OSI (Open System Interconnection) Model defines a network framework to implement the protocol in Seven Layers. The main purpose of the OSI Model is to guide the vendors and developers, the OSI Model doesn’t perform any functions in networking it is just a medium to understand how networking communication takes place.
The OSI Model is divided into seven Layers, every layer individually has its own very important function in the networking communication (making the data or information travel from sender to receiver).
Seven Layers of the OSI model are:
- Layer 7: Application Layer
- Layer 6: Presentation Layer
- Layer 5: Session Layer
- Layer 4: Transport Layer
- Layer 3: Network Layer
- Layer2: Datalink Layer
- Layer1: Physical Layer
The Application Layer (Layer 7)
Application Layer is the Top-Most Layer of the OSI Model. All the Manipulation of Data/Information is done in this layer only. Application Layer Provides some services to users such as E-Mail, File Transfer, Directory Services, Network Resources, Enabling Remote Access, Network Management Activities, Client/Server Processes, Information Location. This Layer is responsible to get the user or software access to the network.
Application Layer contains the variety of protocols which is used by the users, one of the common Application Layer Protocol used is HTTP (Hyper Text Transfer Protocol), which is the base of the World Wide Web (WWW).
Elaborating this layer in depth we get that the Application Layer of the OSI Model marks the spot where users actually communicate to the computers and come into play only when it’s clear that access to the internet will be needed soon.
So basically, the Application Layer is working as the interface between the actual application program and the next layer down by providing ways for the application to send the information down through the protocol stack.
The Presentation Layer (Layer 6)
The Presentation layer gets its name from its purpose: It presents data to the Application layer and is responsible for data translation and code formatting. One very effective way of ensuring a successful data transfer is to convert the data into a standard format before transmission. Computers are configured to receive this generically formatted data and then reformat it back into its native state to read it.
So, by providing translation services, the Presentation layer ensures that data transferred from the Application layer of one system can be read by the Application layer of another one.
With this in mind, it follows that the OSI would include protocols that define how standard data should be formatted, so key functions like data compression, decompression, encryption, and decryption are also associated with this layer. Some Presentation layer standards are involved in multimedia operations as well.
The Session Layer (Layer 5)
The Session layer is responsible for setting up, managing, and dismantling sessions between Presentation layer entities and keeping user data separate. Dialog control between devices also occurs at this layer.
Communication between hosts’ various applications at the Session layer, as from a client to a server, is coordinated and organized via three different modes:
Simplex is simple one-way communication, kind of like saying something and not getting a reply. Half-duplex is actual two-way communication, but it can take place in only one direction at a time, preventing the interruption of the transmitting device. But full-duplex is exactly like a real conversation where devices can transmit and receive at the same time.
The Transport Layer (Layer 4)
The Transport layer segments and reassembles data into a single data stream. Services located at this layer take all the various data received from upper-layer applications, then combine it into the same, concise data stream. These protocols provide end-to-end data transport services and can establish a logical connection between the sending host and destination host on an internetwork.
The Transport layer is responsible for providing mechanisms for multiplexing upper-layer applications, establishing sessions, and tearing down virtual circuits. It can also hide the details of network-dependent information from the higher layers as well as provide transparent data transfer.
The Network Layer (Layer 3)
The Network layer, or layer 3, manages device addressing, tracks the location of devices on the network, and determines the best way to move data. This means that it’s up to the Network layer to transport traffic between devices that aren’t locally attached. Routers, which are layer 3 devices, are specified at this layer and provide the routing services within an internetwork.
The Data Link Layer (Layer 2)
The Data Link layer provides for the physical transmission of data and handles error notification, network topology, and flow control. This means that the Data Link layer will ensure that messages are delivered to the proper device on a LAN using hardware addresses and will translate messages from the Network layer into bits for the Physical layer to transmit.
The Physical Layer (Layer 1)
Finally arriving at the bottom, we find that the Physical layer does two things: it sends bits and receives bits. Bits come only in values of 1 or 0.
The Physical layer communicates directly with the various types of actual communication media. Different kinds of media represent these bit values in different ways. Some use audio tones, while others employ state transitions—changes in voltage from high to low and low to high. Specific protocols are needed for each type of media to describe the proper bit patterns to be used, how data is encoded into media signals and the various qualities of the physical media’s attachment interface.