# Chapter 2 Network Models

Chapter 2 of *Data Communications and Networking* by Behrouz A. Forouzan, titled **"Network Models"**, introduces the two primary networking models: the **OSI (Open Systems Interconnection)** model and the **TCP/IP model**. Here are detailed notes on this chapter:

#### 1. **Layered Tasks**

* **Concept of Layering**: Layering simplifies networking tasks by dividing them into smaller, manageable processes.
* Example: A postal system where tasks like writing, mailing, and delivering are carried out in layers, similar to how communication systems operate.

#### 2. **The OSI Model**

* **Introduction**: The OSI model, established by the International Standards Organization (ISO), is a seven-layer framework that allows diverse systems to communicate.
* **Layered Architecture**:
  * Communication happens between corresponding layers (peer-to-peer processes).
  * Data is passed through all layers from the application on the source to the destination, with headers and trailers added at each layer (encapsulation).
* **Layers of the OSI Model**:
  1. **Physical Layer**: Deals with the transmission of raw bits over a communication channel.
  2. **Data Link Layer**: Responsible for error detection, flow control, and framing.
  3. **Network Layer**: Handles the logical addressing and routing of packets.
  4. **Transport Layer**: Ensures reliable process-to-process delivery of the message.
  5. **Session Layer**: Manages and controls the dialogs between computers.
  6. **Presentation Layer**: Transforms data into a format that the application layer can accept.
  7. **Application Layer**: Provides services directly to the end users such as file transfers, email, etc.

#### 3. **The TCP/IP Model**

* **Introduction**: Unlike the OSI model, the TCP/IP model is used practically on the Internet.
* **Layers of the TCP/IP Model**:
  1. **Link Layer**: Corresponds to the physical and data link layers of the OSI model.
  2. **Internet Layer**: Equivalent to the network layer of the OSI, handling logical addressing and routing.
  3. **Transport Layer**: Manages data delivery between processes.
  4. **Application Layer**: Handles high-level protocols such as HTTP, FTP, etc.

#### 4. **Addressing**

* Different types of addresses used in network communications:
  1. **Physical Address**: The MAC (Media Access Control) address used for node-to-node communication.
  2. **Logical Address**: IP addresses used for source-to-destination delivery of data across multiple networks.
  3. **Port Address**: Identifies specific processes on the host device.
  4. **Specific Address**: User-friendly addresses like email addresses or URLs that are converted into port and logical addresses during communication.

#### 5. **Summary**

* The OSI model, though theoretical, helps in understanding how communication protocols should function.
* The TCP/IP model is practical and used widely in the Internet today.
* Understanding both models is crucial for analyzing and working with modern networking technologies .

This chapter lays the foundation for further exploration of specific layers and their protocols in subsequent chapters.