hop 3 of 32 · 4 min
Wires, and how they are arranged
The physical layer moves bits. Everything above it depends on how well, and on how the wires are arranged.
Bandwidth is not latency
Bandwidth is how much per second. Latency is how long one bit takes.
Light in fibre covers about 200,000 km per second. Coimbatore to Mumbai and back is roughly 2,600 km, so about 13 milliseconds before anything else happens. To California and back is about 130.
Upgrading your connection raises bandwidth and does nothing at all to latency. That is why a content delivery network exists: you cannot make the distance shorter, so you move the content closer.
Topologies
| Topology | One cable cut | Cables for n machines |
|---|---|---|
| Bus | Usually breaks everything | n |
| Ring | Breaks the loop | n |
| Star | Isolates one machine | n |
| Mesh | Survives easily | n(n-1)/2 |
Every office uses star: one cable per machine to a central switch. The switch is a single point of failure and that is the accepted trade, because 4,950 cables for 100 machines is not a trade anyone makes.
Multiplexing
One medium, many conversations:
- Time division: each gets the whole channel for a slice of time.
- Frequency division: each gets a band of frequencies for all the time. This is how radio stations coexist.
- Wavelength division: the same idea in fibre, using colours of light.