Computer network model
A computer network model refers to a conceptual or theoretical framework that describes the structure, components, and interactions of a computer network. It provides a way to understand and organize the various elements involved in networking, including devices, protocols, and communication channels.
There are several widely used computer network models, with the most notable ones being the OSI model (Open Systems Interconnection) and the TCP/IP model (Transmission Control Protocol/Internet Protocol). These models are used as reference frameworks to guide the design, implementation, and troubleshooting of computer networks.
OSI Model
The OSI model is a conceptual model developed by the International Organization for Standardization (ISO) to standardize network communications. It consists of seven layers, each responsible for specific functions and protocols. The layers, from top to bottom, are as follows:
- Application Layer
- Presentation Layer
- Session Layer
- Transport Layer
- Network Layer
- Data Link Layer
- Physical Layer
Each layer has its own set of protocols and performs specific tasks, such as data encryption (Presentation Layer), establishing and managing connections (Transport Layer), routing (Network Layer), and managing physical connections (Physical Layer).
Example of OSI Model
Let's use the example of accessing a web page in a browser to illustrate the functions of each layer of the OSI model:
1. Application Layer:
At the application layer, the user interacts with the web browser (e.g., Google Chrome) to request a specific web page by typing a URL (e.g., www.example.com) and pressing Enter. The browser uses protocols such as HTTP or HTTPS to establish a connection with the web server.
2. Presentation Layer:
The presentation layer is responsible for data representation and encryption. In the case of accessing a web page, the presentation layer may handle tasks such as converting the HTML content of the web page into a format suitable for display on the user's device.
3. Session Layer:
The session layer establishes and manages the session between the client (browser) and the server. It facilitates the exchange of information and coordinates the start and end of communication sessions. For example, it may manage session cookies for maintaining state during a browsing session.
4. Transport Layer:
The transport layer ensures reliable data delivery between the client and the server. It breaks the data into smaller units called segments and provides mechanisms for error detection, retransmission, and flow control. In the case of web browsing, the transport layer may use the TCP protocol to ensure the reliable delivery of HTTP requests and responses.
5. Network Layer:
The network layer handles the logical addressing and routing of data packets. It determines the optimal path for the data to reach its destination. For web browsing, the network layer may use IP (Internet Protocol) to assign IP addresses to the client and server, and route the packets between them across different networks.
6. Data Link Layer:
The data link layer is responsible for the reliable transmission of data frames between directly connected devices. It adds physical addressing (MAC addresses) to the frames and performs error detection and correction. In the case of web browsing, the data link layer may handle Ethernet frames for communication between the client's device and the local network router.
7. Physical Layer:
The physical layer deals with the actual transmission of bits over the physical medium. It includes the cables, connectors, and signaling mechanisms. For web browsing, the physical layer encompasses the physical Ethernet or Wi-Fi connections between the client device and the local network infrastructure.
Throughout the process of accessing a web page, data flows through each layer of the OSI model, with each layer performing specific functions to ensure the successful communication between the client and server. The layers work together to enable the retrieval and display of the requested web page in the user's browser.
TCP/IP Model
The TCP/IP model is another widely used network model, which is the basis for the internet. It is a four-layered model that describes the protocols and processes involved in transmitting data across a network. The layers, from top to bottom, are as follows:
- Application Layer
- Transport Layer
- Internet Layer
- Network Interface Layer
The TCP/IP model is more closely aligned with the actual protocols used on the internet, such as HTTP (Hypertext Transfer Protocol), TCP (Transmission Control Protocol), IP (Internet Protocol), and Ethernet.
It's important to note that while the OSI model has seven layers and the TCP/IP model has four layers, they are not entirely equivalent. The TCP/IP model's layers can be mapped to the OSI model to some extent, but they do not align perfectly.
These network models provide a structured approach to understanding and discussing network concepts and facilitate interoperability between different network devices and technologies.
Example of TCP/IP model
Let's take a simple example of how data flows through the layers of the TCP/IP model when you browse a website using a web browser like Google Chrome.
