Coarse Wave Division Multiplexing (CWDM)

Fibre Cables and CWDM

The key consideration when deploying CWDM are the characteristics of the optical fibres. Older single mode fibre (ITU standard  G652A and B) does not have a constant loss budget - across the "usable" wavelength range of 1250-1650 nanometres, there is increased loss/kilometre at either end, and a significant spike around 1383nm (the water peak).  The more recent G652.C and D standards define low water-peak fibre, which has a more constant loss profile across the 1250-1650nm spectrum.

While Citylink has specified low water-peak fibre for new installations for many years, there is still a significant amount of G652A cable in the network.  The Citylink fibre network is a mesh of short runs of fibre - any given FibreLINK service across the city could pass through several different cables, and it is likely that even a recently commmisioned FibreLINK circuits will include lengths of both normal and low water peak fibres.

  * Diagram courtesy of the ITU

FibreLINK Service Specification

The Citylink FibreLINK service specification offers guaranteed performance of ≤ 0.40 dB/km loss at 1310nm and ≤ 0.30 dB/km at 1550nm - it does not offer performance guarantees at intervening wavelengths.  So how well a given FibreLINK will carry the complete 18 CWDM wavelengths (as specified in ITU G.694.2) will require direct measurement to determine the unusable frequencies other than 1310nm and 1550nm.

Factors to Consider When Deploying CWDM

For those considering CWDM, CityLink’s FibreLINK service is only specified for operation at 1310nm and 1550nm. However we do offer the following advice:

1. Consider using only the top eight wavelengths (1471nm-1611nm) for CWDM, and keep a 1310nm wideband pass through channel in the bottom of the range.  That will allow you to run existing legacy non-CWDM equipment (such as a 10GE channel), and will avoid the worst of the 1383nm water peak.
2. If you are using the wavelengths near the water peak, then you may need to invest in higher powered optics.  As 40Km optics aren't a great deal more expensive than 20Km optics, this may not be too much of a issue - make sure you have an arrangement with your vendor that allows you to try higher powered optics if the standard optics don't work.