Back to our networking concepts for example if we decided to use new fiber optic cables instead of older coaxial cables the delivery step the other steps in the entire process are not impacted. The OSI model divides network communications into seven layers as shown in the figure in order from the top:
7 – Application
6 – Presentation
5 – Session
4 – Transport
3 – Network
2 – Data Link
1 – Physical
We will briefly discuss each layer to give you a high-level understanding of what happens at each layer without getting into too much detail.
- Application Layer
The Application layer (Layer 7) provides an interface for applications to access network services such as file sharing, message handling, database access and more. Protocols such as HTTP, HTTPS, FTP, DHCP, DNS, SMTP and many more operate here. Please keep in mind that the Application layer is not the application itself such as Microsoft Word or Adobe Photoshop but its connection to the network services. However, some user applications such as web browsers and email clients have integrated network functions with the application layer.
- Presentation Layer
The Presentation layer (Layer 6) handles data formatting and translation. The Data from the Application layer is “presented” by protocol conversion, data encryption and decryption, data compression and decompression and data representation. For example, a web browser that connects to a secure Web server may need to encrypt and decrypt the data before it is transferred to the Web server.
- Session Layer
The Session layer (Layer 5) sets up and holds ongoing communications called a “session” across the network so that applications on both sides can exchange data for as long as the session lasts. Synchronization and check pointing occur here as well as in the example of an audio or video stream used by a web-conferencing application.
- Transport Layer
The Transport layer (Layer 4) manages the data transfer from one application across the network. One of the processes that happen here is the segmenting of the data streams in to small units called “segments” for travel over the network. The two primary protocols that operate at this layer is TCP and UDP. TCP is the connection-based, higher overhead protocol that uses hand-shaking thus is more reliable. UDP is the connection-less, less overhead and less reliable protocol.
- Network Layer
The Network layer (Layer 3) handles logical network addressing such as translating Internet Protocol (IP) addresses into physical addresses (Media Access Control, MAC) addresses. It is responsible for performing the best route calculations to reach a certain destination and is the workhorse of the all networking. IP, ICMP, ARP, IPSec among other known protocols operate here. The data unit at this layer is called a “packet”.
- Data Link Layer
The Data Link layer (Layer 2) works with “frames” as its data unit and it acts as a conduit between the Network layer and the Physical layer. Media Access Control (MAC) addresses can be found here as well as communication methods such as CSMA/CD and token passing. Network cards also operate here since they are programmed with MAC addresses as well as include the physical interface to the network.
- Physical Layer
The Physical layer (Layer 1) is where the conversion of data into bit signals of 0 or 1 to be transferred over the medium. The type of signals can be pulses of light as in the case of fiber optic cabling, electrical pulses in the case of twisted-pair cable or radio waves in the case of wireless communications.
In summary the OSI model is an organized and helpful way to separate networking activities and associate them with protocols and functionality. It helps explain how data is formatted as it moves through the layers and aids in understanding the hardware, software and protocols at each step of the overall communication process.