Media Lab

Data Communication, Lecture 4. Bend light inside a fibre, budget the losses of a cable, and explore the radio spectrum.

Core (bottom) and cladding (top) · ray hits the boundary

Refract or reflect?

Critical angle Refracted angle θ₂ Numerical aperture

Snell: n₁ sin θ₁ = n₂ sin θ₂. Above the critical angle no refracted ray exists, so all the light reflects back into the core.

Received power along the link · dashed line = receiver sensitivity

Will the signal arrive strong enough?

Total loss Received power Margin Repeaters needed

Try 0.1 km on copper, then 0.1 km on fibre. Then stretch the fibre to 200 km.

Radio spectrum, 3 kHz to 300 GHz (logarithmic scale)

What lives at this frequency?

Band Propagation Wavelength λ λ/4 antenna

Low frequencies travel far along the ground but carry little data. High frequencies carry a lot of data but need a clear line of sight.