Computer Networks · Academic Reference · OSI / TCP-IP

DATA COMMUNICATION NETWORKS

📡 Communication Models ⚡ Physical Layer 🔗 Transmission Media 📶 Wireless Communication
01

Communication Models

// OSI · TCP/IP · Data Flow · Network Components

OSI vs TCP/IP Models
OSI Reference Model (ISO 7498)
7-LAYER MODEL — CLICK LAYER FOR DETAILS # LAYER NAME PDU KEY PROTOCOLS 7 APPLICATION HTTP·FTP·DNS·SMTP DATA 6 PRESENTATION SSL/TLS·JPEG·MPEG·ASCII DATA 5 SESSION NetBIOS·RPC·SIP DATA 4 TRANSPORT TCP·UDP·SCTP SEGMENT 3 NETWORK IP·ICMP·ARP·OSPF·BGP PACKET 2 DATA LINK Ethernet·PPP·802.11·HDLC FRAME 1 PHYSICAL RS-232·V.35·802.3·DSL BITS
TCP/IP Model (DoD / IETF) + Comparison
TCP/IP ↔ OSI MAPPING APPLICATION HTTP·FTP·DNS·SMTP·SSH Combines OSI 5+6+7 L7 App L6 Pres L5 Sess TRANSPORT TCP · UDP · SCTP L4 Transport INTERNET IP · ICMP · ARP · OSPF L3 Network NETWORK ACCESS Ethernet·Wi-Fi·V.35·DSL Combines OSI 1+2 L2 Data Link L1 Physical
Data Encapsulation / De-encapsulation
SENDER TRANSMISSION MEDIUM RECEIVER BITS / SIGNALS ON WIRE / AIR L7 App: [DATA] L4 Transport: [TCP H|DATA] L3 Network: [IP H|TCP H|DATA] L2 Data Link: [FR H|...|FR T] L1 Physical: 01100101... L1 Physical: bits → signal L2 Data Link: frame check L3 Network: route packet L4 Transport: reassemble L7 App: data delivered FRAME ← DATA UNIT TRANSFORMATION → DATA → SEGMENT → PACKET → FRAME → BITS ENCAPSULATION ↓ DE-ENCAPSULATION ↑ Each layer adds its own header (and trailer at L2) — removed in reverse order at the receiver
Data Communication System Components
SOURCE generates data PC · Phone · Sensor TRANSMITTER encodes signal Modem · NIC · Encoder TRANSMISSION MEDIUM carries signal Copper · Fibre · Air RECEIVER decodes signal Modem · NIC · Decoder DESTINATION receives data PC · Phone · Server 5 FUNDAMENTAL COMPONENTS OF ANY DATA COMMUNICATION SYSTEM
02

// Digital & Analog Transmission · Data Rate · Nyquist-Shannon

Digital vs Analog Signal Transmission
DIGITAL SIGNAL (NRZ-L) +V 0V 1 0 1 1 0 1 0 0 ←— 1 BIT PERIOD (T) —→

Binary data encoded as discrete voltage levels. High = 1, Low = 0. Immune to analogue noise but needs higher bandwidth for same data rate.

ANALOG SIGNAL (AM MODULATED) +A -A carrier wave (grey) + modulated signal (purple) — bit 1=high amp, bit 0=low amp

Continuous signal varies in amplitude, frequency or phase. Amplitude Shift Keying (ASK) shown: high amplitude = 1, suppressed = 0.

ENCODING COMPARISON — SAME BIT STREAM: 1 0 1 1 0 1 NRZ-L MANCH. 4B/5B (5 code bits for 4 data bits — prevents DC wander) 8B/10B (10 code bits for 8 data bits — DC balanced, used in PCIe·SATA·GbE) 1 0 1 1 0 1
Nyquist–Shannon Capacity Calculator
NYQUIST THEOREM (NOISELESS CHANNEL)
Bandwidth B 20 MHz
Signal Levels M 4
Cnyq = 2 × B × log₂(M)
= 2 × 20 × log₂(4)
= 80 Mbps
SHANNON THEOREM (NOISY CHANNEL)
Bandwidth B 20 MHz
SNR (dB) 20 dB
Csha = B × log₂(1 + SNRlinear)
SNRlinear = 10^(SNRdB/10) = 100
= 20 × log₂(1 + 100)
= 132.9 Mbps
NYQUIST CAPACITY
80 Mbps
Theoretical max (noiseless)
SHANNON CAPACITY
132.9 Mbps
Hard upper bound (with noise)
CHANNEL CAPACITY vs BANDWIDTH BANDWIDTH (MHz) → CAPACITY → Nyquist (M=4) Shannon (SNR=100) Actual rate = min(Nyquist, Shannon) 0 25 50 75 100
03

