A budget for light
You have a fixed pocket of money for the week. Each thing you buy takes some. If you spend more than you have, you are stuck.
A fibre link has a fixed amount of light it can lose. Fibre, joins and plugs each use a little. Add them up and check you are still in budget.
Key words
| Word | What it means |
|---|---|
| Loss budget | the most loss a link may have and still work |
| Attenuation | the loss of signal strength as it travels |
| Decibel | the unit for loss, written dB |
| Connector pair | two plugs joined together at a patch panel or outlet |
| Splice | a permanent join of two fibres |
| Margin | the spare loss left over in the budget |
| Datasheet | the maker's page of facts about a cable or device |
The cells students fill in are left blank in this preview.
By the end of this lesson
Note
I can add up whole-number losses on a fibre link and say whether it is within its budget.
Starter: decibels
Have a go
Starter: decibels
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
What takes light on a link
Three things use up the budget. The fibre itself loses a little each metre. Every connector pair loses some where two plugs meet. Every splice loses a little where two fibres join.
Add them all. Then compare the total with what the equipment allows.
Teaching figures only
Important
Here each connector pair and each splice costs 1 dB, and the fibre loss is given. Real links use the maker's datasheet and the standard, with decimals. Rounding up like this is the safe way to learn.
Worked example: one link
| Item | How many | Loss each | Loss |
|---|---|---|---|
| Fibre run (given) | 1 | 3 dB | 3 dB |
| Connector pairs | 2 | 1 dB | 2 dB |
| Splices | 1 | 1 dB | 1 dB |
| Total | 6 dB | ||
| Allowed by the equipment | 8 dB | ||
| Margin | 2 dB |
The cells students fill in are left blank in this preview.