol. The requirements are to support the capabilities that are recommended in the IMT-2000 Framework document and specified as Capability Set 1 (CS-1). This ITU-T Recommendation covers requirements for four communication groups of the NNI: Call and Bearer Control, Mobility Management, Virtual Home Environment (VHE) Service Control, and Packet Data Services and Internet Access Control. The requirements for inter-network security are described at a high level, and its functionality is incorporated into applicable communications group protocols. The requirements specified in this ITU-T Recommendation are non-information flow related, and they should be viewed as complementary to the information flows of ITU-T Recommendation Q.1721. They include general NNI protocol requirements, NNI functional models, NNI reference points, state models for selective functional entities, and the choice of various protocol suites.