The top layer is the application layer 7. It includes application programs for electronic mail, browsing the Web, data transfer or remote login.
These applications have to respect a certain standardized syntax and semantics, which are defined in the presentation layer 6. Protocols for file transfer (FTP), remote login (telnet), electronic mail (SMTP), gopher, wais, WWW (HTTP) etc. are defined here. In addition, data conversion between different platforms (e.g. ASCII to EBCDIC) or data encryption may take place.
The World Wide Web uses another protocol to transfer the contents of Web sites from one place to another. It is called HTTP/1.1 (HyperText Transfer Protocol, Version 1.1). Using this protocol a WWW-client (e.g. a Browser) can ask a WWW-server to send a certain Web page. As soon as the client receives the data, he has to interpret them in a sensible way. This process of interpretation is governed by HTML, the HyperText Markup Language. It is a sort of ``programming language'' which resembles SGML (Structured Generalized Markup Language), a standardized (ISO 8779) document formatting language.
The session layer 5 is responsible for communication between processes on different hosts. In practice the real task of this layer is rather obscure and therefore of no major importance. In fact, the Internet does not know any presentation or session layer, which were invented some twenty years after the development of ARPANET. Therefore the tasks described above are all included in the application layer.
The transport layer 4 provides services for opening network connections, transferring data, and closing connections. On the Internet this is done by the Transfer Control Protocol (TCP). Any data to be transferred have to be split up in small packets. A header containing information about the source and destination port, a sequence number for correct rearrangement of the packets at the destination, and a checksum for detection of data loss is attached to each packet.
The network layer 3 is the level of the Internet Protocol (IP). Its main task is the routing of datagrams to their destination. Since there is usually no direct connection between source and destination host, the data are sent from one host to the other, each finding for each datagram individually the best way to its destination. This technique makes it possible to by-pass defective or overloaded connections.
The data link layer 2 has to provide services for flawless data transmission. For this task the data are once again split into small frames which usually carry a checksum at the end. The receiving host checks the incoming frames for data loss and acknowledges correct transmission or asks for retransmission. Another important task of this layer is to control the data flow. This means that the speed at which data are sent and the speed at which the receiver processes the data have to be synchronized.
The physical layer 1 describes the physical connection between the hosts in a network, basically how single bits (digital) or data units (analog data transfer) can be transmitted. Data transmission can be effected using twisted pair, coaxial or light-wave cables, radio transmission or satellite links.