1. End devices, also referred as hosts: The sources and destinations of the communication (i.e., devices that generate and transmit messages or receive and interpret messages). The devices the end users are most familiar with. These devices act as the interface between the end users and the underlying network. Examples: Computers, IP phones, mobile phones, PDAs, etc. 2. Intermediary devices: Devices that give network access to the attached end devices and transport the messages between hosts. Usually, transparent to the end users. Also, these devices accomplish communication functions in order to ensure the success of the communication process. Examples: Hubs, switches, routers, modems, firewalls, etc. 3. Transmission media: The physical media that connects the devices, enabling the exchange of messages between them. It may be wired, that is, some copper cable (allows the propagation of electric voltage or current pulses) or optical fiber cable (allows the propagation of light pulses), or wireless, that is, some radio link (allows the propagation of electromagnetic waves). 4. Services: Network-aware software applications (e.g., a web browser) that request network resources (e.g., data) in order to enjoy the end user of the application some provided service (e.g., World Wide Web). 5. Processes: Software that runs on network devices in order to support the communication functions - in accordance with the established, also in software, communication rules or protocols - and facilitate the provision of services to the end users. In contrast with the services, processes are transparent to the end users. 6. Messages: Well-known applications. Includes telephone calls, e-mail, web pages, etc. Devices and media are the physical elements or hardware of the network, whereas the services and the processes are the computer programs or software of the network. We have to note that modern computer networks are converged data or information networks, meaning that can carry any type of information (i.e., text, voice, graphics, video streams, computer data and files) between a wide range of devices (e.g., computers, IP phones, PDAs, etc.), and provide previously separate and distinct conventional communication methods and services, as well as new, alternative communication forms (e.g., telephony, e-mail, instant messaging, file transfer, printer sharing, hard disk sharing, collaboration tool, video conference, etc.) onto a common platform.
