Computer Network and Its Applications: Complete Lesson for Ethiopian Students

1.1.1 What is Data Communication?

Before we talk about networks, we need to understand what data communication really means. Let me explain it to you step by step.

When we communicate, we are sharing information. Right? Now, this sharing can happen in two ways: it can be local (like talking face to face) or it can be remote (over a distance). The term telecommunication comes from the Greek word “tele” which means far. So telecommunication simply means communication at a distance — and it includes telephony, telegraphy, and television.

The word data refers to information presented in whatever form is agreed upon by the parties creating and using the data. So data communication is the exchange of data between two devices via some form of transmission medium such as a wire cable.

Now, here is something important. For data communication to occur, the communicating devices must be part of a communication system. This system is made up of a combination of hardware (physical equipment like cables, routers) and software (programs that control the communication). You cannot have data communication with just hardware alone, or just software alone. Both must work together.

Four Fundamental Characteristics of Data Communication

Now listen carefully. Your exam will definitely ask about these four characteristics. The effectiveness of a data communication system is measured by four things:

1. Delivery — The system must deliver data to the correct destination. The data must be received by the intended device or user, and only by that device or user. Imagine sending a private message on Telegram and it goes to the wrong person. That means delivery has failed.

2. Accuracy — The system must deliver the data accurately. If the data has been altered during transmission and left uncorrected, it becomes unusable. For example, if you send “1000 Birr” and the receiver sees “9000 Birr” because bits changed — that is a accuracy failure.

3. Timeliness — The system must deliver data in a timely manner. Data delivered late is useless. For video and audio, this means delivering data as it is produced, in the same order, and without significant delay. This kind of delivery is called real-time transmission. Think about a live video call — if the audio arrives 10 seconds late, the conversation becomes impossible.

4. Jitter — This one is tricky but very important for exams. Jitter refers to the variation in the packet arrival time. It is the uneven delay in the delivery of audio or video packets. Let me give you the exact example from your textbook:

1.1.2 Communication Basics

Now let us understand what forms communication can take. Data communications include much more than just sending emails. Let me list them for you:

• Writing and talking — The most basic forms of human communication
• Nonverbal communication — Facial expressions, body language, gestures
• Visual communication — Images, pictures, painting, photography, video, film
• Electronic communication — Telephone calls, electronic mail, cable television, satellite broadcasts

Notice the progression here. From simple face-to-face communication, humans evolved to electronic communication. And in electronic communication, we need both hardware and software working together. Without the hardware (cables, routers, switches), there is no physical path. Without the software (protocols, drivers), the hardware cannot understand what to do with the data.

1.1.3 Data Representation Techniques

This is a very important section for your exam. The question “How is data represented in a computer?” appears very frequently.

Information today comes in different forms: text, numbers, images, audio, and video. But here is the key idea — all of these must be converted to bit patterns (sequences of 0s and 1s) before they can be transmitted or stored in a computer. Let us look at each form one by one.

Text Representation

Text is represented as a bit pattern — a sequence of bits (0s or 1s). Different sets of bit patterns have been designed to represent text symbols. Each set is called a code, and the process of representing symbols is called coding.

There are two important coding systems you must know:

1. ASCII (American Standard Code for Information Interchange) — Developed in the United States some decades ago. It uses 7 bits to represent 128 characters. Today, ASCII constitutes the first 127 characters in Unicode and is also referred to as Basic Latin.

2. Unicode — This is the prevalent coding system used today. It uses 32 bits to represent a symbol or character used in any language in the world. This means Unicode can represent Amharic characters, Chinese characters, Arabic script, and every other language — something ASCII could never do.

Numbers Representation

Numbers are also represented by bit patterns. However — and this is important — a code such as ASCII is NOT used to represent numbers. Why? Because if we use ASCII, mathematical operations become very complicated. Instead, the number is directly converted to a binary number to simplify mathematical operations.

For example, the number 13 in binary is 1101, not the ASCII code for “1” followed by ASCII code for “3”.

Images Representation

Images are also represented by bit patterns. In its simplest form, an image is composed of a matrix of pixels (picture elements), where each pixel is a small dot.

The size of the pixel depends on the resolution. For example:

• An image can be divided into 1,000 pixels — lower resolution
• The same image can be divided into 10,000 pixels — better resolution, but needs more memory

After dividing the image into pixels, each pixel is assigned a bit pattern. The size of this bit pattern depends on the type of image:

Black and white images (like a chessboard): Only 1-bit pattern is enough. 0 = black, 1 = white (or vice versa).

