Fundamentals of Data Communication and Computer Networks Study Notes

Fundamentals of Data Communication

A communication system is an integrated assembly of components designed to transmit information from one point to another. The system is composed of five essential components:

• Message: This is the data or information to be communicated. Common formats include text files, audio files, video files, and other digital representations.

• Sender: This is the device that initiates the data transmission. It can be a computer, mobile phone, telephone, laptop, video camera, or a workstation.

• Receiver: This is the device designated to receive the message. Similar to the sender, it can be a computer, telephone, mobile phone, or workstation.

• Transmission Medium / Communication Channels: These are the physical or logical paths that connect two or more workstations. They are categorized into:     * Wired Media: Physical cables connecting devices.     * Wireless Media: Non-physical connections (e.g., radio waves).

• Protocols (Set of Rules): These are the governing rules that make communication understandable between the sender and the receiver. Without protocols, the data might be received but not understood.     * Example: Sonali sends a message to Chetan. If Sonali writes in Hindi and Chetan cannot understand Hindi, the conversation is meaningless. The shared language (Hindi) acts as the protocol here.

Definition and Elements of a Computer Network

A computer network is defined as a set of communication elements connected by communication links. These elements are often referred to as nodes.

Communication Elements (Nodes)

Nodes are devices capable of sending and/or receiving data generated by other nodes on the network. Examples include:

• End Devices: Computers, printers, mobile phones, workstations, laptops, file servers, and web servers.

• Intermediary Devices: Devices that interconnect end devices or multiple individual networks, such as routers and switches.

Communication Links (Media)

A link is the physical or wireless medium used to transport a signal carrying information. Types include:

• Optic Fiber

• Coaxial Cable

• Twisted Pair

• Wireless: Radio, microwave, and satellite.

Network Criteria

To evaluate the effectiveness of a network, three primary criteria are considered:

• Performance: This depends on the network elements and is primarily measured using:     * Delay: The time taken for data to travel.     * Throughput: The amount of data successfully transmitted.

• Reliability: This refers to the frequency of failure and the time it takes a link to recover from a failure. It is measured in terms of:     * Availability/Robustness: The consistency of network uptime.     * Failure rate: The frequency of component malfunctions.

• Security: This involves protecting data against corruption or loss. Protection is required against:     * Errors: Internal system issues or transmission faults.     * Malicious users: External threats or unauthorized access.

Network Representation and Transmission Technology

Network Representation

A diagram that illustrates how devices in a network are connected is known as a topology.

Transmission Technology

• Point-to-Point Links: These connect individual pairs of machines. A point-to-point link-based system provides dedicated links between two specific devices. This is often called unicasting when there is exactly one sender and exactly one receiver.

• Broadcast Network: In this system, the communication channel is shared by all machines on the network. A packet sent by any machine is received by every other machine.     * Processing: If the packet is intended for the receiving machine, it processes it; otherwise, the packet is ignored.     * Example: Wireless networks.

• Multicasting: This is a specific type of broadcast system that supports transmission to a specific subset of machines rather than all of them.

Modes of Data Flow

• Simplex: The direction of data is strictly one-way. One device acts only as a sender and the other only as a receiver (e.g., Mainframe to Monitor).

• Half-Duplex: Data can travel in both directions, but not at the same time. While one device sends, the other must receive (e.g., Station to Station with direction changing at Time 1 and Time 2).

• Full-Duplex: Data can be transmitted in both directions simultaneously at all times.

Key Performance Metrics

Bandwidth

Bandwidth is defined in two ways:

• Bandwidth in Hertz ($Hz$): The range of frequencies contained in a composite signal or the range of frequencies a channel can pass.     * Example: A subscriber telephone line has a bandwidth of $4\,kHz$.

• Bandwidth in Bits per Second ($bps$): The number of bits per second that a channel, link, or network can transmit. It determines how much data can be sent and received at once.     * Example: A fast Ethernet network has a maximum bandwidth of $100\,Mbps$.

Throughput

Throughput is a measure of how fast data is actually sent through a network.

• If a link has a bandwidth of $B\,bps$, the throughput $T\,bps$ is typically such that T < B.

• Example: A link might have a bandwidth of $1\,Mbps$, but the connected devices can only handle $200\,kbps$, making the throughput $200\,kbps$.

Latency (Delay)

Latency is the total time required for an entire message to arrive at its destination, starting from the moment the first bit is sent from the source. It consists of four components and is influenced by the "traceroute" data follows between computers.

• Ping: A tool used to check latency by sending $4$ packets of data to an address and calculating the total response time.

Classification of Computer Networks by Scale

Networks are primarily distinguished based on their geographical span:

Network Types

1. Personal Area Network (PAN): Restrained to a single person's workspace, typically within a range of $10\,metres$. Examples include USB, phones, tablets, and printers.

2. Local Area Network (LAN): Spans a building (offices, schools, colleges) under a single administrative system. Used for sharing resources like file servers, printers, scanners, and internet.

3. Metropolitan Area Network (MAN): Expands throughout a city. It connects multiple LANs within a municipal area (e.g., connecting all offices of an organization in a single city).

4. Wide Area Network (WAN): Covers large areas like provinces or entire countries. WANs use expensive, high-speed backbone equipment to provide connectivity to MANs and LANs.

Network Topologies

Topology refers to the physical interconnection of network elements.

Bus Topology

Devices connect to a single backbone cable.

• Only one machine (the master) can transmit at a time.

• Collisions occur if two machines transmit simultaneously; they must wait a random time to retry.

• Example: IEEE 802.3, Ethernet.

• Cables: For $n$ devices, cables required = $n + 1$ ($n$ for device connections, $1$ for the backbone).

Ring Topology

Computers are connected in a closed loop.

• Data travels in one direction.

• Uses a token passing scheme to control network access; only the holder of the token can transmit.

• Nodes act as repeaters (signal boosters).

• Disadvantage: Failure of one machine causes the entire network to fail.

• Cables: For $n$ devices, links required = $n$.

Star Topology

Every node is connected to a central hub or switch.

• All traffic passes through the central node.

• Easily extensible and simple to set up in LANs.

• Cables: For $n$ devices, links required = $n$.

Mesh Topology

Every node is connected to every other node via dedicated links.

• Highly fault-tolerant and reliable.

• Complex and expensive to set up; used mostly in WANs.

• Cables: For $n$ devices, links required = $\frac{n \times (n - 1)}{2}$.

Comparison: Star vs. Mesh

Communication Models

Client-Server Communication

• Centralized Control: A central server manages resources, security, and monitoring.

• Scalability: Servers can be upgraded to handle increased demand.

• Specialized Roles: Servers function specifically as web, database, or file servers.

• Security: Easier to implement at the server level.

• Examples: Email servers, web applications.

Peer-to-Peer (P2P) Communication

• Decentralization: No central server; no single point of failure.

• Ease of Setup: Simple to configure for small applications; lower cost.

• Redundancy: Every peer can serve as a backup for others.

• Distribution of Load: Workload is shared across all peers.

• Example: File-sharing applications.

Comparison: Analog vs. Digital Communication