Physical And Data Link Layers
Signals, Media and Bandwidth
It has no idea what the bits MEAN. That is the
JrCodex·8 min read
Jr Codex Computer Networks Notes
Level: Beginner Prerequisites: Module 1, Chapter 5: Measuring a Network Time to complete: ~15 minutes
Table of Contents
- Turning Bits Into Signals
- Line Coding
- Guided Media
- Wireless
- What Actually Limits Capacity
- Duplex and Multiplexing
- Summary & Next Steps
1. Turning Bits Into Signals
The Physical Layer's One Job
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Take a 1 or a 0 and make it a physical
phenomenon the other end can detect:
copper a voltage
fibre a pulse of light
wireless a modulated radio wave
And do the reverse at the other end.
It has no idea what the bits MEAN. That is the
data link layer's problem (Chapter 2).
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Analogue and Digital
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All physical signals are ANALOGUE — continuous
voltages, continuous light intensity.
DIGITAL transmission means agreeing on
THRESHOLDS: "above 2.5V is a 1, below is a 0".
That thresholding is what makes digital robust.
A signal degraded by noise still crosses the
right side of the threshold, and can be
REGENERATED perfectly — where an analogue signal
accumulates every distortion along the way.
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2. Line Coding
The Problem With the Obvious Encoding
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NRZ (Non-Return to Zero): high = 1, low = 0.
1 0 1 1 0 0 0 0 0 0 0 0 1
▔▔__▔▔▔▔________________▔▔
Sending twelve zeros means a long flat line. The
receiver's clock DRIFTS, and it loses count of
how many zeros passed.
A CLOCK SYNCHRONISATION problem, caused by the
encoding itself.
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The Fixes
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MANCHESTER ENCODING
Every bit is a TRANSITION in the middle:
1 = low-to-high, 0 = high-to-low.
Guarantees a transition per bit, so the clock
always resynchronises.
COST: twice the signal rate for the same data
rate. Used by classic 10 Mbps Ethernet.
4B/5B
Encode every 4 data bits as 5 transmitted
bits, choosing codes that never contain long
runs of zeros.
COST: 25% overhead — much better than 100%.
Used by 100 Mbps Ethernet.
8B/10B, 64B/66B
The same idea, more efficient. 64B/66B is only
3% overhead, and is what gigabit and faster
links use.
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The Trade-off, Stated Once
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Encoding schemes trade OVERHEAD for
SELF-CLOCKING and DC balance.
Faster standards use lower-overhead codes because
their clocks are better and their transceivers
more sophisticated. The progression from 100%
overhead to 3% is most of the story of how
Ethernet got a thousand times faster.
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3. Guided Media
TWISTED PAIR (Cat5e, Cat6, Cat6a)
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Pairs of copper wires twisted together.
WHY TWISTED: the two wires carry the same signal
in opposite polarity. Interference hits both
equally, and subtracting them CANCELS the noise.
Tighter twists cancel better — which is the whole
difference between the categories.
Cat5e 1 Gbps, 100 m
Cat6 10 Gbps, 55 m
Cat6a 10 Gbps, 100 m
+ cheap, flexible, ubiquitous
- distance-limited, susceptible to interference
COAXIAL
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A central conductor inside a shield.
+ better shielding than twisted pair
- bulky
Now mostly cable television and cable internet.
FIBRE OPTIC
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Light through glass. Total internal reflection.
SINGLE-MODE a very narrow core; one light path
Tens of kilometres. Long haul.
MULTI-MODE a wider core; many paths, which
spread the pulse over distance
Hundreds of metres. Within
buildings.
+ enormous bandwidth, very low loss, IMMUNE to
electromagnetic interference, no signal leakage
(harder to tap)
- more expensive to terminate; fragile if bent
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Why Long Distance Is Always Fibre
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Copper attenuates rapidly — a signal is
unusable after ~100 m.
Fibre attenuates so little that transatlantic
cables need repeaters only every ~50-100 km.
There is no copper alternative at that scale.
Every intercontinental link is fibre.
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4. Wireless
The Fundamental Difference
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A cable is a PRIVATE medium. Radio is a SHARED
one.
