Physical Layer & Cabling

1. Under the legacy 5-4-3 network design rule for shared Ethernet hubs, what does the number ‘3’ represent?

The 5-4-3 rule caps how many hubs you can daisy-chain before signal timing breaks down:

5  physical segments (max)
4  repeaters / hubs connecting them (max)
3  segments that may be "populated" (have active user devices)

The number 3 = the maximum number of populated segments containing active user nodes. The other two segments must be unpopulated link spans — just cabling between hubs.

Why it exists: on shared Ethernet, every hop through a hub adds delay to a frame. Too many hops and the round-trip time exceeds the slot time, so collisions go undetected and frames get lost. The rule keeps the collision domain small enough to work. It’s legacy — modern switched networks don’t need it.

2. What is the operational difference between Baseband and Broadband network transmissions?

• Baseband — uses the entire bandwidth of the medium to carry one digital signal at a time. Standard Ethernet is baseband (that’s the “Base” in 100Base-T).
• Broadband — splits the medium into multiple frequency channels, each carrying a separate analog signal concurrently. That’s how cable TV and DOCSIS internet share one coax line.

Baseband:   [ one digital signal using all the bandwidth ]
Broadband:  [ ch1 TV ] [ ch2 TV ] [ ch3 internet ] [ ch4 phone ]

Think of baseband as a single-lane road and broadband as a multi-lane highway where each lane is a different channel.

3. What is Signal Attenuation, and how is it practically corrected over long cable distances?

Attenuation is the progressive loss of signal strength as it travels down a medium, caused by the medium’s resistance.

Strong signal ───────────────→ weak signal
   [source]     distance        [destination]

The longer the cable, the weaker the signal — until it’s too weak to decode reliably.

The fix: a repeater (or hub) receives the degraded signal, regenerates/amplifies it back to full strength, and resends it. This lets signals cross long distances without corruption.

• Attenuation = strength loss over distance.
• Repeaters/hubs = regenerate the signal.

4. Which of the following describes the phenomenon of Crosstalk in copper network cabling?

Crosstalk is electromagnetic interference where the signal from one wire pair bleeds into an adjacent pair inside the same cable.

Pair 1: ──██████░░░░────  (signal "leaks" sideways)
Pair 2: ──░░░░██████────  (picks up Pair 1's bleed)

Why it happens: every wire carrying current radiates a small electromagnetic field. In a multi-pair cable, that field couples into neighboring pairs, distorting their signals and causing bit errors.

The fixes:

• Twisting the pairs together (twisted pair) cancels most of the bleed — hence the name.
• Shielding (STP, foil) blocks external interference.

That’s exactly why UTP cabling is twisted: each wire’s twist keeps its own field contained.

5. What do the physical specifications of a 100Base-FX network standard indicate?

Decompose the name:

100    → 100 Mbps
Base   → Baseband signaling
FX     → Fiber-optic cabling

So 100Base-FX = 100 Mbps over baseband fiber-optic cabling.

The IEEE naming convention is consistent:

• 100Base-T — 100 Mbps, baseband, twisted pair (copper)
• 10Base-T — 10 Mbps, baseband, twisted pair
• 10Base-2 / 10Base-5 — 10 Mbps, baseband, coaxial (Thinnet/Thicknet)

Speed first, then signaling type, then media.

6. How did legacy Thinnet (10Base2) and Thicknet (10Base5) coaxial cables compare in maximum run length?

	Thinnet (10Base2)	Thicknet (10Base5)
Diameter	Thin, flexible	Thick, stiff
Max length	185 m	500 m
Typical use	Workstation segments	Network backbone

• Thinnet — thinner coaxial, cheaper and easier to route, but only reaches 185 meters.
• Thicknet — thicker, more expensive, harder to bend, but reaches 500 meters, so it served as backbones.

Same 10 Mbps speed in both; the trade-off was flexibility vs. reach.

7. What is the structural and functional difference between an RJ11 connector and an RJ45 connector?

	RJ11	RJ45
Size	Smaller	Wider
Pins	4–6	8
Pairs supported	1–2	4
Use	Analog telephone lines	Ethernet (twisted-pair)

RJ11 (phone):  [• • • •]     4-6 pins
RJ45 (ethernet): [• • • • • • • •]   8 pins

RJ11 is the classic telephone jack — small, 4–6 pins. RJ45 is the bigger Ethernet plug, housing 8 pins for the 4 twisted pairs inside a standard Cat5e/6 cable. They look similar at a glance, but they don’t fit each other’s jacks.

8. Which multiplexing type relies on combining different colors of laser light down a single physical strand of fiber-optic cabling?

WDM — Wavelength Division Multiplexing.

Each wavelength of light is a different color, and each color is a separate channel:

one fiber strand:
  [ red λ1 ] [ orange λ2 ] [ green λ3 ] [ blue λ4 ] → combined → split apart

• FDM — frequency bands on copper/radio.
• TDM — time slots (each sender gets a turn).
• WDM — optical wavelengths on fiber. A single fiber carries dozens of independent “colors,” each at full speed — the technology behind terabit fiber links.

9. What advantage does Orthogonal Frequency Division Multiplexing (OFDM) have over standard Frequency Division Multiplexing (FDM)?

OFDM packs subcarriers mathematically perpendicular (orthogonal) to each other, so overlapping channels don’t interfere — eliminating the need for guard bands.

FDM:   [ch1]│guard│[ch2]│guard│[ch3]     ← wasted guard band space
OFDM:  [ch1][ch2][ch3][ch4][ch5]...      ← no guards, peaks/crossings cancel

Why orthogonality works: each subcarrier’s peak aligns with the others’ zero crossings, so even though spectra overlap, one channel’s signal reads as zero in another’s sample window. No crosstalk without wasted spectrum.

Result: far better spectral efficiency — the reason Wi-Fi (802.11a/g/n/ac) and 4G/5G use OFDM to squeeze maximum data out of a fixed bandwidth.

10. When comparing wireless frequencies, what are the trade-offs of using the 5 GHz band instead of the 2.4 GHz band?

	2.4 GHz	5 GHz
Speed	Lower	Faster
Channels	3 non-overlapping	Many more
Congestion	High (microwaves, Bluetooth, neighbors)	Low
Range	Longer, better wall penetration	Shorter
Obstacle resistance	Better	Worse

Why: higher frequency → shorter wavelength → more bandwidth available and more distinct channels → faster data and less interference. But higher frequencies attenuate faster in air and are blocked more easily by walls — so 5 GHz is fast but short-range.

Rule of thumb: 5 GHz for speed at close range; 2.4 GHz for range and through-wall reliability.