# Unit 4: Information Communication & Networking

## 4.1 Basic Terminology of Information Communication

### Basic Structure of Communication System
A communication system enables the transmission of information from a source to a destination. The basic structure consists of:
- **Sender/Source**: The device or entity that generates the message to be sent.
- **Transmitter**: Converts the message into a signal suitable for transmission over the medium.
- **Communication Channel/Medium**: The physical or wireless path through which the signal travels from sender to receiver.
- **Receiver**: Extracts the signal from the medium and converts it back into the original message.
- **Destination**: The final recipient of the message.
- **Noise**: Any unwanted interference that can distort the signal during transmission.

### Transmission Modes
Transmission modes define the direction of signal flow between two linked devices.
- **Simplex**: Communication is unidirectional. Only one device can transmit, and the other can only receive (e.g., Radio, Television broadcasting, Keyboard to Monitor).
- **Half-Duplex**: Communication is bidirectional, but not at the same time. Both devices can transmit and receive, but when one is sending, the other must wait (e.g., Walkie-Talkies).
- **Full-Duplex**: Communication is bidirectional and simultaneous. Both devices can send and receive data at the same time (e.g., Telephone networks, Mobile phones).

### Synchronous and Asynchronous Transmission
- **Asynchronous Transmission**: Data is transmitted one byte (or character) at a time. It uses start and stop bits to indicate the beginning and end of a byte. It is typically used for low-speed communication. There is irregular spacing between characters.
- **Synchronous Transmission**: Data is sent as a continuous stream of bits, grouped into frames or blocks, without start and stop bits. The sender and receiver must be synchronized using a common clock signal. It is faster and used for high-speed data transfer.

### Serial and Parallel Communication
- **Serial Communication**: Data bits are transmitted sequentially, one bit at a time, over a single communication line. It is slower but ideal for long-distance communication because it requires fewer wires and is less prone to interference.
- **Parallel Communication**: Multiple data bits (typically a byte or word) are transmitted simultaneously over multiple parallel wires. It is faster but generally used for short distances (e.g., CPU to memory) because of issues like crosstalk and higher cost over long distances.

## 4.2 Transmission Media and Connectors

### Transmission Media
Transmission media carry data from one point to another.
- **Twisted-Pair Cable**: Consists of pairs of insulated copper wires twisted together to reduce electromagnetic interference (EMI) and crosstalk.
  - **Unshielded Twisted Pair (UTP)**: Does not have any extra shielding. Commonly used in LANs and telephone networks. It is cheap and easy to install but susceptible to interference.
  - **Shielded Twisted Pair (STP)**: Has a metallic shield around each pair or the entire group of wires to provide better protection against EMI. It is more expensive and bulkier than UTP.
- **Coaxial Cable**: Consists of an inner copper conductor surrounded by an insulating layer, a metallic shield (mesh), and an outer plastic jacket. It offers higher bandwidth and better shielding than twisted-pair. Used in cable TV and older LANs.
- **Fiber Optic Cable**: Transmits data as pulses of light through thin strands of glass or plastic (core). It is surrounded by cladding that reflects light inward. Fiber optics offer very high bandwidth, are immune to EMI, and are suitable for long-distance communication (e.g., Internet backbones).

### Connectors
- **RJ-45 (Registered Jack 45)**: An 8-pin connector used universally for networking with UTP and STP cables (Ethernet). It snaps into networking devices like switches, routers, and computer network interface cards (NICs).

## 4.3 OSI Model

The Open Systems Interconnection (OSI) model is a conceptual framework that standardizes the functions of a communication system into seven distinct layers.

| Layer | Name | Functioning & Working | Supported Protocols | Supported Hardware |
|-------|------|-----------------------|---------------------|--------------------|
| **7** | **Application** | Provides network services directly to the user applications. It serves as the window for users to access network services. | HTTP, FTP, SMTP, DNS, POP3 | Gateways, Firewalls |
| **6** | **Presentation** | Translates, encrypts, and compresses data. It ensures data is in a usable format and is where data encryption occurs. | SSL, TLS, JPEG, ASCII | Gateways, Firewalls |
| **5** | **Session** | Establishes, manages, and terminates sessions (connections) between applications. | NetBIOS, PPTP | Gateways |
| **4** | **Transport** | Ensures reliable, end-to-end data delivery. Segments data from the upper layers and reassembles it. Handles error recovery and flow control. | TCP, UDP | Gateways |
| **3** | **Network** | Determines the best path (routing) for moving data packets from source to destination across multiple networks. Uses logical addressing (IP addresses). | IP, ICMP, IPSec, IGMP | Routers, Layer 3 Switches |
| **2** | **Data Link** | Provides node-to-node data transfer. Formats data into frames and handles physical addressing (MAC addresses). Detects and corrects errors that occur in the Physical layer. | Ethernet, PPP, Switch, Frame Relay | Switches, Bridges, NICs |
| **1** | **Physical** | Transmits raw bit streams over the physical medium. Deals with the physical characteristics of the network (cables, voltages, signals). | 10Base-T, 802.11 (Wi-Fi) standards | Hubs, Repeaters, Cables, Connectors |

