der consid- eration. However, this gives a quick reference and a way for a first comparison among different models. Thus, it is useful to briefly describe the functions of the seven layers of the OSI model, keeping in mind that it was standardized assuming a computer network while present days public networks, even if based on packet switching, convey a much more complex set of services [13]. 1. Layer 7, application layer: This layer supports application and end-user processes. Communication partners are identified, quality of service is defined, user authen- tication and privacy are considered, and any constraints on data syntax are identi- fied. Everything at this layer is application specific. This layer provides application services for file transfers, e-mail, and other network software services. 2. Layer 6, presentation layer: This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. * It is to be noted that the acronym TCP in this context is used as "Terminal Connection Point" is also used with the meaning of "Transmission Control Protocol." describing the internet protocol suite. The two meanings indi- cate very different concepts SO that no difficulty should arise to identify the correct meaning from the context. Telecommunication Networks 3. Layer 5, session layer: This layer establishes, manages, and terminates connections between specific applications. The session layer sets up, coordinates, and termi- nates conversations, data exchanges, and content broadcast between the applica- tions at each end. 4. Layer 4, transport layer: This layer provides end-to-end data transfer between the hosts; end-to-end message integrity and flow control are delegated to this layer. Layer 3, network layer: This layer provides routing technologies, creating logical paths in case virtual circuits are used; otherwise the layer functions process indi- vidual datagrams at each network node. Routing and forwarding are functions