What is the OSI model ?

The OSI model:

* Allows various “open” systems to communicate.
* The Open Systems Interconnection model was created by the International
Standards Organization in the late 1970's.
* Serve as a blueprint for all network communication technologies.
* Dividing up all the processes of networking activity into seven layers.
* Each layer has its own distinct functions and services.


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 data stream 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 heterogeneous 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

Layers 4 to 7 are true end-to-end layers, i. e. the layer on the source system carries on a communication process with the same layer on the destination system. In the lower layers the protocols are between a system and its immediate neighbour, for example the source system and a system "on the way" to the destination.

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.
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. ictglobal.com

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