Comprehensive Study Notes: Introduction to Networks & Wireless Summary & Study Notes

🏗️ Layered Network Architecture

Layering divides networking complexity into clear responsibilities: Application, Transport, Network, Link, and Physical. Each layer offers services to the layer above and uses the layer below. This modularity makes updates and debugging easier because changes inside one layer are largely transparent to others.

📦 Encapsulation & Protocol Stack

Data is wrapped as it descends the stack: application message → transport segment (adds header $H_t$) → network datagram (adds header $H_n$) → link frame (adds header $H_l$). This is encapsulation, and each header carries control information used by its layer.

🔁 Important Layers & Examples

• Application: HTTP, SMTP, IMAP (user-facing protocols).
• Transport: TCP (reliable, ordered byte stream), UDP (unreliable datagrams).
• Network: IP (routing, host-to-host delivery).
• Link: Ethernet, 802.11 (WiFi) — neighbor-to-neighbor frame transfer.
• Physical: bits on the wire/air.

⏱️ Packet Delay Components

End-to-end nodal delay has four parts: processing, queueing, transmission, propagation. Key formulas:

• Transmission delay: $d_{trans} = \frac{L}{R}$ (L = packet bits, R = link bps).
• Propagation delay: $d_{prop} = \frac{d}{s}$ (d = distance, s ≈ speed of light). Queueing varies with congestion; packet loss occurs when router buffers overflow.

📈 Throughput & Bottlenecks

Throughput is bits/time delivered end-to-end. The slowest link (bottleneck) limits throughput. When sending from a server at rate $R_s$ to a client at $R_c$, effective throughput is constrained by the minimum link capacity on the path.

🌐 Addressing, Routing & DNS

• IP addresses: 32-bit (IPv4) identifiers in network byte order.
• DNS maps domain names to IP addresses (e.g., example.com → 34.239.29.130).
• Routing: Networks (blocks) are advertised and routes chosen (static or adaptive). The Internet is a network-of-networks (ISPs, IXPs).

🔌 Sockets, Ports & Connections

A socket is an endpoint: (IP address, port). Ports let the kernel demultiplex services (e.g., port 80 → webserver). TCP provides point-to-point, full-duplex, reliable byte streams.

🧪 Practical Notes

Use tools like Wireshark to observe packets and practice the material. Labs will focus on protocol analysis and capture.

📡 What is Wireless & RF Basics

Wireless uses electromagnetic waves. RF (Radio Frequency) spans roughly 3 kHz–300 GHz. Signals carry analog (continuous) or digital (discrete bits) information. Key signal properties: frequency, amplitude, and phase.

🌊 Wavelength & Frequency Relationship

Wavelength $\lambda$ relates to frequency $f$ by $\lambda = \frac{c}{f}$, where $c$ ≈ 3×10^8 m/s. Examples: 900 MHz → $\lambda$ ≈ 0.33 m; 2.4 GHz → $\lambda$ ≈ 0.125 m.

📉 Power, Gain, and Decibels

Power ratios use decibels: $\text{gain(dB)} = 10 \log_{10}\left(\frac{P_{out}}{P_{in}}\right)$. Power in dBm: $P_{dBm} = 10 \log_{10}(P_{mW})$. Rules of thumb: +3 dB ≈ double power; +10 dB ≈ 10× power.

🔋 Transmit Power and Sensitivity

Examples: an nRF52840 might transmit at 8 dBm (~6.31 mW) and have a receive sensitivity of -95 dBm. Different technologies trade transmit power and sensitivity (e.g., LoRa uses higher Tx power and much better sensitivity for long range).

🛑 Propagation & Attenuation

Signal strength reduces with distance (free-space path loss, multipath, obstacles). Free-space loss scales roughly with the square of distance and depends on wavelength; however, indoor/outdoor models and reflections often dominate real-world range.

⚠️ Noise & Errors

Noise sources: thermal, induced, interference, and impulse. Lower received energy increases BER (bit error rate). Distortion (frequency-dependent delays) can cause phase changes and inter-symbol interference.

