UNIT 1: Introduction to Computer Networks

 

UNIT 1: Introduction to Computer Networks

A computer network is a group of computers connected together to share resources and information. Networks are the backbone of modern communication, allowing data to travel between computers and devices across the globe.

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1.1. Introduction to Computer Networks

Definition: A computer network is a collection of computers and other devices (like printers, routers, switches) that are connected together to share data and resources, such as the internet, files, and hardware.

Types of Computer Networks:

1. LAN (Local Area Network):
   • A network that covers a small geographic area, such as a home, office, or campus.
   • Devices are connected via cables or wireless.
2. WAN (Wide Area Network):
   • A network that spans a large geographic area, often country-wide or global.
   • The internet is the largest example of a WAN.
3. MAN (Metropolitan Area Network):
   • Covers a city or a large campus.
   • It is larger than a LAN but smaller than a WAN.
4. PAN (Personal Area Network):
   • A network for personal devices, such as connecting a smartphone, laptop, and tablet.

Components of a Computer Network:

1. Hardware:
   • Devices like computers, routers, switches, and cables.
2. Software:
   • Programs that manage the network, including protocols and applications like network management software.
3. Protocols:
   • Rules that define how data is transmitted over a network (e.g., TCP/IP, HTTP).

Advantages of Computer Networks:

• Resource Sharing: Share printers, files, and internet connections.
• Cost Efficiency: Reduces cost by sharing resources.
• Communication: Enables communication through emails, instant messaging, etc.
• Access to Remote Resources: Accessing data or services remotely, like cloud storage.

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1.2. Network Models

A network model defines how different components of a network communicate and interact with each other. It provides guidelines to ensure that devices in a network can communicate effectively.

1.2.1. OSI Model (Open Systems Interconnection Model)

The OSI Model is a conceptual framework that standardizes how communication should occur in a network. It divides network communication into 7 layers to simplify troubleshooting and to ensure devices from different manufacturers can work together.

The 7 Layers of OSI:

1. Physical Layer:

   • Deals with the transmission of raw binary data over physical mediums (wires, fiber optics).
   • Examples: Ethernet cables, fiber optics, wireless signals.

2. Data Link Layer:

   • Provides error detection and correction and manages the frame for data transmission.
   • Responsible for data packet flow and managing how data is placed on the physical medium.
   • Examples: Ethernet, MAC addresses.

3. Network Layer:

   • Responsible for routing and forwarding data packets across networks.
   • It handles the addressing and routing of data.
   • Example: IP (Internet Protocol), Routers.

4. Transport Layer:

   • Ensures reliable data transmission between devices and provides error handling and flow control.
   • Breaks down data into smaller packets for transport and then reassembles it on the other side.
   • Examples: TCP, UDP.

5. Session Layer:

   • Establishes, manages, and terminates communication sessions between applications on different devices.
   • Example: Managing a session for a file transfer.

6. Presentation Layer:

   • Deals with data translation, encryption, and compression.
   • Ensures that the data is in a usable format for the application layer.
   • Example: Data encryption (e.g., SSL/TLS).

7. Application Layer:

   • The top layer, where end-user software resides, such as email, web browsers, and file transfer applications.
   • Examples: HTTP, FTP, DNS.

OSI Model Diagram

       Application
       ----------------
       Presentation
       ----------------
       Session
       ----------------
       Transport
       ----------------
       Network
       ----------------
       Data Link
       ----------------
       Physical

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1.2.2. TCP/IP Model (Transmission Control Protocol/Internet Protocol)

The TCP/IP model is a more simplified version of the OSI model and is used to describe how data is transferred over the internet. The TCP/IP model is made up of 4 layers:

1. Application Layer:

   • Corresponds to the top three layers of the OSI model (Application, Presentation, Session).
   • It includes protocols that allow applications to interact with the network.
   • Examples: HTTP, FTP, SMTP.

2. Transport Layer:

   • Responsible for end-to-end communication and error handling.
   • Example: TCP (provides reliable communication) and UDP (provides faster but unreliable communication).

3. Internet Layer:

   • Responsible for addressing and routing packets between networks.
   • Example: IP (Internet Protocol), which assigns unique IP addresses.

4. Network Access Layer:

   • Combines the OSI’s Physical and Data Link layers.
   • Defines the methods for data transmission over physical networks, such as Ethernet or Wi-Fi.

TCP/IP Model Diagram

     Application
     ----------------
     Transport
     ----------------
     Internet
     ----------------
     Network Access

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1.3. OSI Reference Model

The OSI Reference Model (Open Systems Interconnection Model) was developed by the International Organization for Standardization (ISO) to standardize the way networking works and to facilitate communication between different systems and technologies. It divides network communication into 7 distinct layers.

Key Points:

• The OSI Model is a conceptual model; it does not directly relate to any physical hardware.
• It helps different vendors to create compatible devices and software that can communicate across networks.

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1.4. TCP/IP Model

The TCP/IP Model is the practical model used for internet communication. It is based on the Transmission Control Protocol and the Internet Protocol, which are used to transmit data across the internet. The TCP/IP model is simplified and contains 4 layers, as opposed to the 7 layers in the OSI model.

Key Differences Between OSI and TCP/IP Models:

• The OSI model has 7 layers, while the TCP/IP model has 4 layers.
• OSI is a theoretical model, whereas TCP/IP is the practical model used for real-world communication.

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Summary

• Computer networks connect multiple computers for sharing data and resources. Examples of networks include LAN, WAN, MAN, and PAN.
• Network models define how data is transmitted across a network. Two popular models are the OSI Model and the TCP/IP Model.
  • The OSI model has 7 layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.
  • The TCP/IP model has 4 layers: Application, Transport, Internet, and Network Access.
• Both models aim to standardize communication protocols, but the OSI model is more theoretical, while TCP/IP is more practical and widely used in real-world communication.