CWDM/DWDM Networks

Testing forms a critical part of construction, service activation, maintenance, and troubleshooting of CWDM, DWDM and hybrid networks. Advanced fiber characterization of DWDM networks ensures transmission is not impacted by abnormal losses, including ORL, CD, and PMD impairments. We have extensive testing experience using OTDR, OLTS and OSA/OCC test equipment including the post processing and analysis of test results.


xWDM Technology

Coarse and Dense wavelength division multiplexing (CWDM/DWDM) has been used in metro and core networks for well over two decades – more recently, their implementation has extended to access networks especially in mobile x-Haul and FTTx applications.


Different variations exist for enterprise, mobile, DAA and FTTx applications. Most implementations use a single pair of fibers - one fiber for transmit and the other for receive. The most popular channel configuration deployed consists of four to eight wavelengths, ranging from 1471 nm to 1611 nm, spaced 20 nm apart. However, as service providers experience a shortage fibers or “fiber-exhaust”, some networks now operate in the lower O-band spectrum as well, increasing  capacity up to 16 wavelengths. Some systems using specialized fiber, can also operate over the 1371 nm and 1391 nm water peak wavelength region.

Due to spectral attenuation, fiber loss versus wavelength over the complete CWDM spectrum is very significant – this includes some components. While CWDM is an effective technology to increase bandwidth, it also presents several fiber characterization and deployment challenges confusing fiber novices.


Initially developed for core network applications operating in the C-band (1525 nm to 1565 nm) spectrum with the ITU-T G.694.1 recommendation specifying up to 80 channels at 100 GHz spacing (0.8 nm). While 100 GHz channel spacing is still the most popular today, many systems now support 50 GHz (0.4 nm) spacing, increasing channel capacity considerably. DWDM is no longer reserved for core networks only – it is now widely implemented across utility, cable, mobile, and telecom service provider networks with applications extending to 5G mobile fronthaul, Distributed Access Architecture (DAA) for MSO business services, and next-generation FTTx networks.

Testing DWDM networks is far more complex than CWDM based or traditional networks. DWDM channels are spaced very closely together, so wavelength precision is of utmost importance - filters must pass selected channels while rejecting others. Most testing is performed in-service alongside other live traffic, so care must be taken not to disrupt other channels or subscribers. Identification of wavelength-selective components such as muxes and demuxes along with their corresponding losses also requires more precise characterization.

In Summary

Testing and analyzing fibers in a CWDM, DWDM or hybrid architecture can be a daunting exercise. DWDM and CWDM OTDRs are an invaluable tool to verify end-to-end link continuity, including characterization of wavelength-selective components without disrupting adjacent channels. Due to its single ended test capability, troubleshooting newly deployed passive CWDM, DWDM and Hybrid networks also requires less labor resources and is thus more cost effective. 


Spectral attenuation (for CWDM systems) including Mux and DeMux losses however impact an OTDR's test parameters and performance analysis capability. Despite many specialized OTDRs being equipped with multi-pulse acquisition and intelligent event detection algorithms, passive devices like Muxes and filters often go undetected or unidentified – this can be a nightmare for an inexperienced fiber technician tasked to characterize or troubleshoot a CWDM or DWDM enabled network.


Contact us to learn more about specialized DWDM and CWDM testing services and how to save, analyze and report your valuable test data.