The Physical Layer & Signals

September 23, 2026 • 10 min read

Table of contents

A bit is not a number, it is a signal
Guided media: signals inside something
Unguided media: signals through the air
Two numbers that decide everything
Modulation: putting bits on a wave
Attenuation, noise, and the end of the road
The big picture

The TCP/IP post ended with a promise to drop to layer one. So here we are at the bottom: no addresses, no packets, no protocols, no error handling, no idea what the bits mean. A voltage is high or low, a light pulses or does not, a radio wave is at one frequency or another. Everything the internet is, down here is just physics, and physics has rules. This post opens them: what a bit becomes on a wire, what the media can and cannot carry, the two numbers that describe a road, and why moving a single bit is harder than it looks.

A bit is not a number, it is a signal

A bit is an abstraction. On the wire it is never a number, only a physical difference that the receiver has to interpret.

logical bit:      1        0        1        1        0
on copper:       +2.5V   -2.5V   +2.5V    +2.5V   -2.5V
on fiber:         light    dark    light    light    dark
on radio:         shift   no shift  shift   shift   no shift

Two machines agree beforehand on what counts as a 1. That agreement is the only reason the same bytes mean the same thing everywhere. Change the encoding and the bytes change meaning, which is why older Ethernet standards had to stop transmitting preamble bytes when they changed the signaling scheme: to old receivers those bytes would have looked like data.

A bit on the wire is a promise that the receiver will read it back the same way. Everything above layer one is bookkeeping on top of that promise.

The crucial thing to internalize: a bit is analog. The voltage drifts, the light dims, the wave picks up noise. The receiver does not read a bit; it decides, with a threshold and a guess, that a bit was there. It sometimes guesses wrong. Layer one offers no guarantee whatsoever, and that is exactly why layer two exists.

Guided media: signals inside something

Guided media¹ carry signals inside a physical path: the signal has nowhere to go but forward.

Twisted pair² is the reason your house has ethernet ports. Two insulated copper wires carry opposite signals and twist around each other. The twist looks decorative and is not: each twist makes the interference each wire receives at a slightly different time, so the noise partially cancels out, the same idea as noise-canceling headphones. Copper categories differ mainly in how tightly the pair is twisted and how well shielded the cable is, which is why “Cat 6” is not just marketing.

Limits come from physics, not standards:

Distance: the signal weakens with every meter, and eventually the receiver cannot tell a 1 from a 0. That is why copper runs stop at 100 meters.
Crosstalk: a fast-changing signal leaks into neighboring pairs. Better twisting and shielding reduce it; higher frequencies leak more, which is why high-speed standards are fussier about cable quality.
Interference: motors, fluorescent lights, and other cables all add noise.

Coaxial cable³ puts one conductor inside another with insulation between them. The shielding makes it far less noisy than twisted pair, which is why it survived for cable television and early Ethernet, and why it still dominates long cable runs in some countries.

Fiber-optic cable⁴ carries light through a glass strand thinner than a human hair. The interesting part is why the light stays inside the fiber at all. The core and the cladding have slightly different refractive indices, and light hitting the boundary at a steep angle reflects back inside, a phenomenon called total internal reflection⁵. The fiber acts as a light pipe, bouncing light along internally.

copper:  electricity  →  needs a wire, limited by noise and distance
fiber:   light        →  no electricity in the path, immune to
                         electrical interference, tiny loss over
                         kilometers

Fiber’s practical consequence is scale. It carries data for tens of kilometers before any regeneration, uses far less power than electrical repeaters, and supports wavelengths beyond visible light so many independent channels share one strand, each carrying more than a hundred gigabits per second. This is why undersea cables carry almost all intercontinental traffic, and why “the cloud” has a physical shape: long glass lines drawn across ocean floors, landing in a handful of coastal buildings.

Real example: between two cities, a fiber backbone may carry hundreds of thousands of concurrent video calls, while the copper from your home to your provider’s cabinet may manage a few hundred megabits. Same internet, different material, different physics.

Unguided media: signals through the air

Unguided media⁶ have no path. The signal radiates and arrives wherever it arrives, which buys mobility and costs control.

Everything wireless is a slice of the radio spectrum⁷, a limited range of frequencies handed out by regulators because radio waves from different users interfere if they overlap. Within a slice, the trade is always the same: higher frequency carries more data but travels less far and is stopped by walls more easily.

