Fibre Channel Physical Layer
When using multimode optical fibre we use a shortwave optical transmitter.
Multimode fibre means we don’t have a pure or clean transmit source. We are
going to get some modal dispersion.
In the first 2m of cable we get a lot of noise and distortion. Minimum
recommended distance between transmitter and receiver is 2m in multimode.
Single mode fibre is 9 micron. When using single mode optical fibre we use a
longwave optical transmitter that is much more focused and is used for very
There is more 9 micron fibre used in the world than the other sizes because it
is used for the telecom backbone, transatlantic fibres, etc.
You will sometimes see on a cable a reference of 62.5/125, 50/125 or 9/125.
The 125 is the size of the fibre cladding.
We are interested in the size of the glass and then the cladding which is to
protect it. The cladding stops it from falling apart and stops the light escaping.
Orange cables always used to be used for multimode fibre.
Yellow was always used for single mode.
Now, any fibre comes in any colour.
Multimode is used because it is a lower cost.
The connector is cheaper, the transmitter is cheaper, the receiver is cheaper.
With modal dispersion, when we transmit anything that goes down the centre
of the fibre, it will go slightly faster than anything that goes round the edges.
This is why we talk about OM2 (Optimal Multimode Grade 2) which is a graded
index fibre. So the centre of the fibre is slightly more dense than the outer
edges so anything that goes directly down the centre will be slowed down a
little bit more than anything going round the edges. This is how the distance
can be increased without impacting on the delivery.
We can also go up to OM3 to increase distance even further but this is