A torch in the fog
Shine a torch down a long, foggy road. Near you the beam is bright. Far away it is faint, and at some point you cannot see it.
Light in a fibre fades too. Some of it is lost on the way. The cable team must know how much, and why.
Key words
| Word | What it means |
|---|---|
| Attenuation | the loss of signal strength as it travels |
| Decibel | the unit for loss, written dB |
| Absorption | light taken up by tiny particles in the glass and turned into heat |
| Scattering | light knocked off its path by tiny particles |
| Macrobend | a large, gentle-looking bend that lets light escape |
| Microbend | a tiny bend caused by pressure on the fibre |
| Media converter | a box that changes copper signals into light and back |
| Electro-optical conversion | changing an electrical signal into light, or light into an electrical signal |
| Analogue to digital conversion | turning a smoothly changing signal into bits, and back again |
| Synchronous | sent with a shared clock |
| Asynchronous | sent with start and stop bits and no shared clock |
The cells students fill in are left blank in this preview.
By the end of this lesson
Note
I can explain attenuation, read a decibel loss, and name the causes of loss in fibre.
Starter: light in fibre
Have a go
Starter: light in fibre
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
What attenuation is
Attenuation is the loss of signal strength over distance. It is measured in decibels, written dB. The more dB, the more loss.
The scale is not even. Every few dB removes a large share of the light, as the table shows.
Worked example: how much light is left
| Loss | Light left (out of every 100) | Lost |
|---|---|---|
| 0 dB | 100 | None |
| 3 dB | About 50 | About half |
| 10 dB | 10 | 90 out of every 100 |
| 20 dB | 1 | 99 out of every 100 |
The cells students fill in are left blank in this preview.
Check: attenuation and dB
Have a go
Check: attenuation and dB
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.