THM Pre-Security Room | What is OSI Model?
The OSI (Open Systems Interconnection) Model is a framework that helps us understand how networked devices communicate with each other. It provides a systematic approach to how data is sent, received and interpreted. There are 7 layers in the OSI model, and each layer has a unique function.
The Application Layer is where protocols and rules are in place to determine how the user should interact with data sent or received. Users can interact with applications such as email clients, browsers, or file-sharing browsing software. The GUI (Graphical User Interface) and DNS (Domain Naming System) are parts of the application layer.
The Presentation Layer acts as a translator for data to and from the application layer. Security features like data encryption (ex: HTTPS) occur here.
The Session Layer creates a connection to the other computer that the data is destined for. It syncs the two computers to ensure that they are on the same page before data is sent and received. It divides the data sent into smaller chunks of data and sends these chunks one at a time. Sessions are unique, so data can’t travel over different sessions.
The Transport Layer plays an important role in transmitting data over a network. Two different protocols follow when data is sent between devices: TCP and UDP. TCP (Transmission Control Protocol) is a connection-oriented protocol that provides accuracy and reliability delivery of data through error checking, a process that ensures that data sent from the small chunks of data in the session layer has been received and reassembled in the same order. It is also capable of syncing 2 devices to prevent them from being flooded with data. It requires a three-way handshake to establish a connection. TCP is used for file sharing, internet browsing, sending emails, etc because they require data to be complete and accurate. Protocols such as HTTP, POP3, IMAP, and SMTP use TCP.
On the other hand, UDP (User Datagram Protocol) is a connectionless protocol that provides faster delivery of data, but without error-checking and reliability guarantees. It’s suitable for protocols that rely on fast queries such as DNS, for protocols that prioritize real-time communications, such as audio/video conferencing and streaming/broadcast, and for protocols used for discovering devices such as ARP and DHCP. UDP is much faster than TCP and leaves the application layer to decide if there is any control over how quickly packets are sent. But, it doesn’t care if the data is received. Because it doesn’t reserve a continuous connection, unstable connections result in a terrible experience for the user.
The Network Layer is where routing and the re-assembly of data (from small chunks to large chunks) occur. Routing determines the most optimal path in which these chunks of data should be sent. IP addresses are dealt with at this layer. Devices such as routers capable of delivering packets using IP addresses are known as Layer 3 devices.
The Data Link Layer focuses on the physical addressing of the transmission. It receives a packet & IP address for the remote computer from the network layer and adds the physical MAC address of the receiving endpoint. Inside every network-enabled computer is a Network Interface Card (NIC) which comes with a unique MAC address to identify it.
The Physical Layer refers to the physical components of the hardware used in networking, such as ethernet cables. Devices use electrical signals to transfer data between each other in a binary numbering system (1’s and 0's).