9. Wi-Fi and other link layers

IP does not care what the link is, but the link's quirks leak upward as latency, loss, and jitter that TCP must cope with.

9.1 Wi-Fi (802.11) vs Ethernet — the differences that matter

Property	Switched Ethernet	Wi-Fi
Medium	Dedicated per port	Shared radio, half-duplex
Access method	None needed (full duplex)	CSMA/CA — listen, backoff, maybe RTS/CTS
Link-layer ACKs	No (FCS drop only)	Yes — every unicast frame is ACKed and retried at L2
Rate	Fixed, negotiated once	Adapts constantly (MCS) with signal quality
Loss	~0 on healthy cable	Bursty; interference, distance, hidden nodes
Latency	~microseconds, stable	1–50 ms, highly variable (airtime contention)
Addressing	2 MACs per frame	Often 3 or 4 (adds the AP/BSSID)

Key consequences for TCP:

Wi-Fi retries hide loss from TCP — good (fewer spurious congestion events) but retries add latency and jitter, which inflates RTT estimates and can cause bufferbloat in the AP's queue (Ch. 44).
Airtime is the real currency. One slow/distant client transmitting at 6 Mbit/s hogs airtime and starves fast clients ("performance anomaly").
Half-duplex + contention means measured "100 Mbit/s Wi-Fi" delivers ~50–60 Mbit/s of TCP goodput at best, less with many clients.
Roaming between APs drops frames briefly; long-lived TCP connections survive because of retransmission, but real-time media hiccups.

9.2 Other link layers you'll meet

PPP / PPPoE: point-to-point (DSL, some fiber, cellular). MTU 1492 for PPPoE. Carries authentication (PAP/CHAP) and IP assignment.
DOCSIS (cable): shared upstream, asymmetric, historically bufferbloat-prone until "PIE"/AQM mandates.
Cellular (LTE/5G): deep per-device buffers, RTT 20–80 ms that spikes on radio state transitions (idle→connected can add 100+ ms), NAT everywhere, frequent IP changes on handover. TCP sees this as latency and occasional connection death.
MPLS: a "Layer 2.5" label-switching transport carriers use internally; invisible to your IP packets except as one fast hop and possible ICMP oddities.
Loopback (lo): MTU 65536, no real link; used for host-local IPC over 127.0.0.1/::1.

9.3 War story

A video-call app was blamed for "freezing," but only in one conference room. The room's AP shared a channel with a 2.4 GHz wireless projector and a microwave kitchen next door. iw dev wlan0 station dump showed the client rate collapsing to 1 Mbit/s and 40% frame retries during freezes. Moving the AP to a clean 5 GHz channel fixed it. Lesson: on Wi-Fi, "the app froze" is often "the RF environment degraded"; measure PHY rate and retry percentage, not just ping.

Try it yourself

# Linux Wi-Fi diagnostics
iw dev wlan0 link                   # current BSSID, signal, tx bitrate
iw dev wlan0 station dump           # rx/tx bitrate, retries, failed, signal
watch -n1 'iw dev wlan0 link | grep -E "signal|bitrate"'
# macOS
/System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -I