 by TCAP. This implies that TCAP agents should not be drawn in an OSI layer-structure model as if they were a part of an ISDN call-handling layer element. A common agent layer must always be drawn as a separate layer, related by actor-agent interfaces to other layers, as illustrated in Fig. 2.38. 18 The author's most significant personal experience in this respect was when working on a large mobile network system project. As usual, designers had a very fragmented picture of how call handling was supposed to work in the system, due to the way standards were written. One designer had the ambition to understand how all fragments were related, and acquired the deep and overall knowledge of call handling. By investing man-years of efforts, he developed and gave a course that described call handling as (the correspondence to) an LSM model. This course always sold out. It became the most popular course by many hundreds of designers and testers, and a major contributor to the overall success of the project. 2.3 Agents and Actors AAIs used by actors of other actor layers The ISDN call-handling layer (OSI layer) TCAP messages Call-handling layer element in an LE The TCAP common agent handling AAI used DSS1 L3 layer actor for INAP T-ROs Resources MTP L3 ISDN local exchange (LE) Fig. 2.38 Common agent layers, such as TCAP, must be described as separate layers 2.3.3 Agents and Actors in Connectivity Layers 2.3.3.1 General Modeling Principles A completely different type of actors exist in connectivity layers. We refer to any type of functional element that transports data between some kind of inlets and out- lets, without changing anything in the data stream. The type of functions performed by the actors varies over a wide range, but can be classified as either some type of relaying (multiplexing, cross-connecting, switching) or just termination. The term "relaying" is also used to denote elements that relay data between just one inlet and one outlet, such as amplifiers and regenerators, which we will, however, disregard in th