Gray scale images: If the image is not pure black and white, you increase the bit pattern size. The textbook gives this clear example:

Color images: There are several methods. Your textbook mentions two:

• RGB — Each color is made from a combination of three primary colors: Red, Green, Blue. The intensity of each color is measured and a bit pattern is assigned.
• YCM — A color is made from a combination of: Yellow, Cyan, Magenta. This method is commonly used in printers.

Audio Representation

Audio is by nature different from text, numbers, or images. It is continuous, not discrete. Even when we use a microphone to change voice or music into an electric signal, we create a continuous signal. To represent audio digitally, this continuous signal must be converted to digital form through a process called sampling — but your Chapter 1 textbook introduces this concept only briefly here.

Video Representation

Video can either be produced as a continuous entity (like by a TV camera), or it can be a combination of images, each a discrete entity, arranged to convey the idea of motion. So video is essentially a sequence of images displayed rapidly to create the illusion of movement.

1.1.4 Digital Data Transmission Formats

Now we come to a very important topic. How exactly does digital data travel from sender to receiver? There are two main methods: Serial and Parallel transmission. Let me teach you each one deeply.

Serial Transmission

In serial transmission, the various bits of data are transmitted one after the other, sequentially. Only a single bit is transferred at a time using the communication channel.

Key characteristics of serial transmission:

• Requires only one communication line (not n lines)
• All bits travel on a single line in serial fashion
• Bits are reassembled at the destination
• Mostly used by computer peripherals like printers
• Less costly than parallel transmission
• Used for long distance transmission

MSB First vs LSB First (Endianness)

Here is an interesting question: when sending bits serially, which bit should go first? Consider the number 13 (binary: 1101). Should we send the Most Significant Bit (MSB) first (1, then 1, then 0, then 1) or the Least Significant Bit (LSB) first (1, then 0, then 1, then 1)?

• Little-endian = sends the LSB first
• Big-endian = sends the MSB first

Either form can be used, but the sender and receiver must agree on which method. The textbook notes that usually the LSB is transmitted first.

Synchronous vs Asynchronous Serial Transmission

This is a very common exam topic. Let me explain both clearly.

A. Synchronous Transmission — Data is sent in multiple bytes at one time (a block of data).

• No start and stop bits are used
• No gap between data units
• Data transmission speed is fast
• Cost is high
• Used for transfer of data between two computers
• Synchronization between sender and receiver is required (they must have a shared clock)

B. Asynchronous Transmission — Sends only one character at a time (one byte of data at a time).

• Synchronizes two devices using Start Bit and Stop Bit
• Start bit: Usually 0 is used — signals the beginning of data
• Stop bit: Indicates the end of data — more than one bit can be used
• No time relation between consecutive bits
• During idle condition, signal “1” is transmitted

Parallel Transmission

In parallel transmission, all the bits of data are transmitted simultaneously on separate communication lines. Multiple bits (for example, eight bits) are transferred at the same time.

Key characteristics of parallel transmission:

• To transmit n bits, you need n wires or lines
• Used for short distance communication only
• More costly than serial (more wires)
• Faster than serial transmission (bits go simultaneously)
• Data can be transmitted in less time

1.1.5 Transmission Impairments

This is one of the most tested topics in Ethiopian university exams. Pay very close attention to this section!

Signals travel through transmission media, which are not perfect. The imperfection causes signal impairment. This means the signal at the beginning of the medium is not the same as the signal at the end. What is sent is not what is received. Your textbook identifies three causes of impairment:

1. Attenuation

Attenuation means a loss of energy. When a signal (simple or composite) travels through a medium, it loses some of its energy in overcoming the resistance of the medium. That is why a wire carrying electric signals gets warm, if not hot, after a while. Some of the electrical energy in the signal is converted to heat.

To solve this problem, we use amplifiers to amplify the signal and restore its strength.

2. Distortion

Distortion means that the signal changes its form or shape. Distortion occurs in a composite signal (a signal made of different frequencies). Here is why it happens:

Each signal component has its own propagation speed through a medium. Therefore, each component has its own delay in arriving at the destination. If these delays are not exactly the same, the signal components arrive with different phases than what they had at the sender. The shape of the composite signal changes.

