a layer element in the traffic plane (which is described by an element state machine (ESM)) and the resources on which that behavior relies since only the latter are the things to be managed. We touched on this subject already in Sect. 2.3.2.1. As an example, let's assume a layer element that relies on a local routing table (T-RO, which is a typical managed resource in a control layer). Figure 5.2 shows the refinement of that layer element. 5.2 Two Systems and Two planes Actor Actor Layer element Remote Remote layer element Manageable resource Fig. 5.2 Refinement of a layer element in the traffic plane into actor, agent and manageable resources 1. The model to the left shows how we model this resource in the layer structure of the traffic plane. 2. In the middle we separate the layer element into an actor and an agent. The SAG is called SAGT, where "T" denotes that it connects to an agent layer of the traffic plane. We also introduce the ESM of the actor and define a separate abstract machine that handles the table. This machine allows the ESM to use information stored in the table to control traffic system functions. The operations available to the ESM are defined by the rCP. 3. Since local resources are not visible to remote elements, we can move the resource out of the actor, as shown to the right, thereby making it an abstract machine that can also be accessed from the management plane. Let's now add a function that can maintain the table resource (see Fig. 5.3). Actor Traffic node (TS node) Management system (MS) Resource Fig. 5.3 The principle for managing traffic system resources from a management system. AF=adaptation function; CP=control point; ESM=element state machine; mpCP=management plane CP; mrCP=managed-resource CP; OSF=operations system function; rCP=resource control point; SAG=specified agent interface This function is assumed to be part of the management system, and therefore an operations system function (OSF). It controls the contents of the table over a (local) mrCP in the traffic node, via a