Understanding the “OSI Model ” In Networking— All 7 Layers Explained.
OSI layer model & how the networking works around it!
Firstly, I want to clarify that the OSI layer model is a fundamental and highly important concept in networking. It’s too simple to understand. So, let’s dive deeper into it.
Table of Content
- Definition of OSI model
- Easy explanation in my words
- Overview of the OSI model
- IMP points to remember
- Architecture of OSI model
- Explanation of all 7 layers
Definition
The open systems interconnection (OSI) model is a conceptual model created by the International Organization for Standardization (ISO 1984) which enables diverse communication systems to communicate using standard protocols.
In My Words ( simple & short )
The OSI layer model isn’t hardware or software. It’s more like the set of rules or protocols governing how networking devices communicate with each other and share data. That's it.
Overview In Short
Okay, now you know what is the OSI layer model right? It’s nothing but a set of rules that govern how the data will be transferred from one device to another. Now the next part is very easy to understand if you imagine everything okay.
Points to be remembered: 👇
As you saw in the diagram there are 7 layers in the OSI layer model. Each layer has its designated role in it. However, before delving into the details of these 7 layers, you need to remember those points -
- Every device in the networking that communicates with each other has the OSI model’s concept internally. No matter whether the device is the sender or the receiver.
- If the device is the sender, that time the data flow through the OSI model is in the top-down approach. and vice-versa for the receiver, if the device is the receiver then the data flow is bottom-up.
- Now why do I keep these points in the “Points to remember section”? — because once you know that, everything related to the OSI layer model is just to let us know, how the data is flowing through the devices logically.
- The arrows are nothing but the data flow from top to bottom on the sender side and bottom up on the receiver side.
I hope everything is clear up to this point. Now do some imagination for the below points as well, to know how the OSI model and the data flow is working.
- Let’s talk about the first or uppermost layer of the OSI model, which is the Application layer. And here we are considering the sender side.
- This layer is the creator of the data. Now imagine that the data needs to be sent to the receiver side right? So we need to pass it to the below layer because as we said sender-side data flow is top-down manner.
- Now imagine, your data is getting wrapped or encapsulated by the headers of each layer. and getting passed to the next layer downside.
- at the last level i.e. physical layer all data and headers are converted into the signals and pass through the transmission media.
- Now start imagining the receiver side. Here as we know the data flow direction is bottom-up.
- So the data that the sender sends, gets received by the receiver's physical layer. Then it starts to decapsulate layer by layer in the upward direction.
- At last at the application layer of the receiver end, we can see the exact data that the sender sends.
7 Layers Stack of OSI Model
The OSI layer model consists of a total of 7 layers. Each layer has a significant role in it.
So I hope you got the point, What is the OSI Layer Model, Important points to be remembered, Overview of data flow through the devices.
Now let’s start with a profound understanding of all 7 layers of the OSI model
Application Layer
This is the only layer that directly interacts with data from the user.
This layer enables communication between software applications through protocols like HTTP, FTP, SMTP, and DNS.
Software applications like web browsers and email clients rely on the application layer to initiate communications.
FTP — File Transfer Protocol
HTTP/S — Hypertext transfer protocol / Security
SMTP — Simple Mail Transfer Protocol
DNS — Domain naming system
Presentation Layer
The Presentation layer receives data in textual format from the Application layer. It primarily conducts three operations on this data.
- Data Translation
- Data Compression
- Data Encryption
Session Layer
This is the layer responsible for opening and closing communication between the two devices.
The time between when the communication is opened and closed is known as the session.
The session layer ensures that the session stays open long enough to transfer all the data being exchanged, and then promptly closes the session to avoid wasting resources.
This layer is also again responsible for the 3 functions :
- Session Management
→ Session establishment between 2 nodes.
→ Session Opening
→ Session closing - Authentication
- Authorization
Transport Layer
- The transport layer takes data from the above layer converts it into segments and sends it to a further layer. (By adding the port address into it) Note:- The PDU at the transport layer is called the segments.
- The transport layer is a point-to-point OR node-to-node protocol instead of hope to hope protocol.
- Uses 2 popular protocols named TCP and UDP. (Who creates segments).
- It also checks for error control at the receiver's end. and responsible for flow control at the sender side.
→ Flow control — It is the term to maintain the flow of both the nodes (Server and receiver). Transport protocol helps to maintain the flow of data (bit rates).
→ Error Control — Transport layer checks for automatic repeat requests if any data gets lost. Also performs the checksum at the receiver's end. - The transport layer is operated by the Operating System. It is a part of the OS and communicates with the Application Layer by making system calls.
- The transport layer is called as the Heart of the OSI model.
TCP vs UDP differ in their transmission mechanisms:
- TCP facilitates connection-oriented transmission, while UDP enables connection-less transmission.
- UDP typically offers faster transmission compared to TCP.
- UDP lacks feedback mechanisms, whereas TCP provides feedback.
- UDP is favoured for scenarios where receiving all data is not critical, such as online streaming, gaming, and VOIP. Conversely, TCP is preferred for applications like web browsing, email, and FTP, where complete data delivery is essential.
Network Layer
Services Offered by Network Layer :
- Packetization
- Routing
- Forwarding
- The major task of the network layer is to transfer the data from one node to another node which is present in the another network also.
- The network layer generates packets from the segments received by the transport layer.
- The network layer added the header in the segment to generate it into the packet to transfer it further.
- It decides the route to be taken by the packets to travel from the source to the destination among the multiple routes available in a network (also called routing).
- The part consists of the source IP address and the destination IP address.
- The network layer uses the Internet protocol (IP), the Address Resolution Protocol (ARP), and the Internet Control Message Protocol (ICMP) (for error control) protocol.
- Routers & Type 3 Switches are the network layer devices.
Data Link Layer
- The most important part of DLL (Major responsibility) is Error control, Flow Control, Access control and ‘node to node delivery’ (Using MAC address), Using the ARP protocol.
- In DLL there is a frame get generated.
- The DLL added a ‘trailer’ to the packet.
- Data Link layer is handled by the NIC
- The Data Link layer works for the same network only.
- Switch & Bridge are Data Link Layer devices.
NOTE:- The data link layer receives the information in the form of packets from the Network layer, it divides packets into frames and sends those frames bit-by-bit to the underlying physical layer.
Physical Layer
The lowest layer of the OSI reference model is the physical layer. The physical layer contains information in the form of bits. It is responsible for transmitting individual bits from the transmission media.
Functions Performed by Physical Layer -
The following are some important and basic functions that are performed by the Physical Layer of the OSI Model –
- The physical layer maintains the data rate (how many bits a sender can send per second).
- It performs the Synchronization of bits.
- It helps in Transmission Medium decisions (direction of data transfer).
- It helps in Physical Topology (Mesh, Star, Bus, Ring) decisions (Topology through which we can connect the devices).
- It helps in providing Physical Medium and Interface decisions.
- It provides two types of configuration Point Point configuration and Multi-Point configuration.
- It provides an interface between devices (like PCs or computers) and transmission medium.
- It has a protocol data unit in bits.
- Hubs, Ethernet, etc. are used in this layer.
So, I hope all the points are getting cleared. Please do share this blog post and stay stunned for more such super simple and easy-to-understand concepts related to networking, operating systems, core C++, security and more.
