The OSI reference model was originally developed in the 1980's because there were many different network implementations. It became a guide for networking vendors so that they would make compatible products. It is comprised of 7 layers.
Physical (Layer 1) defines the physical characteristics of the network hardware, such as cables, connectors, repeaters, and hubs. It also encodes binary digits and transmits signals on the wire.
Data Link (Layer 2) controls NIC software functions including identification of source and destination nodes via MAC addresses. This layer defines how data is packaged for transport and also provides error control.
Sublayers:
-Logical Link Control (LLC) Layer which defines how data is packaged.
-Media Access Control (MAC) Layer which defines the media access method and provides MAC addresses.
Network (Layer 3) provides logical addressing for data packets and routes data. It implements traffic control measures, determines the route from source to destination based on network conditions, and provides software/logical addressing for data packets.
Transport (Layer 4) is responsible for data transportation and QoS. TCP and UDP are transport layer protocols.
Session (Layer 5) enables two network nodes to have ongoing communication. It defines when the transmission begins and ends.
Presentation (Layer 6) deals with data translation, formatting, syntax reconstruction, encryption, and compression. It prepares data for the application layer.
Application (Layer 7) provides user interaction to the computer and network. It contains the protocols which provide application services.
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