REFERENCE MODELS OF NETWORK
INTRODUCTION :-
The problem of getting a computer of one brand to accept the data created by another manufacturers machine has confirmed users of these machine since their earlier days.
At that time the only practical way around this barrier was to retype the data from the computer into the second.
This is very time consuming procedure and has the chance of lot of errors.
As the computer proliferated, interfaces or adapters we are inverted that translated one machines code into a from that other could understand altering a variety of computers to be hooked together in a network.
But these interfaces were expensive both in agreegate because a substantial number of them needed to establish a large network, and singly, because each type of computer needed an interface of its own.
As a result there was little opportunity to spread the cost of developing give interfaces by selling many copies of it.
Networking would be facilitated and the expense much reduced if computer manufacturers could agree on how to construct their products, so that each could communicate with the other.
A master plan computer to dialogue, the OSI model devices the communications process into seven layer.
The model sets standards that permit a wide variety in the design of computer hardware and software.
OSI MODEL :-
An open system interconnection (OSI) covers all the aspects of ISO (international standard organisation).
An open system is a a model that allows any two different systems to communicate regardless of their underlying architecture.
OSI model is not a protocol but is a model for understanding and designing the network architecture that is flexible robust and inter - operable.
NETWORK ARCHITECTURE MODELS :-
The layered concept of Network architecture was built into many system but the protocols used and interface standards were different for them.
The layered partitioning were also different and were not matching in all of them. So there was an integration in compatibility of architectures developed by different vendors. Was realised later that standardization of Network architecture can solve many problems and save a lot of effort required for developing interfaces for networking different architectures.
Several network architectures were developed by standardization organizations by manufacturers.
Some of the important standard architectures are :
* open system interconnection (OSI) reference model developed by ISO and CCITT.
* Prime computers network (PRIMENET)
* IBM's system network architecture (SNA)
* Digitals Digital Network Architecture (DNA)
* Burroughs Network Architecture (BNA)
Among various layered architectures the OSI model has been accepted as an international standard. Let us discuss it in more detail.
OSI REFERENCE MODEL :-
This layered network architecture model was developed by international organization for standardisation (ISO) and consultative Committee for international telephone and telegraph (CCITT).
The development of open system interconnection was the first step towards standardization.
It provides a general concept of inter process communication in such a way that any open system may communication with anther open system technically without any problems.
Here open system means a system that can be inter connected to other systems according to established standards architecture and protocols.
It is a seven layered architecture model. In the model the communication process is decomposed into hierarchical functional layers.
It also identifies the standards necessary for open system interconnections development.
In this model the LAN communications are divided into seven layers and each layer is insulated from the other by a well defined interface.
The seven layers architecture of OSI models along with Protocols and interfaces.
* The seven layers of OSI model of OSI model are :
1. Physical Layer
2. Data Link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer
The model is designed in a highly structured way. It defines a separate set of Protocols for each layer and hence each layer has a specific independent function. The goals of model are.
GOALS OF OSI MODEL :-
To provide standards for communications between computers /systems.
To remove technical hindrance and stammering of communication between systems.
To define points of interconnection for the exchange of information between systems.
To increase ability to communicate.
To provide a reasonable point of departure from standards in case they do not meet all needs.
To prevent user from description of internal operation during communication, of a single system.
The international organization for standardization (ISO) developing the open system interconnections (OSI) references in 1977.
It has become the most widely accepted model for understanding network communication.
In the OSI model nothing is tangible it is simply conceptual framework which can be used to better understand the complex interaction taking place among the various devices on a network.
The OSI model does not perform any function in the communication process.
The actual work is done by the appropriate software and hardware. The OSI model simply defines which tasks need to be done and which Protocols will handle those tasks.
The OSI model divides communication tasks into smaller pieces called subtasks.
Protocols implementations are computer processes that relate to these subtasks.
Specific Protocols fulfill subtasks at specific layers of the a model.
When these Protocols are grouped together to complete the a whole tasks, is Protocol stack a protocols stack is a group of Protocols arranged on top of each other as part of communication process.
Each layer of OSI model has different Protocols associated with it.
Each layer in the protocol stack receives services from the layer below and provides services to the layer above.
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