3. Noise

Noise is another cause of impairment. Your textbook lists four types of noise:

a) Thermal Noise — The random motion of electrons in a wire creates an extra signal not originally sent by the transmitter. This is always present and cannot be eliminated — only minimized.

b) Induced Noise — Comes from external sources such as motors and appliances. These devices act as a sending antenna, and the transmission medium acts as the receiving antenna. The medium picks up unwanted signals from these devices.

c) Crosstalk — The effect of one wire on another. One wire acts as a sending antenna and the other as the receiving antenna. This is why you sometimes hear another conversation on your phone line.

d) Impulse Noise — A spike (a signal with high energy in a very short time) that comes from power lines, lightning, and so on. Unlike thermal noise which is continuous, impulse noise is sudden and brief but can cause serious damage to the signal.

1.1.6 Modes of Data Transmission

Now let me ask you a question: when you talk on the phone, can both people speak at the same time? Yes! But when you use a walkie-talkie, can both people speak at the same time? No! The difference is the mode of transmission. Let me explain all three modes.

1. Simplex Mode

In simplex mode, data flows in only one direction — from the transmitter to the receiver. The receiver cannot send anything back.

This is useful when data does not need to flow in both directions. Notice that in all examples, one device only sends and the other only receives.

2. Half-Duplex Mode

In half-duplex mode (also called alternating or semi-duplex), data flows in one direction or the other, but not both at the same time. Each end takes turns transmitting.

Important points about half-duplex:

• The entire capacity of the channel is used by whichever device is transmitting at that time
• Both devices share the same channel, but only one uses it at a time
• Used when there is no need for simultaneous communication in both directions

3. Full-Duplex Mode

In full-duplex mode (also called duplex), both stations can transmit and receive simultaneously. It is like a two-way street with traffic flowing in both directions at the same time.

Important points about full-duplex:

• Signals going in both directions share the capacity of the link
• This sharing can happen in two ways: (1) Two physically separate paths — one for sending, one for receiving, or (2) The capacity is divided between both directions
• The capacity of the channel must be divided between the two directions — this means each direction gets less than the full capacity
• Used when communication in both directions is required all the time

1.1.7 Five Components of Data Communication

Every data communication system has five components. Think of it like sending a letter — you need a message, someone to write it, someone to receive it, a postal service to carry it, and rules about how to address it. Let me explain each one:

1. Message — The information (data) to be communicated. Popular forms include text, numbers, pictures, audio, and video. Without a message, there is nothing to communicate.

2. Sender — The device that sends the data message. Examples: computer, workstation, telephone handset, video camera.

3. Receiver — The device that receives the message. Examples: computer, workstation, telephone handset, television.

4. Transmission Medium — The physical path by which a message travels from sender to receiver. Examples: twisted-pair wire, coaxial cable, fiber optic cable, and radio waves.

5. Protocol — A set of rules that govern data communications. It represents an agreement between the communicating devices. This is the most abstract but perhaps the most important component.

1.1.8 Protocols and Standards

Now let us go deeper into protocols. This section is very important because protocols are the backbone of all networking.

What is a Protocol?

A protocol is defined as a set of rules that governs data communications. It defines what is to be communicated, how it is to be communicated, and when it is to be communicated.

When the sender sends a message, it may consist of text, numbers, images, etc. These are converted into bits and grouped into blocks to be transmitted. Often, certain additional information called control information is also added to help the receiver interpret the data. Without protocol, the communicating entities are like two persons trying to talk to each other in different languages without knowing the other’s language.

Three Key Elements of a Protocol

This is a must-memorize topic for your exam:

1. Syntax — The structure or format of the data. It is the arrangement of data in a particular order. Think of it as the grammar of the protocol — how the data is organized.

2. Semantics — The meaning of each section of bits. It indicates the interpretation of each section and tells what action or decision is to be taken based on the interpretation. Think of it as the meaning of words in a sentence.

3. Timing — Tells the sender about the readiness of the receiver. It tells the sender at what rate the data should be sent to avoid overwhelming the receiver. Think of it as the pace of conversation — not too fast, not too slow.

Standards in Networking

Standards are necessary to ensure interconnectivity and interoperability between various networking hardware and software components. Without standards, we would have proprietary products creating isolated islands of users which cannot interconnect.

Your textbook classifies standards into two categories:

1. De Facto Standard — Means “by fact” or “by convention.” These standards are not approved by any organized body but are adopted by widespread use. Example: The QWERTY keyboard layout became a de facto standard because everyone used it, not because any official body mandated it.

2. De Jure Standard — Means “by law” or “by regulation.” These standards are legislated and approved by an officially recognized body. Example: The USB standard is approved by official standards organizations.