Transmission Media

// Guided · Unguided · Copper · Fibre Optic · Wireless

Guided vs Unguided Media
GUIDED MEDIA Signal travels inside a physical conductor / fiber 🔌 Twisted Pair 📡 Coaxial Cable 💡 Optical Fibre Cat5e/6/6A · UTP/STP RG-6/RG-58 · CATV SMF/MMF · DWDM UNGUIDED MEDIA Signal propagates freely through air / space 📻 Radio Waves 🛰 Microwave 🔴 Infrared LF·MF·HF·VHF·UHF Terrestrial·Satellite IrDA · Short range
Guided Media — Select Type
TWISTED PAIR — CROSS SECTION + SIDE VIEW CROSS SECTION (CAT 6A) OUTER JACKET (PVC) SIDE VIEW (twisting reduces EMI) TWO PAIRS SHOWN
Twisted Pair Properties
Bandwidth
500 MHz (Cat 6A)
Data Rate
10 Gbps max
Max Distance
100 m (UTP)
Attenuation
High (copper)
Cost
Low
Unshielded (UTP): 4 twisted pairs, most common for Ethernet (RJ-45). Twisting reduces crosstalk by ensuring interference affects both wires equally and cancels. Cat 5e → 1 Gbps, Cat 6A → 10 Gbps.
Shielded (STP/FTP): Additional foil/braid shield around pairs; better EMI immunity for industrial environments.
10BASE-T 100BASE-TX 1000BASE-T 10GBASE-T
COAXIAL CABLE — CROSS SECTION COAXIAL CROSS SECTION inner conductor (Cu) dielectric (PE) outer shield (braid) PVC outer jacket SIDE VIEW 50Ω (data) or 75Ω (CATV)
Coaxial Cable Properties
Bandwidth
1 GHz+
Data Rate
10 Mbps–2 Gbps
Max Distance
500 m (10BASE5)
Attenuation
Medium
EMI Immunity
Good (shielded)
The coaxial design — signal on inner conductor, return on outer shield — completely contains the electromagnetic field, giving excellent noise immunity.
50Ω: Data networks (10BASE2/10BASE5).
75Ω: Cable TV (CATV), DOCSIS broadband up to 1 Gbps.
10BASE-2 10BASE-5 DOCSIS 3.1 CATV 75Ω
OPTICAL FIBRE — CROSS SECTION + TIR glass core (9µm SM / 50µm MM) cladding (lower n₂) buffer coating outer jacket CROSS SECTION · LC/SC connector cladding (n₂) core (n₁ > n₂) Total Internal Reflection cladding (n₂) light trapped in core when θ > θc
Optical Fibre Properties
Bandwidth
100+ THz
Data Rate
100 Gbps+ (DWDM)
Max Distance
80–120 km unamplified
Attenuation
0.2 dB/km @ 1550 nm
EMI Immunity
Complete (glass)
Light travels through the glass core via Total Internal Reflection (TIR): n₁ (core) > n₂ (cladding), so photons are trapped.
SMF (9 µm core): long-haul, single mode, laser source, low dispersion.
MMF (50/62.5 µm core): short reach, LED/VCSEL, datacentre.
1000BASE-LX 10GBASE-SR DWDM FTTH/PON
04

