Dcn Notes

| | IntroductionThe term “computer network’’ means a collection of autonomous computers interconnected by a single technology. Two computers are said to be interconnected if they are able to exchange information. The connection need not be via a copper wire; fiber optics, microwaves, infrared, and communication satellites can also be used. Networks come in many sizes, shapes and forms. Uses of Computer Networks 1. Business Applications 2. Home Applications 3. Mobile Users 4. Social Issues Example NetworksThere are different kinds of networks, large and small, well known and less well known. They have different goals, scales, and technologies.The InternetThe Internet is not a network at all, but a vast collection of different networks that use certain common protocols and provide certain common services.The ARPANETThe story begins in the late 1950s. At the height of the Cold War, the DoD wanted a command-and-control network that could survive a nuclear war. At that time, all military communications used the public telephone network, which was considered vulnerable. NSFNETBy the late 1970s, NSF (the U.S. National Science Foundation) saw the enormous impact the ARPANET was having on university research, allowing scientists across the country to share data and collaborate on research projects. However, to get on the ARPANET, a university had to have a research contract with the DoD, which many did not have.Internet UsageTraditionally (meaning 1970 to about 1990), the Internet and its predecessors had four main applications: 1. E-mail. 2. News. 3. Remote login. 4. File transfer. | | | | |

| | | | | | | OSI LAYERS | Group | # | Layer Name | Key Responsibilities | Data Type Handled | Scope | Lower Layers | 1 | Physical | Encoding and Signaling; Physical Data Transmission; Hardware Specifications; Topology and Design | Bits | Electrical or light signals sent between local devices | | 2 | Data Link | Logical Link