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Internet Architecture- OSI Model
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Due to the high demand for internet applications, and the ease of network programming using the internet standards, this turorial will be solely on internet programming. This short article will demonstrate the fundamentals of the Internet that make it so unique.
ISO's OSI
The first standard that was proposed was the OSI layered standard by the ISO organization. The following diagram illustrates the architecture of this standard.
Application |
Presentation |
Session |
Transport |
Network |
Data Link |
Physical |
These layers form the foundations of the OSI architecture. Each layer is assigned a specific task as stated in the standard. Each layer is also capable of communicating with the layers above and below it. For example, the data link layer is capable of using the the physical and the network layers. However, it does not have access to the transport layer.
The specific tasks of each layer is as follows:
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Physical |
The physical layer specifically deals with the transmission of the raw bits over a medium. It should be reliable (to some extent) in transmitting the data such that a 1 is received as a 1. |
Data Link |
This layer converts a raw transmission line into an error free line which will be used by the network layer. It will use error detecting methods such as CRC, parity checking etc. |
Network |
The network layer is responsible for the operation of a subnet (small network of computers). Such operations include routing and congestion control. It is also responsible for making a heterogeneous network even though some parts of the network maybe optical and the rest may be ethernet. The network layer should be independant of the type of physical networking equipment connected to a computer. |
Transport |
The transport layer is responsible for breaking data which is sent from the Session layer into smaller parts (packets), such that they can be send to the destination by the layers below. Furthermore, it must ensure that the packets arrive in the correct order and they do arrive at the destination. This is called the error-free point-to-point service of the transport layer. Other services can be used. |
Session |
The session layer provides the means to form sessions at the different computers. It provides token management (which is a way of ensuring that only the computer with the token is doing a specific task). It can also provide dialogue control, which takes care of the direction of data flow (both directions or a one way traffic), and it would also know whose, turn it would be. |
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Presentation |
The presentation layer is concerned with the syntax and the semantics of the data being transmitted. An example would be to ensure that computers using Unicode and ASCII standards can communicate. |
Application |
The application layer serves to aggregate the different standards in terminal type and file transfer into one standard that can be used anywhere. |
This architecture, however failed to be used because of the complexities involved in specifying the task of each layer. As an example the tasks of the session and presentation layer were oftern combined together. However, the OSI layer did form the foundation of the present day internet, the TCP/IP reference model. The TCP/IP reference model uses the concepts of abstraction and layering found in the OSI reference model to form a simpler and more precise way of communication.
TCP/IP
The TCP/IP reference model is the Internet's standard for communication. The following diagram represents this model's architecture.
| Application |
| Transport |
| Internet |
| Host-to-Network |
This reference model greatly resembles that of the OSI model. However, it is simpler by being smaller with fewer layers..
The following provides an overview of the tasks performed by each layer.
Host-to-Network |
The host-to-network layer is the same as the physical layer in the OSI model. It performs the same function. It is also responsible for the tasks performed in the data-link layer of the OSI architecture. |
Internet |
The Internet layer is the main layer in the TCP/IP model. It performs the routing, data fragmentation etc. required to transport data from one computer to another. It offers a best attempt at delivery of data. i.e. It would try its best but if it fails, it fails and would not attempt a recovery! |
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Transport |
There are many choices here, the simplest of which is the UDP transport which is a guaranteed delivery of data, but it does not guarantee the order in which data is received. This is further explained in the packet section. Another transport protocol is the TCP which ensures that data will arrive, and that it will arrive in the correct order. This is further expanded upon in the section on streams. |
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Application |
This is the application which is providing a service to the user and requires networking facilities to be able to provide such services. |
This concludes the section on the Internet architecture.
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