CS331 · Chapter 2 · Part 2 · Annotated SlidesPhysical Layer: Transmission Media
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Overview

Physical Layer: Transmission Media

Guided media (twisted pair, coaxial, fiber) and unguided media (radio, microwave, infrared): how each one is built, what it is good and bad at, UTP vs. STP and the cable categories, RJ-45 pinouts and when you need a crossover cable. Most questions here are comparisons, so the notes keep pointing at the differences.

How to read this page

Every original slide is shown unchanged. Yellow marks on a slide point to a numbered note: hover or tap a mark to jump to its note, and hover a note to see its marks. Questions tagged Past exam appeared on a real paper; Prediction questions are likely angles, not confirmed. Pick an answer to each question: a wrong pick tells you why it is wrong, so you can try again.

All 29 slides
  1. 1 Physical Layer
  2. 2 Transmission medium and the physical layer
  3. 3 Classes of transmission media
  4. 4 Guided media
  5. 5 Twisted pairs
  6. 6 Twisted-pair cable
  7. 7 Twist length: Category 3 vs. Category 5 UTP
  8. 8 Why twisting cancels interference
  9. 9 UTP and STP cables
  10. 10 STP vs. UTP
  11. 11 Categories of unshielded twisted-pair cables
  12. 12 UTP connectors
  13. 13 The RJ-45 connector pinout
  14. 14 Crossover cable
  15. 15 Ethernet cabling: straight-through vs. crossover
  16. 16 Coaxial cable
  17. 17 Optical Fiber
  18. 18 Optical Fiber
  19. 19 Fiber Optics Properties
  20. 20 UTP cabling vs. fiber-optic cabling
  21. 21 Unguided media: wireless
  22. 22 Wireless transmission waves (Figure 7.19)
  23. 23 Electromagnetic spectrum for wireless communication
  24. 24 Radio waves
  25. 25 Note: radio waves
  26. 26 Microwaves
  27. 27 Note: microwaves
  28. 28 Infrared (IR Wireless)
  29. 29 Note: infrared
Slide 1 / 29

Physical Layer

Slide 1: Physical Layer

Notes

  1. Guided (wired): twisted pair, coax, fiber. Unguided (wireless): radio, microwave, infrared.

  2. Know one advantage and one disadvantage of each, plus the frequency ranges of the three wireless types.

Slide 2 / 29

Transmission medium and the physical layer

Slide 2: Transmission medium and the physical layer

Notes

  1. Seen again Ch 1 Part 1, slide 3: The medium as a component of data communication

Exam angle

PredictionQ1

The transmission medium is located above the physical layer.

Slide 3 / 29

Classes of transmission media

Slide 3: Classes of transmission media

Notes

  1. Memory trick Guided = the signal is GUIDED along a physical path. Unguided = it just spreads through the air.

Exam angle

PredictionQ1

Which is an unguided medium?

PredictionQ2

Fiber-optic cable is classified as:

Slide 4 / 29

Guided media

Slide 4: Guided media

Notes

  1. Three guided media, from cheapest and slowest to most expensive and fastest.

Slide 5 / 29

Twisted pairs

Slide 5: Twisted pairs

Notes

  1. Four pairs, each a different colour, inside one jacket.

Slide 6 / 29

Twisted-pair cable

Slide 6: Twisted-pair cable

Exam angle

PredictionQ1

Why are the wires in a twisted pair twisted?

PredictionQ2

More twists per length gives better quality twisted-pair cable.

Slide 7 / 29

Twist length: Category 3 vs. Category 5 UTP

Slide 7: Twist length: Category 3 vs. Category 5 UTP

Notes

  1. Cat 5 is twisted far more tightly than Cat 3, so it carries higher frequencies with less crosstalk.

Slide 8 / 29

Why twisting cancels interference

Slide 8: Why twisting cancels interference

Notes

  1. The receiver looks at the DIFFERENCE between the two wires. Twisting makes the noise equal on both, so it subtracts out.

Exam angle

PredictionQ1

Explain how twisting a pair of wires reduces noise.

Compare with the model answer

In parallel wires one wire is always closer to the noise source and picks up more noise, so a difference remains. Twisting swaps the positions all along the cable, so both wires pick up the same total noise; the receiver uses the difference between them, and the noise cancels.

Slide 9 / 29

UTP and STP cables

Slide 9: UTP and STP cables

Notes

  1. STP adds a metal shield (foil or braid) around the pairs.

Slide 10 / 29

STP vs. UTP

Slide 10: STP vs. UTP

Notes

  1. Trap UTP is not “worse” for every case: it is the default in LANs because it is cheap and easy.

Exam angle

PredictionQ1

Compared with UTP, STP:

PredictionQ2

A factory floor has heavy motors next to where the network cables will run. UTP or STP, and why?

Compare with the model answer

STP. The motors cause electromagnetic interference (induced noise); the metal shield in STP reduces it. UTP would suffer from the EMI. (Fiber would be immune altogether.)

Slide 11 / 29

Categories of unshielded twisted-pair cables

Slide 11: Categories of unshielded twisted-pair cables

Exam angle

PredictionQ1

Which is the first category in the table that is shielded only?

PredictionQ2

CAT6a supports a maximum data rate of:

Slide 12 / 29

UTP connectors

Slide 12: UTP connectors

Notes

  1. RJ-45 (Registered Jack) is the 8-pin connector of twisted-pair Ethernet.

Slide 13 / 29

The RJ-45 connector pinout

Slide 13: The RJ-45 connector pinout

Notes

  1. 10/100 Ethernet uses only 4 pins: 1 & 2 transmit, 3 & 6 receive.

  2. 4, 5, 7, 8 are unused (Gigabit Ethernet uses all 8).