🔊 Signal Quality: SNR & Multipath

SNR (Signal-to-Noise Ratio) is the signal power divided by noise power (often expressed in dB) and indicates link quality. Multipath occurs when reflections cause multiple delayed copies of a signal to arrive, producing constructive/destructive interference and fading.

📶 Spectrum, Channels & Unlicensed Bands

Spectrum is limited and valuable. Common unlicensed bands for IoT: 902–928 MHz, 2.4–2.5 GHz, 5 GHz. Different technologies share spectrum in different ways; coexistence and interference are key concerns.

🔁 Modulation Types & Tradeoffs

• ASK / OOK: amplitude-based (simple, noise-prone).
• FSK: frequency shifts (robust, uses more bandwidth).
• PSK / DPSK: phase changes (energy-efficient, needs more complex hardware).
• QAM: combines amplitude and phase to encode multiple bits/symbol. Tradeoffs include bandwidth use, noise immunity, and hardware complexity.

🧩 Framing & PHY–MAC Interaction

Frames typically include a preamble (sync), header (addresses/type), payload (data + upper-layer headers), and trailer (CRC). Wireless link layers often include PHY control info, robust header protection, and sometimes in-frame rate changes.

✅ Error Control

• Detection: CRCs detect single-bit and burst errors.
• Recovery: retransmissions (ARQ) or Forward Error Correction (FEC) to correct some errors without retransmission. Wireless favors more robust error strategies due to higher retransmission costs.

🕹️ Medium Access Control (MAC) — Intuition

Wireless medium is a shared broadcast channel. MAC protocols decide who transmits when to avoid collisions. Approaches mirror human coordination: listen-before-talk, scheduled turns, or random access.

⚔️ MAC Protocol Families

• Contention-based: ALOHA, Slotted ALOHA, CSMA/CA (WiFi), BLE advertising.
• Contention-free: TDMA, scheduled slot allocations (BLE connections, some LPWANs).
• Frequency split: FDMA.
• Code split: CDMA. Each has tradeoffs in throughput, latency, overhead, synchronization, and robustness to interference.

🔁 ALOHA & CSMA Highlights

• Pure ALOHA: send whenever; peak throughput ≈ 18.4% of channel capacity.
• Slotted ALOHA: synchronized slots; peak ≈ 36.8%.
• CSMA/CA: listen-before-send with collision avoidance (used by WiFi).
• CSMA/CD: collision detection (works on wired media like Ethernet, not wireless).

🛡️ Hidden Terminal & RTS/CTS

Hidden terminal arises when two transmitters cannot hear each other but collide at a receiver. RTS/CTS reduces costly collisions by reserving the medium but adds overhead.

🌍 Real-world Mappings

• WiFi → slotted CSMA/CA.
• BLE → both advertising (ALOHA-like) and connection modes (TDMA-like scheduling).
• LoRa/Sigfox → ALOHA-style LPWANs.
• Cellular LPWANs (LTE-M, NB-IoT) → scheduled/TDMA-like and centralized control.

🧭 How to Approach These Slides (User Guidance)

These slides cover two linked themes: how networks are structured (layers, addressing, routing) and how wireless differs (physics, modulation, MAC). Study the layered model first to place concepts, then map wireless specifics to the layers they affect (PHY → modulation/propagation; Link → framing/MAC; Network/Transport → routing/connection semantics).

❓ Key Questions to Practice

• Which layer provides reliability? (Transport, e.g., TCP.)
• Where does DNS live? (Application.)
• Who decides when a wireless device can transmit? (Link/MAC.)
• Who decides which router to forward a packet to? (Network/Routing.)

🔁 Example: Visiting a Website (Step Checklist)

1. Enter domain name → trigger DNS lookup (Application).
2. Resolve to IP → open socket to IP:80 (Transport + Network).
3. Send HTTP request and receive response → parse and render (Application/Transport).

🧪 Tips for Labs

Use Wireshark to see encapsulation and timing (delays, retransmissions). Observe physical/wireless specifics by noting retries, rates, and RSSI/SNR fields in captures.