Consequences that shape everything above it:
- anyone in range receives your signal, so
encryption is not optional (Module 6)
- two devices transmitting at once INTERFERE
- signal strength falls with distance, so the
data rate falls too
- obstacles absorb and reflect
- you cannot detect a collision while
transmitting, because your own signal drowns
everything ── which is why wifi uses
collision AVOIDANCE, not detection
(Chapter 3)
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The Bands
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2.4 GHz travels further, penetrates walls
better; CROWDED (microwaves, Bluetooth,
baby monitors); only 3 non-overlapping
channels
5 GHz more channels, less interference,
higher rates; shorter range, worse
through walls
6 GHz Wi-Fi 6E and 7; very wide clean
channels; shortest range
The trade is always the same: HIGHER FREQUENCY
means more bandwidth and less range.
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Reading a Wifi Speed Claim
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"Wi-Fi 6, up to 9.6 Gbps" describes ideal
conditions, all streams, all channels, one
device, no walls.
Real single-device throughput is typically a
small fraction of it — and it is SHARED among
everyone on the access point, because the medium
is shared.
Advertised wireless rates are a ceiling nobody
reaches, not a speed you get.
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5. What Actually Limits Capacity
NYQUIST — the noiseless limit
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max rate = 2 × B × log₂(L) bits per second
B = bandwidth in Hz
L = number of distinguishable signal levels
More levels means more bits per symbol — so a
modem sending 16 distinguishable levels carries
4 bits per symbol instead of 1.
SHANNON — the real limit, with noise
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capacity = B × log₂(1 + S/N) bits per second
S/N = signal-to-noise ratio.
This one is a HARD PHYSICAL LIMIT. No encoding,
no cleverness, no amount of engineering exceeds
it.
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import math
def shannon_bps(bandwidth_hz, snr_db):
snr = 10 ** (snr_db / 10) # dB is logarithmic
return bandwidth_hz * math.log2(1 + snr)
print(f"{shannon_bps(20e6, 25)/1e6:>7.1f} Mbps") # good wifi: 166.1
print(f"{shannon_bps(20e6, 10)/1e6:>7.1f} Mbps") # weak wifi: 69.2
print(f"{shannon_bps(20e6, 3)/1e6:>7.1f} Mbps") # very weak: 30.0What Shannon Explains
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Why wifi gets SLOWER as you walk away from the
access point, without ever disconnecting.
Distance lowers the signal-to-noise ratio, which
lowers capacity, so the devices negotiate a lower
data rate. Nothing is broken; you have moved to a
different point on Shannon's curve.
It also tells you the two ways to get more
capacity: MORE BANDWIDTH (wider channels, higher
frequencies) or BETTER SNR (closer, more power,
less noise). There is no third way.
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6. Duplex and Multiplexing
Duplex Modes
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SIMPLEX one direction only (broadcast TV)
HALF-DUPLEX both directions, one at a time
(walkie-talkie, wifi, old
Ethernet hubs)
FULL-DUPLEX both directions simultaneously
(switched Ethernet, phone calls)
Full duplex on a switched Ethernet port is what
ELIMINATED collisions entirely — Chapter 3.
Multiplexing — sharing one medium
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FDM Frequency Division — different frequency
bands. Radio stations, wifi channels.
TDM Time Division — different time slots.
Traditional telephone trunks.
WDM Wavelength Division — different colours of
light down ONE fibre. The reason a single
fibre strand carries terabits: 80+
wavelengths, each at 100+ Gbps.
CDM Code Division — different codes, all at
once. Mobile networks.
STATISTICAL — allocate on demand rather than by
fixed slot. This is PACKET SWITCHING
(Module 1, Chapter 4), and it is why the
internet uses capacity so much more
efficiently.
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7. Summary & Next Steps
Key Takeaways
- Digital transmission is analogue signals plus agreed thresholds, and that thresholding is what allows perfect regeneration where analogue accumulates distortion.
- Line coding trades overhead for self-clocking; the progression from Manchester's 100% overhead to 64B/66B's 3% is much of how Ethernet got faster.
- Radio is a shared medium, which forces encryption, causes interference, and makes collision detection impossible — hence wifi's collision avoidance.
- Shannon's limit is a hard physical bound, and it explains why wifi slows with distance: lower signal-to-noise means lower capacity, with only two ways to raise it.
Concept Check
- Why does twisting a pair of copper wires reduce interference?
- What problem does Manchester encoding solve, and what does it cost?
- Using Shannon's formula, explain why your wifi gets slower as you move away without disconnecting.
Next Chapter
→ Chapter 2: Framing and Error Detection
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