Responsibilities

The physical layer's mission is simple to state — move raw bits across a medium — but doing that reliably means agreeing on a surprising number of details. Before two devices can exchange even one bit, they must agree on how a bit looks as a signal, how fast bits arrive, how to stay in time with each other, how they're wired together, and which direction data can flow. Those agreements are the physical layer's responsibilities, and we'll take them one at a time.

A bit is just an idea — a 0 or a 1. To send it, the physical layer must give it a physical form: a specific voltage on a wire, a pulse of light, or a pattern in a radio wave. Both sides have to agree on the rule, for example "higher voltage means 1, lower voltage means 0." Turning bits into signals like this is called encoding, and it's the first thing the physical layer does.

The data rate (also called the transmission rate) is how many bits the physical layer pushes onto the medium each second, measured in bits per second (bps). A higher data rate means more bits sent per second, which usually means faster transfers. The physical layer, together with the medium, sets the ceiling on how fast data can go — and there are precise terms for measuring it, covered in Bandwidth, Bit Rate, Baud Rate.

Signals arrive as a continuous stream, so the receiver has to know exactly when one bit ends and the next begins. If its timing drifts even slightly, it could read the same bit twice or miss one entirely. Bit synchronization is the physical layer keeping the sender's and receiver's clocks lined up, so the receiver samples the signal at the right instant for every bit.

The physical layer also concerns itself with physical topology — the actual arrangement of how devices are wired to one another. Are they all plugged into one central device? Strung along a single cable? Connected in a loop? The most common layout in homes and offices is the star, where every device connects to a central hub or switch. If you want the full tour of layouts, see Network Topology.

Finally, the physical layer defines which way data can travel between two devices. There are three modes. In simplex, data flows one way only, like a keyboard sending keystrokes to a computer. In half-duplex, both sides can send, but only one at a time — like a walkie-talkie where you take turns. In full-duplex, both sides can send and receive at the same time, like a phone call where you can both talk at once. Here's how they compare: