Data Communication · Lecture 4
Transmission Media
Copper, glass and air: the physical paths that carry signals
Md Mahbub E Noor · Assistant Professor, Department of CSE, University of Barishal
Course: Data Communication · 2nd Year, 2nd Semester
Choosing the path: today's goals
- Classify media as guided (cables) or unguided (wireless)
- Explain how twisting and shielding protect copper from noise
- Use refraction and the critical angle to explain how fibre guides light
- Compare multimode and single-mode fibre
- Match radio frequency bands to propagation methods and uses
Lecture 3
Signals weaken (dB per km), distort and pick up noise.
Today
Each medium is a different answer to those problems, at a different cost.
Reading: Forouzan, Data Communications and Networking, Chapter 7
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
2 / 29
Two families of transmission media
- A transmission medium carries signals from sender to receiver; it sits below the physical layer
- Guided media confine the signal inside a physical conductor or fibre
- Unguided media send electromagnetic waves through air, water or vacuum
- Copper carries electrical signals; fibre carries light; wireless carries radio, microwave or infrared
Where media sit
Physical layer (sender)
Transmission medium
Physical layer (receiver)
Guided
Twisted pair
Coaxial cable
Optical fibre
Unguided
Radio waves
Microwaves
Infrared
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
3 / 29
Twisted-pair cable
- Two insulated copper conductors, usually about 1 mm thick, twisted together
- One wire carries the signal; the other is the reference (return path)
- The receiver uses the difference between the two wires
- Cheapest and most common medium: telephone lines and Ethernet LANs
One pair: two insulated copper wires twisted together
| Wire | Carries |
|---|---|
| Orange | The signal |
| Blue | The ground reference (the return path) |
An Ethernet cable holds four such pairs, each twisted at a slightly different rate.
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
4 / 29
Why twisting cancels noise
- Untwisted, the wire nearer a noise source picks up more noise than the far one
- Twisting swaps which wire is nearer, many times per metre, so both pick up equal noise
- The receiver subtracts: (s + n) − (−s + n) = 2s
- This also cuts crosstalk from neighbouring pairs
Wire A: +signal + noise
Wire B: −signal + the same noise
Receiver: A − B = 2 × signal; the noise cancels
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
5 / 29
Unshielded and shielded twisted pair
- UTP: no metal shield; relies on twisting alone
- STP: adds a metal foil or braided shield that blocks outside interference
- Both end in the 8-pin RJ-45 connector used for Ethernet
- The shield must be grounded properly, or it can make noise worse
UTP
Unshielded: four pairs inside a plastic jacket
Cheap, thin, flexible: almost all office and home Ethernet
STP
Shielded: a metal foil or braid around the pairs
Better noise immunity, but bulkier and costlier: factories, data centres
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
6 / 29
Twisted-pair categories
- The category (Cat) rates how high a frequency the cable can carry cleanly
- Higher bandwidth in MHz allows a higher data rate (Lecture 3)
- Older Cat 3 carried telephone and 10 Mbps Ethernet
- Ethernet over twisted pair is specified for up to 100 m per run (Cat 8: 30 m)
| Category | Bandwidth | Typical Ethernet speed | Max length |
|---|---|---|---|
| Cat 5e | 100 MHz | 1 Gbps | 100 m |
| Cat 6 | 250 MHz | 1 Gbps (10 Gbps to ~55 m) | 100 m |
| Cat 6A | 500 MHz | 10 Gbps | 100 m |
| Cat 8 | 2,000 MHz | 25 to 40 Gbps | 30 m |
Higher categories: tighter twists, better insulation and separators, so they pass higher frequencies with less crosstalk.
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
7 / 29
Worked example: how far can copper go?
- Problem: a receiver needs at least 1% of the transmitted power
- Cat 6 loses roughly 20 dB per 100 m at 100 MHz
- Allowed loss: 10 log₁₀(0.01) = −20 dB
- Length = 20 dB ÷ 0.2 dB/m = 100 m, matching the Ethernet rule
Loss budget
receiver needs ≥ 1% of the power → 10 log₁₀(0.01)
20 dB
Cat 6 at 100 MHz
≈ 20 dB per 100 m → 0.2 dB per metre
0.2 dB/m
Maximum length
20 dB ÷ 0.2 dB/m
100 m
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
8 / 29
Coaxial cable
- A central copper conductor surrounded by an insulator, a metal shield and a jacket
- The shield fully surrounds the signal, so coax handles much higher frequencies than twisted pair
- Rated by RG numbers; connectors include BNC and the screw-on F-type
- Today: cable TV, cable broadband, satellite dishes, CCTV
Inner copper conductor: the signal
Insulator (dielectric)
Braided metal shield: return path and noise barrier
Plastic outer jacket
| Type | Impedance | Use |
|---|---|---|
| RG-59 / RG-6 | 75 Ω | Cable TV, satellite dish, cable internet |
| RG-58 | 50 Ω | Thin Ethernet (10BASE2), radio |
| RG-8 / RG-11 | 50 / 75 Ω | Thick Ethernet (10BASE5), trunk lines |
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
9 / 29
Checkpoint 1 · 3 minutes
Check your understanding
Q1
In differential signalling, wire A = s + n and wire B = −s + n. What does the receiver compute, and what is the result?
