OSI Model

 

OSI REFERENCE MODEL

When people talk about connecting computers together, then they referred the term Networking. So, OSI model is behind this communication to share the resources between them

 

OSI model is a logical model for how systems in the network should communicate to each other. All the model does is to bre

ak down all the components for communication and arrange them into layers.

 

OSI model consists of seven logical layers that allows you to troubleshoot or know about the network.

The layers from top to bottom are for what is closer to the end user. So, what the user is interacting with will be at the top,

 

7. APPLICATION LAYER:

• This is the layer that the user is actually interacting with. Like Chrome, Safari, Firefox or somehow Internet Explorer.
• Not just browsers any application or piece of code or any software such as Outlook.
• This is the actual stuff that user sees or use to interact with the Network.

6. PRESENTATION LAYER:

• This is the layer where the operating system comes into play.
• The Application layer sends the information to the presentation layer.
• Device drivers for the Network Interface Card reside on this layer.
• It is concerned about the format of data exchanging between the two systems.

5. SESSION LAYER:

• This is the layer which deals with the communication creating a session between the two computers.
• When you visit a website on your computer, your computer at the session layer creates a session with the web server that you are trying to get data from.
• It is a session maintained between your computer and host server so that the connection is alive till you close the browser Tab or the page.
• If an application creates different transport streams session layer binds all these streams belonging to the same application.

4. TRANSPORT LAYER:

• It decides that how much information should be sent at one time. Windowing – send information back and forth.
• Receive data from Session layer and divide it into smaller units called as messages which are passed on to the Network Layer.
• On the receiving end, it makes sure that the messages are accepted and arranged in the correct order. These messages are merged and passed on to the upper layers.

3. NETWORK LAYER:

• Routers operates on the Network layer.
• Your IP address is at the network layer.
• It breaks a message into packets and transmits them across the network.
• It makes sure that the packet reaches the correct destination.
• This is implemented on every node in the network.
• A node here can be a computer, a router etc.

2. DATA LINK LAYER:

• This the layer where all the switching occurs or where the switches operate on.
• MAC Addresses exists here.
• It is responsible for transmission of error-free data.
• It divides packets into frames and passes on to the physical layer for transmission.
• On the receiving end, it takes the raw bytes from the physical layer and aggregates them into frames.
• Data-encoding framing error detection and correction are applied here.

1. PHYSICAL LAYER:

• The physical is literally all the physical stuff that a network requires. Like wires, Network Interface Cards and all other devices.
• It is responsible for transmission of raw bits over the communication link.

 

UNDERSTANDING THE FLOW OF INFORMATION:

• Let us assume that there are two systems System A and System B which wants to communicate with each other over a network.

• All the seven layers will be implemented by both the systems.

• All the intermediate nodes will implement only the bottom 3 layers (Network Layer, Data Link Layer and Physical Layer) as their job is just to pass on the data to the next node.

• The top 4 layers are implemented only by the end systems, but the bottom 3 layers are implemented by every node in the path.

• Every layer has some protocols through which it communicates with the corresponding layer in the other system.

• Every layer also communicates with the layer above and below it, or every layer provides some service to the layer above it.

Note: Protocols work between same layers of different machines, whereas services work among different layers within the same machine.

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1. An application, creates data that will be sent, such as an email message. The Application layer places a header fields that contains information about the source and destination ports, and passes the data to the Presentation layer.
   –| Source and Destination Port | Data |–
2. The Presentation layer places layer 6 header information. For example, the text in the message might be converted to ASCII. The Presentation layer will then pass the new data to the Session layer.
   –| Encryption Info | Source and Destination Port | Data |–
3. The Session layer follows the same process by adding layer 5 header information. Session layer will manage the data flow, and passes this data to the Transport layer.
   –| Session Header | Encryption Info | Source and Destination Port | Data |–
4. The Transport layer places information, such as a sequence number to message, and passes it to the Network layer. It allows multiple applications to use the same network at the same time.
   –| Session Header | Encryption Info | Source and Destination Port |Message Secuence | Message Secuence | Data |–
5. The Network layer places information, such as the source and destination address so the Network layer can determine the best delivery path for the packets, and passes this data to the Data Link layer.
   –| Source and Destination IP | Session Header | Encryption Info | Source and Destination Port | Message Secuence | Data |–
6. The Data Link layer places header and trailer information, such as a Frame Check Sequence (FCS) to ensure that the information is not corrupt, the Source and Destination MAC addresses, and passes this new data to the Physical layer for transmission across the media.
   –| Source and Destination MAC | Source and Destination IP | Session Header | Encryption Info | Source and Destination Port | Message Secuence |Data | FCS |–
7. The bit stream is then transmitted as ones and zeros on the Physical layer. It is at this point that the Physical layer ensures bit synchronization. Bit synchronization will ensure the end user data is assembled in the correct order it was sent.
8. Steps 1 to 7 occur in reverse order on the destination device. Device B collects the raw bits from the physical wire and passes them up the Data Link layer. The Data Link layer removes the headers and trailers and passes the remaining information to the Network layer and so forth until data is received by the Application layer.
9. Once the request has reached, the response is snet in the same secuence as steps 1 to 7 while reversing the source and destination information recieved in the request (ie. the source becomes the destination and the destination becomes the source).