Wireless Communication

// Radio Waves · Microwave · Satellite · Propagation

Electromagnetic Frequency Spectrum
ELECTROMAGNETIC SPECTRUM — WIRELESS COMMUNICATIONS 3 kHz 300 kHz 30 MHz 300 MHz 3 GHz 30 GHz 300 GHz 3 THz+ VLF/LF MF/HF VHF UHF SHF EHF AM RADIO FM RADIO LTE / 4G Wi-Fi 2.4 Wi-Fi 5/6 SAT Ku-band 5G mmWave ← LOWER FREQUENCY / LONGER RANGE ····················· HIGHER FREQUENCY / MORE BANDWIDTH → WAVELENGTH: km ←——————————————————————————————→ mm/µm
Radio Wave Propagation Mechanisms
LINE-OF-SIGHT (LOS) PROPAGATION TX RX DIRECT PATH clear sky — no obstacles — highest signal quality
Line-of-Sight Propagation

In LOS propagation, the transmitter and receiver have an unobstructed direct path. Signal strength decreases with distance following the inverse-square law: P ∝ 1/d².

Free-Space Path Loss:
FSPL = (4πd/λ)² where d = distance, λ = wavelength.

Applications: Satellite links, long-range microwave, 5G mmWave (short range), air-traffic radar.

Limitation: Requires optical visibility between antennas; earth's curvature limits range. Repeater towers are needed for long terrestrial links.

REFLECTION PROPAGATION BUILDING TX RX BLOCKED REFLECTION POINT θᵢ=θᵣ ground reflect
Reflection

A radio wave strikes a surface and bounces back. The angle of incidence equals the angle of reflection (θᵢ = θᵣ), just like light in a mirror.

Multipath fading: Multiple reflected copies of the same signal arrive at the receiver with different time delays and phases. Constructive or destructive interference causes signal fading.

Exploited by: OFDM (Wi-Fi, LTE, 5G) uses many sub-carriers to combat multipath. MIMO uses multiple antennas to harness multipath as additional spatial channels.

DIFFRACTION — BENDING AROUND OBSTACLES HILL TX RX DIFFRACTION EDGE Huygens principle: each point on wavefront = new point source
Diffraction

Waves bend around the edges of obstacles according to Huygens' Principle: every point on a wavefront acts as a new point source of secondary wavelets.

Lower frequencies diffract more readily, allowing them to travel beyond hills and around buildings. This is why AM radio (hundreds of kHz) reaches behind hills but 5G mmWave (millimetre wavelength) cannot.

Knife-edge diffraction model: used by engineers to predict signal strength behind terrain obstacles. Fresnel zones determine the degree of diffraction loss.

SCATTERING — FOLIAGE / ROUGH SURFACES FOLIAGE / RAIN TX RX energy dispersed in all directions — only a fraction reaches RX
Scattering

When a radio wave encounters objects smaller than its wavelength — rain drops, foliage, rough surfaces — energy is dispersed in multiple directions, causing severe signal loss.

Rain attenuation becomes significant above ~10 GHz (Ku/Ka satellite bands, mmWave 5G). Engineers add a rain fade margin to satellite link budgets.

Tropospheric scattering: at UHF, temperature irregularities in the troposphere scatter energy forward, enabling beyond-horizon links of 100–2000 km ("troposcatter").

Terrestrial Microwave vs Satellite Link
TERRESTRIAL MICROWAVE RELAY Tower 1 Repeater Tower 2 ~50 km hop ~50 km hop 2–40 GHz · LOS required · ~50 km per hop
SATELLITE MICROWAVE LINK (GEO) Station A Station B GEO SAT ~35,786 km uplink downlink GEO: 35,786 km · ~240ms one-way delay · Ku/Ka band
Terrestrial Microwave vs Satellite — Key Comparison
PARAMETER TERRESTRIAL MICROWAVE SATELLITE (GEO)
Frequency 2–40 GHz Ku (12–18 GHz), Ka (26–40 GHz)
Distance ~50 km per hop (LOS limited) 35,786 km altitude → global
Propagation Delay <1 ms per hop ~240 ms one-way (GEO)
Coverage Point-to-point (relay chain) Broadcast / wide area
Bandwidth High (reusable spectrum) Moderate (transponder limited)
Applications Backhaul, telecom backbone TV broadcast, VSAT, GPS