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‎notes/2025-03-22-network-architecture-en.md

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---
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title: Network Architecture and Models
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lang: en
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layout: post
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audio: false
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translated: false
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generated: true
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---
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Below is a detailed tutorial to help you learn the chapter on **Network Architecture and Models**, focusing on the **OSI Reference Model**, the **TCP/IP Protocol Suite**, their comparison, and the functions and responsibilities of layered architectures. This is tailored to a technical course like *Computer Networks Technology (02141)* in the Chinese Higher Education Self-Study Examination (自考) system. The tutorial is structured to be comprehensive, beginner-friendly, and aligned with the technical depth expected in such a course.
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---
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### Step 1: Understand the Basics of Network Architecture
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Before diving into the models, let’s establish why network architecture matters:
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- **What is Network Architecture?** It’s a framework that defines how data communication happens between devices in a network. Think of it as a blueprint for organizing tasks like sending an email or streaming a video.
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- **Why Layers?** Networks are complex. Breaking them into layers simplifies design, troubleshooting, and standardization.
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---
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### Step 2: Learn the OSI Reference Model (7 Layers)
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The **OSI (Open Systems Interconnection) Model** is a theoretical framework with 7 layers. Each layer has a specific role in communication. Let’s break it down:
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#### 1. Physical Layer
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- **Function:** Handles the physical connection between devices (e.g., cables, switches, signals).
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- **Responsibilities:** Transmits raw bits (0s and 1s) over a medium like copper wires, fiber optics, or wireless signals.
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- **Examples:** USB cables, Ethernet cables, Wi-Fi signals.
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- **Key Concepts:** Bit rate, voltage levels, connectors.
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- **Analogy:** Think of it as the road or wire carrying the data traffic.
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#### 2. Data Link Layer
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- **Function:** Ensures error-free data transfer between two directly connected nodes.
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- **Responsibilities:**
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- Frames data (adds headers/trailers to bits).
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- Detects and corrects errors (e.g., using checksums).
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- Manages access to the shared medium (e.g., Ethernet’s MAC addressing).
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- **Examples:** Ethernet, Wi-Fi (IEEE 802.11), switches.
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- **Key Concepts:** MAC addresses, framing, error detection.
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- **Analogy:** Like a postman ensuring letters reach the next house without damage.
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#### 3. Network Layer
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- **Function:** Routes data between different networks.
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- **Responsibilities:**
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- Determines the best path for data (routing).
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- Uses logical addressing (e.g., IP addresses).
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- **Examples:** IP (IPv4/IPv6), routers.
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- **Key Concepts:** IP addressing, routing protocols (e.g., OSPF, RIP).
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- **Analogy:** A GPS deciding which roads to take to reach a distant city.
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#### 4. Transport Layer
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- **Function:** Provides reliable data transfer between devices.
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- **Responsibilities:**
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- Ensures data arrives in order and without loss (e.g., TCP).
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- Manages flow control and error correction.
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- Offers connectionless service (e.g., UDP).
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- **Examples:** TCP (reliable), UDP (fast, unreliable).
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- **Key Concepts:** Ports, segmentation, congestion control.
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- **Analogy:** A courier service ensuring packages arrive complete and in sequence.
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#### 5. Session Layer
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- **Function:** Manages sessions (connections) between applications.
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- **Responsibilities:**
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- Establishes, maintains, and terminates sessions.
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- Handles session recovery if interrupted.
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- **Examples:** NetBIOS, RPC.
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- **Key Concepts:** Session ID, synchronization.
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- **Analogy:** A phone call setup—connecting, talking, and hanging up.
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#### 6. Presentation Layer
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- **Function:** Translates data between application format and network format.
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- **Responsibilities:**
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- Encrypts/decrypts data (e.g., SSL/TLS).
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- Compresses data.
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- Converts data (e.g., text to ASCII, JPEG encoding).
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- **Examples:** SSL, JPEG, XML parsers.
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- **Key Concepts:** Encryption, data translation.
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- **Analogy:** A translator converting your language for someone else to understand.
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#### 7. Application Layer
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- **Function:** Provides network services directly to user applications.
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- **Responsibilities:**
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- Supports protocols for email, web browsing, file transfer, etc.
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- **Examples:** HTTP (web), SMTP (email), FTP (file transfer).
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- **Key Concepts:** User interface, application protocols.
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- **Analogy:** The app or website you use to interact with the network.
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**Tip:** Memorize the layers in order (Physical → Application) using a mnemonic like “Please Do Not Throw Sausage Pizza Away.”
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---
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### Step 3: Learn the TCP/IP Protocol Suite (4 Layers)
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The **TCP/IP Protocol Suite** is a practical model used in real-world networks (e.g., the Internet). It has 4 layers, which map roughly to the OSI model.
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#### 1. Link Layer
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- **Function:** Combines OSI’s Physical and Data Link layers.
