Wednesday, August 13th 2025 | Notion

Cornell Notes

Cue / Question	Notes
What is the OSI model?	A framework (7 layers) to standardize how devices communicate over networks.
Why does it matter?	Helps troubleshoot problems by isolating them to specific layers. Foundation of CCST/CCNA.
1. Physical Layer	Hardware: cables, hubs, NICs, signals. “The wires.”
2. Data Link Layer	MAC addresses, Ethernet, frames. Ensures error-free transmission on same network.
3. Network Layer	IP addressing, routing. Gets packets from source → destination.
4. Transport Layer	TCP/UDP, port numbers. Ensures correct delivery of data.
5. Session Layer	Opens, manages, ends communication sessions.
6. Presentation Layer	Data translation, encryption, compression. “Prepares data for application.”
7. Application Layer	What users interact with: HTTP, DNS, email, apps.
Memory Trick	“Please Do Not Throw Sausage Pizza Away.” (Physical → Application)

Summary (Bottom Section):

OSI = 7 layers.
Each layer handles a specific role in communication.
Knowing layers = faster troubleshooting & exam success.

OSI Model – Deep Dive Reference

Layer	Detailed Explanation	Practical Applications	Example Scenario
7. Application	Closest to the user; where apps interact with the network. Defines protocols/services (HTTP, DNS, SMTP).	Web browsing, email, file transfers.	If your browser can’t reach a website, check if DNS or HTTP service is available.
6. Presentation	Ensures data is in a usable format. Handles encryption, compression, and translation between systems.	SSL/TLS for secure browsing, file compression, converting character sets.	Encrypted data sent via HTTPS is decrypted here before the user sees it.
5. Session	Establishes, manages, and ends connections between applications. Keeps sessions alive.	Logging into a server, video conferencing sessions, maintaining state between two endpoints.	You log into Zoom → the session layer manages the continuous communication until you end the call.
4. Transport	Ensures complete, accurate delivery. Uses TCP (reliable, connection-oriented) or UDP (fast, connectionless). Adds port numbers.	TCP for email/web; UDP for streaming/gaming.	Downloading a file (TCP) vs. live streaming a soccer match (UDP).
3. Network	Handles logical addressing and routing. Moves packets across different networks. Devices: routers. Protocols: IPv4, IPv6.	IP addressing, routing tables, path selection.	Your packet leaves your home LAN → router finds the path to Google’s server.
2. Data Link	Transfers data between devices on the same network. Uses MAC addresses to identify devices. Divides packets into frames.	Ethernet, Wi-Fi. Switches operate here.	Your laptop sends a frame addressed to your printer’s MAC address.
1. Physical	The hardware layer. Deals with raw bits, voltages, cables, fiber, radio frequencies. Defines specs like bandwidth.	NICs, cables, hubs, wireless signals.	Damaged Ethernet cable = Physical Layer issue.

📝 Key Points to Emphasize in Class

Each layer depends on the one below it.
Problems can be isolated to specific layers (ex: “no signal” → Physical; “bad IP” → Network).
Real-world tech maps directly: cables (Physical), switches (Data Link), routers (Network), firewalls (Transport/Session/Presentation), apps (Application).
OSI is a mental model, not an exact mirror of modern networking, but it’s still tested heavily.

Warm-Up (Certification Practice)

Question: Which OSI layer is responsible for encryption and compression?
- a) Network
- b) Presentation
- c) Data Link
- d) Session
Answer: b) Presentation

Seminar Activity