Introduction: Transport, Session, Presentation & Application Layer
Welcome to Video 5 of our free CompTIA Network+ training series. In this lesson, we cover the upper layers of the OSI model—Transport, Session, Presentation, and Application layers—which are essential for understanding how data flows across a network from one device to another in a structured, secure, and efficient way. These layers govern data transmission, session management, data representation, and user interface integration. You’ll learn about TCP/UDP protocols, port numbers, encryption, session handling, and application communication. Whether you’re preparing for the CompTIA Network+ exam or seeking foundational networking knowledge, this video breaks down complex concepts into simple explanations. Watch, learn, and reinforce your understanding with clear examples and real-world use cases.
Written Content / Lesson Notes
The Transport layer (Layer 4) is the backbone of reliable data transfer. It ensures complete data delivery between host systems using TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). TCP is reliable and connection-oriented, making it suitable for web traffic (port 80) and secure communication (port 443), while UDP is faster and connectionless, used in applications like video streaming and VoIP.
Each application is identified by port numbers. For example:
- HTTP → Port 80
- HTTPS → Port 443
- FTP → Port 21
- SSH → Port 22
- SMTP (Email) → Port 25
The concept of multiplexing allows multiple applications to use the network simultaneously without conflict. Devices like load balancers and firewalls operate at this layer to filter or route traffic based on IP addresses and ports.
Next is the Session layer (Layer 5), which establishes, maintains, and terminates sessions between applications. It ensures consistent communication by managing dialogue and synchronisation between endpoints.
The Presentation layer (Layer 6) formats and translates data. It handles data encryption/decryption, compression/decompression, and ensures data from one system is readable by another. This layer works with file types like MP4, PDF, DOC, and ensures that character encoding and data structure are preserved.
Finally, the Application layer (Layer 7) interacts directly with user-facing software such as browsers, email clients, and file transfer tools. It handles requests and responses between client and server, using protocols like HTTP, FTP, SMTP, and more. This layer is critical for delivering services directly to users in a structured and meaningful format.
Together, these top four layers of the OSI model handle the most user-visible and functionally rich aspects of networking.
