Transmitter

Feeder

Antenna

Free space

Obstruction

Receiver

The Air Interface · Explainer

What Actually Travels Between a Handset and a Mast

A carrier wave, deliberately disturbed. Follow it from the power amplifier to the far receiver and every design decision in the network turns out to be an argument about decibels.

By Spectrum Notes · The Air Interface · 12 min read

Along the link

01Modulation, PlainlyHow information is written onto a carrier, and why the scheme matters.
02What an Antenna Is DoingGain, pattern and polarisation without the mathematics.
03Noise, Interference and the FloorWhy the limit is rarely power and usually noise.
04Why Physics Changes With FrequencyLow bands travel, high bands carry. The trade in one page.

What Actually Travels Between a Handset and a Mast — Figure 1 — The transmitting end. Everything after this point is subtraction.

Interactive · signature

The Link Budget, Step by Step

Add the gains, subtract the losses, and see whether anything is left above the receiver's noise floor. Illustrative figures — the shape of the argument is what matters.

StageContribution · running level

Transmitter output

Power leaving the amplifier.

+43.0 dB43.0 dBm

Feeder loss

Cable between amplifier and antenna.

-2.0 dB41.0 dBm

Antenna gain

Energy focused into a useful pattern.

+17.0 dB58.0 dBm

Free-space path loss

The dominant term; grows with distance and frequency.

-103.6 dB-45.6 dBm

Obstruction and fading

Buildings, terrain and the margin you keep for a bad day.

-12.0 dB-57.6 dBm

Receive antenna gain

A handset antenna is small and barely directional.

+2.0 dB-55.6 dBm

Received power

What actually arrives — compare against the noise floor.

-55.6 dBmvs -100.0

Distance to receiver

4.0 km

200 m20 km

Carrier frequency

MHz · currently 900 MHz

Margin over sensitivity

44.4 dB

Positive means the receiver can hear it. Negative means the link does not close, and no amount of protocol cleverness above will fix it.

EIRP 58.0 dBm · path loss 103.6 dB · sensitivity assumed −100 dBm

The band chart

Where the physics changes, and what lives there

Interactive · signature

Scrub From Shortwave to Millimetre Wave

Choosing a frequency is choosing a physical size — of the antenna that radiates it and of the obstacle it can bend around. Drag the cursor and watch both change.

Spectrum ruler — logarithmic, 3 MHz to 300 GHzDrag or use arrow keys

HF3–30 MHz

VHF30–300 MHz

UHF300 MHz–3 GHz

SHF

SHF+

mmWave

Frequency: 900 MHz
Wavelength λ: 33 cm
Quarter-wave element: 8.3 cm
Band: UHF

300 MHz–3 GHz — The workhorse; balance of range, capacity and antenna size.

HF3–30 MHzBounces off the ionosphere; long range, tiny capacity.
VHF30–300 MHzGood ground coverage; classic broadcast and land mobile.
UHF300 MHz–3 GHzThe workhorse; balance of range, capacity and antenna size.
Low SHF3–6 GHzMore bandwidth, shorter reach, needs denser sites.
Millimetre wave24–100 GHzEnormous capacity, blocked by almost anything.

Figure 2 — Band edges are drawn on a logarithmic axis; each decade is one fifth of the ruler. Why physics changes with frequency →

Interactive · signature

The Protocol Stack, Layer by Layer

Agreements stacked on top of each other, each pretending the layers below it are simple. Pick a layer to see what it owns.

Toward the user↑

L5ApplicationWhat the user actually wanted to do. L4TransportEnd-to-end delivery, ordering and flow control. L3NetworkAddressing and getting a packet across networks. L2Data linkFraming, error control and access to the shared medium. L1PhysicalTurning bits into a waveform and back.

↓Toward the antenna

Layer detailClick a layer

L5 · Application

Owns

What the user actually wanted to do.

In practice

It defines what "working" means: a voice call tolerates loss but not delay, a file transfer the other way round. Everything below is in service of this layer, and knows nothing about it.

L4 · Transport

Owns

End-to-end delivery, ordering and flow control.

In practice

It hides the fact that the path is unreliable, re-asking for what went missing and slowing down when the network pushes back — which is why a lossy radio link often feels like a slow one.

L3 · Network

Owns

Addressing and getting a packet across networks.

In practice

Here the device stops being a radio terminal and becomes an ordinary host. Mobility lives at this boundary: keep the address stable and the session survives the move.

L2 · Data link

Owns

Framing, error control and access to the shared medium.

In practice

It decides who transmits and when, cuts payloads to fit what was granted, and asks again for anything that arrived damaged. Reliability is bought here, and it is paid for in time.

L1 · Physical

Owns

Turning bits into a waveform and back.

In practice

The only layer that touches the outside world. It spends coding and modulation as a bet about the channel; every layer above inherits the errors it fails to prevent.

Full walkthrough: The Protocol Stack, Layer by Layer →

What each generation changed

Engineering changes, not marketing steps

Table 1 — Described as engineering changes. Where a figure would vary by deployment, the principle is given instead.
Generation	What it changed	Access method	How voice is carried
1G	Analogue voice, cellular reuse	One call per channel, divided by frequency	Analogue, circuit-switched
2G	Digital voice, messaging, encryption	Time slots, or spreading codes, depending on the family	Digital, still circuit-switched
3G	Packet data as a first-class citizen	Wideband spreading codes over a shared carrier	Circuit-switched in most deployments
4G	All-IP, broadband to the handset	Orthogonal subcarriers, scheduled in time and frequency	Packets, like everything else
5G	Wider bandwidths, lower latency, network slicing	Scalable orthogonal subcarriers, with beamforming assumed	Packets, with separated service treatment