Standards Organizations

Your textbook lists important organizations you should know:

Standard Creation Committees:

• ISO — International Organization for Standardization
• ITU-T — International Telecommunications Union – Telecommunications Standard
• ANSI — American National Standards Institute
• IEEE — Institute of Electrical & Electronics Engineers
• EIA — Electronic Industries Associates

Forums: ATM Forum, MPLS Forum, Frame Relay Forum

Regulatory Agencies: FCC — Federal Communications Committee

1.1.9 Switching Techniques

Have you ever wondered how your email reaches a friend in another city when there is no direct cable between your computer and theirs? The answer is switching. Let me explain the three techniques.

Switching is a mechanism by which data sent from source towards destination which are not directly connected. Networks have interconnecting devices that receive data from the source, store it, analyze it, and then forward it to the next device closest to the destination.

A. Circuit Switching

In circuit switching, the complete physical connection between two computers is established first, and then data is transmitted. Think of it like making a traditional phone call — first the connection is set up, then you talk.

B. Message Switching

In message switching, the source sends the entire message to a switching office, which stores the data in its buffers. It then looks for a free link to another switching office and forwards the data. This process continues until the data reaches the destination. This is also known as store and forward.

C. Packet Switching

Packet switching places a tight upper limit on block size. The message is broken into small, fixed-size packets. These packets are then transmitted across the network independently.

Key differences from message switching:

• Packets are stored in main memory (fast access), whereas in message switching, data is stored on disk (slow access)
• This improves performance because access time is reduced, thus improving throughput
• There is a fixed size limit on packets (unlike message switching which has no limit on block size)

1.2 Computer Network and Its Applications

A computer network is the interconnection of various computer systems located at different places. Two or more computers are linked together with a medium and data communication devices for the purpose of communicating data and sharing resources.

The computer that provides resources to other computers on a network is known as server. The individual computers that access shared network resources are known as nodes.

1.2.1 Network Types Based on Geographical Area

Local Area Network (LAN)

A LAN connects computers and devices in a limited geographical area such as home, school, computer laboratory, office building, or closely positioned group of buildings.

Key points about LAN from your textbook:

• Each computer or device on the network is a node
• Current wired LANs are most likely based on Ethernet technology
• Covers an organization’s offices, schools, colleges/universities
• Number of systems may vary from as few as 2 to as many as 16 million
• Provides sharing of resources like printers, file servers, scanners, and internet
• Composed of inexpensive networking and routing equipment
• Mostly operates on private IP addresses and generally does not involve heavy routing
• Works under its own local domain and controlled centrally
• Uses either Ethernet (most widely employed, uses star topology) or Token-ring (rarely seen)
• Can be wired or wireless or both at once

Metropolitan Area Network (MAN)

A MAN connects two or more LANs or campus area networks together but does not extend beyond the boundaries of the immediate town or city. Routers, switches, and hubs are connected to create a MAN.

Wide Area Network (WAN)

A WAN covers a large geographic area such as a city, country, or spans even intercontinental distances. It uses a communications channel that combines many types of media such as telephone lines, cables, and air waves.

• Often uses transmission facilities provided by common carriers (telephone companies)
• WAN technologies generally function at the lower three layers of the OSI model: Physical, Data Link, and Network layers

Personal Area Network (PAN)

A PAN is the interconnection of information technology devices within the range of an individual person, within a range of 10 meters. It is the smallest network type.

• May include Bluetooth-enabled devices or infrared-enabled devices
• Examples: laptop connected to a portable printer using Wi-Fi, wireless keyboard and mouse, Bluetooth headphones, wireless printers, TV remotes
• Connectivity range: up to 10 meters

1.2.1.1 Network Architecture

Peer-to-Peer (P2P)

• No single computer controls the network
• Each computer is the same (a peer) to all others
• Suitable for small offices

Client/Server

• The network is controlled by a special high-powered server
• The server is dedicated to running the network
• Types: print and file servers, application servers, communication servers, directory service servers
• Client host requests and receives service from always-on server
• Examples: Web browser/server, email client/server

1.2.2 Network Components

Connection or Cabling System

• Wired: twisted-pair wiring, coaxial cable, fiber optics
• Wireless: infrared or radio wave transmission

Interconnection Devices (Hardware Components)

These are very important for your exam. Let me explain each one clearly:

Hub — Connects network cables coming from client computers. It broadcasts data to ALL connected nodes. This means every device receives every message, even if it was not intended for them. Very inefficient and creates unnecessary traffic.

