Networks

• Hub ("Dumb" Device):
1. Operates at the physical layer.
2. When a data packet arrives at one port, the hub blindly retransmits and broadcasts that packet to EVERY SINGLE CONNECTED PORT, regardless of which computer it was intended for.
3. Creates heavy unnecessary network traffic, frequent data collisions, and severe security vulnerabilities.
• Switch ("Intelligent" Device):
1. Operates at the data link layer.
2. It maintains an internal lookup table of physical MAC addresses of all connected devices.
3. When a packet arrives, it reads the destination MAC address and forwards the packet ONLY to the specific intended recipient device.
• Why Superior? A switch completely eliminates packet collisions, maximizes available network bandwidth, and provides secure private data communication.

1. PAN (Personal Area Network): Covers a tiny personal space within $10\text{ meters}$ radius.
   Example: Connecting a smartphone to Bluetooth wireless earbuds or a smartwatch.
   2. LAN (Local Area Network): Spans a small localized geographic area such as a single room, office floor, or school campus.
   Example: Computers connected in a school computer lab sharing a single laser printer.
   3. MAN (Metropolitan Area Network): Covers an entire city or metropolitan region.
   Example: A municipal cable television distribution network or interconnected bank ATMs across a city.
   4. WAN (Wide Area Network): Spans across countries, continents, and the entire globe.
   Example: The global Internet.

• Star Topology: A network arrangement in which every individual workstation/node is connected directly to a central connecting device (a Hub or Switch) via its own dedicated cable.
• Two Advantages:
1. Fault Isolation: If a cable connecting a workstation breaks, only that single computer is disconnected; the rest of the network continues functioning normally.
2. Easy Troubleshooting: Adding new nodes or diagnosing faulty connections is simple and does not disrupt active network traffic.
• One Disadvantage:
Single Point of Failure: If the central Hub or Switch crashes, the entire network is completely paralyzed.

1. Immense Bandwidth & Speed: Fiber optic cables transmit data as pulses of laser light using Total Internal Reflection (TIR), traveling at the speed of light with bandwidths exceeding terabits per second.
   2. Immunity to Electromagnetic Interference (EMI): Because data is carried as photons of light through glass cores, it is 100% immune to electrical noise, radio interference, and lightning strikes.
   3. Minimal Signal Attenuation: Light travels tens of kilometers through pure silica glass before requiring amplification, unlike copper wires where electrical resistance causes severe signal loss over short distances.

• Router: An intelligent networking device that connects different independent networks together (e.g., connecting a local home Wi-Fi LAN to the global Internet WAN). It inspects incoming data packets, reads logical IP addresses, and directs them along the fastest route.
• Modem (Modulator-Demodulator): A hardware device that converts digital computer signals into analog electrical signals for transmission over telephone or cable lines (Modulation), and converts incoming analog signals back into digital binary for the computer (Demodulation).
• In modern homes, both devices are often integrated into a single hybrid wireless gateway unit.

• MAC Address (Media Access Control):
1. The physical hardware address permanently burned into the Network Interface Card (NIC) microchip by the manufacturer.
2. It is 48 bits long (represented in 12 hexadecimal digits: e.g., 00:1A:2B:3C:4D:5E).
3. It is permanent and unchangeable regardless of where the device travels.
• IP Address (Internet Protocol):
1. The logical software address assigned dynamically to a device by an internet service provider (ISP) or router when connected to a network.
2. It changes whenever the device moves to a new Wi-Fi network (e.g., 192.168.1.1).

• HTTP (Hypertext Transfer Protocol): The standard protocol used to transmit web pages over the internet. Data is transmitted in plain, unencrypted text.
• HTTPS (Hypertext Transfer Protocol Secure): An encrypted extension of HTTP that encrypts all communication between the web browser and the server using SSL/TLS cryptographic protocols.
• Why Mandatory for Banking?
On plain HTTP, malicious hackers can intercept passwords, credit card numbers, and banking credentials using packet sniffers. HTTPS encrypts sensitive credentials into indecipherable cyphertext, ensuring that only the authorized bank server can decrypt the data.

• Apply the Mesh Topology formula:

$$\mathbf{L = \frac{N(N - 1)}{2}}$$

• Here $N = 6$ computers:

$$L = \frac{6(6 - 1)}{2} = \frac{6 \times 5}{2} = \frac{30}{2} = \mathbf{15\text{ physical cable links}}$$

.
Connecting 6 computers in a full mesh network requires $15$ physical cables.