Lesson 2: The OSI Model: Seven Layers

Why this model matters

The Open Systems Interconnection (OSI) reference model splits network communication into seven layers. Each layer has one job and passes its work to the layer next to it. No device runs "OSI" as software. It is a reference model: a shared map that technicians use to describe where something works and where it has failed.

This level serves objective 1.1 of the N10-009 exam (maximum of 90 questions, 90 minutes, passing score 720 on a scale of 100-900). This course is independent of CompTIA and cannot promise a pass.

Task 1: Memorize the seven layers in order

Layers are numbered from the bottom (1) to the top (7). A protocol data unit (PDU) is the name for the chunk of data a layer handles.

Do this now:

1. Write the layer names from 1 to 7 from memory.
2. Use a mnemonic if it helps. Bottom to top: "Please Do Not Throw Sausage Pizza Away."
3. Write the data units next to layers 1 to 4: bits, frames, packets, segments/datagrams. Layers 5 to 7 all handle "data."

Two terms to learn at Layer 4:

• Transmission Control Protocol (TCP) is reliable: it confirms delivery and resends lost data. Its data unit is the segment.
• User Datagram Protocol (UDP) sends without confirmation. Its data unit is the datagram.

Task 2: Place the network interface card (NIC)

A network interface card (NIC) is the hardware that connects a host to a network. A wired port and a Wi-Fi adapter are both NICs. A NIC does two jobs, so it spans two layers:

• Layer 1 (Physical): it converts bits into signals on the medium and converts incoming signals back into bits.
• Layer 2 (Data link): it builds and reads frames, and it carries a media access control (MAC) address.

A MAC address is a 48-bit hardware address assigned to the NIC, written as 12 hexadecimal digits, for example 00-1A-2B-3C-4D-5E. It identifies the NIC on the local network only. The MAC address is a Layer 2 address.

Do this: on a Windows PC, run ipconfig /all and find the line "Physical Address." That value is the NIC's MAC address. Despite the label, place it at Layer 2, not Layer 1.

Task 3: Trace a web request down and up

Encapsulation means each layer wraps the data from the layer above with its own header (control information). Think of placing a letter in an envelope, then that envelope in a larger one, each with its own address label.

Sender, top to bottom, when a user opens https://www.example.com:

1. Layer 7 Application: the browser creates an HTTP request.
2. Layer 6 Presentation: the data is formatted and encrypted.
3. Layer 5 Session: the conversation with the server is opened and tracked.
4. Layer 4 Transport: TCP splits the data into segments and adds source and destination port numbers (destination port 443 for HTTPS).
5. Layer 3 Network: a header with source and destination IP addresses is added. The unit is now a packet.
6. Layer 2 Data link: a header with source and destination MAC addresses and a trailer for error checking are added. The unit is now a frame.
7. Layer 1 Physical: the NIC sends the frame as bits on the cable or over radio.

Decapsulation is the reverse. The receiver works from Layer 1 up to Layer 7:

1. Layer 1 turns signals back into bits.
2. Layer 2 checks the frame and removes its header and trailer.
3. Layer 3 checks the destination IP address and removes the packet header.
4. Layer 4 reads the port number, reassembles the segments and hands the data to the correct application.
5. Layers 5 to 7 manage the session, decrypt the data and deliver the request to the web server software.

Do this: recite the unit names in sending order: data, segment, packet, frame, bits.

Task 4: Place an item or fault at a layer

When a scenario names a clue, map it to a layer with this checklist:

• Cable, connector, signal, no link light: Layer 1
• MAC address, frame, NIC addressing: Layer 2
• IP address, packet, routing between networks: Layer 3
• Port number, TCP, UDP, segment: Layer 4
• Starting, maintaining or ending a conversation: Layer 5
• Encryption, compression, data format: Layer 6
• A protocol the application uses, such as HTTP: Layer 7

Example: a user reports no network access.

1. Check Layer 1 first. Is the cable seated and the link light on?
2. Check Layer 2. Does the NIC show a MAC address and an active link?
3. Check Layer 3. Does the host have a valid IP address?
4. Move up only when the lower layer works. Every layer depends on the ones below it.

In practice, encryption protocols do not map neatly to one layer. For OSI placement questions, use the model's answer: encryption belongs to Layer 6.

What the exam asks about this

Objective 1.1 reads: "Explain concepts related to the Open Systems Interconnection (OSI) reference model." Expect to:

• Match a layer number to its name and its job.
• Place a device, an address type or a fault at a layer in a short scenario.
• Order the data units for encapsulation or decapsulation.
• Apply layer reasoning in performance-based questions, such as matching items to layers.

Key takeaways

• Seven layers, bottom to top: Physical, Data link, Network, Transport, Session, Presentation, Application.
• Data units: bits (1), frames (2), packets (3), segments or datagrams (4), data (5 to 7).
• Encapsulation adds headers going down the sender's layers. Decapsulation removes them going up the receiver's layers.
• A NIC works at Layers 1 and 2. Its MAC address is a Layer 2 address.
• IP addresses are Layer 3. Port numbers are Layer 4. Encryption is Layer 6.
• Troubleshoot from Layer 1 upward.