Network Basics: OSI model

The Open System Interconnection (OSI) Reference Model was developed by the International Standards Organization (ISO). It is an attempt to build a framework of layers, in which various protocols in computer networking fit.

The OSI model consists of seven layers which are:

1. The Physical Layer: transmits raw data bits over a communication channel (mostly mechanical and electrical issues)
2. The Data Link Layer: guarantees to the network layer that there are no transmission errors by breaking the input datastream up into frames and sending back acknowledgement frames
3. The Network Layer: controls the operation of the involved subnet; main issues are routing (determine a way from source to destination) and dealing with problems of heterogenous networks, e. g. different size requirements of transmitted data blocks
4. The Transport Layer: splits up data from the session layer if necessary (segmentation) and ensures that the pieces arrive correctly
5. The Session Layer: allows users on different computer systems to establish a session between them, i. e. they are able to transfer files or log into a remote system; the conditions of communication are laid down, for example full-duplex or half-duplex
6. The Presentation Layer: unlike the layers before it is concerned with the syntax and semantics of the transmitted information; it is concerned with all aspects of information representation such as data encoding, data compression and encryption
7. The Application Layer: contains a variety of commonly needed protocols like handling with different terminal types and file systems; a label to identify the communication process, its origin and destination application is added to the transmitted information

Some of the functions of the physical and the data link layer are combined in the Medium Access Control (MAC) sublayer, which in particular is important to Local Area Networks (LANs). It determines how devices attached to the network gain access to the transmission medium.

A communication between two systems via a relay system (another computer or an interconnection device), causes the following flow of information between the OSI architectures of the involved systems:

Note that the OSI model does not lay down the specific protocols used to communicate between two computers on a specific layer. Although ISO recommends which protocols to use with the OSI model, the model itself is in proper speaking no standard of computer networking.
Which protocol in a single layer is actually used, depends on several factors like the physical network, the needed reliability, etc.

There is also an external source about the OSI Reference Model reachable.