Lighting up the Dark


Whilst there is a lot of fibre in the ground, much of it is in use and any spare unused fibre (also known as Dark Fibre) can be expensive to rent.  For owner/operators,  maximising the use of fibre is key to make the most of this ‘sunk asset.

Following on from my ramblings about WDM <> I’d like to introduce you to an extension concept for High Density CDWM or HD CWDM.  Before we get right into it, let’s first look at the concept of bi-direction transmission over fibre, or bi-di for short.

Generally, fibre connections use 2 fibres – one for transmitting (Tx) and one for receiving (Rx) data:

 Fibre connections

Both transmit and receive use the same single wavelength e.g. 850, 1310, 1550nm which are the typical ‘colours’ used for multimode and singlemode transmission over fibre.  These are also known as ‘grey’ optics as they are not coloured.

With bi-di, we use 2 different colours to transmit and receive on the same single fibre core:


Typically we use 1490/1310nm or 1550/1310nm.  In the former case this means that at end A we transmit at 1490 and receive at 1310, whilst at the other end, end B, we transmit at 1310 and receive at 1490.  You have to be careful to provision transceivers in pairs so this works correctly – it’s a bit like using a crossover cable.

So now if you combine the concepts of bi-di and CWDM, you can optimise CWDM for transmission over a single fibre, by using slightly offset colours for transmit and receive.  This way you can transmit and receive on all standard 18 CWDM wavelengths on the same single fibre.  We call this High-Density CWDM (HD CWDM)

This gives two advantages:

  •  Using a standard fibre-pair, you can now double the capacity of CWDM from 18 to 36 lines of simultaneous data.
  •  Where only a single fibre core is available, it is now possible to use multiple colours for parallel transmission.

Both of these mean that there is no longer the need to install additional fibres between endpoints which can be very costly.

Written By Anthony Clarkson, ProLabs


Contact ProLabs for more information about HD CWDM and other bi-di technologies.