The original OSI model has been in existence for many years. It was originally formulated for use by application and network protocol designers so that they could come up with products that exhibited an open standard. This meant that it was possible to come up with a standard that could be used in the development of services, applications, interfaces as well as network devices. It is referred to as a model and not a specification because it seeks to define layers as well as services for the same layers and their operations. It also lays out how these layers interact amongst themselves especially how a particular layer operates with the one below and above it.
The model has been likened to a representation of the workings of a network especially the phone call. Let us assume you want to make a phone call. In order for your phone call to go through, there has to be some kind of physical connection of an electrical nature between the two contacts or locations. The initial action of unhooking the phone and making a call can be likened to the first physical layer and the second one, which involves data link services. Dialing a number is network service or layer three. The process that goes on at this level is also known as phone number routing.
When your call gets through and someone answers on the other end, this is layer 4 transport as well as session services or layer 5. Layer 5 also involves listening and talking. The language that you end up using to pass on your message to the person on the other side is presentation or layer 6 and application services or layer 7. All modern state of the art data networks use the above design principles based on the model. The PC that you use at home or at work is always connected to a server used for various applications. Communication between the two is made possible by a physical connection in the form of a UTP cable or unshielded twisted pair.
This is usually layer 1 and it basically involves electrical signaling of the campus switch from the desktop computer and on to the server. Data link protocol or Ethernet of layer 2 is used to establish connection to a campus switch. This is made possible by a card known as the desktop network interface. Before data is sent, your desktop has to have an idea of where it is sending the data to. This is where the servers address comes in handy. This is level 3 also known as network service. The fourth layer of the model or transport service ensures that the desktop and server have a logical connection between them.
This guarantees that all traffic flow between the desktop and the server is coordinated. The fourth layer is also charged with error correction and packet re transmission. The fifth layer maintains the logical connection between the server and the desktop and their application processes. The sixth layer undertakes data formatting while layer 7 of the model defines API that is used by applications to come up with other specific applications that are compatible with them.
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This article touched the basics of the topic. I have 2 more resources related to the above. They are osi model and osi model . They are worth a read.
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