vlapp17In the 1990s, there was no standard way to communicate between the different networks, operating systems and computers available. As a consequence of this interoperability problem the International Standards Organization (ISO) developed the OSI (Open Systems Interconnect) model. It was a guide on how to get all computers, networks and operating systems to smoothly communicate with each other. The OSI model describes the network protocol stack which is a set of rules that determine how data is communicated over a network. It consists of seven layers that each have a special function:
| Application | Consists of network aware applications like e.g. email, web browsers, file sharing services etc. |
| Presentation | Its job is to configure the data if needed, e.g. through encryption |
| Session | Its job is to control the communication by for instance coordinating login rights etc. |
| Transport | Guarantees end to end delivery of data (e.g. by verifying existence of destination, using ACK packages, error checking of data) |
| Network | Finds best path to the destination network (responsible for e.g. routing, traffic control) |
| Data Link | Guarantee reliable transmission through managing usage of the (shared) transmission medium and finding physical device on the network |
| Physical | Describes physical part of the network, e.g. cables, responsible for transmission of bits to physical destination (bit encoding, transfer rates) |
TCI/IP is the implementation of the OSI model that is used for the Internet. It consists of many protocols that are all sorted into four layers which correspond to the OSI model. Each layer has a PDU that describes the layer’s information.
| Internet protocol (TCP/IP) | OSI model | Protocols | PDU (Protocol data unit) |
| Application | Application, Presentation, Session | HTTP, FTP, etc. | Data |
| Transport | Transport | TCP, UDP, etc. | TCP: segments, UDP: datagrams |
| Internet | Network Layer | IP, ICMP, etc. | Packets |
| Network Access | Data Link, Physical | Ethernet, ISDN | Data link: frames, physical: bits |
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