Chapter 8 — Wi-Fi: same idea, different physics

In one sentence

Wi-Fi does the same job as Ethernet — deliver frames between devices on a local network — but over a shared, noisy, half-duplex radio channel, which changes its behaviour in ways you will feel.

The problem

On a cable, you have a dedicated wire and can send and receive simultaneously. On radio, everyone shares the same air, you can't listen while you're transmitting, and the signal degrades with distance and interference.

The key differences (and why you care)

	Wired Ethernet	Wi-Fi
Medium	Your own dedicated cable	Shared air, half-duplex
Taking turns	Not needed	CSMA/CA: listen, wait a random time, then transmit
Acknowledgements	None at this layer	Every frame is ACKed and retried at the link layer
Speed	Fixed once negotiated	Changes constantly with signal quality
Loss	Nearly zero on a good cable	Bursty; interference, distance, walls, microwaves
Latency	Microseconds, very stable	1–50 ms, highly variable

Three consequences you will actually experience:

1. Wi-Fi hides loss from TCP — mostly. Because the link layer retries failed frames itself, TCP often doesn't see a loss. Good (TCP doesn't panic), but those retries add latency and jitter, which is why Wi-Fi feels less "crisp" for video calls even at high speeds.

2. Airtime is the real currency, not bandwidth. One distant device transmitting slowly hogs the air, slowing everyone. This is called the "performance anomaly", and it's why one person in the far bedroom can make the whole house's Wi-Fi feel bad.

3. Advertised speeds are fantasy. "Wi-Fi 6, 1200 Mbps" is a theoretical combined figure. Because the channel is shared and half-duplex, real TCP throughput is typically 40–60% of the negotiated rate, less with multiple devices.

The other links you'll meet

Link type	Notes
Fibre / cable to the home	High speed; cable is shared upstream in the neighbourhood
Mobile (4G/5G)	20–80 ms latency that spikes when the radio wakes from idle; deep buffers; your IP changes as you move
VPN tunnels	Your data wrapped inside another packet — which steals bytes from the MTU (Chapter 15). A very common cause of weird bugs.
Loopback (`lo`)	A fake interface inside your own machine, address `127.0.0.1`. Traffic never touches a wire.

Practice (20 min)

A. Look at your actual Wi-Fi quality:

# Linux
iw dev wlan0 link                # signal strength and current bitrate
iw dev wlan0 station dump | grep -E 'signal|bitrate|retries|failed'

# macOS
system_profiler SPAirPortDataType | grep -A10 "Current Network"
# or: wdutil info    (may need sudo)

Two numbers matter most:

Signal: better than −60 dBm is good; worse than −75 dBm is trouble. (It's negative — closer to zero is stronger.)
Tx bitrate: if it's far below your Wi-Fi standard's maximum, you're far from the access point or there's interference.

B. Watch it degrade. Run this while walking away from your router:

# Linux
watch -n1 'iw dev wlan0 link | grep -E "signal|bitrate"'

Watch the bitrate drop in steps as you move. That's the radio adapting, and it explains why your video call gets worse in the far room.

C. Compare latency stability. Ping your router over Wi-Fi and (if you can) over a cable:

ping -c 50 192.168.1.1

Look at the summary line: min/avg/max/mdev. mdev is the jitter. On a cable it'll be a fraction of a millisecond. On Wi-Fi it may be several milliseconds and occasionally much worse. That variability is why Wi-Fi feels different even when "fast".

Common confusions

"More bars = faster?" Bars show signal, not speed. A strong signal with heavy interference or 30 other devices can still be slow.
"5 GHz is always better than 2.4 GHz?" 5 GHz is faster and less congested but has shorter range and is blocked more by walls. 2.4 GHz reaches further but is crowded (and shared with microwaves, Bluetooth, baby monitors).
"Wi-Fi is a network." Wi-Fi is a link layer — a way to carry frames. IP, TCP, and everything above are identical over Wi-Fi and Ethernet.

Check yourself

Why does Wi-Fi acknowledge every frame when Ethernet doesn't?
Why can one slow device degrade everyone's Wi-Fi?
What does mdev in ping output tell you, and why is it bigger on Wi-Fi?

End of Part 2 — Milestone check

I can explain what a MAC address is and what its first 3 bytes mean
I have watched an ARP request and reply in tcpdump
I can explain how a switch learns, and what it does with an unknown MAC
I know the difference between a collision domain and a broadcast domain
I know why a Layer 2 loop is dangerous
I have checked my own Wi-Fi signal and bitrate