The OSI Model

The OSI model is a model for understanding and designing a network architecture that is flexible. It is a layered famework which allow communication across all types of computer systems. It consist of seven separate layers which define a segment of process of moving information across a network. The OSI model provides the basis ofor exploration of data communication.

The seven ordered layers of OSI model are :
1. Physical Layer
2. Data Link Player
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer

When a message travels from one device to other, it travels through different intermediary nodes. These intermediary nodes usually involves the first three layers of OSI model.
At first layer the entire package is converted to a form that can be transferred to the receiving machine. At the receiving machine, the message is unwrapped layer by layer,with each process receiving and removing the data meant for it.
The passing of data anf information through the layer is made possible by an interface between each pair of adjacent layers.

The seven layers further divided in three subgroups.

Layers 1,2 & 3 physical,data link and network layer are network support layers. They deal with the physical aspect of moving data from one device to another.

Layers 5,6 & 7 session,presentation and application layer are user support layers. They allow the interoperability among the softwares.

Layer 4 transport layer ensures the end to end reliable data transmission whereas the data link layer ensures reliable transmission on a single link.

The upper OSI layers are used in softwares.
The lower layers are combination of software and hardware, except physical layer which is mostly hardware.

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Transmission Mode

The direction of a signal between two devices is defined by a transmission mode.
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The transmission mode defines the direction of a signal between two devices.There are three types of transmission mode given as below;

Simplex Mode:
In the simplex mode the communication is unidirectional. Only one can send data and other can receive. No any other possibility is available. A remote is an example of simplex mode.

Half Duplex Mode:
In half duplex  mode both devices can send and receive data, but not at the same time. At a time one device send data and other receives. Walkie talkie is an example of half duplex mode.

Full Duplex Mode:
In full duplex mode both devices can send and receive data at the same time. It has two way traffic at the same time. The telephone network is an example of full duplex mode.

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Components of Data Communication

Data communication consists of five basic components.

1. Messsage

The message is the information or data to be communicated. i can be text, numbers, pictures, sound or video or combination of these.

2. Sender

The sender is the device that sends the message. It can be a computer, work station, camera etc..

3. Receiver

The receiver is the device that receives the message.It can be a computer, work station, television etc..

4. Medium

The transmission medium is the path by which message trevels from sender to receiver. It can be a wire, coaxial cale, fibre optics, laser or radio waves etc..

5. Protocol

A protocol is a set of rules that governs the data communication. It represents the agreement of communicating devices. Without a protocol devices can not communicate to each other.

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