The Humble Fibre Cable

There are some technological innovations that have transformed the modern digital world, MOSFET Transistor’s (silicon chips) is probably the top-ranking in the last 50-60 years. But one that ranks just as high is the fiber optic cable and WDM (Wavelength Division Multiplexing) which enables a single fiber pair cable to transmit terabits of data per second and responsible for the modern cloud technologies we use today.

So how does a single fiber pair transmit such high speeds? Lasers, at each end of the cable is light emitting laser transmitters and receivers, that light is set at a wavelength measured in nanometers. Single mode fibers support wavelengths 1250nm to 1610nm, this means we can potentially use any wavelength between these two numbers, this is the world of WDM which essentially allows you to increase the capacity of existing fibre infrastructure with the use of “colored” optics working at different wavelengths on a single fibre pair. This can save serious money if existing fibre capacity is near full, as fibre infrastructure installations are expensive, especially if civil (trenching, digging etc.) work is required.

Essentially WDM has 2 parts, the MUX’s units at each end of the fibre pair, the MUX unit has ports for each wavelength and a line port for the fibre between the 2 MUX’s. The second part is the SFP’s which transmit the wavelengths, plug the corresponding SFP color to the corresponding MUX color and that’s it, simple.

Below picture is of a SFP 1G CWDM at 1530nm to 100km’s and 18 port MUX. Note, color green is 1530nm on both SFP and MUX unit.

The above example is using CWDM (Coarse Wavelength Division Multiplexing) which is limited to 18 channels between 1250nm to 1610nm, gapping of 20nm between channels, so in theory using 10Gb SFP+’s 1 single fibre can transmit 200Gb/s, not bad but 400Gb/s single SFP’s are now out. Then there is DWDM (Dense Wavelength Division Multiplexing) which is essentially multiplexing within CWDM, so within the 1550 channel there are sub wavelengths 1547.72, 1548.51 etc. A single pair fibre using DWDM can transit in the 10's of terabits' per fibre and constantly increasing especially with 400Gb/s DWDM optics on the horizon.

Should I use CWDM or DWDM?

CWDM SFP’s and MUXs (passive units) are lower cost and simple to deploy but limited to 10Gb’s SFP+ optics, so good for campus or metro deployments.

Whereas DWDM can be complex, very expensive and support 100Gb/s optics, so mainly used by large telco’s, eg Chorus, Enable, Vodafone, over large distances for backhaul as the signal can be amplified.