CompTIA Network+ N10-009

The OSI Reference Model

Covering the Open Systems Interconnection (OSI) model, its seven hierarchical layers, encapsulation, and how data reliably travels across disparate networks.

1.0 Domain

1.1 Objective

7 Layers

20 Quiz Qs

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01 · Fundamentals

Internetworking Models

In the late 1970s, the Open Systems Interconnection (OSI) reference model was created by the ISO to break through the barrier of proprietary networks. It serves as a conceptual blueprint defining how data communicates from an application on one computer through network media to another.

// Exam Objectives Covered

Domain 1.0: Networking Concepts
1.1 Explain concepts related to the OSI reference model
Layer 1 through Layer 7 Functions
Encapsulation and Decapsulation
Media Access Control (MAC) & Logical Link Control (LLC)
Connection-oriented vs Connectionless
TCP Three-way Handshake

The Layered Approach

A reference model addresses all processes required for effective communication and divides them into logical groupings called layers. If a developer needs to design a protocol for a specific layer, they only focus on that layer's functions without worrying about others—a concept known as binding.

Advantages of Reference Models

The OSI model divides complex communication into simpler components, allows multiple-vendor development through standardization, prevents changes in one layer from affecting others, and ensures various types of hardware and software can communicate smoothly.

02 · The Seven Layers

The OSI Model Stack

The OSI model is divided into two groups. The top three layers define how applications communicate with end users. The bottom four layers define how data is transmitted end to end. Remember the mnemonic: Please Do Not Throw Sausage Pizza Away.

Application

Acts as an interface between the application program and the network. Identifies communication partners and resource availability.

Presentation

Presents data to the Application layer. Responsible for data translation, formatting (e.g., ASCII, EBCDIC), compression, and encryption.

Session

Sets up, manages, and tears down sessions. Provides dialogue control (simplex, half-duplex, full-duplex) and keeps application data separate.

Transport

Segments and reassembles data. Provides end-to-end transport, multiplexing, virtual circuits, flow control, and reliable delivery (TCP/UDP).

Network

Manages logical device addressing and determines the best path to move data (routing). Routers operate here.

Data Link

Formats messages into data frames and adds hardware (MAC) addresses. Ensures proper delivery on a LAN. Divided into LLC and MAC sublayers.

Physical

Sends and receives bits (1s and 0s) over physical media. Defines physical topology, voltage, and cabling standards.

03 · Transport Layer

Reliable Communication & Flow Control

The Transport layer is crucial for establishing logical connections. It handles connection-oriented communication, flow control, windowing, and acknowledgments to ensure data integrity.

Connection-Oriented Communication

Before sending data, a transmitting host establishes a virtual circuit via a TCP three-way handshake:

1. SYN: Request for synchronization.
2. SYN/ACK: Acknowledges request and establishes connection parameters.
3. ACK: Final acknowledgment. Connection is established.

Flow Control

Prevents a sender from overflowing a receiver's buffers. If a receiver is overwhelmed, it issues a "not ready" or "Stop" indicator. Once processed, it sends a "ready" indicator to resume transmission.

Windowing

The quantity of data segments (measured in bytes) a transmitting machine can send without receiving an acknowledgment. A larger window size increases throughput efficiency.

Acknowledgments

Guarantees data won't be duplicated or lost via positive acknowledgment with retransmission. The receiver sends an ACK back to the sender. If the sender's timer expires before an ACK is received, the segment is retransmitted.

04 · Lower Layers

Network, Data Link & Physical

The bottom three layers dictate how data is routed across the internetwork, framed for local delivery, and physically transmitted as bits.

Network Layer (Layer 3)

Routers operate here. They use logical addresses (IP) to determine the next-hop router. Routers break up broadcast domains by default. They use routing tables containing network addresses, exit interfaces, and metrics (like hop count) to find the best path.

Data Link Layer (Layer 2)

Handles error notification, topology, and flow control. Uses MAC addresses. IEEE 802 standards divide this layer into two sublayers: LLC (Logical Link Control) and MAC (Media Access Control).

Physical Layer (Layer 1)

Sends and receives bits. Defines electrical, mechanical, procedural, and functional requirements for maintaining and deactivating physical links. Defines physical topologies (bus, star, ring, mesh).

05 · Encapsulation

Data Encapsulation Method

When a host transmits data, it is wrapped with protocol information at each layer of the OSI model. Each layer communicates with its peer layer using Protocol Data Units (PDUs).

Application

Data

Transport

Segments

Network

Packets

Data Link

Frames

Physical

Bits

10 · Written Lab

Practice Written Lab

Fill in the blank with the OSI layer name that best fits each statement. Type your answer and press Enter or click outside the field to check.

What is the name of Layer 7 in the OSI model?

Hint: Where users interact with the computer

What is the name of Layer 6 in the OSI model?

Hint: Presents data, handles formatting and encryption

What is the name of Layer 5 in the OSI model?

Hint: Sets up and tears down sessions

What is the name of Layer 4 in the OSI model?

Hint: Segments and reassembles data (TCP/UDP)

What is the name of Layer 3 in the OSI model?

Hint: Logical addressing and routing

What is the name of Layer 2 in the OSI model?

Hint: Two words. Framing and MAC addresses

What is the name of Layer 1 in the OSI model?

Hint: Sends and receives bits

11 · Review Quiz

Test Your Knowledge

20 review questions covering all the chapter material. Select the best answer for each question.

Question 1 of 20

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