Let us know about Function of OSI Model Layer

OSI model: Open System Interconnection Model: OSI model defines and is used to understand how data is moved from one computer to another in a computer network in the most basic form.

Function of OSI Model Layer

Two computers connected to each other using LAN cable and connectors sharing data by using network interface cards(NIC) forms the network but if one computer has Microsoft windows and the other one has MAC OS, then how these two computers are going to communicate with each other? In order to accomplish successful communication between computers for networks or different architectures seven layers OSI model was introduced by International Organization of Standardization in 1984 containing

  1. Application layer
  2. Presentation layer
  3. Session layer
  4. Transport layer
  5. Network layer
  6. Data link layer
  7. Physical layer

Note that each layer is the package of protocols.

Application layer:

Application layer is used by network applications. Network applications means computer applications that uses internet like google chrome, fire-fox, skype etc. Web browser runs in your pc and it is a network application, it does not live in your application layer but it uses application layer protocols like HTTP’s, HTTP. All these are dependent on application layer protocols to perform.

The protocols which are useful for application layer are HTTPS,HTTP,FTP,FMTP,DHCP,SNMP,TELNET,NNTP,IRC,POP,NFS. These protocols form the basis for various network services like File Transfer, Wen surfing, Emails, virtual Terminals, File transfer is done by using FTP protocol, “Web surfing” is done with the help of HTTPS or HTTP, for emails SMTP protocol is used and for terminals that are virtual, Telnet is used.

Presentation layer: Function of OSI Model Layer

This layer will receive data from application layer, this data will be like characters and numbers, presentation layer will convert these characters and numbers into “machine understandable binary format”. This type of presentation is called translation. Presentation layer will reduce many bits that are used to show the original data, before transmission of the data. This reduction of bits process is called Data Compression. and it can be lossy or lossless.

Data compression reduces the amount of space used to store the original file and when the file size is reduced, it can be received at destination in very less time then data transmission can be faster.

To maintain the integrity of data, before transmission data is encrypted, so encryption enhances the security of sensitive data and at sender side the data is encrypted and at receiver side the data is decrypted. SSL layer protocol is present in presentation layer which is Secure Socket Layer.

Presentation layer performs 3 operations and they are:

Translation, data compression and encryption/ decryption.

Session layer: Function of OSI Model Layer

Session layer helps in setting up and connections are managed. And also enables sending and receiving of data after that termination of connections or sessions happens.

Session layer has APIs, Application Programming Interfaces.

NETBIOS networks basis for input output system is an example of APIs which allows applications on different computers to communicate with each other. Just before a session or connection is created with the server, the server performs a function

called Authentication. Authentication is the process of verifying of who you are. So a system has a user name and a password once entered username and password are matched a session or a connection is established between your computer and the server after authentication of the user, authorization is checked, Authorization is the process used by server to determine if you have permission to access a file, if not you will get a message saying you are not authorized.

So both Authentication and Authorization is performed by Session layer.

Session layer keeps a track of the file that are downloaded, for example a webpage contains text, image etc. These text and images are stored as different files on the web server when you request a website in your web browser the web browser opens a separate session to the web server to download each of these texts and images separately, these files are received as data packets.

Session layer will keep a track of which layer belongs to which file, either a text file or an image file and tracks where the received data packet go, in this case it goes to web browser that is session layer helps in session management.

Layer below session layer is Transport layer.

Transport layer:Function of OSI Model Layer

It controls the communication reliability through segmentation, error control and flow control. In segmentation data which is received from session layer is divided into small parts called segments each segment contains a source and destination port number and a sequence number, port number helps to direct each segment to the correct application and sequence number helps in reassembling the segments in the correct order to form correct message at the receiver

Flow control: Transport layer does control the amount of data being transmitted.

Error control: if some data does not reach the destination, transport layer will use automatic repeat request schemes to transmit back the loss corrupted data, checksum is added to each segment by the transport layer to find out the received corrupted segment.

The protocols of Transport layer are TCP( Transmission control protocol) and UDP(User datagram)

Transport layer performs services like connection-oriented transmission and connectionless transmission.

Connection oriented transmission is done through TCP and  Connection-less transmission is done via UDP.

UDP is faster than TCP. because it does not provide any feedback whether data has been received or sent whereas TCP provides feedback Therefore lost data can be transmitted in TCP.

TCP is used where full data delivery is must.

Transport layer passes data segments to the Network layer.

Network layer:Function of OSI Model Layer

It works for the transmission of the received data segments from one computer to another located that are in different networks, data units  that are present in network layer are called packets, it is the layer where address reside.

This is also a hardware component. The network layer controls the operations of subnet.

The routing packets from source to destination. It selects the shortest path from source to destination to transmit packet from number of routers available. We have to choose one of the routers and with the help of routers, we can transmit the packets.

IP address has to be placed in network layer, for each system there is one IP. The network layer is implemented by network devices such as routers.

Functions of the network layer:

  1. Routing: it determines which route suits from source to destination.

Routing is the method of moving data packets from source to destination and it is based on the logical address

  • Logical address: if you want to send a packet so For each packet, you should know the logical address of that router.

IPv4 and IPv6 Ip addressing which is performed in network layer is called logical addressing, every computer in a network has a unique IP address, network layer assigns sender and receiver’s IP address to each segment to form an IP packet.

3.Path determination: A computer can be connected to Internet server or a computer in a numerous ways choosing the best possible path for determining the delivery from source to destination is called Path determination.

The protocols such as OSPF (Open shortest Path First) and BGP(Border Gateway Protocol) and IS-IS(Intermediate system to intermediate system) to determine the best possible path for data delivery

Network layer protocols:

We have three protocols, one is IPv4(Internet Protocol version4) and ICMP v4(Internet control message protocol) and third is IGMP(internet group management protocol).

IPv4 is sued for packetizing and forwarding and delivery of packets.

ICMP v4: it will handle all the errors during transmission of data

IGMP: Useful for multicasting

Data Link Layer: Function of OSI Model Layer

Data packets are received from Network layer, Data packets contains IP address of sender and receiver, there are two kinds of addressing:

Logical addressing and Physical Addressing

Logical Addressing is done at network layer where sender and receiver’s IP addresses are assigned to each segment to form a data packet.

Physical addressing is done at data link layer where MAC address of a sender and receiver are assigned to each data packet which will form a frame, MAC address is a “12 digit alpha-numeric number” present in network interface card of a computer(NIC) by your computer manufacturer. data units in data link layer are called frames.

Data link layer performs two functions:

Allows upper layer of OSI model to access the media using techniques such as framing.

It controls how data is received and placed from the media using techniques such as Media Access Control and Error Detection.

Physical layer: Function of OSI Model Layer

Physical layer convert these binary digits into signals and transmit over local media, it can be an electrical signal in case of an open cable and LAN cable, light signal in case of optical fiber and radio signal in case of air. Signals generated by physical layer are used to connect two devices at the receiver, and this physical layer receives signal convert it into bits and pass into data link layer as a frame is decapsulated as data moves through higher layers.

Finally the data is moved to application layer, the application layer protocols makes the sender’s message is seen in the application in the receivers computer screen.