Q2
You need 10 Gbps Ethernet over 90 m of twisted pair. Which category?
Q3
A cable loses 0.25 dB/m. With a 20 dB budget, how long can it be?
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
10 / 29
Bending light: refraction and the critical angle
- Light changes direction when it passes between materials of different refractive index n
- Going from denser (higher n) to less dense (lower n), it bends away from the normal
- At the critical angle θc the refracted ray skims along the boundary
- Above θc, all the light reflects back: total internal reflection
Below θc: refracts out
At θc: skims the boundary
Above θc: reflects back
n₁ sin θ₁ = n₂ sin θ₂
sin θc = n₂ / n₁
Angles are measured from the normal (dashed line). Light travels from n₁ (denser, bottom) towards n₂.
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
11 / 29
Worked example: a fibre's critical angle
- Problem: core index n₁ = 1.48, cladding index n₂ = 1.46
- Rays must hit the core–cladding boundary at more than θc ≈ 80.6° from the normal
- So light must travel almost parallel to the fibre axis to stay trapped
- Light entering within about ±14° of the axis is guided; steeper light escapes
Critical angle
sin θc = 1.46 / 1.48 = 0.9865
θc ≈ 80.6°
Numerical aperture
NA = √(n₁² − n₂²) = √(2.1904 − 2.1316)
NA ≈ 0.24
Acceptance angle (in air)
sin θa = NA → θa = arcsin 0.24
≈ ±14°
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
12 / 29
Try it: trap light in a fibre
Interactive
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
13 / 29
Optical fibre: light trapped in glass
- A thin glass or plastic core surrounded by cladding of slightly lower refractive index
- Light hits the boundary above the critical angle and reflects, again and again
- A protective buffer and jacket surround the cladding
- Data is sent as pulses of light, usually from a laser or LED; a photodiode detects them
- Light travels one way, so a link normally uses a pair of fibres
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
14 / 29
Propagation modes: how many paths?
- Multimode step-index: many paths of different lengths; pulses spread the most
- Multimode graded-index: index falls gradually from the centre, so paths re-align; less spreading
- Single-mode: a core so narrow that light takes essentially one path; almost no modal spreading
- Less spreading means pulses can be closer together: higher data rates over longer distances
Multimode, step-index (50 or 62.5 µm core)
Multimode, graded-index (50 or 62.5 µm core)
Single-mode (≈ 9 µm core)
pulse at the far end →
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
15 / 29
Why fibre carries the world's data
- Fibre forms the backbone of the Internet, mobile networks and undersea cables
- Common connectors: SC (push-pull), ST (twist-lock) and the small LC
- Fibre-to-the-home (FTTH) now brings fibre into many homes and offices
- Copper remains for short, cheap connections to individual devices
Advantages
Enormous bandwidth: terabits per second per fibre with many wavelengths
Very low loss: about 0.2 dB/km, so tens of km between amplifiers
Immune to electromagnetic noise and lightning
Light, thin and hard to tap without detection
Disadvantages
Installation and splicing need skilled work and special tools
Light travels one way: links need fibre pairs
Transceivers and connectors cost more than copper's
Fragile if bent too tightly
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
16 / 29
Worked example: copper versus fibre
- Copper (Cat 6 at 100 MHz): about 0.2 dB per metre
- Single-mode fibre at 1550 nm: about 0.2 dB per kilometre
- Same 20 dB loss: 100 m of copper, or 100 km of fibre
- Fibre goes about 1,000 times further for the same loss
Cat 6 copper · 100 m
−20 dB
0.2 dB/m × 100 m → 1% of the power arrives
Single-mode fibre · 100 km
−20 dB
0.2 dB/km × 100 km → 1% of the power arrives
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
17 / 29
Checkpoint 2 · 3 minutes
Check your understanding
Q1
Core n₁ = 1.50, cladding n₂ = 1.45. Find the critical angle.
Q2
Which fibre type keeps pulses sharpest over long distances, and why?
Q3
A 50 km single-mode link loses 0.2 dB/km. Total loss, and what fraction of the power arrives?
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
18 / 29
Try it yourself: Media Lab
- 1 · Light in a fibre: change n₁, n₂ and the angle; find total internal reflection
- 2 · Link budget: copper, coax or fibre; how far before you need a repeater?
