Scope and field of application This Recommendation defines those functionalities of subnetworks which relate to the OSI connection-mode network service in terms of: a) the actions and events which occur at the interfaces to a subnetwork; b) the parameters associated with each action and event@ and the form which they take; c) the interrelationship between@ and the valid sequences of@ these actions and events@ for a given connection; d) the interrelationship between different connections established through the same subnetwork. This Recommendation also defines the ways different types of subnetworks support the OSI connectionmode network service@ by including within the subnetwork part or all of the functionalities of subnetworks which relate to the OSI connection-mode network service. The principal objective of this Recommendation is to provide guidance for consideration of interworking between subnetworks@ in relation with the support of the OSI connection-mode network service. This Recommendation does not specify products@ or implementations of those functionalities in real network equipment@ nor does it constrain the distribution of those functionalities among the pieces of network equipment considered within a given subnetwork (e.g. PDNs@ IWFs@ ISDNs@