Card 01/ 09
The Physical Layer: Bits on Copper, Glass and Radio
Every network ends in a signal on a medium, and the medium sets speed, distance and noise.
The physical layer, layer 1 of the OSI model, turns bits into signals and back: voltage on a copper cable, pulses of light in a glass fibre, or radio waves through the air. It decides how fast bits can move and how far, and it knows nothing about addresses or about what the bits mean.
Picture a new office where video calls stutter in one meeting room and nowhere else. The room is wired with an old cable that negotiates 100 megabits per second instead of a gigabit, or it sits at the edge of a Wi-Fi access point's range. Nothing about the call, the addresses or the server is wrong. The fault is in how bits are carried, and no setting higher in the stack can fix it.
A bit is a single one or zero, and a byte is eight bits. Speeds are quoted in bits per second, so a gigabit link carries about 125 megabytes per second before any headers are counted.
| Medium | Typical speed | Typical reach | Degraded by |
|---|---|---|---|
| Copper twisted pair (Cat 6) | 1 to 10 Gbit/s | Up to 100 m | Electrical noise, long runs, damaged cable |
| Multimode fibre | 10 to 100 Gbit/s | Up to a few hundred metres | Dirty connectors, tight bends |
| Single-mode fibre | 10 to 400 Gbit/s | Kilometres to tens of kilometres | Dirty connectors, tight bends |
| Wi-Fi | Tens of Mbit/s to a few Gbit/s, shared | Tens of metres indoors | Walls, distance, other networks on the same channel |
The figures are typical rather than limits, and the exact numbers depend on the standard and the equipment at both ends. Two points hold everywhere. First, the medium caps the speed: no tuning higher in the stack makes a 100 megabit link carry a gigabit. Second, Wi-Fi is shared: everyone on one channel takes turns, so its speed is divided between them, while a switched cable gives each device its own link.
The physical layer is not where addresses live. A cable carries every bit put on it to the far end, and a radio signal reaches every device in range; deciding which device the data is for is the job of the layer above.
Cables, connectors, network cards and the radios in access points all belong here, along with the hub, an old device that repeated every signal out of every port.
So when one place, one cable or one distance is slow and everything else is fine, suspect the physical layer first: check the link speed the card reports and the signal strength before looking at anything higher.
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