Switch — Wired similarly to a hub but makes each network connection a private one. It uses MAC address to send data only to the intended recipient. Much more efficient than a hub.

Bridge — A hardware and software combination used to connect different media types of networks. Also uses MAC address. Think of it as connecting two different types of LANs.

Router — A special computer that directs communicating messages when several networks are connected together. It uses IP address (not MAC address) to make forwarding decisions. Routers operate at the network layer and are essential for WAN communication.

Gateway — An interface that enables dissimilar networks to communicate, such as a LAN with a WAN, or two LANs based on different topologies or network operating systems. A gateway can translate between different protocols.

Software Components

Network software is highly structured. Most networks are organized as a stack of layers, each one built upon the one below it. Key terms:

• Network architecture — A set of layers and protocols
• Protocol stack — A list of protocols used by a certain system, one protocol per layer
• Interface — Defines which operations and services the lower layer makes available to the upper layer

Design issues for layered architecture include: addressing (specifying a specific destination), determining logical channels and their priorities, and error control (because physical circuits are not perfect).

1.2.3 Network Topology

Network topology is one of those topics that appears in almost every exam. Let me explain each topology thoroughly with its advantages and disadvantages.

A network topology is the way computer systems or network equipment are connected to each other. The term refers to the geometric representation of the relationship of all the links and linking devices (nodes) to one another.

Before we look at specific topologies, understand two important terms:

• Physical topology — How the wires are actually run; the way the network is laid out physically
• Logical topology — How the signal actually travels through the network

Physical and logical topologies can be different in the same network. For example, a LAN can be physically wired as a star but logically function as a bus, depending on how the hub is wired.

1. Bus Topology

In bus topology, all devices share a single communication line (cable). All devices are connected to this shared line.

Problem: When more than one host sends data at the same time, there is a collision. The bus topology either uses CSMA/CD technology or designates one host as Bus Master to solve this.

Advantages: Requires less cable; easy to install and extend

Disadvantages: Not secured; can become congested; a fault in the bus cable stops ALL transmissions — the damaged area reflects signals back, creating noise in both directions; difficult to add new devices

2. Star Topology

Each device has a dedicated point-to-point link only to a central device (hub, switch, or router). There is no direct traffic or link between devices.

Advantages: Easy to install, reconfigure, and less expensive; each device needs only one link and one I/O port; robust — if one link fails, only that link is affected; easy fault identification and isolation

Disadvantages: Failure of the central device causes the entire network to fail (single point of failure); requires more cable than ring or bus topologies

3. Ring Topology

Each host connects to exactly two other machines, creating a circular network structure. Each device has a dedicated point-to-point connection only with the two devices on either side.

Advantages: Relatively easy to install and reconfigure; fault isolation is simplified

Disadvantages: Unidirectional traffic; a break in the ring can disable the entire network (can be solved by using a dual ring)

4. Mesh Topology

Every device has a dedicated point-to-point link to every other device in the network. Often used in MANs and WANs.

Two types of mesh:

• Full Mesh — All hosts have a point-to-point connection to every other host. Most reliable but most expensive. For every new host, n(n-1)/2 cables are needed.
• Partially Mesh — Not all hosts have point-to-point connection to every other host. Some hosts connect in an arbitrary fashion. Used where reliability is needed for some hosts but not all.

Advantages: Privacy/security (dedicated lines); no traffic problems (dedicated links); robust (one link failure does not incapacitate the system); easy fault identification and isolation

Disadvantages: Needs more resources (cables and ports); expensive to implement

1.2.4 OSI Reference Model

The OSI model is one of the most fundamental topics in networking. You will see questions about it in every exam. Let me teach it thoroughly.

The OSI (Open Systems Interconnection) model was created by the International Organization for Standardization (ISO). The design principles are:

• A layer should be created where a different abstraction is needed
• Each layer should perform a well-defined function
• Layer boundaries should minimize information flow across interfaces
• The number of layers should be not too large or not too small

The Seven Layers of OSI Model

Now let me explain each layer in detail:

Layer 1 — Physical Layer: Concerned with transmitting raw bits over a communication channel. It deals with the physical characteristics of the medium — voltage levels, timing of voltage changes, data rate, maximum transmission distance.

Layer 2 — Data Link Layer: Performs flow control and transforms a raw transmission facility into a line that appears error-free using ARQ (Automatic Repeat Request). It also handles framing — dividing the bit stream into manageable frames.