1. Application Layer:
You open Google Chrome and type in the URL of a website, such as "www.example.com". At the application layer, the browser sends an HTTP request to the web server hosting the website.
2. Transport Layer:
The transport layer takes the HTTP request from the application layer and divides it into smaller segments, if needed, for efficient transmission. It adds necessary information like source and destination port numbers. In this case, the browser uses the TCP protocol, which provides reliable and ordered delivery of data. The TCP protocol assigns a source port number (e.g., 12345) to the request.
3. Internet Layer:
The internet layer, also known as the network layer, takes the TCP segment and adds the source and destination IP addresses. It creates an IP packet. The browser uses the DNS (Domain Name System) to resolve the domain name "www.example.com" to an IP address (e.g., 192.0.2.10) of the web server. The IP packet is then sent to the appropriate network.
4. Network Interface Layer:
The network interface layer, also called the data link layer, takes the IP packet and encapsulates it into a frame that can be transmitted over the physical network medium, such as Ethernet. The frame includes the MAC (Media Access Control) addresses of the source and destination devices. The frame is then transmitted over the network medium.
At the receiving end, the process is reversed:
5. Network Interface Layer:
The receiving device receives the frame from the network medium and extracts the encapsulated IP packet.
6. Internet Layer:
The receiving device checks the destination IP address of the IP packet and determines that it is intended for itself. It then removes the IP header and forwards the packet to the transport layer.
7. Transport Layer:
The transport layer receives the TCP segment, removes the TCP header, and reassembles the original HTTP request. It then delivers the request to the application layer of the receiving device, which in this case is the web server.
8. Application Layer:
The web server processes the HTTP request, retrieves the requested web page (e.g., index.html), and prepares an HTTP response.
The process continues in reverse order as the web server sends the HTTP response back to the browser, following the same path through the layers.
This example demonstrates how the TCP/IP model breaks down the communication process into different layers, with each layer responsible for specific tasks, ultimately enabling communication between the web browser and the web server.
Easier example of TCP/IP model
Let's use a more straightforward example to explain how data flows through the layers of the TCP/IP model when you send an email from your computer to a friend.
1. Application Layer:
You compose an email using an email client, such as Microsoft Outlook or Gmail. At the application layer, your email client prepares the email, including the sender's and recipient's email addresses, subject, and message content.
2. Transport Layer:
The transport layer takes the email message from the application layer and adds necessary information for reliable delivery. It uses the SMTP (Simple Mail Transfer Protocol) to encapsulate the email message into a TCP segment. The TCP segment includes source and destination port numbers. The email client assigns a source port number (e.g., 587) to the email.
3. Internet Layer:
The internet layer takes the TCP segment and encapsulates it into an IP packet. It adds the source and destination IP addresses. The email client determines the recipient's IP address based on the recipient's email domain (e.g., gmail.com). It uses DNS (Domain Name System) to resolve the domain name to the appropriate IP address.
4. Network Interface Layer:
The network interface layer encapsulates the IP packet into a frame suitable for transmission over the physical network medium. It includes the MAC addresses of the sender's and recipient's devices. The frame is then sent over the network medium.
At the receiving end, the process is reversed:
5. Network Interface Layer:
The receiving device receives the frame from the network medium and extracts the encapsulated IP packet.
6. Internet Layer:
The receiving device examines the destination IP address of the IP packet and determines that it is intended for itself. It removes the IP header and forwards the packet to the transport layer.
7. Transport Layer:
The transport layer receives the TCP segment, removes the TCP header, and extracts the original email message. It delivers the email message to the application layer of the recipient's email client.
8. Application Layer:
The recipient's email client receives the email message and displays it in the inbox for the recipient to read.
In this simplified example, we focused on the core steps involved in sending an email using the TCP/IP model. The process involves the application layer for composing the email, the transport layer for segmenting and reliable delivery, the internet layer for addressing and routing, and the network interface layer for physical transmission. Each layer performs specific functions to ensure the successful transmission of the email from the sender to the recipient.
