Network Architecture and Models Study | Generated by AI
This chapter lays the foundation for understanding how computer networks are structured and how data is transmitted across them. In the context of the Computer Networks Technology course (自考 code 02141 or similar), it emphasizes theoretical models like OSI and TCP/IP, which are core to exam questions on layering, protocols, and data flow. Focus on memorizing layer names, functions, and mappings between models. Expect multiple-choice, short-answer, or diagram-based questions testing comparisons and responsibilities.
1. Introduction to Layered Architectures
Networks use layered architectures to simplify complex communication by dividing tasks into modular layers. Each layer:
- Handles specific functions (e.g., error checking, routing).
- Interacts only with adjacent layers via standardized interfaces.
- Uses encapsulation (adding headers/trailers) when sending data down the stack and decapsulation when receiving.
Benefits:
- Modularity: Easy to develop, test, and update individual layers.
- Interoperability: Devices from different vendors can communicate.
- Scalability: Layers can evolve independently (e.g., new transport protocols).
Responsibilities (general across models):
- Lower layers: Focus on hardware and reliable data transfer (physical transmission, error detection).
- Upper layers: Handle user-facing tasks (e.g., file transfer, web browsing).
- Data flows down the sender’s stack (encapsulation) and up the receiver’s stack (decapsulation).
2. OSI Reference Model
The Open Systems Interconnection (OSI) model is a conceptual 7-layer framework developed by ISO in 1984. It’s theoretical, not implemented directly, but used as a standard for understanding protocols. Mnemonic: Please Do Not Throw Sausage Pizza Away (Physical → Application).
| Layer Number | Layer Name | Key Functions and Protocols | PDU (Protocol Data Unit) | Devices/Examples |
|---|---|---|---|---|
| 7 | Application | Provides network services to user apps (e.g., email, file transfer). Interfaces with software. | Data | HTTP, FTP, SMTP; Web browser |
| 6 | Presentation | Translates data formats (e.g., encryption, compression, ASCII to EBCDIC). Ensures syntax compatibility. | Data | JPEG, SSL/TLS |
| 5 | Session | Manages sessions/connections (e.g., setup, synchronization, dialog control). Handles checkpoints for recovery. | Data | NetBIOS, RPC |
| 4 | Transport | End-to-end reliable delivery (e.g., segmentation, flow control, error recovery). | Segment (TCP) / Datagram (UDP) | TCP, UDP; Ports (e.g., 80 for HTTP) |
| 3 | Network | Logical addressing and routing (e.g., path determination across networks). Handles congestion. | Packet | IP, ICMP, OSPF; Routers |
| 2 | Data Link | Node-to-node delivery on same network (e.g., framing, error detection via CRC, MAC addressing). | Frame | Ethernet, PPP; Switches, NICs |
| 1 | Physical | Bit transmission over physical medium (e.g., signaling, cabling, topology). Deals with hardware specs. | Bit | RJ-45, Fiber optics; Hubs, Cables |
Key Notes:
- Layers 1-2: Media-focused (LAN/WAN).
- Layers 3-4: Host-to-host (internetworking).
- Layers 5-7: User-oriented (application support).
- Exam Tip: Draw the stack and label PDUs/headers (e.g., TCP segment has TCP header + data).
3. TCP/IP Protocol Suite
The TCP/IP model (or Internet Protocol Suite) is a practical 4-layer model developed in the 1970s for the ARPANET (basis of the internet). It’s implemented worldwide and maps loosely to OSI. Mnemonic: LITA (Link → Application).
| Layer Number | Layer Name | Key Functions and Protocols | PDU | Devices/Examples |
|---|---|---|---|---|
| 4 | Application | Combines OSI Layers 5-7: User services (e.g., web, email). | Data/Segment | HTTP, FTP, DNS; Apps like browsers |
| 3 | Transport | End-to-end (OSI Layer 4): Reliable/unreliable delivery. | Segment/Datagram | TCP (reliable, connection-oriented), UDP (best-effort) |
| 2 | Internet | Routing and addressing (OSI Layer 3): Logical paths across networks. | Packet | IP (IPv4/IPv6), ICMP; Routers |
| 1 | Link (or Network Access) | Physical + Data Link (OSI Layers 1-2): Hardware delivery on local network. | Frame/Bit | Ethernet, Wi-Fi; Switches, Cables |
Key Notes:
- No dedicated session/presentation layers; handled within Application.
