The International Standards organization ( ISO) developed the Open System Interconnect (OSI) model in year 1983. This model connects systems that are open to communicate with each other regardless of the protocols that each system uses. This model allows computers from different manufacturers to communicate with each other without requiring any logical changes to the hardware and software.
The OSI model is represented in seven layers the define the entire process of communication between two computers on a network.
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| OSI Reference Model |
Physical Layer
The physical layer is the first layer in the OSI Model. This layer transmits data in the form of raw bits using physical media such as twisted-pair cables, coaxial cables, and fiber optic cables. During transmission, the data is converted into an electrical/optical signal before it is sent to the receiving device. the receiving device converts the signal into data.
The network components such as Ethernet, Token ring, repeaters, hubs, and amplifiers are part of this layer.
Data Link Layer
The data link layer maintains the device address that determines the particular device on the network. This is know as the physical address or the Media Access Control (MAC) address, which is unique to a Network Interface Card (NIC).
The data link layer receives the data from the network layer, packages it into frames (a unit of data), and then sends it bit-by-bit on to the physical layer.
- To make the data error free, the data link layer breaks the data into frames.the frames are then transmitted sequentially from the sender system.
- Controls the flow of data received from the sender this is referred to as flow control.
Network layer is responsible for the delivery of data from the source to the destination computer. This layer reside above the data link layer. This can be achieved by arranging the data into data packets and adding a header to it. This layer follows two processes: routing and forwarding.
- Routing is the process of finding the path through which the data packet can be delivered.
- forwarding is the process of using the path provided by routing and forwarding it to the destination computer.
- This network components, such as Routers, are the part of this layer. The routable protocols that work at this layer are TCP/IP, IPX/SPX and AppleTalk. Non-routable protocols are NetBEUL, NetBIOS etc.
The transport layer is the 4th network layer of OSI model. this layer is responsible for ensuring error-free end-to-end data transfer to the destination computer.
- This layer acknowledges the successful transmission of data packets.
- This network component gateway is part of this layer. The protocols used at this layer are NetBEUI, TCP etc.
- Re-transmits the data packets if they are not error free.
- Manages the flow of data packets between computers across multiple networks.
Session Layer
The session layer is the 5th network layer of OSI model. The session layer allows applications to organize and to manage data exchange. In addition, the session layer allows users to logon to remote computers on a network for transferring files. Session layer helps in establishing interaction between two computers on a network.
This layer provides the following services:
- Dialogue control: Manage data traffic on a network. There are two dialogue control methods
- Half-duplex : This method provide the facility that each device can either send or receive data.
- Duplex: This method provide the facility that each device can receive and transmit data simultaneously.
- Synchronization and recovery: Sometimes, the data that you want to transfer may not be transferred in one attempt. If, you want to transfer some files that take an hour to complete. During this process, if the source machine crashes, the complete data needs to be transferred again. this problem will continue....To solve this issue, the session layer inserts checkpoints into data stream.
- Token management: A session connection consists of tokens. a token is an attribute assigned to a session.The session layer allows traffic in only one direction.
* NetBIOS protocol work on this layer. The network component gateway is a part of this layer.
Presentation Layer
The presentation layer is the 6th network layer of OSI model. This layer specifies that how the information is presented to the application layer and is concerned with the syntax of the transferred data. This layer encodes the data in a standard format to solve this problem. But not every computer uses the same data encoding scheme.
The data may be encoded by the two methods:
- Direct translation method: The American Standard Code for Information Interchange (ASCII) code from the source computer is translated into Extended Binary Coded Decimal Interchange Code (EBCDIC) on the destination computer.
- Indirect translation method: The ASCII code is translated into standard format at the source computer and then translated to EBCDIC on the destination computer.
*The presentation layer performs functions such as Encryption and decryption, and Data compression
The application layer is the last (7th) network layer of OSI model. This layer is closest to the end user. This layer help the applications to access network resources. Application layer provide the network services such as file transfer and e-mail to the end users or to the computer application.
The computer applications can be separated as follows:
- Embedded Applications: This type of application consist of other network applications that are built in with the application. As for example, MS-Word, MS-Excel etc. allows you to activate email or a Web site reference from the MS-Word, MS-Excel document itself.
- Network Applications: Programs designed to work on a network. As for example, you can access a remote computer using the Telnet program.
- Standalone Applications: These application are programs designed for individual access, as for example, the Solitaire, FreeCell, Chess Titans etc.. game available with Windows.
*This layer the protocols are used such as HTTP, FTP, SMTP, Telnet, AppleTalk and SNMP. All these are very useful for establish the network connection and provide data services for the end user.
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What is OSI reference model
Reviewed by Neel Kamal
on
August 30, 2018
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Reviewed by Neel Kamal
on
August 30, 2018
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