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- **Responsibilities:** Handles hardware-level data transfer and framing.
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- **Examples:** Ethernet, Wi-Fi, PPP.
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- **Key Concepts:** Same as OSI’s Physical + Data Link.
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#### 2. Internet Layer
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- **Function:** Moves packets across networks (like OSI’s Network layer).
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- **Responsibilities:**
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- IP addressing and routing.
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- **Examples:** IP (IPv4/IPv6), ICMP (ping).
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- **Key Concepts:** Packet switching, IP headers.
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#### 3. Transport Layer
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- **Function:** Same as OSI’s Transport layer.
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- **Responsibilities:**
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- Reliable (TCP) or fast (UDP) data delivery.
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- **Examples:** TCP, UDP.
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- **Key Concepts:** Ports, reliability vs. speed trade-off.
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#### 4. Application Layer
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- **Function:** Combines OSI’s Session, Presentation, and Application layers.
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- **Responsibilities:**
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- Handles all user-facing protocols and data formatting.
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- **Examples:** HTTP, FTP, SMTP, DNS.
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- **Key Concepts:** End-user services.
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**Tip:** Think of TCP/IP as a simplified, real-world version of OSI.
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---
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### Step 4: Compare OSI and TCP/IP Models
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Here’s how they stack up:
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| **Aspect** | **OSI Model** | **TCP/IP Model** |
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|-------------------------|--------------------------------|--------------------------------|
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| **Number of Layers** | 7 | 4 |
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| **Nature** | Theoretical, detailed | Practical, implemented |
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| **Layer Mapping** | - Physical → Physical | - Link → Physical + Data Link |
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| | - Data Link → | |
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| | - Network → Network | - Internet → Network |
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| | - Transport → Transport | - Transport → Transport |
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| | - Session/Presentation/Application → | - Application → Session + Presentation + Application |
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| **Development** | Designed before protocols | Protocols came first |
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| **Usage** | Teaching, reference | Real-world (Internet) |
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| **Flexibility** | Rigid, distinct layers | More flexible, overlapping |
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**Key Insight:** OSI is like a detailed textbook; TCP/IP is the working engine of the Internet.
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---
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### Step 5: Understand Layered Architecture Functions and Responsibilities
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Each layer has a **specific job** and interacts with layers above and below it:
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- **Encapsulation:** As data moves down the stack (sender side), each layer adds its header (metadata). On the receiver side, each layer removes its header (decapsulation).
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- **Peer-to-Peer Communication:** Layers “talk” to their counterparts on another device (e.g., Transport layer on your PC talks to Transport layer on a server).
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- **Abstraction:** Lower layers hide complexity from upper layers (e.g., the Application layer doesn’t care about cables).
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**Example Flow (Sending an Email):**
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1. **Application:** You write an email (SMTP formats it).
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2. **Presentation:** Email text is encoded (e.g., UTF-8), maybe encrypted.
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3. **Session:** A connection to the mail server is established.
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4. **Transport:** TCP breaks the email into packets, ensures delivery.
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5. **Network:** IP routes packets to the server.
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6. **Data Link:** Ethernet frames the packets for the local network.
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7. **Physical:** Bits travel over Wi-Fi or cable.
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Reverse happens at the receiver!
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---
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### Step 6: Study Tips for 自考 (Self-Study Exam)
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1. **Memorize Key Terms:** Know layer names, functions, and examples (e.g., TCP = Transport, reliable).
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2. **Draw Diagrams:** Sketch OSI (7 layers) and TCP/IP (4 layers) side by side to visualize mapping.
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3. **Practice Questions:**
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- What’s the role of the Network layer in OSI?
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- How does TCP/IP’s Application layer differ from OSI’s?
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- Explain encapsulation with an example.
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4. **Use Analogies:** Relate layers to real life (e.g., Physical layer = roads, Transport layer = couriers).
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5. **Review Protocols:** Focus on common ones (IP, TCP, UDP, HTTP) and where they fit.
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---
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### Step 7: Test Your Understanding
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Answer these to check your grasp:
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1. What layer handles IP addressing? (Network/Internet)
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2. Which OSI layer encrypts data? (Presentation)
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3. How does TCP/IP’s Link layer relate to OSI? (Combines Physical + Data Link)
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4. Why is OSI not used directly in the Internet? (Too theoretical; TCP/IP is practical)
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### Additional Resources
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- **Books:** “Computer Networking: A Top-Down Approach” by Kurose & Ross (simplified explanations).
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- **Online:** Cisco’s networking tutorials, YouTube channels like “PowerCert Animated Videos.”
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- **Practice:** Use tools like Wireshark to see real network traffic and identify layers.
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---
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This tutorial gives you a solid foundation for the chapter. Study each section, revisit tricky parts, and practice explaining concepts aloud (great for 自考 prep). Good luck! If you need clarification on any part, just ask!

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