Computer Networks

The goal of a computer network is to fulfill tasks in a more efficient way. Some tasks can only be handelt by a network. The following list contains some of these use cases:

Communication and Information Cluster: Providing devices and connection to handle communication.
Function Cluster: Providing devices and programs that are only available on some computer of the network.
Data Cluster: The logical coupling of separate data streams, which is supported through system structures that insure safety, consistency and topicality of data.
Availability Cluster: Providing a minimum viable system, especially if some components of this system fail (error tolerance).
Performance Cluster: A lot of computer work at the same problem by splitting the it into smaller sub problems. The performance is limited by the maximum achievable parallelism.
Load sharing Cluster: Distributing incoming request across multiple computer to minimize the load on the individual.

OSI-Reference-Model

The Open System Interconnection reference model is a protocol that defines layer which provide a certain set of functionality to the layer directly above it.

Physical Layer: Bit sequences are transmitted.
Data Link Layer: The bit sequences are treated as data packets, which are assigned addresses (MAC address, Media Access Control) and a bit sequence for error detection. Additionally, controlled access to Layer 1 is ensured. (Ethernet, ATM)
Network Layer: The transmission path in the network is determined. The appropriate addresses (IP address, Internet Protocol) are assigned to the data packets, and they are transmitted. Error detection and correction are also performed. (IP)
Transport Layer: The transmission quality and type between sender and receiver are negotiated, and further error handling is carried out. Large messages are segmented into packets. (TCP)
Session Layer: The connection during a session is maintained. Login, password verification, dialog control, and logout processes are handled.
Presentation Layer: Different data encodings are converted. Data is compressed or decompressed, as well as encrypted and decrypted.
Application Layer: Application-specific services, such as email, file transfer, and use of the World Wide Web, are implemented here (TCP, HTTP, FTP, SMTP, Telnet).