OSI Reference Model

So we have seen the different kind of networks available (not all, other we will cover later). We get a small idea what is the meaning of each type of network. As we procced we will be learning more about the structure and their standatds. So in order to understand these variations in structure and the standards we will be starting what are the layers in the OSI and the TCP/IP model and do all these depend on each other.To start with we will proceed with the OSI layer model.

OSI Reference Model

This model was developed on the proposal given by International Standards Organization which was in a sense first effort to standardize the protocols used in the various layers. It is well known that OSI has seven layers.Earlier networking was completely vendor-developed and proprietary, with protocol standards such as SNA and DECnet. OSI was a new industry effort, attempting to get everyone to agree to common network standards to provide multi-vendor interoperability. It was common for large networks to support multiple network protocol suites, with many devices unable to talk to other devices because of a lack of common protocols between them. Many of the protocols and specifications in the OSI stack are long-gone or have been superseded, such as token-bus media, CLNP packet delivery, FTAM file transfer, and X.400 e-mail. Some of them are still alive like X.500 directory srtucture because the original unwieldy protocol has been stripped away and effectively replaced with LDAP.

 

The principles that were applied to arrive at the seven layers can be briefly summerized as follows:

1. A layer should be created where a different abstrction is needed.
2. Each layer should perform a well defined fuction.
3. The function of each layer should be chosen with an eye towards defining internationally standardized protocols
4. The layer boundaries should be chosen to minimize the information flow across the interfaces.
5. The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity and small enough that the architecture does not become unwieldy.

    

   Note: OSI model itself is not a network architecture because it does not specify the exact services and protocols to be used in each layer. It just tells what each layer should do.

   The collapse of the OSI project in 1996 severely damaged the reputation and legitimacy of the organizations involved, especially ISO. This model is now overshadowed by the TCP/IP model.

   In the next sections we will be covering what layers are there in the network and there fuctionalities but this will be in reference to the TCP/IP model.