. Hosting networks are identified in T-RE tables by their NWi. Hosted networks are identified in T-AP tables by their NWi. This is because we want the model to be valid regardless of how a network system will be realized. Layer interfaces, as we have seen in Chap. 2, are defined only with respect to which operations and abstract service primitives (ASPs) they support. What is missing, therefore, is the connectivity support that may be required to allow two layer elements to address each other and communicate over a layer interface. For vertically-partitioned networks, this is actually not an AMLn problem, but some- thing to be solved by implementors. For horizontally-partitioned networks, and when one wants to simulate the behavior of a model, communication over layer interfaces of concern must be specified. In principle, it can be done in two ways: 1. If one wants the two layer elements to be remote to each other, one must define an interlayer protocol that can carry the events (i.e., instances of ASPs) of the interface between the elements. An additional connectivity stratum is then also needed to support that protocol. This is actually a refinement of the original model (since it creates additional protocols and layers), and may therefore be defined as a part of the model itself, but on a lower network level. We describe this kind of refinement in details Chap. 4. 2. In simulation, and on the lowest network level in solution models, layer ele- ments are no longer remote, but run as applications on the same processor or on a common processing platform, which are the elements that provide the connec- tivity for layer interfaces. There are many different design languages and platforms available for this kind of connectivity. However, no matter which method is used to configurate layer ele- ments correctly in that context, identification of layer interfaces will be required. We call such an identifier layer-interface identifier (Lli). A layer structure can be refined to a model that can be simulated or executed by