Exam angle

PredictionQ1

In 10/100 Ethernet, which RJ-45 pins are used to transmit?

PredictionQ2

Which pins receive data?

Slide 14 / 29

Crossover cable

Slide 14: Crossover cable

Notes

  1. Crossover swaps 1↔3 and 2↔6, so TX on one side meets RX on the other.

Slide 15 / 29

Ethernet cabling: straight-through vs. crossover

Slide 15: Ethernet cabling: straight-through vs. crossover

Notes

  1. Different kinds of device (PC ↔ switch, router ↔ switch) → straight-through.

  2. Same kind of device (PC ↔ PC, switch ↔ switch, hub ↔ hub, hub ↔ switch) → crossover, or TX meets TX.

  3. Memory trick “Unlike → straight, like → cross.” (Router ↔ PC counts as “like”: both are end devices.)

Exam angle

PredictionQ1

Which cable connects two PCs directly?

PredictionQ2

Which cable connects a PC to a switch?

PredictionQ3

Two switches are connected with a straight-through cable.

Slide 16 / 29

Coaxial cable

Slide 16: Coaxial cable

Notes

  1. Coax = a central copper wire + an outer copper mesh as a shield. Used in cable TV and old bus Ethernet (10Base5, 10Base2).

  2. Seen again Ch 4, slide 18: Which media each Ethernet generation uses

Slide 17 / 29

Optical Fiber

Slide 17: Optical Fiber

Notes

  1. Core (carries the light) → cladding (different optical property, reflects the light back in) → jacket (protection).

Exam angle

PredictionQ1

The layer of a fiber that keeps the light inside the core is the:

Slide 18 / 29

Optical Fiber

Slide 18: Optical Fiber

Notes

  1. Light travels by reflecting off the core/cladding boundary.

Slide 19 / 29

Fiber Optics Properties

Slide 19: Fiber Optics Properties

Notes

  1. Trap Fiber is NOT cheaper or easier to install. Those are its two disadvantages.

Exam angle

PredictionQ1

Which is a disadvantage of fiber-optic cable?

PredictionQ2

Fiber-optic cable is not affected by electromagnetic noise.

PredictionQ3

Give three advantages of fiber over copper.

Compare with the model answer

Higher bandwidth (hundreds of Gbps), lower attenuation so repeaters can be 50 km or more apart, immunity to noise and crosstalk, smaller size and weight, and better security because no light leaks out.

Slide 20 / 29

UTP cabling vs. fiber-optic cabling

Slide 20: UTP cabling vs. fiber-optic cabling

Exam angle

PredictionQ1

The maximum distance of a UTP segment in the table is about:

PredictionQ2

Two buildings 2 km apart must be linked, and a high-voltage line runs between them. Which cable?

Compare with the model answer

Fiber. UTP only reaches about 100 m and is not immune to EMI or electrical hazards; fiber reaches tens of kilometres and is completely immune.

Slide 21 / 29

Unguided media: wireless

Slide 21: Unguided media: wireless

Notes

  1. Three unguided media, from lowest to highest frequency.

Slide 22 / 29

Wireless transmission waves (Figure 7.19)

Slide 22: Wireless transmission waves (Figure 7.19)

Notes

  1. Unguided: the signal goes through the air and anyone with a receiver can pick it up (security matters).

Slide 23 / 29

Electromagnetic spectrum for wireless communication

Slide 23: Electromagnetic spectrum for wireless communication

Notes

  1. Radio < 300 MHz < microwave < 300 GHz < infrared < 400 THz < visible light.

  2. Memory trick Order by frequency: Radio → Microwave → Infrared → Light (“Really Many Infrared Lights”).

Exam angle

PredictionQ1

Which has the highest frequency?

Slide 24 / 29

Radio waves

Slide 24: Radio waves

Notes

  1. Radio: below 300 MHz, omnidirectional, long distances, passes through walls. Used for AM/FM and TV.

Slide 25 / 29

Note: radio waves

Slide 25: Note: radio waves

Notes

  1. Radio = MULTICAST / broadcast (one sender, many receivers: radio and TV).

Exam angle

PredictionQ1

Radio waves are used for which type of communication?

Slide 26 / 29

Microwaves

Slide 26: Microwaves

Notes

  1. Microwave: 300 MHz – 300 GHz, unidirectional (line of sight), can link two buildings point to point.

  2. Trap Radio is OMNIdirectional; microwave is UNIdirectional (line of sight).

Exam angle

PredictionQ1

Microwaves propagate:

PredictionQ2

Higher frequencies pass through walls more easily.

Slide 27 / 29

Note: microwaves

Slide 27: Note: microwaves

Exam angle

PredictionQ1

Cellular telephones and satellite networks mainly use:

Slide 28 / 29

Infrared (IR Wireless)

Slide 28: Infrared (IR Wireless)

Notes

  1. Infrared: 300 GHz – 400 THz, very large bandwidth, line of sight, short range (can’t pass walls).

  2. Remote controls and some peripherals (IrDA protocol). Doesn’t work in sunlight. Mostly replaced by Bluetooth.

Exam angle

PredictionQ1

Why can’t infrared be used outdoors in the sun?

PredictionQ2

Which wireless medium cannot penetrate walls and is used in TV remote controls?

Slide 29 / 29

Note: infrared

Slide 29: Note: infrared

Exam angle

PredictionQ1

Compare radio waves, microwaves and infrared by direction and typical use.

Compare with the model answer

Radio (< 300 MHz): omnidirectional, long range, multicast such as radio and TV. Microwave (300 MHz – 300 GHz): unidirectional line of sight, unicast such as cellular, satellite and wireless LANs. Infrared (300 GHz – 400 THz): line of sight, short range inside a room, such as remote controls.