- 3 · Radio spectrum: slide through frequencies; see band, wavelength and uses
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
19 / 29
Unguided media: the radio spectrum
| Band | Range | Propagation | Typical uses |
|---|---|---|---|
| VLF | 3–30 kHz | Ground | Long-range navigation |
| LF | 30–300 kHz | Ground | Radio beacons |
| MF | 300 kHz–3 MHz | Sky | AM radio |
| HF | 3–30 MHz | Sky | Shortwave, ship and aircraft radio |
| VHF | 30–300 MHz | Sky and line of sight | FM radio, VHF TV |
| UHF | 300 MHz–3 GHz | Line of sight | TV, mobile phones, Wi-Fi 2.4 GHz |
| SHF | 3–30 GHz | Line of sight | Satellite, Wi-Fi 5 GHz, microwave links |
| EHF | 30–300 GHz | Line of sight | Radar, 5G mmWave, satellite |
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
20 / 29
Three ways radio waves travel
- Ground propagation: low frequencies hug the Earth's surface, reaching far at low data rates
- Sky propagation: waves bounce off the ionosphere, reaching other continents
- Line-of-sight propagation: high frequencies travel straight; antennas must see each other
- The frequency decides which method dominates
━ Ground wave (below 2 MHz)
━ Sky wave (2–30 MHz)
━ Line of sight (above 30 MHz)
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
21 / 29
Worked example: why towers are tall
- Line-of-sight antennas must see each other over the curve of the Earth
- The maximum distance grows with the square root of antenna height
- Doubling the distance needs four times the height
- In practice, the atmosphere bends waves slightly, extending range by about 15%
d ≈ 3.57 (√h₁ + √h₂) km
h₁, h₂ = antenna heights in metres; ignores atmospheric bending
Two 100 m towers
3.57 × (√100 + √100) = 3.57 × 20
≈ 71 km
Phone (1.5 m) to a 30 m mobile tower
3.57 × (√1.5 + √30) = 3.57 × 6.70
≈ 24 km
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
22 / 29
Try it: why towers are tall
Interactive
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
23 / 29
Radio waves, microwaves and infrared
Radio waves
3 kHz – 1 GHz
Direction: omnidirectional: spread in all directions
Walls: pass through walls
Uses: AM and FM radio, TV broadcast, maritime radio
Microwaves
1 – 300 GHz
Direction: unidirectional with dish or horn antennas; Wi-Fi and phones use wide-beam antennas
Walls: higher frequencies are blocked more easily
Uses: mobile networks, Wi-Fi, satellite, point-to-point links
Infrared
300 GHz – 400 THz
Direction: short-range line of sight
Walls: blocked by walls
Uses: TV remotes, short-range device links
Omnidirectional antennas suit broadcasting to many receivers; directional dishes concentrate energy towards one receiver, increasing range and reducing interference.
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
24 / 29
Comparing the media
| Medium | Data rate | Distance per span | Noise immunity | Cost | Typical use |
|---|---|---|---|---|---|
| Twisted pair | Up to 10 Gbps (Cat 6A) | 100 m | Moderate (twisting) | Low | Desk-to-switch LAN |
| Coaxial | Hundreds of Mbps to Gbps, shared | Hundreds of m (amplifiers) | Good (shield) | Moderate | Cable TV and broadband |
| Fibre | Tbps per fibre (many wavelengths) | Tens of km per span | Immune to EMI | High to install | Backbones, undersea, FTTH |
| Radio / microwave | Mbps to Gbps | Metres to tens of km | Interference, weather | No cabling | Mobile, Wi-Fi, backhaul |
| Infrared | Kbps to Mbps | A few metres | Blocked by walls | Very low | Remote controls |
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
25 / 29
See it in action: copper, glass and air
- Watch twisting and subtraction cancel noise on a cable
- See light trapped by total internal reflection, then escape
- Compare pulses after multimode and single-mode fibre
- When a “Pause & predict” card appears, answer before the video moves on
Click the video to play or pause · Press → for full screen
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
26 / 29
Checkpoint 3 · 3 minutes
Check your understanding
Q1
Connect two campus buildings 2 km apart at 10 Gbps, with the cable route running beside power lines. Which medium?
Q2
Connect 40 desks to a switch in the same room. Which medium and category?
Q3
Connect a village across a wide river, with no way to lay cable. What would you use?
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
28 / 29
Key takeaways
- Guided media (twisted pair, coax, fibre) confine the signal; unguided media radiate it
- Twisting plus differential signalling cancels noise; shields block it
- Fibre guides light by total internal reflection: sin θc = n₂ / n₁
- Single-mode fibre keeps pulses sharp; it carries data furthest and fastest
- Radio propagation: ground, sky or line of sight, decided by frequency
Next lecture
Lecture 5 · Analog-to-Digital Conversion
How a microphone's continuous voltage becomes bits: sampling, quantisation and encoding (PCM), with fully solved examples.
Reading: Forouzan, Section 4.2 · Explore Media Lab
Md Mahbub E Noor
Lecture 4: Transmission Media
University of Barishal
29 / 29