Layer 3 — Network Layer: Controls the operation of the subnet. Key functions include routing (determining the best path), flow control, and internetworking (connecting different networks together).

Layer 4 — Transport Layer: Performs assembling and disassembling of messages. It isolates the upper layers from changes in the network hardware. It determines the type of service (reliable vs unreliable) provided to the upper layers.

Layer 5 — Session Layer: Establishes sessions between applications. Provides dialog control — determining whose turn it is to transmit. It also manages synchronization points in the data stream.

Layer 6 — Presentation Layer: Concerned with the syntax and semantics of the information exchanged. It handles data format translation, encryption/decryption, and compression.

Layer 7 — Application Layer: Contains a variety of commonly used protocols. Examples include HTTP (Hyper Text Transfer Protocol for WWW), file transfer (FTP), e-mail (SMTP), and network news. This layer is closest to the end user.

1.3 Transmission Media

Now let us look at the physical paths that carry our data. This is the last major topic of Chapter 1, and it is very important for your exam.

Transmission media (also called communication channels) are the connecting cables or connection media used to transmit data from one device to another.

There are two broad categories:

• Guided (Wired) — Signals are guided along a physical path (cable)
• Unguided (Wireless) — Signals travel through air/space (your textbook focuses on guided media)

1.3.1 Guided Transmission Media

A. Twisted Pair Cable

Consists of two insulated copper wires arranged in a regular spiral pattern. The twisting is very important — it decreases the crosstalk interference between adjacent pairs in a cable.

Key characteristics:

• Can transmit both analog and digital signals
• Limited in distance, bandwidth, and data rate
• Attenuation is a very strong function of frequency — higher frequencies attenuate more
• Maximum data rates: 1 Mbps without conditioning, 10 Mbps with conditioning

Types of Twisted Pair:

1. Unshielded Twisted-Pair (UTP):

• Wrapped inside a plastic cover (for mechanical protection only)
• Ordinary telephone wire
• Cheapest and easiest to install
• No metallic shielding — more susceptible to interference

2. Shielded Twisted-Pair (STP):

• Has a metal foil or braided-metal-mesh cover that encases each pair of insulated wires
• Metal braid/sheathing reduces interference
• More expensive than UTP
• Harder to handle (thick, heavy)

B. Coaxial Cable

Consists of a solid wire core surrounded by one or more foil or wire shields, each separated by plastic insulator. The inner core carries the signal, and the outer shield provides the ground.

Key characteristics:

• Carries signals of higher frequency (100 KHz to 500 MHz) than UTP
• Outer metallic wrapping serves as both shield against noise and the second conductor completing the circuit
• Higher bandwidth — up to 400 Mbps
• Versatile — used in TV distribution, aerial to TV systems

Types of Coaxial Cable:

• Thicknet — Thicker, used up to 500 meters
• Thinnet — Thinner, used up to 185 meters

Advantages: Better than twisted pair; used as shared cable network; broadband transmission; higher bandwidth

Disadvantages: Expensive compared to twisted pair; not compatible with twisted pair cables

C. Optical Fiber Cable

A fiber-optic cable is made of glass or plastic and transmits signals in the form of light. This is the most advanced transmission medium.

Structure: A signal-carrying glass core (5 to 100 microns in diameter) surrounded by a layer of pure silica called cladding, which prevents light from escaping. For reference, a sheet of paper is about 25 microns thick and a human hair is about 75 microns thick.

How optical fiber works:

• Light travels at 3 × 10⁸ m/s in free space (fastest possible speed)
• Light slows down in denser media like glass
• At the interface between core (denser) and cladding (less dense), light bends away from the normal when entering a less dense medium (refraction)
• Beyond the critical angle → total internal reflection occurs
• Light bounces back and forth along the core, guided by the cladding

Types:

• Multimode step-index — Multiple paths (modes) for light; refractive index changes abruptly between core and cladding

Light sources:

• LED (Light Emitting Diode) — Cheaper, wider operating temperature range, lasts longer
• ILD (Injection Laser Diode) — More efficient, greater data rate

Advantages: Immune to electrical and magnetic interference; suitable for harsh environments; secure transmission (virtually impossible to tap); very high transmission capacity; broadband

Disadvantages: Installation problems; connecting two fibers is difficult; light can reach receiver out of phase; connection losses are common; difficult to solder; most expensive

📚 Chapter 1 — Final Exam Summary