- TCP/IP is “protocol family” – e.g., IP is core, with TCP/UDP on top.
- Exam Tip: Emphasize real-world use (e.g., TCP ensures reliability via acknowledgments, while UDP is lightweight for video streaming).
4. Comparison of OSI and TCP/IP Models
Use this table for quick revision. OSI is theoretical (reference), TCP/IP is practical (implementation).
| Aspect | OSI Model | TCP/IP Model |
|---|---|---|
| Layers | 7 (detailed, conceptual) | 4 (simplified, practical) |
| Development | ISO (1984), top-down design | DoD/Internet (1970s), bottom-up |
| Focus | General networking standards | Internet-specific protocols |
| Implementation | Not directly implemented; reference for standards | Widely used (basis of modern internet) |
| Layer Mapping | 1: Physical → Link 2: Data Link → Link 3: Network → Internet 4: Transport → Transport 5-6-7: Session/Presentation/Application → Application |
Application absorbs OSI 5-7; Link absorbs 1-2 |
| Protocols | Theoretical (e.g., no single IP) | Specific (e.g., IP, TCP, HTTP) |
| PDU Flow | Strict per-layer headers | Flexible (e.g., IP packet includes transport data) |
| Strengths | Comprehensive, easy to teach | Efficient, scalable, vendor-neutral |
| Weaknesses | Overly complex, not practical | Less detailed for upper layers |
Key Differences:
- Granularity: OSI separates session/presentation; TCP/IP merges them into Application for simplicity.
- Addressing: OSI uses service access points (SAPs); TCP/IP uses ports/IP addresses.
- Reliability: Both have transport reliability, but TCP/IP’s TCP is connection-oriented like OSI’s Transport.
- Exam Tip: Questions often ask for mappings (e.g., “Which OSI layer corresponds to TCP?”) or advantages (e.g., TCP/IP’s adaptability led to the internet’s growth).
5. Layered Architecture Functions and Responsibilities
Core Principles:
- Abstraction: Each layer hides lower-layer details (e.g., Transport doesn’t care about physical cables).
- Service Primitives: Layers provide services like CONNECT, DATA, DISCONNECT to upper layers.
- Error Handling: Lower layers detect errors; upper layers recover (e.g., Transport retransmits lost packets).
- Addressing: Hierarchical – physical (MAC), logical (IP), service (ports).
Data Transmission Example:
- Application data → Transport adds segment header (ports, seq. no.) → Network adds packet header (IP addresses) → Link adds frame header/trailer (MAC) → Physical sends bits.
- Reverse at receiver.
Common Pitfalls:
- Don’t confuse layers: Physical is bits/signals; Data Link is frames/error-free frames.
- TCP/IP Link layer = OSI 1+2 combined.
Study Tips for 自考 Exam
- Memorization: Use mnemonics and draw layered diagrams daily. Focus on 80% exam weight on OSI/TCP/IP descriptions.
- Practice:
- Compare layers in essays (200-300 words).
- MCQs: “Which layer handles routing? (A) Network”.
- Diagrams: Label a PDU traversal.
- Resources: Review textbook figures for encapsulation. Time yourself: 30 mins for comparison table.
- Common Questions: Explain why OSI has 7 layers vs. TCP/IP’s 4; Describe Transport layer duties.
Master this chapter for a strong start – it’s referenced in later topics like protocols (e.g., IP in Chapter 3). Next, tackle physical layer details.
References:
- Computer Networking: A Top-Down Approach by Kurose & Ross (for TCP/IP depth).
- OSI Model Explained - GeeksforGeeks (visual aids).