## 4.4 Network Topologies

Topology refers to the physical or logical layout of a computer network.

- **Bus Topology**: All devices are connected to a single central cable (the backbone or bus). It is easy to install, but if the main cable fails, the entire network goes down.
- **Star Topology**: All devices are connected to a central hub or switch. All communication passes through the central device. It is easy to troubleshoot and manage. If a single cable fails, only that device is affected, but if the central hub fails, the network goes down.
- **Ring Topology**: Devices are connected in a closed-loop circle. Data travels in one direction (unidirectional) from one node to the next until it reaches its destination. If one node or cable fails, the entire network can fail (though dual-ring topologies mitigate this).
- **Mesh Topology**: Every device is connected to every other device (Full Mesh) or at least multiple devices (Partial Mesh). Provides high redundancy and reliability. If one link fails, data can be rerouted. It is expensive and complex to install.
- **Hybrid Topology**: A combination of two or more different topologies (e.g., Star-Bus or Star-Ring). It offers flexibility and scalability, inheriting the advantages and disadvantages of its component topologies.

## 4.5 Types of Computer Networks

- **LAN (Local Area Network)**: A network that spans a small geographical area, such as a single room, building, or campus. Typically owned and managed by a single person or organization. Offers high data transfer rates. (e.g., an office network).
- **MAN (Metropolitan Area Network)**: Covers a larger area than a LAN but smaller than a WAN, such as a city or a large campus. It may connect multiple LANs. (e.g., a city cable television network).
- **WAN (Wide Area Network)**: Covers a vast geographical area, spanning cities, countries, or continents. The Internet is the largest example of a WAN. It usually utilizes public, leased, or private communication links.

## 4.6 Internet & Intranet

### URL (Uniform Resource Locator)
A URL is the address of a unique resource on the Internet. It specifies the location of a web page or file and the protocol used to access it (e.g., `https://www.example.com/page.html`).

### Internet
The Internet is a globally connected network of millions of computers and networks using standard protocols (TCP/IP). It is a public network accessible to anyone, containing a vast amount of information, resources, and services like the World Wide Web, email, and file sharing.

### Intranet
An Intranet is a private, secure network used within a specific organization. It uses the same technologies and protocols as the Internet (TCP/IP, web browsers) but is isolated from the public Internet. It is used by employees to share company information, internal tools, and resources securely.

### Comparison between Internet and Intranet

| Feature | Internet | Intranet |
|---------|----------|----------|
| **Accessibility** | Public, accessible to anyone globally. | Private, accessible only to authorized members of an organization. |
| **Security** | Less secure due to its public nature. | Highly secure, protected by firewalls and access controls. |
| **Ownership** | Not owned by any single entity. | Owned by a specific organization. |
| **Size/Scale** | Global, massive scale. | Limited to the organization's needs. |
| **Content** | General information, public websites. | Company policies, internal portals, proprietary data. |

## 4.7 Networking Devices

Networking devices are hardware components used to connect computers and other devices to form a network.

- **Repeater**: Operates at the Physical layer (Layer 1). Its primary function is to regenerate and amplify weakened signals before retransmitting them, extending the distance over which data can travel without loss of quality.
- **Switch**: Operates primarily at the Data Link layer (Layer 2). It connects multiple devices within a LAN and uses MAC addresses to forward data packets only to the specific intended destination device, reducing collisions and improving network efficiency.
- **Router**: Operates at the Network layer (Layer 3). It connects multiple different networks together (e.g., a LAN to the Internet) and uses IP addresses to route data packets along the best path to their destination.
- **Wireless Access Point (WAP)**: A device that allows wireless devices (like laptops and smartphones) to connect to a wired network using Wi-Fi standards. It typically connects to a router or switch via a wired connection and projects a Wi-Fi signal.