Wi-Fi⁸: short range, high speed, walls are enemies. It powers local networks.
Cellular: the same idea in cells that hand phones from tower to tower as they move, tuned by frequency: low bands reach far, high bands go fast and stop at the next building.
Microwave and satellite: line-of-sight beams and orbital relays for distances where cable is impossible. A satellite link is famous for a property that will appear in the next post: about 300,000 km round trip, and a speed of light that refuses to negotiate.

Wireless is not “slower internet”. It is the same bits through air, and air is a harder medium than glass.

Two numbers that decide everything

Two words describe a road, and confusing them makes conversations about internet speed pointless.

Bandwidth⁹ is how much data fits per second, measured in bits per second or multiples of it. It is the width of the pipe.

Latency¹⁰ is how long one piece of data takes to arrive, measured in milliseconds. It is the length of the pipe.

a fat, short pipe      → fast throughput, quick answers
a thin, long pipe      → slow throughput, slow answers
a fat, long pipe       → fast throughput, slow answers
                        (this is the common real-world surprise)

Fiber and long distances are why the first two rows are not the two things you get. Seattle to São Paulo is one of the shortest intercontinental routes and still costs about 120 ms each way, and no amount of bandwidth improves it, because the light is already taking as long as it takes. That delay is why video calls feel awkward and why players on other continents are tough.

The two combine into a number that decides how much you can have “in flight” at once. The bandwidth-delay product¹¹ is bandwidth times round-trip time, and it is the amount of data that fits in the pipe between a request and its answer.

100 Mbps × 100 ms = 10 Mbit ≈ 1.25 MB in flight at once

that is why throughput feels capped when latency is high,
even on a very fast connection

Modulation: putting bits on a wave

Sending a stream of 1s and 0s is one option. Real links do something more efficient: they let bits change properties of a wave, a technique called modulation¹².

amplitude:  change the strength of the wave
frequency:  change how fast it oscillates
phase:      shift where the wave starts

Early schemes changed one property per bit: on-off, a shift in frequency, a shift in phase. QAM, used everywhere from cable modems to Wi-Fi, changes amplitude and phase together, so one symbol carries several bits. The efficiency is not free: denser constellations need cleaner signal, so higher-order modulation works close to the transmitter and falls apart with noise and distance.

Digital links have a second problem: the receiver must know where each bit starts. A long run of identical bits gives nothing to synchronize against. Line encodings solve it by adding structure: Manchester encoding¹³ guarantees a transition in every bit period, using it to mean both the value and the clock.

NRZ:        1 1 1 1 0 0 0        ← long runs, no way to resync
Manchester: ↑↓ ↑↓ ↑↓ ↑↓   ↑↓      ← a transition every bit

Layer one is the only layer where physics dictates the vocabulary. Everything above can be renamed by a committee. Voltages and frequencies cannot.

Attenuation, noise, and the end of the road

Every signal on every medium gets worse as it travels, through two named effects.

Attenuation¹⁴ is loss of strength with distance: the wave simply weakens. Noise¹⁵ is anything else added along the way that was not part of the message, from electrical interference to thermal jitter to another radio station.

Both are cumulative, which is why physical limits exist and why cables are sold in categories. Three techniques push the limit rather than eliminate it:

Amplification boosts the signal, and also boosts the noise along with it. It works a few times, then the noise dominates.
Regeneration (repeaters, and later regenerators) reads the signal, decides what it was, and transmits a clean fresh copy. This is the only technique that truly resets the damage, and it is why copper links have repeaters every hundred meters and fiber has none for tens of kilometers.
Coding and equalization spend extra bits to undo predictable damage. This is why a modern copper or radio link can move more data through a worse channel than an old one did.

The big picture

A bit is a signal, not a number: a voltage, a pulse of light, or a shift in a wave, all of them analog and all of them read by a receiver making a threshold guess. Guided media (twisted pair, coaxial, fiber) move that signal inside a path, limited by attenuation, crosstalk, and distance; unguided media (radio, microwaves, satellite) radiate it, trading reach for speed. Bandwidth and latency are different numbers, and a fat long pipe gives you both a fast download and slow answers. Modulation puts many bits in each symbol, line encodings keep the receiver in sync, and regeneration is the only thing that truly undoes accumulated damage.

sender: bits → symbols → modulation → signal
                                    ↓  copper · glass · air
receiver: signal → threshold decision → bits
                    (sometimes wrong)

Nothing down here knows what your data is. The next post adds the first thing that does: addresses for the machine on the other end of this cable, and the switch that delivers a frame to exactly that machine.

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