| OSI HISTORY |
The OSI (open Systems interconnection) model was developed by the International Standards Organization (ISO) in 1984 in an attempt to provide some standard to the way networking should work. It is a theoretical layered model in which the notion of networking is divided into several layers, each of which define specific functions and features. However it must be noted that this model only represents a general guideline for developing usable network interfaces and protocols. Sometimes it may become very difficult to distinguish between each layer as some systems do not rigorously adhere to the model. Despite all this, however the OSI model has earned the honour of being "the model" upon which all good network protocols are based.
The OSI Model is based upon 7 layers (Application layer, Presentation Layer, Session Layer , Transport Layer, Network Layer, Data Link Layer and the Physical layer). Each one of these are used to perform particular tasks, which include "supporting end user processes, encrypting the data as-well as providing transparent data transfer over a network" (Croucher P, "Communications and Networks", 1999 British Library publication).
The architectural framework of each layer is implemented throughout with software and hardware to establish how data can be transmitted transparently.
Figure 1 below highlights the responsibility of each layer according to the OSI model.
|
Layer |
Responsibility |
|
Application (Layer 7) |
The application layer consists of application programs and user interfaces. It is at this layer that user's interact with all of the systems that the model defines. It supports many features that allow exchange of information across users. For example it provides application services for transfer of e-mails, and other network software services. The applications that use these services include FTP and Telnet which exist entirely on this layer. In Addition to this the layer focuses on aspects such as "communication partners, quality of service, constraints on data syntax ,authentication and privacy". (www.internet-guide.co.uk/tcp-ip) |
|
Presentation (Layer 6) |
The purpose of the presentation layer is to represent data in such a form that it can be exchanged through a network. A typical example of this would be encrypting the data. This layer is also referred to as the syntax layer. |
|
Session (Layer 5) |
The Session layer is used for establishing, managing and terminating connections on a network. Additionally this layer is also concerned with setting up, coordinating, terminating and managing conversations between the user and network. It is at this level that the user and machine names are interpreted. |
|
Transport (Layer 4) |
The Transport layer controls the quality and reliability of the data transmission. At this layer each data packet is sequenced and acknowledged . In addition to this layer also provides transparent transfer of data between end users and the host. It is also responsible for end to end error recovery and flow control. |
|
Network (Layer 3) |
The network layer is used for routing data across configured network nodes. This layer uses many technologies to enable the data to be transmitted which are known as switching and routing. These technologies create logical (known as virtual circuits) paths for transmitting data across a network from a node to node transmission. Additionally other functions of this layer include addressing, error handling, congestion control and finally packet sequencing. |
|
Data Link (Layer 2) |
The Data link layer is primarily concerned with packaging and un-packaging data packets so that they can be encoded and decoded into bits which will enable the data packets to be transmitted through a network. The Data Link layer works as the transmission protocol by supporting error handling in the physical layer and corruption (through check summing). Additionally it is also known that the Data Link Layer is divided in to two sub-layers; these are known as The “Media Access Control” and the “Logical Link Control Layer”. The MAC sub-layer controls the flow of how a computer sends data across a network and how the network will receive it. Apart from this, it also gives permission to transmit data. Whilst the Logical Link Control Layer controls frame synchronization and flow control. |
|
Physical (Layer 1) |
The Physical layer establishes the physical connection between a computer and the network. This layer also controls the transmission of information as it specifies the mechanical and electrical characteristics of the protocol in terms of connector size, pin assignments, and voltage levels.
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