Network fundamentals OSI model
Published: 2019-12-03 01:59:52

Network fundamentals OSI model

We all use the internet but how many of us can say we have a good understanding of how all of it works? Using the popular OSI model this article will help demystify how data is transferred across the internet.

The Open Systems Interconnection model (OSI model) is a model used to show how the network system are supposed to communicate with each other. The OSI model is divided into 7 layers, each layer are considered to be a bunch of protocols of how data are to be transported. The 7 layers from top to bottom are Application layer, Presentation layer, Session layer, Transport layer, Network layer, DataLink layer, and Physical layer. The OSI the model describes how webpage, email, etc. from the Application layer are deconstructed down the model to the basic electrical signals of 1s and 0s in the Physical layer and transported across vast distance then reconstructed back up the model to what we see every day on the internet. 

Application Layer

The application layer is the topmost layer and is the layer we see in our everyday use such as google chrome, email, and file transfers. The job of this layer is to take data and present it to the user, HTTP/S is a protocol in the Application layer it is what browser use to request webpages. Some other protocols are FTP for file transfer, SMTP for email, and Telnet for Virtual Terminals. The Application layer is what connects the PC to the internet. 

The functions of the Application layer are: Network Virtual Terminal, FTAM-File transfer access, and management, Mail Services, Directory Services.

Presentation Layer

The Presentation Layer receives data from the Application layer in the form of letters and data, presentation layer then converted it to binary in the step called translation, after it compresses the data for faster transmission and encrypts it for security/ error detection. When receiving data from the Session layer it checks the encryption for error and decompresses it and translated it to a useful form for the Application layer. SSL (Secure Sockets Layer) is a protocol of the Presentation layer; it is used to establish an encrypted link between a web server and a browser. The presentation layer is what translates between pictures and text to binary.

The functions of the presentation layer are Translation, Encryption/Decryption, and Compression. 

Session Layer

Session layer creates a session with the webserver when there is a request for information, one important role Session layer play is session management where it keeps track of what is currently using the internet and where the data it receives should go. For example, it makes sure the web browsing and Skype calls are on a different session, so closing one doesn’t affect the other. Another import job of the Session layer is authentication and authorization where it checks if the user allows access to the server. 

The functions of the session layer are: Session establishment, maintenance and termination, Synchronization, and Dialog Controller.

Transport Layer

The transport layer controls the reliability of the communication, through segmentation, flow control, and error control. Data in the Transport layer is broken down into smaller segments where each gets a sequence number and a port number. The sequence number is used to help reassemble the data back together and the port number is used to direct the data to the correct application. Flow control is used to reach the optimal amount of the data being transmitted, if the transmission is too high it will send a message to lower the transmission and if it’s too low it will ask to increase it. Error control helps to minimize the amount of error in a transmission each segment will have a checksum to check for corruption and if there is a missing segment it will request to resend the segment. Two important protocols in the transport layer are TCP and UDP. TCP is connection-oriented transmission where it will check for any error or missing data and ask to resend it. UDP is a connectionless transmission where there will be no feedback on if the data is received or not. TCP is used in email, file transfer, a webpage where full delivery of data is a must, UDP is used for video streaming because it’s much faster than TCP and dropping some data are ok. 

The functions of the transport layer are: Segmentation and Reassembly, Service Point Addressing. 

Network Layer

The network layer is where pathfinding is done, using the IP address the network layer will try to find the best route it can take to send the data. The data of this layer is called a packet where it will have the senders IP address and the destination’s IP address. Using that information and the routing protocol the network layer will be able to reliably route the packet to the destination.  For example, an IP address of 192.168.0.1 with the subnet mask of 225.225.225.0 will deliver the packet to the network of 192.168.0.0 and the network will send it to host 1. There are many routing protocols such as OSPF, BGP, RIP, etc. base on the different protocols it will have different pathfinding. The job of the Network layer is finding the best path to the destination. 

The functions of the network layer are: Routing and Logical Addressing.
 
Data Link Layer

Data Link layer deal with physical addressing where it will use the MAC address to send data between two devices. Data of this layer is a frame it has the MAC address of both source and destination, MAC address is a 12 digit alphanumeric number that is unique to each device. Many media make up the internet such as Ethernet cable, wireless, satellite transmission, etc. Data Link layer will encapsulate the packet from the Network layer so that it can be sent across different media using the protocol appropriate for the media. The Datalink layer get the two devices to agree on certain rules so that they can communicate. Data Link layer is divided into two sub-layers: Logical Link Control (LLC) and Media Access Control (MAC) sub-layers. 

The functions of the Data Link layer are Framing, Physical addressing, Error control, Flow Control, and Access control. 

Physical Layer

Physical Layer is the physical representation of the internet. It is responsible for the actual physical connection between the devices.  They are the physical media that make up the internet. The physical layer converts bits to an electrical signal for copper wire, light for fiber optic or radio for air and at the receiving end converts it back.  
The functions of the physical layer are bit synchronization, bit rate control, physical topologies, and transmission mode. Hub, Repeater, Modem, cables are Physical Layer devices. 

The OSI model is used to debug connection problems; usually, it starts from the bottom and goes up layer by layer. The OSI model is a very general overview of how the internet works.