5.1 Introduction to Computer Networks
Last updated
Last updated
In this section, we will explore the fundamentals of computer networks, including networking models, protocols, networking devices, and transmission media. Understanding the OSI and TCP/IP models will help you grasp how data is transmitted and routed across networks. The physical layer, which forms the foundation of all network communication, is also covered to explain how signals travel between devices.
A networking model defines how different network devices communicate with each other. Two primary models are used to understand and design networks: the OSI model and the TCP/IP model. Along with the models, protocols and standards define the rules and guidelines for communication between devices, ensuring compatibility and efficient data transfer.
1.1 OSI Model (Open Systems Interconnection Model)
The Open Systems Interconnect (OSI) model is a conceptual framework designed to help understand how data flows over a network. It serves as a reference model, acting as a roadmap to explain what occurs across and within a network. The OSI layer model isn’t hardware or software. It’s more like the set of rules or protocols governing how networking devices communicate with each other and share data. Every device in the networking that communicates with each other has the OSI model’s concept internally. No matter whether the device is the sender or the receiver.
Development and Purpose
The OSI model was developed by the International Organization for Standardization (ISO) to assist developers in comprehending modern computer network technology in a connection-oriented manner. It enables technology vendors to create both software programs and digital communication products aligned with a clear framework that defines how a network functions.
Adoption and Relevance
Since its creation in 1984, the OSI model has been widely adopted by major network companies worldwide. Although the modern Internet primarily relies on the simpler TCP/IP model, the OSI model remains relevant. Its comprehensive structure extends beyond TCP/IP, offering valuable insights for troubleshooting network problems when issues arise.
The 7-Layer Model
The OSI model divides the communication process into seven layers, each assigned a specific role in supporting the layers above and below it:
Each layer performs its functions independently, ensuring modularity.
Together, the layers provide users with a big picture understanding of how networks operate.
The OSI layers are:
1. Physical Layer
The lowest layer of the OSI structure, the Physical Layer, is responsible for transporting raw data across physical hardware such as Ethernet cables. Common protocols at this layer include RS232, ATM, and FDDI.
Administrators often use this layer to check cable connections, including:
Type of cable used
Type of connector
Cable length
2. Data Link Layer
The Data Link Layer ensures the reliable transfer of data frames between physically connected devices by correcting errors from the Physical Layer.
At this layer, data is organized into frames and transferred between network nodes. It is divided into two sublayers:
Media Access Control (MAC): Tracks data frames using source and destination MAC addresses.
Logical Link Control (LLC): Manages error control, multiplexing, and line protocol identification.
3. Network Layer
The Network Layer handles the routing of data across networks. It ensures that data packets are forwarded to their destination via the shortest route.
Key responsibilities include:
Managing IP addresses of the sender and receiver.
Mapping physical and logical addresses.
Operating routers.
Protocols in this layer include IP, TCP/UDP, AppleTalk DDP, and IPX.
4. Transport Layer
The Transport Layer (Layer 4) coordinates data transfers between hosts and ensures end-to-end delivery of messages.
Functions include:
Dividing data from the Session Layer into segments for the Network Layer.
Error detection and correction.
Managing flow control and realigning segmented data.
This layer supports connection-oriented communication via TCP and connectionless communication via UDP.
5. Session Layer
The Session Layer establishes, maintains, and terminates communication sessions between devices.
Key tasks include:
Creating communication channels (sessions).
Synchronizing data flow with checkpoints to prevent data loss.
Managing protocols like NetBIOS, RPC, SQL, and NFS.
6. Presentation Layer
The Presentation Layer ensures data is formatted for compatibility between applications, devices, and networks.
Its main functions are:
Translation: Converting data into formats understood by the application.
Data Compression: Reducing data size without loss.
Encryption and Decryption: Ensuring secure data transmission.
7. Application Layer
The Application Layer is the most visible to end users. It is where applications like web browsers, email clients, and communication tools operate.
Key functions include:
Identifying resources and communication partners.
Synchronizing communication.
Supporting protocols such as HTTP/HTTPS, SMTP, POP3, and FTP.
Applications like Skype, Outlook, and web browsers rely on this layer for network-related tasks like sending emails and reading messages.
1.2 TCP/IP Model (Transmission Control Protocol/Internet Protocol)
The TCP/IP model is the foundation of the internet and most modern networks. It is simpler than the OSI model, with only four layers:
Link Layer: Corresponds to the OSI's Physical and Data Link layers, managing the physical connection.
Internet Layer: Corresponds to the OSI's Network layer, responsible for routing and addressing data (e.g., IP protocol).
Transport Layer: Ensures reliable data transfer (e.g., TCP, UDP).
Application Layer: Handles high-level protocols and application services (e.g., HTTP, FTP).
Networking devices are hardware components that help manage data flow, direct traffic, and connect different devices in a network. Some common networking devices are hubs, bridges, switches, and routers.
2.1 Hub
A hub is a basic networking device that connects multiple devices in a local area network (LAN). It broadcasts data to all connected devices, regardless of which device the data is intended for.
Function: Hubs operate at the Physical Layer (Layer 1) of the OSI model. They simply repeat electrical signals to all ports.
Limitation: Hubs cause network collisions and do not offer any form of intelligent traffic management.
2.2 Bridge
A bridge connects two network segments and filters traffic between them based on MAC addresses. It helps reduce network collisions and can segment a network to make it more efficient.
Function: Bridges operate at the Data Link Layer (Layer 2) and use MAC addresses to forward data.
Limitation: Bridges can only connect two networks within the same protocol.
2.3 Switch
A switch is more advanced than a hub and is used to connect devices in a LAN. It can intelligently forward data only to the device it is intended for, based on MAC addresses.
Function: Switches operate at the Data Link Layer (Layer 2) and can improve network efficiency by reducing collisions.
Limitation: Switches still operate within a single network and cannot route data between different networks.
2.4 Router
A router is used to connect different networks, such as a LAN to a WAN (Wide Area Network) or the internet. Routers determine the best path for data to travel from one network to another.
Function: Routers operate at the Network Layer (Layer 3) of the OSI model and use IP addresses to route data packets between networks.
Example: A router in a home network that connects the local network to the internet.
Transmission media is the physical path that carries data between network devices. It can be wired or wireless.
3.1 Wired Transmission Media
Twisted Pair Cable: Composed of pairs of copper wires twisted together, this is used for short-to-medium range communication (e.g., Ethernet cables).
Coaxial Cable: A type of cable with a central conductor, insulation, shielding, and an outer cover, used for longer distances and higher frequencies (e.g., cable TV).
Fiber Optic Cable: Uses light signals to transmit data over long distances with minimal signal degradation, ideal for high-speed internet connections.
3.2 Wireless Transmission Media
Radio Waves: Used in wireless communication technologies such as Wi-Fi and Bluetooth.
Microwaves: High-frequency radio waves used for point-to-point communication, including satellite communication.
Infrared: Uses infrared light to transfer data over short distances, typically for devices like remote controls or short-range file sharing.
Networking Models: The OSI and TCP/IP models help standardize and structure network communication into layers.
Networking Devices: Devices like hubs, bridges, switches, and routers are crucial for data flow management, connecting and directing traffic between devices and networks.
Transmission Media: Data can travel over physical (wired) or electromagnetic (wireless) paths, with options ranging from twisted pair cables to fiber optics for wired transmission, and radio waves to infrared for wireless communication.
Understanding these core concepts provides a solid foundation for studying computer networks and helps in designing and troubleshooting network systems.
