What is the OSI model ?
OSI stands for open system interconnection , is a conceptual model used to describe the function of a networking system or computing system without regard to its underlying internal structure and technology. It was developed by ISO( International Standard Organization) in 1984.
The original objective of the OSI model was to provide a set of design standards for equipment manufacturers so they could communicate with each other. The OSI model defines a hierarchical architecture that logically partitions the functions required to support system-to-system communication.
Layers of OSI model
It has seven layers. All these 7 layers work collaboratively to transmit the data from one host/device to another across the globe.
7. Application layer
The application layer is used by end-user applications such as web browsers and email clients. It provides protocols that allow applications to send and receive information and present meaningful data to users. A few examples of application layer protocols are the Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), Post Office Protocol (POP), Simple Mail Transfer Protocol (SMTP), and Domain Name System (DNS).
6. Presentation layer
It receives data from the application layer in the form of character and number and translate the data into machine readable format to transmit over the network.
The functions of the presentation layer are :
Translation : For example, ASCII to EBCDIC.
Encryption/ Decryption : Data encryption translates the data into another form or code. The encrypted data is known as the cipher text and the decrypted data is known as plain text. A key value is used for encrypting as well as decrypting data.
Compression: Reduces the number of bits that need to be transmitted on the network which speed up the data transmission.
5. Session layer
The session layer permits two parties to hold ongoing communications called a session across a network. The applications on either end of the session can exchange data or send packets to another for as long as the session lasts. The session layer handles session setup, data or message exchanges, and terminate session. It also monitors session identification so only permitted user/device can participate and security services to control access to session information. A session can be used to allow a user to log into a remote time-sharing system or transfer a file between two machines.
Another session layer service is synchronization. Consider the problems that occur when transferring a file between two machines and the system crashed and not being able to complete the transfer. This process must be restarted from the beginning. To avoid this problem, the session layer provides a way to insert checkpoints into the data stream, so that after a crash, only the data after the last checkpoint has to be repeated.
4.Transport layer
The transport layer receives data from the session layer and breaks it into smaller unit called “segments” and pass it to the network layer. It is responsible for reassembling the segments on the receiving end, turning it back into data that can be used by the session layer. The transport layer carries out flow control, sending data at a rate that matches the connection speed of the receiving device, and error control, checking if data was received incorrectly and if not, requesting it again.
In order to deliver the message to correct process, transport layer header includes a type of address called service point address or port address. Thus by specifying this address, transport layer makes sure that the message is delivered to the correct process.
The services provided by the transport layer :
Connection Oriented Service: In this type of transmission, the receiving device sends an acknowledgement, back to the source after a packet or group of packet is received. This type of transmission is reliable and secure. TCP is connection oriented service.
Connection less service: In this type of transmission, the receiver does not acknowledge receipt of a packet. This approach allows for much faster communication between devices. Connection-oriented service is more reliable than connectionless Service. UDP is connection less service.
3. Network layer
Network layer transmits data in from one device/host to another device/host which located at different network. Data unit in this layer is called packet.
Network layer performs
Logical Addressing: In order to identify each device on internetwork uniquely which is also known as ip addressing.
Determine best path for data transmission from source to destination, and transmits the data through the path known as routing.
Router operates in this layer.
2.Data Link Layer
In this layer data is transmitted node to node (usually in directed connected device such as switch to pc or pc to pc which is connected through physically). Packets are divided in this layer known as frame.
DLL has two layers . Logical Link Control(LLC) and Media Access Control(MAC) also known as physical address of device. MAC is globally unique. MAC is 48 bits (6 bytes) , first half of the address uniquely identifies the vendor known as OUI(Organization Unique Identifier), last half identifies for the device id.
Switches operate in this layer though Some switches also operate at Layer 3 in order to support virtual LANs that may span more than one switch subnet, which requires routing capabilities.
Functions of Data Link Layer :
Framing : It provides a way for a sender to transmit a set of bits that are meaningful to the receiver.
Physical addressing : Adds physical address(MAC) to the frame
Error Control : Detects and retransmits damaged or lost frames.
Flow Control : Data must be constant in both side otherwise it may be get corrupted. flow control coordinates the amount of data that can be sent before receiving acknowledgement
Access Control : determine which device has control over the channel at a given time, when a single communication channel is shared by multiple devices.
1. Physical Layer
Physical layer is the bottom layer of the OSI model. This layer is concerned with transmitting raw data bits across the network. For transmitting data it converts the digital bits into electrical, radio or optical signal. It can include specifications such as voltages, pin layout, cabling, and radio frequencies. Fast Ethernet, RS232, and ATM are protocols with physical layer components. Layer 1 Physical examples include Ethernet, FDDI, B8ZS, V.35, V.24, RJ45.
