aders. 2 We will use the layer in Fig. 2.1 as the starting point for presenting some basic concepts that are associated to OSI RM layers. Provided layer interfaces Layer Type of provided layer interfaces Collapse node Layer element structure Protocol Types of used layer interfaces Used layer interfaces Fig. 2.1 The OSI RM layer concept; L=layer dimension; N=node dimension We say that the model to the left depicts an intralayer structure, showing pro- tocols, individual layer elements, and layer interfaces. When we collapse such a structure to a single layer symbol (to the right), layer interfaces become types and the actual distribution of layer elements becomes undefined. The protocol specifi- cations belong to this model, since they are used for creating intralayer structures. Through the OSI RM we have learnt that a layer interface is what connects layer element of one layer to a layer element of another. We do not intend to change this simple principle for identifying what belongs to different layers. Thus, by defi- nition, two layer elements that are interconnected over a mutual layer interface belong to different layers. A layer interface is, similar to protocols between machines in a network, an interface used for communication. In the OSI RM tradition, however, while proto- cols define the communication between nodes, elements that are interconnected ITU-T calls its documents "recommendations." We will however use the term "standard" for all documents published by standardization bodies. If not, look for any text book on data communication. With few exceptions, they all recapitulate OSI RM on some level of details. The ITU-TX.200 recommendation and the ISO 7498 stand- ard are official descriptions of the OSI RM. 2.1 Concepts Based on the OSI RM over a layer interface always exist in the same node. We will later change this assumption from "always" to "for the most part," since there are an increasing number of layer interfaces that are implemented as protocols as well. However, sticking to the OSI RM for 