Getting Started of Basics.

In order to network work perfectly, there is predefined reffernce model for purpose.

The Open System Interconnection (OSI) model was created by the International Organization of Standardization (ISO). It was layered model that was created to enable different networks to communicate reliably between disparate systems. The OSI model porvides a framework for creating and implimentingnetworking standardsand devices and describe how network applications on different computers can communicate through the network media.

The OSI model has seven hierarchical layers. Each layers describe the different network function. The layers are as show in above picture.

The layers are usually numbered from the last one, which means the Physical lyer is the first layer and the Application layer is the seventh layer.

A  mnemonic can be used to remember the seven layers in order: 

Please Do Not Throw Sausage Pizza Away

Or you can just try to remember first alphabet of each layer from top to bottom, i.e.

A P S T N D P

Here is a description of each layer in the OSI model in brief.

1. Physical Layer:

Defines how to move bits from one device to another. It deals with the physical characteristict of the transmission medium - either wired or wireless, describing connectors, pins, cables and network interface cards. In other words, this layer deals with hardware of network.

2. Datalink Layer:

Creates packets into bytes and byte into frames. Each frames has a header and trailer. A header contains a source MAC address and destination MAC address. ( MAC is Media Access Contol, the Physical address of network device. It is hard-burned by manufacturer. MAC is mad of 48 bits and splits of two parts. First part is manufacturer id and second part is individual device id. In networking term it is called second layer or data link layer address.) A trailer contains the frame Check Sequence Field, used for error detection.

3. Network Layer:

This layer provides logical addressing used by routers for path determination. Logical addressing is used to identify a host on a network. Each devices are seperated by IP address, which is 32 bit logical addressing shceme. IP address must be unique in the network.

4. Transport Layer:

This layer is responsible for establishing reliable or unreliable delivery and error recovery and flow control. The transport layer is sometimes said to be responsible for end-to-end or host-to-host transport. Transport layer is multifunction layer performing, Process-level addressing, Multiplexing and Demultiplexing, Segmentation, Packing, Reassembly, Connection Establishment, Management and Termination, Acknowledgement and Retransmission and Flow Control.

5. Session Layer:

Determines how to establish, control and terminate a session between the two systems. Session layer also authenticates user and provides time slot.

6. Presentation Layer:

This layer is responsible for Data encryption/decryption, Character/string conversion, data compression and graphics handling.The presentation layer mainly translates data between the application layer and the network format. Data can be communicated in different formats via different sources. Thus, the presentation layer is responsible for integrating all formats into a standard format for efficient and effective communication. 

7. Application Layer:

This layer is 7th layer of OSI model and directly connected with user. This layer is responsible for providing a UI (user interface) to user and enables network applications to communicate with other applications. The application layer provides many services, including SMTP, File Transfer, Web Surfing, Web Chat, Email Client, Data Sharing, Virtual Terminals, Various files and data sharing.

So here's brief details of OSI layer and its layers. Now we will move to TCP/IP reference model.