Layer 1 — Physical

What This Layer Does

The Physical layer moves raw bits from one device to another. That is its entire job.

A bit is a single one or zero. Everything a computer sends — text, photos, video, a bank transfer — is ultimately a long sequence of bits. The Physical layer is where those bits stop being an idea inside a computer and become something real in the world: voltage on a copper wire, pulses of light in a glass fiber, or radio waves through the air.

The layer defines the unglamorous, physical facts of the connection. What the cable is made of. What shape the plug is. How strong the signal should be. How fast a one can change into a zero and still be recognized at the far end. Two devices that disagree on any of this cannot communicate at all, no matter how well configured everything above them is.

Notice what the Physical layer does not do. It does not know what the bits mean. It does not know who sent them or where they are going. It does not check whether they arrived correctly. It carries them, and that is all.

Common Examples

• Twisted-pair copper cable with RJ-45 connectors, used for wired Ethernet
• Fiber-optic cable, which carries bits as pulses of light
• Wi-Fi radio transmission
• Network interface cards, wired and wireless
• Repeaters and hubs, which strengthen or copy signals and nothing more

Hubs are worth a moment. A hub copies every incoming signal to every other port without looking at it, because looking at it would require understanding addresses, which is a Layer 2 job. Hubs have been replaced almost everywhere by switches. They survive in this book because they are the purest example of a device that does Layer 1 and nothing else.

Simple Real-World Example

A laptop is connected to a home wireless network. When the laptop sends data, its wireless adapter converts the bits into radio waves at a specific frequency and transmits them. The router's antenna receives those radio waves and converts them back into bits.

The radio waves themselves carry no idea of what they contain. They could be part of a video call or part of a software update. At this layer that distinction does not exist yet.

   Laptop                                    Router
  ┌────────┐    1 0 1 1 0 0 1 0 ~~~~~~~~~  ┌────────┐
  │  bits  │ ─────────────────────────────>│  bits  │
  └────────┘     radio waves in the air     └────────┘

What Can Go Wrong?

• The cable is unplugged. The most common network fault in the world, and the first thing an experienced person checks.
• The cable is damaged. A crushed, bent, or badly terminated cable can pass some bits and corrupt others, which produces slow, erratic behavior rather than a clean failure.
• The Wi-Fi signal is weak. Distance, walls, and interference from other devices all weaken radio signals. The connection may appear to exist while barely working.
• The network adapter is disabled. Software can switch the hardware off. Airplane mode is the friendly version of this fault.

Every one of these produces the same symptom for the user: nothing works. That is why Layer 1 is checked first.

Remember This

Layer 1 moves bits using cables, light, electricity, or radio waves.